Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Transmitter Coil System for Airborne Surveys
Introduction
This invention relates to transmitter coil systems used in the field of
airborne
geological mapping.
Background
Geophysical electromagnetic ("EM") techniques can be effective in determining
the
electrical conductivity of soils, rocks and other conductive material at
depths up to
about one kilometer. Conductivity distribution with depth is of great interest
in
mapping base metals and other deposits and other geological formations.
Geophysical EM methods generally involve the generation of a magnetic field by
applying a periodic current to a transmitter coil system located above the
earth's
surface. This primary magnetic field induces electrical currents in the
ground, and
the secondary magnetic field produced by these currents is measured to provide
information about ground conductivity in the region where the induced current
has
been transmitted. To enhance speed of data capture, the transmitter coil is
preferably carried by an airborne vehicle, and transmitted and received
signals are
measured many times per second.
The secondary magnetic field signal may be measured using a receiver coil
system
which can measure up to three orthogonal components of the magnetic field time
derivative dBldt, The received analog signal may then be amplified, filtered,
and
digitized by a high-resolution high-speed analog-to-digital converter ("ADC"),
and the
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data is then stored along with positioning information obtained from a Global
Positioning System ("GPS"). The data is later processed to generate
geophysical
conductivity contour maps.
EM measurements can be done either in the frequency domain or the time domain.
In frequency-domain electromagnetic (FDEM") measurements, the transmitter coil
generally continuously transmits an electromagnetic signal at fixed multiple
frequencies, while the receiver coil may measure the signal as a function of
time.
The measured quantities may include either signal amplitude and phase, or
equivalently, the in-phase and in-quadrature amplitudes as a function of
frequency.
In time-domain electromagnetic ("TDEM') systems, a pulse of current may be
applied
to the transmitter coil during an on-period, generating the primary or
transmitted EM
field, and then switched off during an off period. The secondary signal may be
measured at the receiver coil as a function of time. The signal amplitude
decay
during the off-period, combined with modeling of the conductivity and geometry
of
geological bodies in the ground, may be utilized to yield conductivity contour
maps.
US Patent No. 7,157,914 shows an example of a TDEM system.
Airborne methods, typically using helicopters, are used for large area surveys
and
have been used for exploration of conductive ore bodies buried in resistive
bedrock,
geological mapping, hydrogeology, and environmental monitoring. The data is
acquired while the helicopter flies at nearly constant speed (for example, up
to
30m/s) along parallel equally spaced lines (for example, 50m to 200m apart
from
each other) at close to constant height above the ground (for example, about
30m).
Measurements can be taken at regular intervals, for example in the range lm up
to
100m.
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For a point far away from the transmitter coil, the magnetic field is
proportional to the
magnetic dipole moment of the coil and inversely proportional to the cube of
the
distance from the coil. The magnetic dipole moment of a coil is the product of
NTA
where N is the number of turns. I is the current, and A is the coil area. The
inductance of a coil is proportional to N2xD, where N is the number of turns
and D is
the diameter of the coil. The voltage induced in the receiver coil by a
magnetic field B
is given by NxAxdBidt, where the coil sensitivity NxA is the product of the
coil
number of turns N and the coil area A, and dB/eft is the time-derivative of
the
magnetic field.
Whenever the survey objective is to map near surface conductivity, a small
magnetic
dipole moment with fast turn-off may be appropriate, in which case the number
of
turns in the transmitter coil is generally smaller, thus yielding a reduced
magnetic
dipole moment and inductance. Conversely, for the detection of conductors at
greater depths, it may be desirable to have a longer off-period, and more
importantly,
to increase the transmitter coil magnetic dipole moment.
Whenever an increase in the magnetic dipole moment may be warranted, it is
necessary to increase either the current I, the number of turns N, or the area
of the
transmitter coil A. The electrical power supply from a single engine
helicopter may be
limited by the helicopter generator unless an auxiliary power supply is used.
Also, a
limiting factor for the amount of current in the transmitter coil is the
electrical
resistance of the coil and tow cable. For a fixed-length of cable, the power,
P, from
the helicopter electrical supply is dissipated approximately as the square of
the
current times the resistance (P=12xR). Decreasing the resistance will increase
the
current by the square root of the decrease. Decreasing the resistance in the
loop
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may be accomplished by heavier gauge wire with its corresponding increase in
weight as the electrical resistance is approximately proportional to the
length times
the resistivity divided by the cross sectional area of the wire. The weight of
the
transmitter coil is also proportional to the length of the cable, and
therefore is
proportional to the number of turns N or the square root of the transmitter
coil area A.
Since the weight of the transmitter coils increases as the square of the
current I, and
linearly with the number of turns N, and as the square root of the area A, for
a given
towing weight capacity of the helicopter, the one way to increase the magnetic
dipole
moment of the transmitter coil may be to increase the area A. Another factor
to
consider when optimizing the transmitter coil I, N, and A is the requirement
of a short
turn-off time in time-domain measurements, which can require a low inductance
of
the transmitter coil, the inductance being proportional to the square of N and
to the
square root of the transmitter coil area.
The transmitter coil generally needs to be supported by some form of support
frame
or structure which is sufficiently robust to withstand the rigours of take-off
and
landing of the helicopter, as well as withstand the forces applied to the
system whilst
it is dragged along the survey path below the helicopter in flight and at the
same time
keeping the coil in its circular design configuration. It is important that
whatever
support structure is used for supporting the coil, it needs to be as robust as
possible,
but also as aerodynamic as possible.
In addition, in order to keep weight down, the support structure should be as
lightweight as possible. These various requirements are often in conflict with
each
other, that is, the more robust the support structure, the greater the weight.
Clearly,
where a large diameter coil is required, the diameter of the support frame is
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increased commensurately, which increases the stresses the frame is subjected
to in
flight, and on take-off and landing.
Increasing the transmitter coil diameter will usually reduce aerodynamics and
increase drag of the system. Large structures will be stressed during take-off
and
landing, and therefore there is generally a limit for the size of rigid
structures that can
be deployed without breaking apart. Reinforcing the structure so that it does
not
break during take-off and landings may mean an increase in the weight of the
structure.
Inevitably, during take-off and landings the support structure is not lifted
horizontally.
This is because the structure, when dragged behind the towing aircraft needs
to be
horizontal as the aircraft moves forward. Typically the tow rope is angled at
approximately 60 to the horizontal, This means that when the support
structure is
suspended vertically below the aircraft, as occurs during take-off and
landing, the
support structure is at approximately 30' to the horizontal. On landing the
lowermost
edge of the structure contacts the ground first, and then the remainder of the
structure is lowered onto the ground. There is a tendency for the structure to
bend as
it is lowered, and, given the large diameter of the structure, those bending
forces can
be quite significant. The reverse occurs as the structure is lifted during
take-off.
Also, in many locations the ground where the structure is landed is not level
and will
often have undulations or obstructive objects such as rocks and bushes which
will
tend to cause bending stresses to the structure as it is laid down.
Additionally, maintaining the transmitter coil shape during flight can be very
important
to provide a fixed magnetic dipole moment, in order not to degrade the quality
of the
measurements. Thus, the requirement for an increased magnetic dipole moment
can
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require careful balancing of strength and weight aspects of the support frame.
When
the support frame is stressed to the point where it deforms, it is desirable
that the
frame automatically re-adopts its pre-deformation shape when the force causing
deformation is removed or reduced.
Summary of the invention
According to the invention there is provided a tow assembly for an airborne
electromagnetic surveying system comprising a generally circular support ring
comprised of one or more inflatable tubes which, when inflated, form a semi-
rigid
inflated ring, attachment means for attaching a transmitter coil to the
support ring,
suspension means for suspending the ring from an airborne vehicle, and
connection
means for connecting and supporting a receiver concentrically within the ring.
Preferably the support ring is comprised of a series of individually
inflatable tubular
sections, joined end to end, which, when all inflated form a generally
circular semi-
rigid frame. Alternatively the ring can be formed of a single tubular section
shaped so
as to form a circular semi-rigid frame when inflated. The support ring may
have a
diameter of between about 15 m and 25 m, although it is possible for the ring
to have
a diameter outside of that range. The inflatable tube or tubes preferably have
a
diameter of between 200mm and 750mm. The ring may be formed of an air
impervious synthetic rubber such as StronganTmDuotex m PVC or Hypalonlm-
NeopreneTm, or Vaimex TM Panama (manufactured by Valmex Mehler) for example.
The tube or tubes may be inflated to a pressure of about 35 kPa. The design of
the
support ring, and the pressure to which it is inflated, will be such so as to
ensure that
the ring maintains is circular form in flight, but it is sufficiently flexible
to deform under
impact loads such as may occur on take-off or landing, or resting on uneven
ground.
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The attachment means may comprise a series of loops attached to the support
ring
through which the transmitter coil is threaded in order to connect the
transmitter coil
to the support ring. The loops may be openable to receive the coil. Velcro TM
type
(Tenable loops may be used to hold the transmitter coil to the support ring.
Alternatively the attachment means may comprise a secondary tube attached to
the
support ring, the secondary tube having a continuous passageway therethrough
for
receiving the transmitter coil.
The suspension means may comprise a series of ties attached to the support
ring at
spaced apart locations, the ties being connected, either directly or
indirectly, to a tow
rope for suspending below an airborne vehicle in use.
In a preferred form of the invention the tow assembly comprises the inflatable
support ring, an inflatable concentric inner ring, and a series of radially
extending
inflatable struts which connect the support ring to the inner ring, the
receiver being
concentric with the two rings, and located concentrically within the inner
ring.
Preferably a bucking coil is mounted to, and concentric with, the inner ring.
The connection means may connect the receiver above the plane of the support
ring,
below the plane of the support ring, or co-planar with the support ring. The
connection means may comprise a series of spaced apart radially extending ties
connected between the support ring and the receiver. The receiver may comprise
a
coil concentric with the support ring, the coil preferably being housed within
a circular
substantially rigid tubular structure. The ties preferably connect the support
ring to
the circular tubular structure. The connection means may comprise a series of
struts
extending radially between the support ring and the tubular structure which
houses
the receiver. The struts are preferably formed as inflatable tubes.
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These and further features of the invention will be made apparent from the
description of preferred embodiments of the invention described below by way
of
example. In the description reference is made to the following drawings, but
specific
features shown in the drawings should not be construed as limiting on the
invention,
Brief Description of the Drawings
Figure 1 shows a side view of a tow assembly according to the invention whilst
being towed behind a helicopter;
Figure 2 shows a plan view of the tow assembly without depicting the towing
equipment;
Figure 3 shows a side view of the tow assembly;
Figure 4 shows a perspective view of the tow assembly;
Figure 5 shows a plan view of the fabric panels which make up the tow
assembly,
prior to welding to form a tubular shape;
Figure 6 shows a similar view to that of Figure 5, after assembly, indicating
the
various inflatable chambers which make up the tow assembly; and
Figure 7 shows a view of the attachment means for attaching the transmitter
coil to
the support ring.
Detailed Description of Preferred Embodiments
As shown in the drawings, a tow assembly 10 is attached by means of a tow rope
12
to a helicopter 14 for conducting airborne electromagnetic surveys of the type
previously discussed, The tow assembly 10 is connected to the tow rope 12 by
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means of a series of connecting lines 16, the arrangement being such that when
the
tow assembly is towed behind a helicopter at normal operating speed for a
survey,
the tow assembly will be generally horizontal, as shown in Figure 1, It will
be noted
that the tow rope 12 is at an angle of approximately 600 to the horizontal.
The tow assembly 10 comprises an outer ring 18, an inner ring 20, and a series
of
eight connecting spokes 22 which connect the outer ring 18 to the inner ring
20. The
spokes, and inner and outer rings are all formed from a series of inflatable
chambers
which, when inflated, result in a wheel-like structure as depicted in the
drawings. The
wheel-like structure is sufficiently rigid so that when towed, as shown in
Figure 1, the
structure will substantially maintain its shape, thereby ensuring that
accurate results
are obtained during the survey. The outer diameter of the outer ring is
approximately
20 meters, and the cross sectional diameter of the ring is approximately
40.0mm
although for operational reasons these dimensions can be changed. The outer
diameter of the inner ring is approximately 6 meters.
As shown in Figure 3, the components of the tow assembly are not co-planar,
that is,
when horizontal the inner ring 20 is located above the plane of the outer ring
18.
Preferably the inner ring is between about 400 mm and 1000 mm above the outer
ring. The non-planar form of the assembly is selected for structural
integrity, which is
important for such a large device being suspended from a moving aircraft. The
inner
ring located above the outer ring will tend to transmit compressive forces
outwards,
causing increased rigidity in the outer ring, and ensuring the assembly will
act as a
structurally integral unit in flight. Also the aerodynamics of the structure
with a raised
inner ring is considered to be preferable compared to a purely planar
structure.
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The tow assembly 10 includes at least one, and preferably two, tails or fins
24
located towards the back of the assembly for keeping the tow assembly properly
aligned with the direction of travel of the helicopter. That is, the tails 24
will ensure
that the tow assembly does not rotate about the tow rope as the assembly is
towed
in use.
The tow assembly also includes three coils for the purpose of transmitting,
bucking
and receiving electromagnetic fields. A transmitter coil 26 is mounted around
the
outside of the outer ring 18, and is held in position by a series of openable
loops 28
which form part of the outer ring as shown in Figure 7. The openable loops 28
are
preferably made in a double sandwich hook and loop (Velcro TM) construction.
This
allows the transmitter coil to be laid out around the outside of the outer
ring, and then
attached to the outer ring by the loops 28. The double sandwich attachment
system
ensures that the coil is held securely. The openable loops 28 will preferably
be
spaced apart from each other at a spacing of approximately 800mm.
A receiver coil 30 is held concentric with and inwards of the inner ring 20.
The
receiver coil is preferably attached to the inside of the inner ring by a
series of ties 32
which hold the receiver coil securely in position, and maintain its
concentricity. The
ties 32 may be made from cord, or wire rope, for example.
A bucking coil 34 is mounted to the outside of the inner ring 20 by an
openable loop
system similar to that which holds the transmitter coil in position. The
bucking coil 34
is essentially co-planar with the receiver coil 30. As shown, the bucking coil
is
mounted on the upper side of the inner ring 20.
The transmitter, receiver and bucking coils are electronically connected in
known
manner to the helicopter 14, the helicopter having the necessary transmitting
and
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receiving and data recording apparatus mounted thereto. The manner in which
the
electromagnetic fields are generated and measured need not be discussed in
more
detail herein since that process is well documented in publically available
literature.
Turning now to Figure 5, one pattern of fabric cut-outs is shown that will
produce a
tow structure 10 of the type discussed herein. As will be clear from the
drawing, the
outer ring is formed of 40 panels of generally rectangular fabric which are
welded
together to form a circular inflatable ring. It is envisaged that the outer
ring will be
formed of approximately 16 separate chambers, each having at least one
inflation
valve 42 and one pressure release valve 44. The inner ring will be formed of
16
panels, welded together to form the inner ring which will be comprised of 8
separate
chambers. The spokes will be formed having two separate chambers.
The fabric from which the tow assembly is formed will be an impervious
synthetic
rubber of the type conventionally used to manufacture inflatable boats, for
example.
The type of material preferred is a 1200gsm polymer, preferably a 2 x 2 Panama
having a tensile strength of 4000/4000N and a tear strength of 400/400 Nlcm.
Valmex Mehler manufactures suitable materials. Other materials could equally
well
be used, and the material, welding, and valve technology is well known in the
inflatable boat arts. As mentioned, each inflatable chamber will include an
inflation
valve and a pressure relief valve which can be used to ensure that each
chamber is
inflated to its optimal pressure, between about 22 and 50 kPa, or 3 to 7 psi,
typically
about 35 kPa.
One distinct advantage of the tow apparatus as described is that assembly and
commissioning is a relatively quick operation. When deflated the entire
structure can
be folded to a bundle about 1600 x 1600 x 900 mm in size. To assemble, the
bundle
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is unfolded, and each of the chambers is inflated, generally in stages to
ensure no
part of the assembly is unduly stressed during inflation. Once all chambers
are
inflated to the correct pressure, the different coils will be mounted in
position, and the
tow structure tied to connection points on the assembly.
Optionally the pressure within the inflatable ring may be controlled from
within the
helicopter. This might be required, for example, where it is desirable for the
ring to be
at less than full operational pressure during take-off or landing. In this
instance the
helicopter may carry a means to pressurise the ring (for example, a high
pressure air
cylinder) which is connected to one or more inflation valves 42, and a remote
control
device (not shown) for opening one or more of the inflation and pressure
release
valves, as required. The system would then include a pressure gauge to enable
the
pilot or operator to ensure that the ring was at the pressure required for the
purpose
at hand.
As mentioned previously, many survey operations are carried out in remote and
inhospitable regions. The ability therefore to assemble and disassemble
quickly and
efficiently provides a significant advantage over other more rigid tow
structures. The
small size of the deflated assembly provides a significant advantage for
transportation. Also, because the synthetic rubber is a well used and
understood
material, repairs to the structure in the event of damage are relatively easy
to carry
out by non-specialist workers.
There may be many variations to the above described embodiment without
departing
from the scope of the invention. For example, rigid struts may be provided
between
the inner and outer rings. The inner ring could also be manufactured from a
non-
inflatable relatively rigid material.
