Note: Descriptions are shown in the official language in which they were submitted.
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"METHOD AND APPARATUS FOR CONDUCTING
ELECTROMAGNETIC EXPLORATION"
BACKGROUND TO THE INVENTION
THIS invention relates to a method and apparatus for conducting
electromagnetic exploration, i.e. geophysical survey.
It is known in electromagnetic exploration systems to make use of a high
power transmitter which generates a primary, time-varying electromagnetic
field by means of a transmitter loop. The primary field excited currents in
the earth which in turn generate a secondary field. The secondary field
detected by a receiver can be used in analysis of, for instance, the earth's
composition. It is recognised that the sensitivity and dynamic range of the
system are limited by the ratio between the primary and secondary fields
detected by the receiver.
Exposure of the receiver to the primary field interferes with the accuracy
and accordingly the usefulness of the detected signal. It is accordingly
recognised that it is desirable to null or buck the primary field at the
receiver
in order to limit the interference. For instance, the known AeroTEM system
makes use of a transmitter consisting of a flat primary coil or loop which is
fed with current in one direction to produce a primary field in one axial
direction. A flat nulling or bucking coil of smaller diameter is arranged at
the
centre of the primary coil and is fed with current in the opposite direction
to
produce a nulling field opposed axially to the primary field, thereby to null
the primary field at the axis and in the axial direction.
CONFIRMATION COPY
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However, nulling of the primary field in the axial direction alone enables the
receiver to detect the secondary field in that direction only because
components of the primary field in other directions still swamp the receiver.
It is accordingly not possible with the known system to perform accurate
detection of the secondary field in a three-dimensional or vectorial sense. In
addition, it has been shown that such the accuracy of the known system is
sensitive to various perturbations, for example axial, radial or rotational
displacement of the nulling coil relative to the primary coil. This in turn
means that the mechanical coil structure must be robust and stiff, thereby
detracting from its aerodynamics. This may be particularly problematical in
situations where the coil structure is to be conveyed by an aircraft such as
a helicopter.
The present invention seeks to provide an improved system.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a method of
conducting electromagnetic exploration of the earth's surface, the method
comprising the steps of providing at least one primary coil, powering the
coil to generate a primary electromagnetic field, exposing the earth's
surface to the primary field, providing a receiver to detect a secondary field
generated by the earth as a result of currents generated therein by the
primary field, providing a plurality of spaced apart nulling coils and
powering the nulling coils in a manner to null the primary field in a three-
dimensional volume surrounding the receiver.
Nulling the primary field in a three-dimensional volume surrounding the
receiver enables the receiver to detect the secondary field vectorially.
The nulling coils may be powered by the same current as the primary
coil(s), by arranging them in series with the primary coil. It is however
within
the scope of the invention for the current feed to the nulling coils to be
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controlled, eg by appropriate current shunts, such that specific nulling
coil(s) are driven by a fraction only of the primary current, i.e. the current
fed to the primary coil(s).
In one particular configuration envisaged by the invention the nulling coils
are arranged coaxialiy, on the axis of the primary coil(s), in a vertically
spaced, stacked relationship. There may be multiple primary coils arranged
coaxially in vertically spaced relationship.
According to another aspect of the invention there is provided apparatus for
conducting electromagnetic exploration, the apparatus comprising at least
one primary coil, means for powering the coil to generate a primary
electromagnetic field and for exposing the earth's surface to the primary
field, a receiver to detect a secondary field generated by the earth as a
result of currents generated therein by the primary field, a plurality of
spaced apart nulling coils and means for powering the nulling coils in a
manner to null the primary field in a three-dimensional volume surrounding
the receiver, thereby enabling the receiver to detect the secondary field
vectorially.
Still further according to the invention there is provided a nulling coil
apparatus comprising a plurality of nulling coils arranged in a
predetermined configuration relative to a receiver and to one or more
primary coils producing a primary field, and means for powering the nulling
coils such that in combination they produce a field serving to null the
primary field in a volume surrounding the receiver, thereby enabling the
receiver to detect, vectorially, a secondary field generated by the earth in
response to exposure thereof the primary field,
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According to an aspect, there is provided a method of conducting
electromagnetic
exploration of the earth's surface, the method comprising the steps of
providing at least one
primary coil, powering the coil to generate a primary electromagnetic field,
exposing the
earth's surface to the primary field, providing a receiver to detect a
secondary field
generated by the earth as a result of currents generated therein by the
primary field,
providing a plurality of spaced apart nulling coils and powering the nulling
coils in a manner
to null the primary field in a three-dimensional volume surrounding the
receiver wherein the
nulling coils are arranged coaxially, on the axis of the primary coil(s), in a
vertically spaced
relationship.
According to another aspect, there is provided an apparatus for conducting
electromagnetic
exploration of the earth's surface, the apparatus comprising at least one
primary coil, means
for powering the coil to generate a primary electromagnetic field and for
exposing the earth's
surface to the primary field, a receiver to detect a secondary field generated
by the earth as
a result of currents generated therein by the primary field, a plurality of
spaced apart nulling
coils and means for powering the nulling coils in a manner to null the primary
field in a three-
dimensional volume surrounding the receiver, wherein the nulling coils are
arranged
coaxially, on the axis of the primary coil(s), in a vertically spaced
relationship.
According to another aspect, there is provided a nulling coil apparatus for
use in
electromagnetic exploration of the earth's surface, the apparatus comprising a
plurality of
nulling coils arranged in a predetermined configuration relative to a receiver
and to one or
more primary coils producing a primary field, and means for powering the
nulling coils such
that in combination they produce a field serving to null the primary field in
a volume
surrounding the receiver, thereby enabling the receiver to detect,
vectorially, a secondary
field generated by the earth in response to exposure thereof the primary
field.
Other features of the invention will appear from the description given below.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example only,
with reference to the accompanying drawings.
In the drawings:
Figure 1 diagrammatically illustrates a first embodiment of the
invention; and
Figure 2 diagrammatically illustrates a second embodiment of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The apparatus 10 seen in Figure 1 includes a single, flat primary coil 12
and a plurality, in this case two, flat nulling or bucking coils 14 arranged
in a
vertically spaced, stacked relationship, coaxially with the primary coil. The
primary coil is powered with current in the sense indicated by the arrow 16
while the nulling coils 14 are powered with current in the opposite sense
indicated by the arrow 18. The primary field produced by the primary coil 12
is indicated by the arrow 20. The nulling field produced by the nulling coils
14 is indicated by the arrow 22. As described below in more detail, the
geometry of the coils and their powering is such that the field 20 at least
approximately cancels the field 22 in a volume at the common axis of the
coils.
The nulling coils may have a single turn or multiple turns. In addition it is
possible to control the field curvature of the nulling field produced by the
nulling coils by varying their vertical separation and radius. In practice,
for a
given number of turns in the nulling coils, the coil radius and vertical coil
separation will be selected in order to provide optimal nulling of the primary
field 20 over the nulled volume.
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In a practical arrangement use was made of an eight-turn primary coil 12
with a nominal diameter of 10nn. The table below gives optimal
arrangements, derived mathematically, for the nulling coils.
No. Diameter of Separation Comments
bucking bucking of bucking
turns coils (mm) coils (mm)
2 x 1 1813 886 Z-bucking effective over 100 mm,
Radially very narrow
2 x 2 3806 1700 Z-bucking well matched over 150
mm, radial error < 10 nT over 15
mm radius.
2 x 3 6280 2226 Z-bucking well matched over 250
mm, radial error < 10 nT over 30
mm radius.
From this table it can be seen that where the nulling coil arrangement
consists of two single turn nulling coils 14 of diameter 1.813m and spaced
apart vertically by 0.886m, Z-bucking or nulling, i.e. nulling in a vertical,
axial sense, was very effective but the nulled volume had a limited radial
extent. At the other end of the scale, with three-turn nulling coils of
diameter
6.28m separated vertically by 2.226m, there was again good nulling in the
axial sense but in this case the nulled volume was of greater radial extent.
Stacked nulling coils arranged according to the table were subjected to
perturbation analyses in which the effect of radial, rotational and axial
perturbations applied to the single and multiple turn nulling coil
configurations were investigated. These analyses also illustrated that the
diameter of the axially nulled volume is considerably greater, for each
version, and that radial perturbations, i.e. displacements have a less
pronounced detrimental effect than in the case of the known AeroTEM
system referred to above. While the system is also tolerant of rotational
perturbations, it is quite sensitive to axial perturbations.
Figure 2 illustrates an embodiment which includes a pair of stacked primary
coils 12 and, again, a pair of stacked nulling coils 14. The primary coils are
arranged as a Helmholtz pair in which the combined magnetic field, along
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the common axis of the coils, is constant. With this configuration of primary
coils, vertical nulling of the primary field can be carried out particularly
effectively by arranging the nulling coils as an opposing Helmholtz pair, i.e.
with a number of turns scaled in proportion to the ratio of the radii of the
primary and nulling coils, and with current flowing in the nulling coils in a
direction opposite to that in which it flows in the primary coils.
The diameter of the primary coils was selected to be 7.07m, each primary
coil having eight turns, in order to provide a dipole moment similar to that
of
the 10mm diameter, single primary coil arrangement of Figure 1, for
comparison purposes. For this primary coil configuration, the table below
gives optimal configurations and geometry for the nulling coils.
No. Diameter of Separation Comments
bucking bucking of bucking
turns coils (mm) coils (mm)
2 x 1 884 442 Z-bucking effective over 25 mm,
R-bucking not effective
2 x 2 1768 884 Z-bucking < 10 nT for r < 75 mm,
R-bucking < 10 nT for r < 75 mm.
2 x 3 2562 1326 Z-bucking < 10 nT for r < 150 mm
R-bucking < 10 nT for r < 100 mm.
2 x 4 3526 1768 Z-bucking < 10 nT for r < 180 mm
R-bucking < 10 nT for r < 125 mm.
This table shows that Z-bucking or nulling, i.e. nulling in the vertical or
axial
sense, is effective for single turn as well as two-, three- and four-turn
nulling
coils, and that R-bucking or nulling, i.e. nulling in the radial sense, is
effective for the multiple turn nulling coil configurations. The
configurations
with multiple turns show nulled volumes of increasing radial extent with
increasing numbers of turns.
It is however noted that the nulling coils in these versions are of smaller
diameter than in the corresponding versions described above with
reference to the Figure 1 embodiment. In addition, it is shown that axial
perturbation or displacement is more readily accommodated than with the
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embodiment of Figure 1 while the system is also tolerant of radial and
rotational perturbation.
As illustrated above, the use of multiple nulling coils in particular,
optimised
configurations can provide effective nulling in the vertical or axial sense,
making both embodiments particularly suitable for use with an axially
positioned, vertically mounted receiver/detector. The analyses also indicate
that with both embodiments it is possible to provide a nulled volume of
significant radial extent with a controlled radial field gradient. Both
embodiments are reasonably tolerant of radial and rotational perturbations
while the embodiment of Figure 2 is more tolerant of axial perturbations
than the embodiment of Figure 1.
It is perceived that the embodiment of Figure 1, being vertically more
compact, will be better suited to airborne conveyance while the
embodiment of Figure 2, being vertically bulkier and hence more difficult to
incorporate in an aerodynamic structure, will be better suited to terrestrial
i.e. surface-conveyance.
The nulling coils in both embodiments may, as indicated above, be
powered in series with the primary coil(s). Alternatively, suitable current
shunts or other controllers can be used to feed fractions of the primary
current to individual nulling coils.
