Patent Publication Number: US-RE42217-E

Title: Airborne electromagnetic time domain system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       This application claims priority to U.S. Provisional Patent Application Ser. No.  60 / 427 , 577  filed on Nov.  20 ,  2002 ; and this application is also related to co - pending broadening reissue divisional U.S. patent application Ser. No.  12 / 827 , 650 , filed Jun.  30 ,  2010 . Both this Reissue application Ser. No.  12 / 343 , 321  and its Divisional application Ser. No.  12 / 827 , 650  are reissues of U.S. Pat. No.  7 , 157 , 914 .    
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to the field of airborne geological mapping. This invention further relates to an apparatus for conducting geological surveying using an electromagnetic time domain method. 
     BACKGROUND OF THE INVENTION 
     Time Domain Electromagnetic (TDEM) surveying is a rapidly developing area of geophysical surveying. It encompasses ground based and airborne applications. TDEM geological mapping involves equations for calculating the value of electromagnetic fields that are time dependent. Geological data is then inferred from the electromagnetic field data based on resistivity factors, in a manner that is known. 
     The TDEM method was originally designed for exploration of conductive ore bodies buried in resistive bedrock, but at the present time it is also used extensively in general geological mapping, in hydrogeology, in environmental investigations etc. 
     The method involves generating periodic magnetic field pulses penetrating below the Earth surface. Turning off this magnetic field at the end of each pulse causes an appearance of eddy currents in geological space. These currents then gradually decay and change their disposition and direction depending on electrical resistivity and geometry of geological bodies. The electromagnetic fields of these eddy currents (also called transient or secondary fields) are then measured above the Earth surface and used for mapping and future geological interpretation in a manner that is known. 
     The common technical means to generate magnetic field pulses is a known transmitter generally consisting of a loop of wire or a multi-turn coil connected to the output of a known electrical current pulse generator or transmitter driver. The typical size of a transmitter coil is a few meters in diameter for an airborne device and up to hundreds of meters for ground systems. Generally, the bigger the transmitter coil diameter the stronger its magnetic moment, which then results in deeper and more accurate investigations. 
     An additional multi-turn coil or an x-y-z coil system usually serves as a receiver or sensor for the secondary electromagnetic field. Magnetometers are also applicable for this purpose. Received signals are digitised by a known analog to digital converter (ADC) and processed and stored by computer. 
     The advantage of airborne TDEM systems is the speed with which ground that can be covered in geological surveying. However, there are a number of technical problems in designing airborne TDEM systems based on prior art. 
     The transmitted electromagnetic fields generally generate eddy currents not only in the Earth but also in the proximate metallic parts including those of the system and the aircraft body. The secondary fields of these currents behave as a noise due to typical instability of the system geometry and thermal changes in conductors. This noise impacts the survey data by generally decreasing their reliability for extrapolating geological data therefrom. 
     The most common way to minimise this noise is by keeping the receiver at an adequate distance from the transmitter driver. The result of this spaced apart relationship between the transmitter driver and the receiver is that the secondary fields of the eddy currents in the Earth are comparable with secondary fields of local metal parts and therefore noise level is negligible. This type of solution is used in the TDEM systems branded “GEOTEM” and “MEGATEM” of (FUGRO AIRBONE SURVEYS LTD) GEOTERREX PTY. LTD. This particular solution includes a bird towed behind a fixed-wing aircraft on a tow cable approximately 130 meters long. 
     Another prior art TDEM system consists of a helicopter towed system manufactured by T.H.E.M Geophysics Inc. This system uses a helium balloon to keep its sensor suspended at a distance apart from the transmitter system. 
     One of the disadvantages of these prior art solutions is that there is relatively poor horizontal resolution of the system due to the relatively long distance between transmitter coil and receiver sensor. Another disadvantage is difficulties of system mechanical management in start/landing and in flight manoeuvres. 
     Another prior art method currently used to minimise this kind of noise is to cancel the transmitter primary field localised in metal parts of the system using special coils producing in this local area a magnetic field having opposite direction to the main field of the transmitter coils. This technology is used in the AEROTEM™ branded solution of Aeroquest Ltd. in order to minimise the secondary fields in the metal parts of the transmitter electronics, which instead they locate in the towed bird. This solution requires a high level of system mechanical rigidity. In turn, it leads to heavier frame construction. The heavier frame results in a number of disadvantages. In particular the heavier frame makes transportation of the bird difficult. The production costs and fuel costs associated with manufacturing and use of the AEROTEM™ solution are also relatively high. 
     More importantly, because of the need for a rigid frame having a relatively significant weight, a frame with a generally smaller transmitter coil diameter is selected resulting in a lower transmitter dipole moment. This generally results in insufficient transmitter dipole moment to make deeper measurements. 
     Another problem with the prior art solutions is that they do not easily permit exploiting optimal system geometry, that is the receiver in the centre of the transmitter coil. A relatively large voltage is induced in the receiver coil by each of the magnetic field pulses. But this relatively high voltage in turn renders the receiver preamp saturated and therefore inoperative during system measurement time for a short period after this pulse. This is an important and necessary time for making measurements of the Earth&#39;s response. 
     As a result, the solution of existing systems is to place the system receiver at a distance away from the transmitter where the transmitted pulse is much lower since the strength of this field diminishes as the inverse cube of the distance. However, this then results in a departure from the optimal system geometry. 
     In the case of the AEROTEM™ system, the method of dealing with this large voltage pulse while maintaining optimal system geometry, i.e. receiver in centre of transmitter coil, is to place the receiver coil inside a bucking coil carrying the anti-phased transmitter current so as to cancel a large part of the voltage pulse induced in the receiver coil during the transmitter “ON TIME” while not substantially affecting the reception of the secondary field from the Earth. 
     This approach works well to solve the problem of this on-time voltage pulse problem, however, the process of accurately bucking this signal again mandates the rigid geometry of all parts including the receiver coil. This rigid mounting precludes the proper vibration isolation of the receiver coil thus unwanted mechanical vibration influences the receiver coil so as to induce electrical interference thereby reducing sensitivity. 
     Another technical problem is how to produce maximum magnetic moment in the transmitter coil using minimum weight, size and electrical power. In the above-mentioned systems a significant part of the total weight is used for the structure and power sources. 
     Another problem is the air drag of the bird during flight. Complicated support structures with large effective surface areas create excessive drag. This limits possible flight speed increasing survey cost. 
     Another limitation of the previously mentioned systems is the limitation on the maximum transmitter diameter and therefore obtainable dipole moment. A maximum diameter for these systems is generally attained relatively quickly because the rigidity criterion mandates significant weight of the structure. This stiffness factor forces this type of design to reach the maximum allowable weight for helicopter use before a desirable diameter is attained. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a TDEM system that provides improved sensor resolution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the apparatus of the present invention in an airborne position, in this case towed from a helicopter. 
         FIG. 2  illustrates the tow assembly of the present invention in a perspective view. 
         FIG. 3  illustrates the tow assembly of the present invention in an elevation view. 
         FIG. 4  illustrates the tow assembly of the present invention in a top view thereof, and further showing a bottom view of the receiver section of the tow assembly. 
         FIG. 5  illustrates the structure of the transmitter section of the tow assembly in a partial cut-away view of a joint section thereof. 
         FIG. 5a  illustrates the structure of the transmitter section of the tow assembly in a partial view thereof at a joint section. 
         FIG. 5b  illustrates the structure of the receiver section in a cut-view thereof. 
         FIG. 5c  is a further cut-away view of the receiver section. 
         FIG. 6  is a view of the stabilizer section of the tow assembly, in accordance with one embodiment thereof. 
         FIG. 7  is a chart illustrating the survey data generated by the tow assembly of the present invention in operation. 
         FIG. 8  is a system resource chart illustrating the resources of the system of the present invention. 
         FIG. 9  is a program resource chart that illustrates the resources of the computer product of the present invention. 
     
    
    
     In the drawings, one embodiment of the invention is illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended as a definition of the limits of the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention consists of an airborne TDEM survey system  10 . The TDEM survey system  10  includes an aircraft  12  and a tow assembly  14 .  FIG. 1  illustrates an aircraft  12  that is a helicopter, however, other aircraft such as airplanes having desirable take-off and landing attributes from a geological survey perspective could also be used. 
     It should be understood that in one aspect of the present invention the tow assembly  14  is separate from the aircraft  12  but then attached thereto by a suitable attachment means. Provided that the flexible frame discussed below is provided, the tow assembly  14  could be integrated with an aircraft  12  to produce a geological surveying aircraft including a tow assembly  14  in accordance with the present invention. 
     The tow assembly  14  of the present invention generally includes a flexible frame  15 , as illustrated in FIG.  2 . The flexible frame includes a transmitter section  16  and a receiver section  18 . In accordance with the present invention, the receiver section  18  is in most implementations substantially disposed in the center of the transmitter section  16 . This generally provides the optimal geometry referred to above. 
     One aspect of the present invention is the ease in which the tow assembly  14  can be assembled, disassembled and therefore transported from one location to another. Another aspect of the present invention is that the flexible frame  15  overall can be adjusted in terms of its size to suit for particular applications. 
     To this end, the transmitter section  16 , in a particular implementation of the present invention, as shown in  FIG. 4 , consists of a substantially octagonal support frame  20 . The support frame  20  consists of a plurality of substantially tube sections  22 . As best shown in  FIG. 5a , the various tube sections at the corners are interconnected by means of elbow sections  24 . 
     The tube sections  22  can consist of a single piece, or multiple pieces that can be interconnected. The tubing used in the present invention consists of composite material tubing such as fiberglass or Kevlar  a high strength material, such as a para- aramid synthetic fiber, e.g., KEVLAR . Alternatively, the components (described below) of the support frame  20  can be made of carbon fiber for increased strength, preferably with non-conductive areas along the length of one or more of the components to avoid the anomaly that would be caused by a complete conductive loop. 
     One embodiment of the support frame  20  of the present invention described consists of tube sections  22  and elbow sections  27   (not shown) whereby adding additional tube sections  22  or multiple pieces together providing one of the tube sections  22 , as well as additional elbow sections  24 , provides a support frame  20  having a greater surface area. It should be understood that tube sections  22  and elbow sections  24  can be added or removed to increase or decrease the surface area. 
     While the support frame  20  shown in the Figures has an octagonal shape, it should be understood that the present invention also contemplates support frames  15  having other polygonal shapes, although a polygonal shape approximating a circular shape is generally preferred. It should be understood that the modular pieces together providing the support frame  20  can be modified to provide a support frame  20  having a substantially circular profile. Also, in applications of the present invention where transportation and adjustment of the size of the flexible frame  15  is not required, the support frame can be provided in a single unitary construction, as opposed to the modular construction described above. 
     It should be understood that the construction of the support flame  20  described herein enables a relatively large surface area while the support frame  20  of the present invention is also relatively lightweight. By way of example only, it was found that the construction described herein easily permitted an increase of the transmitter loop diameter (or more than) up to  26  meters while permitting maneuvering of the aircraft  12  with the tow assembly  14  in tow. 
     The support frame  20 , as best shown in  FIG. 3 , is suspended using rope  26  from its corners (in the polygonal construction thereof). In a circular construction of the support frame  20 , the support frame  20  would be suspended by rope at substantially equidistant points along the circumference thereof. 
     The rope  26  is then attached to a central tow cable in a manner that is known. 
     The support frame  20  bears a known multi-turn transmitter coil  28  so as to provide the transmitter function of the transmitter section  16 . In the embodiment of the invention shown in  FIG. 3 , the transmitter coil  28  is strung along the bottom of the support frame  20  by attaching the transmitter coil from multiple points along the support frame  20  by a suitable form of attachment. Alternatively, the transmitter coil  28  can be disposed inside the support frame  20 . 
     In another aspect of the support frame construction of the present invention, the invention also provides flexibility in the ability to make changes in receiver loop turns and loop area, and also by adding receiver coils in other axes, without change to the to disclosed tow assembly  14  configuration. 
     In accordance with the present invention, a known electronic transmitter driver  32  that feeds the transmitter coil  28  is installed in the aircraft  12 . The transmitter driver  32  is connected to the transmitter coil  28  as illustrated in FIG.  8 . This connection is generally provided by wiring the transmitter coil  28  to the transmitter driver  32  along the central tow cable and at least one of the ropes  26  supporting the support frame  20 . 
     The flexible frame  20  also includes a stabilizer as shown in FIG.  1 . The stabilizer  36 , as best shown in  FIG. 6 , generally has a stabilizer frame  37  that supports an aerodynamically shaped stabilizer tube  38 . The stabilizer  36  is generally made of plastic and is connected to the support frame  20  at a point by means of a suitable attachment. 
     In an embodiment of the present invention, as best shown in  FIG. 4 , a series of tension ropes  40  are attached to the support frame  20  at various points and then connected to a central hub  42 . In the particular embodiment of the support frame  20  shown in  FIG. 4 , having an octagonal shape, the tension ropes  40  are attached to the corners of the support frame  20 . The tension ropes  40  provide some rigidity to the support frame  20 . 
     As best shown in  FIG. 4 , the receiver section  18  also consists of a plurality of interconnected receiver tube sections  44  together providing a receiver frame  45 . These receiver tube sections  44  are also made of plastic and are similar in construction to the tube sections  22  and elbow sections  24  that provide the structure of the support frame  20  in the particular embodiment thereof described herein. The tube sections  44  generally provide, however, a receiver section  18  having a much smaller surface area than that of the receiver section  18  or support frame  20 . As best shown in  FIG. 5a , the various receiver tube sections  44  are interconnected by means of receiver elbow sections  46 . 
     Much as in the case of the support frame  20 , the receiver frame  45  has a modular construction whereby additional receiver tube sections  44  and receiver elbow sections  46  may be added to provide a receiver frame  45  having a greater or lesser surface area. Also similarly, the receiver frame  45  can in accordance with the present invention be provided in accordance with alternate polygonal structures or in fact a circular structure. In addition, a unitary construction as opposed to a modular construction may be desirable. 
     In accordance with one embodiment of the present invention, the receiver frame  45  is mounted on the tension ropes  40  by leading the tension ropes  40  through a series of loops  48  disposed on the receiver frame  45  as best shown in FIG.  4 . 
     The receiver frame  45  is provided with a sensor coil  50 . Sensor coil and or sensor loop are synonymous terms being used interchangeably throughout. In accordance with an embodiment of the present invention, the sensor coil  50  is disposed inside a shell  52  disposed inside the receiver frame  45 , as shown in  FIGS. 5b and 5c . The shell  52  consist of plastic tubing similar to the tubing the receiver tube sections  44  and receiver elbow sections  46 , but having a smaller circumference. 
     In addition, the shell  52  is elastically suspended using a series of elastics  54  (one shown only) attached to points  56  along the inner wall of the receiver frame  45  tubing and elastically supporting the shell  52 . The sensor coil  50 , in turn, is elastically supported by a series of elastics  54  (one shown only) attached to points  56  along the inner wall of the shell  52 . 
     The elastic suspension of the sensor coil  50  inside the shell  52  minimizes the effect of vibration. 
     In one particular embodiment of the present invention, the sensor coil  50  output is connected to a non linear preamplifier  63  mounted in a box on the shell  52  outer surface (not shown). This is illustrated in FIG.  8 . 
     The result of the above is that metallic parts except wires and the preamplifier  63  are generally concentrated in the aircraft  12  far enough from field generating and the sensitive components of the flexible frame  12 . This results in relatively small parasitic eddy currents whereby useful signals dominate. 
     A further result of the tow assembly construction described above, is that the tow assembly consists generally of the tubular fiberglass parts described above whereby generally more than a half of the bird weight belongs to transmitter coil wires. 
     Generally a transmitter coil  30  having relatively thick wires with low resistance that can reach higher intensity of the transmitting magnetic field is used. Of course, the overall weight must not exceed values that would otherwise unduly burden the aircraft  12  or negatively affect maneuverability. 
     In addition to the fiberglass or carbon fiber tubing, the tow assembly  14  uses the ropes discussed above. This reduces the need for additional plastic or metal spokes. The ropes reduce air drag and allows for higher flight speed. 
     As best illustrated in  FIG. 8 , the system of the present invention also includes a signal-processing computer  58 . The computer  58  includes a known analog to digital converter device (ADC)  60 . The output of the preamplifier is connected in sequence to a known amplifier  62 , low pass filter  64  and then the ADC  60 , in a manner that is known. The ADC converts the analog data produced by the sensor coil  50  and preamplifier in combination to produce digital data for digital data conversion as described below. 
     The signal from the sensor coil  50 , which is proportional to dB/dt, goes through the amplifier  62  and low pass filter  62 . The ADC  60  continuously converts the signal to digits. The computer  58  includes a microprocessor (not shown) and is linked to a memory. A computer program  66  is installed on the computer  58  for analyzing the digital data to produce the survey data illustrated in FIG.  7 . The computer program can produce arbitrary output waveforms including square, trapezoidal and triangular waveforms in order to meet the particular survey requirements. The computer program  66  also permits pulse repetition rate to be dynamically altered to lower repetition rates being more suitable for very conductive targets or higher for less conductive targets.  FIG. 9  illustrates the resources of this computer program. 
     The sensor coil  50  parameters define the necessary sensitivity so that the signal does not exceed the input range of the non linear preamplifier. 
     The preamplifier  63  is a differential amplifier with a specially designed, fast recovery, dual-mode gain. In relation to the TDEM process, the differential amplifier has a high linear gain of the signal within a set range equal to the expected measurement signal level with the pulse off and rapidly turns the amplified signal to low gain when the signal exceeds this limit during the “on” pulse. In that way the preamplifier limits output voltage during “ON TIME” pulse and provides low distortion and has fast recovery and high gain during off time. 
     This in turn allows the sensor coil  50  to be placed in the optimal position in the center of the transmitting section  16  without the need for any bucking of the primary transmitted pulse. This then allows the use merely of vibration isolation of the sensor coil  50  (as described above) thus increasing our signal to noise ratio. 
     By using this dual-mode gain amplifier method over the bucking method, a transmitter loop diameter and corresponding size of the support frame, as well as the number of loop turns can be selected to suit particular geological targets simply and on site. 
     Alteration of these parameters in the context of a bucking system is generally discouraged because the bucking system would be  lose effectiveness in the advent of such alteration. Thus the bucking method is generally less flexible than the present invention. 
     In another aspect of the invention, the support frame  20  is also adapted to measure the signal during the on-time so as to provide in-phase information. This has been found to improve survey data, for example, in the case of ore bodies of relatively high conductance, for example, nickel. This is achieved by taking signal off of the transmitter coil  28 , or alternatively a separate receiver coil is looped tightly to the transmitter coil for this purpose. 
     In another aspect of the invention a current measuring unit (not shown) is added to the system of the present invention. The current measuring unit measures the residual currents circulating in the transmitter coil  28  during the “OFF” interval thereby enabling the system to minimize distortions caused by these residual currents to the earth response to the electromagnetic field pulse. This is especially important in the time immediately after the transmit pulse when current leakage and current oscillations may exist for a short time. These currents cause errors in the received signals. One implementation of the current measuring unit consists of an air-core transformer and preamplifier which is then connected to an AD converter. The transformer is preferably designed like a Rogowski coil which includes wide dynamic and frequency ranges, high stability and linearity of its characteristics and easy calibration. The primary winding of the transformer is connected in serial with the transmitter coil so that the current flowing through the coil generates emf=M*dl/dt at the secondary winding of the transformer. The signal-processing computer  58  is connected to the transformer and therefore sample signal therefrom much as the receiver signal and uses this data for further correction of the receiver signal. In one particular implementation thereof the current measuring unit is housed in a box (not shown) and is mounted on the tow cable. 
     Other modifications are possible. For example, additional receiver coils oriented in the X-axis and/or the Y-axis can be added. The use of a mechanically flexible relationship between the transmitter coil and the receiver coil. This simplifies and greatly reduces the necessary weight of the support structure as well as allowing the user to use a much larger loop diameter thus giving the system higher dipole moment. The ability to rotate the entire structure 90 degrees so that the transmitter flies in the X-axis direction thus allowing for better detection of vertical conductive bodies.