CA2883011C

CA2883011C - Transmitter coil system for airborne surveys

Info

Publication number: CA2883011C
Application number: CA2883011A
Authority: CA
Inventors: Oliver Martin WRIGHT; Richard Stephen WRIGHT
Original assignee: Nrg Exploration Cc
Current assignee: Nrg Exploration Cc
Priority date: 2013-04-15
Filing date: 2014-03-01
Publication date: 2021-01-19
Anticipated expiration: 2034-03-01
Also published as: CA2883011A1; AU2014253658B2; WO2014169317A1; AU2014253658A1; ZA201501277B

Abstract

A tow assembly for an airborne electromagnetic surveying system comprises a generally circular support ring comprised of one or more inflatable tubes which, when inflated, form a semi-rigid inflated ring. Attachment straps are provided for attaching a transmitter coil to the support ring, and suspension lines are provided for suspending the ring from an airborne vehicle. Connection arms are provided 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 cross sectional diameter of between 200mm and 750mm.

Description

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

2 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.

3 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

4 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 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 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.

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.

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 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 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.

The tow assembly 10 includes at least one, and preferably two, tails or fins 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 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 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 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.

Claims

Claims:

1 A tow assembly for an airborne electromagnetic surveying system comprising a 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 exterior of the support ring, suspension means for suspending the support ring from an airborne vehicle, and connection means for connecting and supporting a receiver coil concentrically within the support ring.

2. A tow assembly for an airborne electromagnetic surveying system according to claim 1 wherein the system includes the transmitter coil, the receiver coil, and a bucking coil supported concentrically within the support ring.

3. A tow assembly for an airborne electromagnetic surveying system according to any one of claims 1 to 3 wherein the support ring is comprised of a series of individually inflatable tubular sections, joined end to end, which, when all inflated form the semi-rigid inflated ring.

4 A tow assembly for an airborne electromagnetic surveying system according to any one of claims 1 to 4 wherein the support ring is formed of a tubular section shaped so as to form the semi-rigid inflated ring when inflated

5. A tow assembly for an airborne electromagnetic surveying system according to any one of claims 1 to 5 wherein the support ring has a diameter of between 15 m and 25 m.

6 A tow assembly for an airborne electromagnetic surveying system according to any one of claims 1 to 6 wherein the inflatable tube or tubes have a diameter of between 200mm and 750mm.

7. A tow assembly for an airborne electromagnetic surveying system according to any one of claims 1 to 6 wherein the suspension means comprises a series of ties attached to the support ring at spaced apart locations, the ties being connected to a tow rope for suspending below an airborne vehicle in use.

8. A tow assembly for an airborne electromagnetic surveying system according to any one of claims 1 to 7 wherein the connection means connects the receiver above the plane of the support ring, below the plane of the support ring, or co-planar with the support ring.

9. A tow assembly for an airborne electromagnetic surveying system according to any one of claims 1 to 9 wherein the connection means comprises a series of spaced apart radially extending ties connected between the support ring and the receiver.

10.A tow assembly for an airborne electromagnetic surveying system according to claim 1 wherein 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 coil being concentric with the two rings, and located concentrically within the inner ring.

11.A tow assembly for an airborne electromagnetic surveying system according to claim 10 wherein a bucking coil is mounted to the inner ring, concentric therewith