Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION:
method and apparatus for detecting rock movement
NAMES) OF INVENTOR(S):
Moustafa Abdel Kader Mohamed
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus
for detecting rock movement.
BACKGROUND OF THE INVENTION
Scientists have been trying to perfect techniques for
co:Llecting data regarding rock movement, such as that which is
caused by the flow of water in a river or stream, which is
necessary for a thorough study of erosion, flooding,
sedimentation and the like. One technique that scientists have
employed involves collecting rocks from the site under
investigation and implanting a small magnet inside each rock.
The rocks are then placed strategically upstream of their
anticipated path of travel. After an event of flooding, the
new positions of the rocks are traced by detecting the magnet
field of the magnets inside each rock. The movement which the
ro~~ks have undergone can then be deduced from the changes in
the positions of the rocks. One disadvantage of this method
is that it is costly to collect rocks, implant a magnet in each
rock, strategically place the rocks back at the site under
investigation and detect the rocks having magnetic implants
after the event. Another disadvantage of this method is that,
in adding magnets to the :rocks and placing the rocks back into
the environment, there is a human intervention that can skew
the data received.
A proportion of rocks at any site under investigation have
a naturally occurring remnant magnetization that is strong
enough to be detected. The signal level which can be obtained
using existing technology for detecting the remnant
magnetization in a rock that is capable of being detected is
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approximately 10 microvolts. By working out a statistical
avE=rage of the percentage of rocks at a site under
investigation that are capable of being detected, rough
estimates can be made as to the real number of rocks being
moved. This method of detection of rock movement requires a
considerable amount of study to arrive at a statistical average
as to the percentage of rocks at the site under investigation
having a remnant magnetism of sufficient strength to generate
a :signal of magnitude of 10 millivolts. It also requires the
use of expensive amplifiers to condition the signals.
SZTL~lARY OF THE INVENTION
What is required is a method and apparatus that will
permit more accurate and less costly detection of rock
movement .
According to one aspect of the present invention there is
provided a method for detecting rock movement. The first step
involves using at least one electromagnetic induction coil as
a sensor. The second step involves monitoring changes in
induced electromotive force in at least one electromagnetic
induction coil over a selected time interval.
This new approach is based upon changes in magnetic
permeability in medium proximate to the coil as a result of
ro~~k movement in accordance with Faraday's law of
electromagnetic induction.
Magnetic induction B is given by
B = ~.H
where ~, is the magnetic permeability of a given medium and H
is the applied magnetic field. The magnetic flux ~ through one
loop of an electrical coil is given by
B.A.cosO
where B is as defined above, A is the cross sectional area of
the loop, and 8 is the angle between the vector of B and the
direction perpendicular to the plane of the loop. As the
magnetic flux ~ changes with time t an electromotive force EMF
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is generated which is proportional to the product of the rate
of change of the magnetic flux and the number of loops in the
electrical coil N
EMF = -N.(d~/dt)
The method of detecting rock movement, as described above,
bared upon induced electromotive force due to the change of
magnetic permeability has been found to be much more sensitive
to rock movement than the previously described method that
re:Lied upon detection of rocks with remnant magnetism. The
method detects changes i.n the magnetic permeability of the
medium proximate to the sensor as an obj ect passes close to the
sensor. The method is capable of generating signals which are
typically between 3 and 10 millivolts as a rock passes by the
sensor; which is about 1000 times greater than the signals
generated by detecting remnant magnetism.
Although beneficial results may be obtained through the
use of the method to determine the existence and extent of the
movement of rocks, as described above, it is also desirable to
detect the speed of such movement. Even more beneficial
results may, therefore, be obtained when a plurality of
electromagnetic induction coils are mounted to a non-magnetic
support, preferably in rows. By knowing a distance between the
rows and a time interval that it took the rock to travel that
distance, a calculation can be made as to speed of motion of
a :rock.
According to another aspect of the present invention,
thare is provided an apparatus for detecting rock movement
which includes a non-magnetic support and at least one
electromagnetic induction coil mounted to the support. Means
are provided for measuring changes in electromotive force of
at least one electromagnetic induction coil. Means are
provided for measuring a time interval.
Although beneficial results may be obtained through the
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use of the apparatus, as described above, in order to determine
speed of rock movement a plurality of electromagnetic induction
co_Lls must be mounted to the support, preferably in rows.
Although beneficial results may be obtained through the
use of the apparatus, as described above, it is preferred that
the electromagnetic induction coils be constructed in a manner
that increases their sensitivity to rock movement (ie. signal
st=rength) and avoids electromagnetic induction caused by
vibration. Even more beneficial results may, therefore, be
obi~ained when the electromagnetic induction coil includes a
central magnetic core, a plurality of loops encircling the
central magnetic core, with the electromagnatic induction coil
being set in epoxy within an iron casing.
Although beneficial results may be obtained through the
use of the apparatus for detecting rock movement, as described
above, there are some additional features that are beneficial
in view of the remote locations at which such apparatus are to
be installed. Even more beneficial results may be obtained
when means are provided for transmitting data from the remote
site to a monitoring station. This enables the data to be sent
from the remote site directly to the location where the data
wi:l1 be processed and examined. Even more beneficial results
may be obtained when a solar panel is provided to recharge a
rechargeable power source that provides power to the apparatus .
BRIEF DESCRIPTION OF THE DRA~NINGS
These and other features of the invention will become more
apparent from the following description in which reference is
made to the appended drawings, wherein:
FIGURE 1 is a top plan view, in section, of an
electromagnetic induction coil sensor constructed in accordance
with the teachings of the present invention.
FIGURE 2 is a side elevation view, in section, of the
electromagnetic induction coil sensor illustrated in FIGURE 1.
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FIGURE 3 is a block diagram of an apparatus for detecting
rock movement constructed in accordance with the teachings of
the present invention.
FIGURE 4 is a graphic representation as to how rock
5 movement can be detected with an electromagnetic induction coil
by detecting changes in magnetic permeability as a rock
approaches and then moves away from the apparatus.
FIGURE 5 is a block diagram of the method used in
detecting and analyzing rock movement.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment, an apparatus for detecting rock
movement generally identified by reference numeral 100, will
now be described with reference to FIGURES 1 through 4.
With reference to FIGURE l, sensor 10 is constructed of
the following components which are assembled in a coaxial
configuration. A strong, circular, permanent magnet 12 is
fitted within a casing 14 composed of soft iron. Magnet 12 has
a circular hole 15 positioned at its centre. In a typical
construction, magnet 12 will be approximately 7.5 cm in outer
diameter and approximately 1.25 cm deep, with hole 15
approximately 2.5 cm in diameter in the centre. The magnetic
field strength of magnetic 12 is approximately 2 kiloGauss at
the centre of hole 15.
Coil 16 is placed coaxially within hole 15 in magnet 12.
The dimensions of coil 16 will be slightly less than 2.5 cm
outer diameter, 0.3 cm inner diameter, and 1.25 cm deep; so as
to avoid direct contact between coil 16 and magnet 12. The
electrical resistance of such a coil will be approximately 500
ohms and the inductance will be approximately 220 milliHenry.
Coil 16 has a plurality of loops 17.
A soft iron core 18 is fitted coaxially within the entire
depth of coil 16 and touching the inside surface of the soft
iron casing 14 in which the assembly is contained. Iron core
18 is used to increase the magnetic flux at the centre of coil
16, compared to the same assembly with air instead of iron core
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18 at the centre of coil 16. It has been found that the
presence of iron core 18 increases the magnetic flux at the
centre of coil 16 by a three or four fold compared to the
magnetic flux at the centre of coil 16 for an otherwise
identical assembly in which there is no iron core within the
coil. Iron casing 14 also serves to enhance the magnetic flux.
The use of soft iron for the construction of casing 14 was
found to increase the magnetic flux at the core by
approximately two fold when compared with the magnetic flux for
apparatus 10 contained within a non-magnetic material. The
casing also shields and protects the enclosed components of
apparatus 10.
It will be obvious to one skilled in the art that the
dimensions or proportions of sensor 10 and of each component
therein can be varied to change the electromagnetic properties
of sensor 10. The principals of operation are not changed by
varying the dimensions.
with reference to FIGURE 2, a side view of the assembled
magnet 12, soft iron casing 14, coil 16, and soft iron core 18
is shown. Electrical leads 20 are connected to coil 16 and
pass through casing 14 to be connected to a data logger or
other recording device. The remaining space within the
assembly is filled with an epoxy or similar non-magnetic and
non-conducting waterproof resin, generally indicated by
reference numeral 21. Resin 21 prevents motion of the
components within the assembly relative to each other. It is
a matter of importance to prevent the motion of the components
of the assembly relative to each other. Motion such as that
caused by mechanical vibrations can create electromagnetic
noise or spurious signals which would adversely affect the
detection limits and accuracy of sensor 10 and thereby reduce
the quality of the data being collected. Resin 21 also
provides the necessary water seal and protection of the
components from damage by humidity, water, or impingement by
rocks or small particles of solid matter.
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With reference to FIGURE 3, a block diagram is shown for
one of the possible embodiments of apparatus 100. A plurality
of sensor 10 are shown mounted on a support 30 in rows 32 and
34. Each of rows 32 and 34 has sensors 10 a through z.
Referring to FIGURE l, each of the electromagnetic induction
coil sensors 10 will be as described above, and will include
central magnetic core 18, coil 16 with a number of loops 17
encircling central magnetic core 18. Referring to FIGURE 3, the
length and width of support 30, and consequently the length of
rows 32 and 34 will be determined by the width of the flow path
at the location at which the array will be used. Support 30
to which sensors 10 are mounted, is constructed from non-
magnetic materials. There is consistent spacing between
sensors a through z in each of rows 32 and 34. The distance
between rows 32 and rows 34 and between sensors a through z
within the rows is selected to be larger than the distance
which a rock (35 or 37) is expected to be capable of travelling
in the interval between measurements. If the expected speed
of flow of the water which will cause the rock to move is
designated V and the time interval between measurements is
designated by dt, then the distance between each of rows 32 and
each of rows 34 will be a multiple of the product of V and dt.
The value of dt will be determined by the scanning time
constant of the signal detection and recording system. The
distance between sensors 10 between rows 32 and 34 and between
sensors a through z within the rows is selected to allow the
collection of sufficient data from across the site under
investigation.
Referring to FIGURES 3 and 5, the signals from each sensor
10 will be collected 4 using a data logger 38 coupled to the
plurality of electromagnetic induction coil sensors 10. Data
logger 38 has processing capabilities and serves as a
microprocessor including means for monitoring changes in
electromotive force of the electromagnetic induction coil
sensors 10 and means for measuring a time interval over which
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those changes occur. The signals from each sensor 10 are
collected through a multiplexer and digitized 5 using an analog
to digital converter 36. The digitized signals and the time
at which each signal was received will be recorded 6 to data
logger 38. The data acquisition, conversion and recording
system will be electrically powered. The electrical power may
be supplied from an external source or may be supplied from a
dedicated source such as a rechargeable battery 40 which can
be recharged using a solar panel 42 or other means . The signals
may be transmitted to a remote location using a transmission
device such as a cellular telephone link 44.
With reference to FIGURE 4, as a rock approaches a sensor
the magnetic permeability of the medium proximate to the sensor
changes. The change in the magnetic permeability is detected
as an induced potential 50. As the rock then moves away from
the sensor the signal decreases in magnitude. The signal from
the detector changes with time 52. The maximum value for the
signal 54 shows at what time the rock was closest to the
sensor.
The use and operation of apparatus 100 will now be
described with reference to the preferred method and with
reference to FIGURES 1 through 5. Referring to FIGURES 1 and
5, the first step 1 of the method involves providing a
plurality of electromagnetic induction coils, as described
above, for use as sensors 10. Referring to FIGURE 3, the
second step 2 involves mounting the plurality of
electromagnetic induction coil sensors 10 in a first rows 32
containing sensors a through z and a second row 34 containing
sensors a through z to a non-magnetic support 30. Referring
to FIGURE 4, the third step 3 involves monitoring changes in
induced electromotive force. Referring to FIGURE 3 as either
of rocks 35 or 37 approach support 30 the magnetic permeability
of the medium proximate to sensors a through z in first row 32
and sensors a through z in second row 34 changes. Referring
to FIGURE 4, the change in the magnetic permeability is
detected as an induced
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potential. The signal from the detector changes with time as
rocks 35 and 37 moves toward, past and then away from the
sensor. As the rock moves toward the sensor the signal
increases, reaches a maximum value as the rock passes the
sensor, and then decreases in magnitude as the rock moves away
from the sensor. The maximum value for the signal shows at
what time the rock was closest to the sensor. Referring to
FIGURE 3, the speed of movement of the rock can be calculated
using data from the sensors 10 in rows 32 and 34 by detecting
a change in induced electromotive force in the electromagnetic
induction coils over a measured time interval. The signals
detected from each sensor are measured in real time. The time
at which rocks 35 or 37 is closest to sensors a through z in
first row 32 is recorded. The subsequent movement of rocks 35
or 37 will then cause them to travel away from first row 32 of
sensors a through z toward second row 34 of sensors a through
z. The time at which rocks 35 or 37 is closest to second row
34 of sensors a through z is recorded. Rock 35 is moving at
right angles to support 30 along a path indicated by reference
numeral 39, which will result in rock 35 moving directly over
sensor b in first row 32 and then sensor b in second row 34.
The time it took rock 35 to move from sensor b in first row 32
to sensor b in second row 34 can then be calculated. The
distance between rows 32 and 34 is known. From the distance
between rows of sensors 32 and 34 and the time taken by rock
to travel from one to the other the speed of movement of the
rock can be calculated. Rock 37, on the other hand is going
on an angular path, generally indicated by reference numeral
41, which will take rock 37 in close proximity with numerous
30 sensors starting with sensors a and b in first row 32 and
ending with sensors y and z in second row 34. The speed and
direction of rock 37 can be determined by plotting the signals
7 for the various sensors along path 41.
35 The strength of the signal recorded from a sensor 10 is
proportional to the change in magnetic permeability in
accordance with Faraday's law of electromagnetic induction.
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The detected signal strength and shape depend upon all of the
following factors: the speed, the angle of approach, the
di:~tance from the sensor and the internal composition of a
given rock. Due to the random nature of all of these factors,
5 each rock will have its own signature signal. This helps in
keeping track of a given rock to calculate its speed.
It will be apparent to one skilled in the art that
modifications may be made to the illustrated embodiment without
10 departing from the spirit and scope of the invention as
hereinafter defined in the Claims.
