Note: Descriptions are shown in the official language in which they were submitted.
CA 02399257 2002-08-22
-1-
TITLE: A DATA ACQUISITION SYSTEM
BACKGROUND TO THE INVENTION
The present invention relates to a data acquisition system.
The invention has been developed primarily for induced polarisation (IP)
geophysical surveys utilising controlled sources and natural sources for
identifying
metalliferous regions within a geological body, and will be described herein
after with
reference to that application. However, it will be appreciated that the
invention is not
limited to this particular field of use and is also applicable to other
geological,
geophysical, geotechnical, environmental and other surveys.
DISCUSSION OF THE PRIOR ART
The purpose of exploration geophysics is to produce images of sub-surface
physical properties. The value of the images and other analysis produced and
manifested
is dictated by the available spatial resolution and the property accuracy at
all points.
While this information can also be used to surmise or interpret chemistry and
rock type,
this is of secondary importance since such interpretations will be limited to
the accuracy
of the physical property image.
Particularly in the field of mineral exploration it has been known to gather
data
indicative of the geophysical properties of a body of earth by injecting a
large square
wave current or sinusoidal wave into the surface of that body and measuring
one of a
number of parameters. It has also been known to measure the parameters with a
plurality
of interconnected transducers disposed at a respective plurality of spaced
apart locations,
usually in a linear array, across the surface of the body. Given the size of
the bodies
under investigation it is easily appreciated that the logistic of setting up
for and
performing a survey are immense and, accordingly, expensive both in terms of
capital
and labour costs.
Most existing systems are prone to error due to noise levels inherent in the
mode
of measurement that occurs. This error is sometimes such that it renders
useless the
entire data set gathered in respect of a survey of a given body. However, due
to the mode
of data capture and the quantity of data gathered, the processing of that data
does not
occur for some time and sometimes not until the survey hardware has been
collected and
moved to another location.
CA 02399257 2002-08-22
-2-
In answer to this limitation, the prior art systems gather large amounts of
data
over a large time period. This, however, is even more time consuming and
results in
even greater delays before any results can be obtained due to the greater
amount of data
processing that is required. It also has the effect of increasing the cost of
the hardware
involved. In some cases the noise is so extreme that little or no useful data
can be
collected.
The present applicant developed a geological data acquisition system that
utilises
synchronisation between an input current signal that is injected into the body
and the
measured output signals to provide for better noise cancellation properties.
Accordingly,
the risk of the data being corrupted is reduced. This system is described in
Australian
Patent Application no 58341/99, the disclosure in which is incorporated herein
by way of
cross-reference.
It has also been known to utilise a psuedo-remote referencing system for
geological surveys. That is, the survey data is collected from the body under
investigation, in accordance with existing methods, to provide an initial
survey result. In
addition, one or more sensors are disposed adjacent to, but theoretically not
part of, the
array associated with the body. The data gained from these additional sensors
is obtained
in synchronism with the other measurements to allow the calculation of a
correction
factor that is applied to the initial result. These sensors are often located,
albeit
unknowingly, too close to the body under investigation and the correction made
is
corrupted due to the presence of the input current signal in what was intended
to be
remotely collected data. Therefore, prior systems of this kind are notoriously
unreliable,
are of limited effectiveness and the validity of the calculated correction is
highly
questionable. As a result, this form of survey is rarely used.
Moreover, any source of noise does not necessarily give rise to a simultaneous
effect at the local and the remote measurement sites. Accordingly, the
correction referred
to above, when applied to the results, often adds to the noise rather than
reducing it.
Any discussion of the prior art throughout the specification should in no way
be
considered as an admission that such prior art is widely known or forms part
of common
general knowledge in the field.
CA 02399257 2002-08-22
-3-
DISCLOSURE OF THE INVENTION
It is an object of the present invention to overcome or ameliorate at least
one of
the disadvantages of the prior art, or to provide a useful alternative.
According to a first aspect of the invention there is provided an induced
polarisation (lP) data acquisition system, the system including:
a first plurality of measurement nodes for obtaining synchronised measurement
signals indicative of the electric field strength at a corresponding plurality
of spaced apart
locations at or adjacent to a surface of a first geological body;
a reference node for obtaining reference signals indicative of the magnetic
field .
strength at a location at or adjacent to a surface of a second geological body
that is spaced
apart from the first geological body, wherein the reference signals are
synchronised with the
measurement signals; and
a processing centre being responsive to the measurement signals and the
reference
signals for deriving a transfer function for the first body.
1 S Preferably, the transfer function is proportional to the measurement
signals and
inversely proportional to the reference signals. More preferably, the
measurement signals
are taken along a substantially horizontal first axis and the reference
signals are taken
along a substantially horizontal second axis, where the first axis is normal
to the second
axis. Even more preferably, the transfer function is frequency dependent and
the
measurement signals are representative of electric field strength and the
reference signals
are representative of magnetic field strength.
Preferably also, the transfer function is used to determine a predicted noise
signal
at the local site. More preferably, the predicted noise signal is determined
by applying
the transfer function to measurement signals and reference signals that are
subsequently
obtained in the presence of a current signal in the first geological body,
where the
predicted noise signal is subtracted from those subsequent measurement signals
to
provide corrected measurement signals. That is, there are two sets of
measurement
signals and two corresponding sets of reference signals. The first sets are
used to
determine the transfer function, and these are collected in the absence of a
current signal
in the first body. The second set, on the other hand, are obtained later an in
the presence
of the current signal.
CA 02399257 2002-08-22
-4-
Preferably, the transfer function is calculated for each of the measurement
nodes.
However, in other embodiments, the transfer function is calculated for the
first body as a
whole. More preferably, there are a plurality of spaced apart measurement
nodes.
According to a second aspect of the invention there is provided a method of
acquiring induced polarisation (IP) data, the method including:
obtaining synchronised measurement signals indicative of the electric field
strength at a corresponding plurality of spaced apart locations at or adjacent
to a surface
of a first geological body;
obtaining reference signals indicative of the magnetic field strength at a
location at
or adjacent to a surface of a second geological body that is spaced apart from
the first
geological body, wherein the reference signals are synchronised with the
measurement
signals; and
being responsive to the measurement signals and the reference signals for
deriving a transfer function for the first body.
1 S According to a third aspect of the invention there is provided a data
acquisition
system for deriving survey data that is indicative of one or more geophysical
properties of
a first geological body, the system including:
a plurality of measurement nodes far obtaining synchronised measurement
signals
indicative of predetermined first characteristics of the body at a
corresponding plurality of
spaced apart locations at or adjacent to a surface of the first body;
a reference node for obtaining reference signals indicative of one or more
predetermined second characteristics of a second geological body that is
spaced apart from
the first geological body, the second node being disposed at or adjacent to a
surface of the
second body and the reference signals being synchronised with the measurement
signals;
and
a processing centre being responsive to the measurement signals and the
reference
signals for deriving the survey data.
Preferably, the measurement nodes each include a transducer for providing the
measurement signal and a sampling circuit and memory for respectively sampling
and
storing the measurement signal. More preferably, each measurement node
includes a
transmitter for allowing that node to communicate with at least one adjacent
node to
CA 02399257 2002-08-22
-S-
transfer the stored measurement signal to the processing centre. That is, the
nodes are
connected in series so that the information contained within the node furthest
from the
processing centre is transferred through all intervening nodes prior to being
received by
the processing centre. However, in other embodiments, the nodes are connected
in
parallel and each node communicates directly with the processing centre.
Preferably also, each measurement node is linked to at least one adjacent node
by an
electrical cable. However, in other embodiments, each transmitter is a
wireless
transmitter and each node includes a corresponding receiver. That is, this
wireless link is
analogous to the serial connection referred to above. In other embodiments,
the
processing centre includes a receiver that is responsive to the transmitter at
each node,
that is, analogous to a parallel connection of the nodes and the processing
centre.
1n a preferred form, the system includes a plurality of reference nodes that
are arranged in
an array that is similarly configured to the array of measurement nodes. More
preferably,
the first plurality is greater than the second plurality.
Preferably, each sampling circuit obtains a plurality of time spaced apart
samples of the
first signal. More preferably, these spaced apart samples are synchronised
with the
sampling of the reference signals. Even more preferably, the memory stores
data
indicative both of the sampled first signals and the timing of the sample.
This latter
feature is referred to as "time stamping".
Preferably, the first and the second characteristics are different. For
example, in some
embodiments, an input signal is applied to the first body and the first
characteristic is the
respective voltage at the first nodes that has been induced by the applied
signal and the
second characteristic is the magnetic field strength at one or more of the
second nodes. In
other embodiments, however, the first and the second characteristic are the
same.
According to a fourth aspect of the invention there is provided a method for
deriving survey data that is indicative of one or more geophysical properties
of a first
geological body, the method including:
synchronously obtaining, with a plurality of measurement nodes, respective
measurement signals indicative of predetermined characteristics of the body at
a
corresponding plurality of spaced apart locations at or adjacent to a surface
of the first
body;
CA 02399257 2002-08-22
-6-
obtaining, with a reference node, reference signals indicative of
predetermined
characteristics of a second geological body that is spaced apart from the
first geological
body, the second node being disposed at or adjacent to a surface of the second
body and
the reference signals being synchronised with the measurement signals; and
S being responsive to the measurement signals and the reference signals for
deriving the survey data.
According to a fifth aspect of the invention there is provided a data
acquisition
system for deriving survey data that is indicative of one or more geophysical
properties of
a first geological body, the system including:
a plurality of measurement nodes for obtaining respective measurement signals
indicative of predetermined characteristics of the body at a corresponding
plurality of
spaced apart locations at or adjacent to a surface of the first body;
a reference node for obtaining reference signals indicative of predetermined
characteristics of a second geological body that is spaced apart from the
first geological
body, the second node being disposed at or adjacent to a surface of the second
body;
a transmitter being responsive to one of the measurement signals and the
reference signals for transmitting a wireless signal;
a processing centre being responsive to the wireless signal and the other of
the
measurement signals and the reference signals for deriving the survey data.
Preferably, the measurement signals and the reference signals are digital
signals. More
preferably; the measurement signals and the reference signals are synchronised
with each
other. More preferably, the measurement signals from each of the first nodes
are
synchronised with each other. Even more preferably, there are a plurality of
spaced apart
second nodes for providing respective reference signals that are synchronised
with each
other.
Preferably also, one or more of the reference signals and one or more of the
measurement
signals includes respective magnetotelluric data and the processing centre is
responsive to
the to magnetotelluric data to reduce the effect of non-plane wave noise
sources on the
survey data.
CA 02399257 2002-08-22
_7_
According to a sixth aspect of the invention there is provided a method for
deriving survey data that is indicative of one or more geophysical properties
of a first
geological body, the system including
obtaining, with a plurality of measurement nodes, respective measurement
signals
S indicative of predetermined characteristics of the first body at a
corresponding plurality of
spaced apart locations at or adjacent to a surface of the first body;
obtaining, with a reference node, reference signals representative of
predetermined
characteristics of a second geological body that is spaced apart from the
first geological
body, the second node being disposed at or adjacent to a surface of the second
body;
being responsive to one of the measurement signals and the reference signals
for
transmitting a wireless signal;
being responsive to the wireless signal and the other of the measurement
signals
and the reference signals for deriving the survey data.
According to a seventh aspect of the invention there is provided a data
acquisition
1 S system for deriving survey data that is indicative of one or more
geophysical properties of
a first geological body, the system including:
a first plurality of measurement nodes for obtaining respective measurement
signals indicative of predetermined characteristics of the body at a
corresponding
plurality of spaced apart locations at or adjacent to a surface of the first
body;
a reference node for obtaining reference signals representative of
predetermined
characteristics of a second geological body that is spaced apart from the
first geological
body, the second node being disposed at or adjacent to a surface of the second
body; and
a processing centre being responsive to the measurement signals and the
reference
signals for deriving the survey data in real time.
According to an eighth aspect of the invention there is provided a method for
deriving survey data that is indicative of one or more geophysical properties
of a first
geological body, the method including:
obtaining, with a first plurality of measurement nodes, respective measurement
signals indicative of predetermined characteristics of the body at a
corresponding
plurality of spaced apart locations at or adjacent to a surface of the first
body;
CA 02399257 2002-08-22
_8_
obtaining, with a reference node, reference signals indicative of
predetermined
characteristics of a second geological body that is spaced apart from the
first geological
body, the second node being disposed at or adjacent to a surface of the second
body; and
being responsive to the measurement signals and the reference signals for
deriving the survey data in real time.
According to a ninth aspect of the invention there is provided a data
acquisition
system for deriving survey data that is indicative of one or more geophysical
properties of
a first geological body, the system including:
a first plurality of measurement nodes for synchronously obtaining respective
measurement signals indicative of predetermined characteristics of the body at
a
corresponding plurality of spaced apart locations at or adjacent to a surface
of the first body,
wherein the predetermined characteristics include one or more of: the electric
field strength
at the node along one or more axes; the magnetic field strength at the node
along one or
more axes; the voltage at the node relative to a given datum;
a reference node being disposed at or adjacent to a surface of a second body
for
obtaining reference signals indicative of the magnetic field strength at that
node along at
least one axis, wherein the second body is spaced apart from the first body
and the
reference signals are synchronised with the measurement signals; and
a processing centre being responsive to the measurement signals and the
reference
signals for deriving the survey data.
According to a tenth aspect of the invention there is provided a method for
deriving survey data that is indicative of one or more geophysical properties
of a first
geological body, the method including:
synchronously obtaining, with a plurality of measurement nodes, respective
measurement signals indicative of predetermined characteristics of the body at
a
corresponding plurality of spaced apart locations at or adjacent to a surface
of the first body,
wherein the predetermined characteristics include one or more of the electric
field strength
at the node along one or more axes; the magnetic field strength at the node
along one or
more axes; the voltage at the node relative to a given datum;
disposing a reference node at or adjacent to a surface of a second body for
obtaining reference signals indicative of the magnetic field strength at that
node along at
CA 02399257 2002-08-22
-9-
least one axis, wherein the second body is spaced apart from the first body
and the
reference signals are synchronised with the measurement signals; and
being responsive to the measurement signals and the reference signals for
deriving the survey data.
According to an eleventh aspect of the invention there is provided a data
acquisition system for deriving induced polarisation (IP) survey data that is
indicative of
one or more geophysical properties of a geological body, the system including:
a plurality of nodes for obtaining respective signals indicative of the
electric and
magnetic field strength at a corresponding plurality of spaced apart locations
at or
adjacent to a surface of the body;
a processing centre being responsive to the signals and a predetermined
transfer
function for deriving the survey data.
According to a twelfth aspect of the invention there is provided a method for
deriving induced polarisation (1P) survey data that is indicative of one or
more
geophysical properties of a geological body, the method including:
obtaining signals, with a plurality of respective nodes, indicative of the
electric
and magnetic field strength at a corresponding plurality of spaced apart
locations at or
adjacent to a surface of the body;
being responsive to the signals and a predetermined transfer function for
deriving
the survey data.
According to a thirteenth aspect of the invention there is provided a data
acquisition system for deriving survey data that is indicative of one or more
geophysical
properties of a geological body, the system including:
a first array of measurement nodes that are spaced apart across a surface of
the
body for providing first measurement signals indicative of one or more
predetermined
characteristics of the body;
a second array of measurement nodes that are spaced apart across the surface
of
the body, the second array being disposed adjacent to the first array for
providing second
measurement signals indicative of one or more predetermined characteristics of
the body;
a reference current electrode being disposed in the body;
CA 02399257 2002-08-22
- 10-
a moveable current electrode that is placed in the body sequentially between
the
nodes in the first array to provide respective input currents in the body that
flow between
the electrodes; and
a processing centre being responsive to the measurement signals during the
provision of the input currents for determining the survey data.
Preferably, the first array and the second array are linear and parallel. That
is, the
measurement nodes in the first array are dispbsed in a substantially straight
line that
extends across the body and the measurement nodes in the second array are
disposed in a
substantially straight line that is spaced apart from the first array. More
preferably, the
measurement nodes in the first array are substantially evenly spaced apart. 1n
other
embodiments, the arrays are non-linear.
Preferably also, the system includes a third array of measurement nodes that
are
spaced apart across the surface of the body, the third array being disposed
adjacent to the
first array on the opposite side to the second array for providing third
measurement
signals indicative of one or more predetermined characteristics of the body,
wherein the a
processing centre is also responsive to the third measurement signals during
the provision
of the input currents. That is, the first array is disposed intermediate the
second and third
arrays. In other embodiment, use is made of a greater number of arrays that
are
symmetrically disposed each side of the first array.
According to a fourteenth aspect of the invention there is provided a method
for
deriving survey data that is indicative of one or more geophysical properties
of a
geological body, the system including:
spacing apart across a surface of the body a first array of measurement nodes
for
providing first measurement signals indicative of one or more predetermined
characteristics
of the body;
spacing apart across the surface of the body a second array of measurement
nodes
that are disposed adjacent to the first array for providing second measurement
signals
indicative of one or more predetermined characteristics of the body;
disposing a reference current electrode in the body;
CA 02399257 2002-08-22
-11-
placing a moveable current electrode in the body sequentially between the
nodes
in the first array to provide respective input currents in the body that flow
between the
electrodes; and
being responsive to the measurement signals during the provision of the input
currents for determining the survey data.
According to a fifteenth aspect of the invention there is provided an induced
polarisation (IP) data acquisition system, the system including:
a first plurality of measurement nodes for obtaining measurement signals
indicative of the electric field strength at a corresponding plurality of
spaced apart
locations at or adjacent to a surface of a first geological body, the
measurement signals
including a time stamp;
a reference node for obtaining reference signals indicative of the magnetic
field
strength at a location at or adjacent to a surface of a second geological body
that is spaced
apart from the first geological body, wherein the reference signals include a
time stamp; and
a processing centre being responsive to the measurement signals, the reference
signals and the time stamps for deriving a transfer function for the first
body.
According to a sixteenth aspect of the invention there is provided a method of
induced polarisation (IP) data acquisition, the method including:
obtaining, via a first plurality of measurement nodes, measurement signals
indicative of the electric field strength at a corresponding plurality of
spaced apart
locations at or adjacent to a surface of a first geological body, the
measurement signals
including a time stamp;
obtaining, via a reference node, reference signals indicative of the magnetic
field
strength at a location at or adjacent to a surface of a second geological body
that is spaced
apart from the first geological body, wherein the reference signals include a
time stamp; and
a processing centre being responsive to the measurement signals, the reference
signals and the time stamps for deriving a transfer function for the first
body.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described, by way of
example only, with reference to the accompanying drawings in which:
CA 02399257 2002-08-22
-12-
Figure 1 is a schematic representation of a geological data acquisition system
according to the invention;
Figure 2 is a time series plot for the output of two magnetometers, one in the
local
field and one in the remote field;
Figure 3 is an enlarged portion of the plot of Figure 2;
Figure 4 is a plot of the induced polarisation time series data for the first
body and
the calculated telluric noise;
Figure 5 is an enlarged portion of the plot of Figure 4;
Figure 6 is a comparison of the induced polarisation decay signal for a system
not
operated in accordance with the preferred embodiment and a system operated in
accordance
with the preferred embodiment;
Figure 7 is a schematic representation of an alternative geological data
acquisition
system according to the invention;
Figure 8 is a schematic representation of a geological body that is under
1 S investigation through use of an array according to one aspect of the
invention;
Figure 9 is a schematic top view of the geological body under investigation
with
one type of array of nodes;
Figure 10 is a schematic top view of the geological body under investigation
with
another type of array of nodes;
Figure 11 is a schematic top view of the geological body under investigation
with
a further type of array of nodes; and
Figure 12 is a schematic top view of the geological body that is the subject
of an
IP survey according to an aspect of the invention.
Figure 13 is a schematic illustration of an alternative embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to Figure 1, there is illustrated an induced polarisation (1P) data
acquisition system 1. System 1 includes a plurality of linearly spaced apart
measurement
nodes 2 for obtaining synchronised measurement signals indicative of the
electric field
strength at a corresponding plurality of spaced apart locations at or adjacent
to a surface 3
of a first geological body 4. A reference node 5 obtains reference signals
indicative of
CA 02399257 2002-08-22
-13-
the magnetic field strength at a location at or adjacent to a surface 7 of a
second
geological body 8 that is spaced apart from body 4. The reference signals are
synchronised with the measurement signals. A processing centre 9 is responsive
to the
measurement signals and the reference signals for deriving a transfer function
for body 4.
System 1 is configured for deriving survey data that is indicative of one or
more
geophysical properties a body 4 and use is made of the transfer function
referred to above
to more accurately derive the survey data. That is, processing centre 9, which
takes the
form of a local desktop computer 12, is responsive to the measurement signals
and the
reference signals for deriving the survey data.
Each of nodes 2 includes a sampling circuit (not shown) and memory (not shown)
for
respectively sampling and storing the measurement signals at a given time. In
this
embodiment, the sampling of the measurement signal is synchronised at all
nodes in the
linear array of nodes. Moreover, the measurement signals are sampled at each
node
about 48,000 times over the 10 minute duration of the survey. In other
embodiments
different sample rates are used. For example, in some embodiments the memory
capacity
of the node is not sufficient to store that many samples and, as such, the
operator has to
be content with a lower sample rate or a shorter survey.
Each node 2 also includes a transmitter (not shown) for allowing the contents
of the
respective memories to be passed to a local Central Recording Unit (CRU) 13.
In this
embodiment, the transmitters are connected in parallel to a common
communication bus
(shown schematically) so that the measurement data, together with a time stamp
that is
indicative of the time the sample occurred, are able to be communicated to CRU
13. In
other embodiments, the transmitters are connected in a daisy chain arrangement
such that,
in effect, each transmitter communicates with the or each adjacent node to
transfer the
sampled measurement signals. That is, the nodes are connected so that the
information
contained within the node furthest from CRU 13 is transferred through all
intervening
nodes prior to being received by the CRU 13. The connection between the nodes
and
between the nodes and CRU 13 is by way of electrical cabling of the required
category
and specification.
In other embodiments, the nodes are connected such that each node communicates
directly with CRU 13. While in some embodiments this connection is also
achieved
CA 02399257 2002-08-22
-14-
through the use of cabling, in other embodiments each transmitter is a
wireless
transmitter that sends the required data and information directly to CRU 13.
It will also be appreciated that, in some embodiments, each node includes a
receiver
corresponding to the transmitter to allow the daisy chain form of
communication. When
S used in this form, the power requirement for each individual transmitter is
reduced as it
only need operate at a power level sufficient to communicate with the or each
adjacent
node.
It will be appreciated by those skilled in the art that the nodes need not be
linearly spaced.
The measurement signals gained from the nodes 2 are used to provide data for
an induced
polarisation (IP) survey of body 4. That is, an input current signal, that
roughly
resembles a square wave, is injected into body 4 via two electrodes (not
shown) that are
generally located at the periphery of body 2. The quantum of the current is in
the order of
5 Amps, although this is somewhat dependent upon the resistivity of body 4 and
more
particularly the surface resistivity of body 4. While the frequency of the
input current in
1 S this embodiment is about 0.1 Hz, in other embodiments different
frequencies are used.
The choice of frequency is usually dependent upon the capacitive effect of
body 4. That
is, the more capacitive, the lower the frequency used. However, regard is also
had to the
amplitude of the input current.
The measurement data, gained from determining the voltage differences between
the
adjacent nodes 2, is transmitted to CRU 13. From there, that data is formatted
and passed
to computer 12 where it is processed, in real time, to provide the IP survey
data.
System 1 also includes a node in the form of a magnetometer 1 S that provides
additional
measurement data indicative of the magnetic field strength at the surface of
body 4 along
both an x and a y-axis. These axes are both parallel to surface 3 and normal
to each
other. In other embodiments, the magnetic field strength along three axes is
provided.
Moreover, in further embodiments, use is made of a number of spaced apart
magnetometers. That is, the preferred embodiment is also suitable for
magnetotelluric
(MT) and electromagnetic (EM) surveys.
The apparatus bounded by broken line 16 approximates the data acquisition
system
disclosed in Australian Patent Application no 58341/99 in the name of the
present
applicants.
CA 02399257 2002-08-22
-15-
Node 5 is included in a reference array 18 that is, in this embodiment,
similar to the
combination of nodes 2 and 15. Node 5 includes a magnetometer 17 while the
remaining
nodes in that array measure the electric field strength at the corresponding
locations.
These nodes are sampled and the reference data stored in respective memory
prior to
S being communicated to a remote CRU 19, together with a time stamp. The
sampling at '
the nodes in array 18 is synchronised with the sampling of nodes 2, and the
synchronisation is achieved by accessing time data from one or more GPS
sources.
A number of time spaced and synchronised samples are taken at each node in
both arrays
to provide the required data for the survey.
Array 18, being a reference array, need not include the same number of nodes
as the
measurement array 3. For the purpose of this example, the arrays include the
same
number of nodes. However, the more usual approach is for system 1 to include
about 30
nodes 2, and array 18 to include about ten nodes. In another embodiment,
however,
system 1 includes about 1000 nodes, and array 18 includes about 100 nodes.
Body 8 is disposed remotely from body 4 and, in this embodiment, the
separation
between the two is about 30 km. In other embodiments a different separation is
used.
The extent of the separation is dependent upon terrain and geography, amongst
other
things. Preferably, however, the separation is at least a few kilometres. The
maximum
separation between the arrays is determined by the transmission distances of
the radio
transmitter used. However, with the use of repeater stations and/or satellite
connections,
the range is effectively limitless. That is, it is now possible to control the
data acquisition
and the derivation of the survey data from a site remote from both body 4 and
body 8.
CRU 19 is controlled by and communicates with a remote desktop computer 21.
This
computer's primary functions include some processing of the reference data and
the time
stamps, and compressing the processed information for communication to
computer 12.
Moreover, computer 21 is responsive to computer 12 for initiating a survey and
for
ensuring synchronisation of the sampling.
The communication between the computers is established by a wireless link 22.
In this
embodiment, link 22 includes a local radio Ethernet modem 23 and a remote
radio
Ethernet modem 24 that are connected to computers 12 and 21 respectively.
Modem 23
drives and receives from an omni-directional antenna 25 that is located at or
near body 4,
CA 02399257 2002-08-22
-16-
while modem 24 drives and receives from a parabolic antenna 26 that is located
at or
adjacent to body 8. The data rate of the wireless communication is 11
Mbit/sec.
However, in other embodiments, alterfiative rates are used.
Link 22 provides for real time two-way communication between computers 12 and
21
and, hence, the operation of the local and remote nodes is able to be
coordinated in real
time. In this embodiment, computer 12 is the point of central control for the
survey, and
is the means by which an operator initiates a survey. That initiation will not
only trigger
CRU 13 to commence data logging at a predetermined sample rate at nodes 2, but
also
the communication over link 22 of command signals to computer 21. Those
command
signals will include information about the type and quantity of reference data
required
from array 18, as well as the synchronisation information.
In this embodiment, both CRU 13 and 19 include GPS capability and this is
utilised to
provide the time stamping and synchronisation. The accuracy of the GPS system
has
been found to be sufficient for the purposes of the surveys being conducted.
As the reference data provided by array 18 is able to be communicated from
computer 21
to computer 12 in real time, there is significant advantage gained. For
example, when the
survey is initiated the first set of samples gained from nodes 2 and array 7
are used to
determine whether all nodes 2 are functioning correctly. This verification
step will
usually involve about 10,000 samples taken over a 5 minute interval. With this
information in hand it is possible to either have the malfunction corrected
or,
alternatively, to ignore any data gathered at the node concerned.
Once sufficient verification of nodes 2 has been achieved, the survey proper
is
commenced. The survey requires sufficient data to be gathered from the
reference and
measurement arrays so that the subsequent analysis will be valid within the
commercial
and theoretical limits being imposed upon the operator of system 1. As will be
explained
further below, the present embodiment is able to provide much greater accuracy
than
prior art systems and yet not have to take as many samples from nodes 2. This
is due to
the use of the remote referencing provided by array 18. Moreover, the data is
available in
real time to the operator.
In some respects the speed of system 1 arises from a distributed computational
power, in
that there are two processing centres, one being computer 12 and the other
computer 21.
CA 02399257 2002-08-22
-17-
However, through use of truly remote reference signals, and the
synchronisation of the
sampling of the reference and measurement signals, even greater advantage is
gained.
Conventional wisdom suggests that having a reference array requires the
gathering of
more data and, as a consequence, even more processing is required to derive
the survey
results from that data. This, in turn, introducing an even greater delay in
the delivery of
the survey results. However, in this embodiment, there arises a synergy
between the
measurement signals and reference signals that allows far less data to be
gathered as only .
synchronised data is obtained. This, in turn, allows system 1 to provide real
time analysis
of the results as well as real time quality assurance of the data. The reduced
data
requirements also reduce pressure on the storage requirements both at the
nodes, the
CRU and the local computer.
As will be appreciated by a person skilled in the art, based upon the teaching
herein, computers 12 and 21 include software for allowing an operator to
control the
survey. In some embodiments this is possible through the use of "off the
shelf' software ,
that has been appropriately configured, while in other embodiments use is also
made of
in-house or proprietary software. In this embodiment use is made of "off the
shelf'
software components, in combination with an in-house operating system known as
"Dirt
Burglar".
Once array 18 has been set up it need not be manned, or at least it need not
be
manned by skilled personnel. That is, array 18 is more or less self sufficient
and useable
for any number of surveys, whether those surveys be in respect of body 2 or
another body
remote from body 2. All that is required is the ability to communicate with
computer 21,
as that computer is responsive to incoming commands for commencing the
acquisition of
reference data. This allows the survey costs to be minimised as there is no
need for
remote operation and the reference array is reusable.
The preferred embodiment is particularly advantageous as it accommodates real
time remote reference noise cancellation for induced polarization,
electromagnetic and
magnetotelluric surveys.
In use, the first step in conducting a survey involves setting up the physical
components of system 1. This includes locating nodes 2 in a spaced array that
extends
across surface 3, as best shown in Figure 9. The array is linear and the nodes
are
CA 02399257 2002-08-22
-18-
connected in a daisy chain to CRU 13. The array of nodes includes three
parallel
longitudinal branches that are transversely spaced apart and connected by
intermediate
transverse links. In other embodiment's use is made of more than three
longitudinal
branches. The size of the array is limited in some cases by the available
nodes.
However, in other cases it is limited by the size of the memory contained in
each node,
the rate of data transmission possible from the nodes to CRU 13 and the sample
rate for
the given survey.
In the event that there are insufficient nodes 2 to allow all of body 4 to be
investigated in a single sweep, it is possible to conduct a first survey and
then move
nodes 2 to a new position on surface 3 and conduct another survey, the results
of which
are combined with those of the first. In further embodiments more than two
surveys are
combined.
Alternative embodiments include nodes 2 being laid out in different
configurations to that of Figure 9. By way of example, the embodiment of
Figure 10
includes three parallel longitudinal branches that are transversely spaced
apart and which
are interconnected at a common end. The other example, as shown in Figure 11,
has
nodes that are disposed in three parallel longitudinal branches that are
transversely spaced
apart and which are separately connected to processing centre 9. The Figure 11
embodiment, for a given rate of data transmission between nodes, allows for a
greater
length of array. However, it does require that CRU 13 has at least three
channels for
receiving data from the respective branches and for sending command signals to
the
nodes in those branches.
For 1P surveys the usual longitudinal spacing between nodes is about 100
metres,
while the usual transverse spacing between branches of nodes is about 200
metres.
However, as would be appreciated by the skilled addressee, these parameters
are varied in
accordance with the geology and other factors.
Magnetometer 15 is also disposed on surface 3, although this is optional for
an IP
survey such as that which is being performed by this embodiment. In other
embodiments, a plurality of spaced apart magnetometers are used, particularly
for MT
surveys.
The modem 23 and antenna 25 are also configured at this stage.
CA 02399257 2002-08-22
- 19-
At the remote or reference site, magnetometer 5 is set up for taking
measurements
of the magnetic field strength along the x and the y axis. In the event that
an MT survey
is also required, then an array of nodes 18 are also provided for obtaining
measurements
of the electric field strength at the respective locations. The spacing
between the nodes
18 is preferably the same as the spacing between nodes 2.
Modem 24 and antenna 26 are set up and a communication link established
between computers 12 and 21.
As best shown in Figure 12, system 1 includes a generator 41 that is
responsive to
processing centre 9 for providing a square wave input current signal (not
shown). This
current signal is injected, as required, into body 4 via electrodes 43 and 44
that are
connected by cable 45. Generator 41 includes an ammeter (not shown) for
sampling the
current signal in synchronism with the measurement signals from the local
nodes 2 and
the reference signals from the remote nodes 5 and 18. This sampled current
signal is
provided to computer 12 as an input signal that is indicative of the current
signal.
With all these physical components now in place, the operator of system 1
conducts a pre-survey investigation. This occurs through the operator
utilising computer
12 to:
1. Verify that all nodes 2 and magnetometer 15 are initiated and ready to
commence
sampling;
2. Verify that magnetometer 5 and all nodes 18 are initiated and ready to
commence
sampling;
3. That CRU 13 and CRU 19 are configured for the survey to be undertaken;
4. Actuate generator 41 to supply a predetermined square wave current into
body 4.
5. Actuate nodes 2 to make a plurality of synchronised samples for providing
the
desired measurement signals.
6. Actuate magnetometer 5 to make synchronised samples to provide respective
reference signals; and
7. Actuate the ammeter to sample to current signal in synchronism with the
sampling of the measurement and reference signals.
This survey runs for about 5 minutes at a sample rate of about 100 Hz. The
data
sampled is feed back to computer 12 and presented in a graphical form to the
operator. In
CA 02399257 2002-08-22
-20-
this instance, the operator is provided with a time series representation of
the samples
from magnetometer 5 along both the x-axis and the y-axis to allow an error
check to be
carried out. This error check takes the form of a comparison of the averages
of the
measurements along those axes, over a given period. These averages are also
displayed
S to the operator and, if they differ beyond a predetermined threshold, it is
deemed that
there is an error in the measurement and a fresh set of data is obtained. If,
once the fresh
data has been obtained, the threshold is again exceeded, further investigation
is taken into
the physical condition of the magnetometer and the surrounding environment.
If the averages of along those axes differ by less than the threshold the
operator,
via computer 12, analyses the data that was sampled and supplied by
magnetometer 5 and
the ammeter. Particularly, a Fourier analysis is done to determine whether the
sampled
signal shares unexpectedly high levels of frequency components with the input
signal. If
so, the remote body 8 is not truly remote in that it is insufficiently
decoupled from body 4
to provide a low noise reference. That being the case, a new remote reference
body will,
have to be found.
Importantly, this determination of suitability of body 8 is done in real time
in that
computer 12 and/or computer 21 commence the processing of the data as soon as
it is
received. Accordingly, the operator is provided with real time feedback not
only on the
quality of the reference measurements but also on the suitability or otherwise
of the
remote body as a reference site. Moreover, the Fourier analysis is undertaken
automatically by computer 12 and the threshold earlier determined. This
relieves the
operator from having to make these decisions in the field. It also greatly
automates the
survey while not preventing the operator from delving further into the detail
of the
measurements, if required. It should also be noted that when body 8 is not
sufficiently
remote, this is usually clearly illustrated to the operator simply by
overlaying the time
series data from magnetometer 5 with the time series data from the ammeter.
The use of the remote referencing of the preferred embodiment allow
considerable time and resources to be conserved with the simple real time
error checks
that have been described above. In either case, the survey will not be carried
out unless
there is sufficient decoupling between the remote body and the local body. For
prior art
CA 02399257 2002-08-22
-21 -
IP survey systems, even if the error were discovered, this would not have
occurred until
post processing of the data occurred.
With the initial verification complete, the operator then deactivates
generator 41.
That is, the injected current signal is halted. Following this, the operator
conducts a
S second round of data gathering, this time in the form of a MT style survey.
More
particularly, this involves having:
1. Nodes 2 at the local body, body 4, obtaining measurement signals that are
indicative
of the electric field strength along an x-axis; and
2. Magnetometer 5 at the remote body, body 8, obtaining reference signals that
are
indicative of the magnetic field strength along a y-axis.
The measurement and reference signals are sampled over a twenty minute period
at about 400 Hz and communicated to computer 12. In other embodiments,
different
times and sample rates are used in accordance with the practical trade off
that the
operator wishes to make between cost and accuracy.
The measurement and reference signals are used as a basis for calculating a
transfer function for body 4. The transfer function, T( f ), is: ' .
T( f ) = K.EX( f )/HY( f ) Equation 1
Where K is a constant, EX( f ) is the electric field strength along the x axis
at the local
body 4 and Hy( f) is the magnetic field strength along the y axis at the
remote body 8. For
a constant geology, this function holds true. The accuracy is limited usually
by the
quality of the measurement and reference signals and, as such, in this
embodiment, the
survey time and sample rate are high to provide a not only a large quantity of
data to
minimise errors, but also to provide for a broad frequency range for the
transfer function.
Once sufficient data has been gathered to allow the determination of the
transfer
function to the desired accuracy, magnetometer 5 is placed on standby. This is
achieved
either automatically, or by the operator selecting the appropriate command
sequence with
computer I2. The computer then initiates a f nal verification that all the
nodes are in
service, and provides command signals to those nodes, including timing
information,
such that the subsequent measurement and reference signals will be
synchronised. In
some embodiments, the nodes will be provided with timing signals by CRU 13 and
19
CA 02399257 2002-08-22
-22-
which, in turn, include GPS referencing hardware. Tn other embodiments, the
nodes have
independent access to timing information.
The timing information is in the form of a start time for the survey and a
sample
rate. In other embodiments, however, the timing information is a series of
times at which
samples are obtained by the respective nodes.
Once the command signals have been sent and the relevant nodes have confirmed
receipt of the command signals, computer 12 actuates generator 41 to once
again inject
the current signal into body 4 in preparation for an IP survey. Electrodes 43
and 44 are
initially disposed as shown in Figure 12 and the data is gathered from the
relevant nodes.
For the IP survey being conducted, this data includes measurement signals from
nodes 2
that are indicative of the electric field strength along the x-axis and
reference signals from
magnetometer 5 that are indicative of the magnetic field strength along the y-
axis. Once
sufficient number of measurement and reference signals have been obtained and
provided
to computer 12, electrode 44 is moved from adjacent to the first node 2, as
shown in
Figure 12, and relocated to be intermediate the first node and the second
node. An
additional set synchronised of measurement and reference signals are then
obtained and
provided to computer 12. Electrode 44 is then relocated to be intermediate the
second
node and the third node and another set of synchronised measurement and
reference
signals are obtained. This sequence continues up until measurement and
reference
signals have been obtained for the positioning of electrode 44 between all the
nodes
intermediate the electrodes.
As the measurement and reference data is collected and collated by computer
12,
the originally determined transfer function is used, in light of the
measurement and
reference signals obtained in the presence of the injected current, to
calculate a predicted
noise correction factor. This factor takes the form of a time varying noise
signal that is
then subtracted from the measured EX signal at the local source. This
correction factor
provides superior noise cancellation effects.
As the physical location of a source of noise will determine the timing of
that
noise at the local and the remote bodies, a simple subtraction of the
simultaneously
collected samples of like properties at those spaced apart bodies is
inaccurate and
CA 02399257 2002-08-22
- 23 -
misleading. The problems of this conventional approach are avoided through use
of the
preferred embodiment, which:
1. First verifies the remoteness of the remote body;
2. Determines a transfer function for the bodies that makes use of a
predetermined
S parameter at the local body and an interrelated but different parameter at
the remote
body;
3. Acquires the data required for the desired survey;
4. Calculates a predicted noise signal based at the local body upon the survey
measurements and the transfer function; and
S. Subtracts this predicted noise from the relevant signals to provide the
corrected
survey data.
Moreover, these additional steps are completed quickly and effectively by the
preferred embodiment through the use of wireless communications. The use of
synchronisation between the measurement and reference signals also allows a
minimum
1 S of data to be collected to authenticate the survey and, as such, even in
the event of errors
- due to, for example, a nearby lightning strike - the survey is easily and
quickly rerun.
By way of example, a time series plot of the sampled output from the X-axis of
respective magnetometers 1S and S are overlaid in Figure 2. In this example,
the two
magnetometers are remote from each other by 13.2 km. An enlarged portion of
the
Figure 2 plot is shown in Figure 3, and illustrates more clearly the
differences beriveen
the magnetic field strength along that axis. As shown, these differences,
while apparent,
are relatively small notwithstanding the considerable distance between the two
measurement sites. Following from this, it is possible to predict, with
sufficient
accuracy, the local magnetic field using a truly remote magnetic field
reference.
2S The data shown in Figure 2 and Figure 3 is available for viewing by the
operator,
if required, as soon as it is gathered and communicated to computer 12. The
data will
arrive either via CRU 13 for the data provided by magnetometer 1 S, or
sequentially via
CRU 19, computer 21 and link 22 for the data provided by magnetometer 1 S.
The real time acquisition of the reference data "on demand" by CRU 19 reduces
the amount of data actually required to be collected. It also provides the
operator with
confidence that the nodes in the arrays at bodies 4 and 8 are synchronised, as
the operator
CA 02399257 2002-08-22
-24-
is able to view both the reference and measurement datasets. This eliminates
wasted
survey time.
Time synchronisation, in conjunction with wireless LAN such as link 22, allows
the use of measurement arrays having in the order of 1000 channels without
necessitating
movement of the CRU 13, computer 12 and array 7. ' , .
Another embodiment of the invention, system 30, is illustrated in Figure 7,
where
corresponding features are denoted by corresponding reference numerals. In
this
embodiment, computer 12 and computer 21, as well as communicating with each
other
via link 22, also communicate separately with a central control centre 31 via
communication links 32 and 33. In this embodiment both links 32 and 33 are
wireless
links. Moreover, while both computers 12 and 21 are operational for processing
and
communicating data, they are both controlled by an operator that is located at
centre 31.
Depending upon the location of the transmission sites, the communication links
include satellite, cell phone or radio links. This allows real time quality
control of the
survey by a central administrator, notwithstanding that that administrator is
located in
another state or country to the actual survey.
System 1 has been developed primarily for combined magnetotelluric (MT) and
Induced Polarisation (I1') surveys. It has been found that the noise affecting
these two
types of surveys have a peculiar antithetic relationship, in that the noise in
the data for the
IP survey is the predominant signal of the data in the MT survey.
One known way to minimise the noise in IP data using the results of MT survey
data that is collected at the end of the IP data collection is illustrated in
the Australian
application referred to above. Another alternative is used in system 1 and 30,
and
includes the collection of synchronous remote magnetic field information
(using wireless
LAN) with local integrated electric field information. The spectral
information produced
from the routine MT processing in conjunction with the remote data is
processed, in real
time, to predict the local telluric response at any of the IP measuring
dipoles. This
predicted noise is subtracted from the recorded time series, or from the
calculated spectral
responses, to provide much cleaner data.
An example is illustrated in Figures 4 to 6. The psuedo square wave in Figure
4
is the IP time series data. That is, the data is a voltage that resembles the
input current
CA 02399257 2002-08-22
-25-
signal. As will be appreciated by the skilled addressee, it is the decay
characteristics that
are of particular interest and these are relatively small in comparison to the
overall
voltage signal provided and, hence, highly susceptible to noise corruption.
The other plot
extending through the centre of Figure 4 is the "predicted" telluric noise
calculated from
the remote reference magnetic data and the MT processing for that node and for
the
defined time.
For the sake of clarity, the area in Figure 4 that is bounded by a rectangle,
is
shown enlarged in Figure 5. This shows a close correlation between the
predicted and
recorded noise.
After removal of predicted noise from the time series the processing provided
by
computer 12 yields IP decays which are much improved. For example, the plot in
Figure
5 shows two repeat readings - that is, using the same transmitter and receiver
pairs - for a
given decay. The left panel shows the results from the standard processing
stream and
the right pane shows the results with the telluric cancellation scheme of the
preferred
embodiment. As suggested above, both these plots are available to be accessed
by an
operator in real time. This allows the operator to gain a greater
understanding and
appreciation of the operation of system 1 and to perform quality control steps
both
initially and throughout the survey. It also allows the operator to be located
other than at
the site of the survey.
The noise cancellation scheme referred to above works well with a remote
reference. However, an alternative embodiment of the invention (not shown)
gains
similar benefit through the use of local magnetotelluric results. That is, the
array consists
of multiple E and H sensors at each dipole location, to allow non plane-wave
signals -
that is, local noise - to be isolated from each node.
The quality of the survey data provided by the preferred embodiments is
enhanced
by noise reduction strategies using MT data. Those embodiments also collect
reference
data at multiple : ites to overcome local, non-plane wave noise sources. The
collection of
truly remote data, with radio links and GPS synchronisation, aids in the
rejection of non-
coherent local noise signals.
CA 02399257 2002-08-22
-26-
Importantly also, the real monitoring of synchronised remote data allows in-
field
processing to reduce noise and thus improve the quality assurance and quality
control of
the survey and the survey data.
Use of magnetometer to reduce IP noise also allows for a reduction in the
resistivity noise.
While the use of a truly remote array provides significant advantage over the
prior
art, there is even more to be gained through the use of IP noise reduction
using local MT
measurements. That is, the latter provides substantially the same processing
and noise
reduction benefits while also negating the need for a remote array.
Through use of the embodiments of the invention and the IP noise reduction
that
is provided, it is possible to collect reliable data from areas where prior
systems have
failed. For example, there are areas in Chile, and other places, where a very
resistive near
surface layer prohibits high current inputs into the ground thereby minimising
the
received signal. In conventional systems the noise component cannot be
adequately .
removed thus making acquisition useless. The noise correction of the preferred
embodiments, however, provides a far more robust and versatile system.
For EM type surveys, the noise prediction allows EM noise reduction by direct
subtraction and performed in real time.
Refernng to Figure 8 there is illustrated a schematic view of three parallel
arrays
51, 52 and 53 that are extend longitudinally across a geological body 54. Each
array
includes a plurality of spaced apart nodes (represented by downwardly
extending arrows)
that sample a predetermined characteristic of the body at the respective
locations. As
described with reference to the other embodiments, the nodes are linked in a
daisy chain
configuration. In this embodiment, as with the above embodiment, the each
array is
configured to provide sampled data for an IP survey. That is, each of the
nodes samples
the electric field strength at that node at a given rate over a given period,
and has these
samples sent to a local computer for subsequent analysis of the samples.
As described with reference to the IP survey of Figure 12, the movable
electrode
is located successively between the nodes and additional samples taken. So
too, in the
Figure 8 embodiment, is the moveable electrode (not shown) progressed between
the
nodes in array 51. However, at each location of the moveable electrode, not
only are
CA 02399257 2002-08-22
-27-
samples obtained by the nodes in array S 1, but also by the nodes in the
flanking arrays 52
and 53. This effectively allows a greater volume of body 54 to be investigated
with a
single pass of the moveable electrode 'along body 54. That is, this embodiment
provides
in a single pass of the moveable electrode more information than would have
been gained
from three passes of the prior arrays.
Particularly, the volume of body 54 investigated by array 51 is approximated
by
the elliptical cross section bounded by line 55. The volume of body 54 that is
investigated by arrays 52 and 53 is approximated by the respective elliptical
cross section
bounded by lines 56 and 57. It will be appreciated that these elliptical cross
sections
extend longitudinally along body 54.
The fact that measurement signals are obtained from the three arrays in
synchronism allows accurate mapping of the subsurface topography in both the x
and y
directions. Moreover, as the number of moves of the moveable electrode is
reduced, in
this case by a third, it provides a significant time and cost saving to the
survey. This is in
addition to the benefits described above to do with error checking and noise
reduction
through use of a remote reference.
This arrangement is such that there are a number of arrays disposed about body
54, where at least one of those arrays is active, in that the moveable current
electrode is
being progressed between the nodes of the active array. The remainder of the
arrays are
passive, in that the moveable current electrode is not progressively disposed
intermediate.
the respective nodes in the passive arrays. Importantly, however, the nodes in
the passive
arrays sample in synchronism with the nodes in the active array to allow
additional data
to be gained while minimising the time consuming task of relocating the
moveable
electrode. The passive arrays are also referred to as shadow arrays.
In other embodiments use is made of two additional passive arrays that are
disposed parallel with the other arrays and which flank arrays 52 and 53
respectively.
While the samples collected from these additional arrays is also useful, it
will not provide
the same depth o I' penetration into body 54 as is the case for the other
arrays. Even
greater numbers o f passive arrays are used in other embodiments, particularly
where the
transverse spacin ~ between the arrays is small.
CA 02399257 2002-08-22
- 28
The arrays shown in Figure 12 are coextending and have the same number of
nodes. Moreover, corresponding nodes in the arrays are aligned along a
transversely
extending axis. In other embodiment, however, the nodes in adjacent arrays are
offset
longitudinally. Preferably the offset is half the spacing between adjacent
nodes in the
same array.
A further embodiment of the invention is shown in Figure 13. More
particularly,
induced polarisation (TP) data acquisition system 71 includes a first
plurality of
measurement nodes for obtaining measurement signals indicative of the electric
field
strength at a corresponding plurality of spaced apart locations at or adjacent
to a surface
of a first geological body 72. The measurement signals each include a time
stamp that
indicates the time at which the respective signals were obtained. A reference
node obtains
reference signals indicative of the magnetic field strength at a location at
or adjacent to a
surface of a second geological body 73 that is spaced apart from body 72 by
many
thousands of kilometres. The reference signals also include a time stamp that
indicates the
time at which the respective reference signals were obtained. A processing
centre, in the
form of a remote monitoring site 74 that is near body 73, is responsive to the
measurement signals, the reference signals and the time stamps for deriving a
transfer
function for the first body. In other embodiments, site 74 is spaced apart
from body 73
by many hundreds or thousands of kilometres.
It will be appreciated from the drawing that this embodiment includes many of
the
features of the other embodiments although these have been omitted for the
purposes of
clarity. For example, the measurement signals are collected from body 72 in
accordance
with one of the methodologies described above.
The remote monitoring site 74 is a permanent magnetotelluric reference site
and
is used in conjunction with the telluric cancellation procedures discussed
above.
Accordingly, in this embodiment, the need for a second mobile manned
acquisition
system is also removed.
Site 74 includes two (X Y) spaced apart B-Field magnetometers 75 (only one
shown). In other embodiments, however, use is made of three or more
magnetometers 75
or, alternatively two or more (X Y Z) B-Field magnetometers.
CA 02399257 2002-08-22
-29-
Also located at site 74 is a two-channel electric field data acquisition
system (not
shown). In other embodiments, use is made of a three-channel data acquisition
system.
The reference signals and the data gained from the two-channel data
acquisition
system are provided to a computer 76 at site 74. While this computer is shown
as a
desktop computer, it is in other embodiments a laptop computer or a computer
network.
There is a Satellite Internet/Internet Connection between the remote site and
the
survey site to establish a network of computers for allowing data and commands
to be
transferred between the sites. In Figure 13 there are shown two examples of
the possible
communication between the sites, one which involves direct communication via
wireless
link 79 with a satellite 80, and the other which involves a combination
landline and
wireless link 81. In other embodiments alternative communications Iinks are
available.
A mobile survey unit 82 that is located at site 72 and communicates wirelessly
with site 74 - via satellite 80 - by means of wireless link 83.
The communication between the remote and local site includes, in addition to
the'.
transfer of data measured at the sites, commands for timing the acquisitions,
including
start times, end times, sampling rates, and the Like. This command
communication is
enabled by network acquisition software. In other embodiments there are more
than the
two sites shown in Figure 13.
All the measurement signals and reference signals are time stamped in a
predetermined format to provided additional functionality. In this embodiment,
the
timing software operates to an accuracy of 1 to 50 msecs, depending on the
characteristics of the synchronization source and network paths.
Jn other embodiments, however, use is made of GPS (Global positioning System)
timing software to time stamp reference data. This allows accuracies of up to
about 20
nsecs in a network of units - including a minimum of two units, the remote and
base. As
will be appreciated by those skilled in the art, the accuracy available is
dependant upon
the GPS Time reference hardware used.
The computer network created by the link between the sites operates data
server
software (incorporating XML) to provide the following functionality: the
ability to send
data between sites for a specified time range with or without data
compression; and data
CA 02399257 2002-08-22
-30-
compression schemes including Wavelet flossy and lossless) and Vector
quantization
types.
The major advantages of havirig.a fixed reference site are:
1. The fixed site does not need to be manned at all, and if so, certainly not
on a full
time basis. Accordingly the labour costs are reduced.
2. There is less equipment to mobilise, also reducing the cost of labour.
3. Diminished survey setup times, as the fixed reference is already in place.
4. Ability to use either current or old survey data to gain a survey result.
That is, as
long as the measurement signals have been time stamped, and the reference
signals were sampled in the relevant time window, it is possible to
retrospectively
remove telluric noise from a survey. This is particularly advantageous where
the
original measurement signals include too much noise until modified through use
of the correspondingly time stamped reference signals.
5. Increased safety with no one-man remote crews needing to be deployed.
6. Increased security of expensive equipment, in that there is no longer a
need to
leave that equipment in unmanned remote locations.
7. The ability to have the remote reference site distant from the processing
centre
and the survey site. This allows the remote reference site to be chosen on the
basis that it is well isolated from the survey site, without having to be
concerned
with regularly having skilled personnel travel to the survey site.
8. The centralised control of surveys from the processing centre. In the
preferred
embodiment shown in Figure 13, the remote site is in an isolated region, while
the
processing centre is located within a city. The communication between the two
is
achieved by a land-based telephone system. If, however, higher data transfer
rates
are required, the communication is achieved by way of satellite or other
broadband services.
9. Over all, less expensive surveying costs.
The preferred embodiments gain the advantages through intelligent use of both
communications a~ ~d recording equipment. To cater with this additional
sophistication,
use is also made of auto detection of malfunctioning equipment.
CA 02399257 2002-08-22
-31-
The Figure 13 embodiment also requires the calculation of time delay factors
between sites, and this is accommodated by the computer network used. In the
Figure 13
embodiment, use is made of linear approximation (distance * c ) and cross-
correlation
between remote and base. However, in other embodiments where more than one
base
station is established, use is made of triangulation.
Although the invention has been described with reference to specific examples,
it
will be appreciated by those skilled in the art 'that it may be embodied in
many other
forms.
