WO2012151695A1

WO2012151695A1 - Geophysical survey transported system of impulsive electromagnetic type, system fabrication process and corresponding detection methods

Info

Publication number: WO2012151695A1
Application number: PCT/CA2012/050299
Authority: WO
Inventors: Pascal Mouge; Rock Samson
Original assignee: Novatem Inc.
Priority date: 2011-05-06
Filing date: 2012-05-07
Publication date: 2012-11-15
Also published as: CA2739630A1

Abstract

The geophysical survey transported system of TDEM type includes at least one inductor element emitting of a primary magnetic field induced during the measurement period and at least one receiver element receiving the secondary magnetic field generated in return by the ground exposed to the primary field induced during the measurement period. The receiver element of the system is only slightly sensitive to the primary field induced. A fabrication processes of such system and a mining and/or environmental detection method using such system are also disclosed. The system can be hung under, in front or above a vehicle, to realize efficient geophysical measurements.

Description

GEOPHYSICAL SURVEY TRANSPORTED SYSTEM OF IMPULSIVE

ELECTROMAGNETIC TYPE, SYSTEM FABRICATION PROCESS AND

CORRESPONDING DETECTION METHODS

CROSS REFERENCE TO PRIOR APPLICATIONS The present application claims priority over Canadian patent application No. 2,739,630 filed on 6 May 2011, and over U.S. Provisional Patent Application No. 61/544,430 filed 7 October 2011, both of which are hereby incorporated by reference.

TECHNICAL FIELD

The technical field relates generally to transported systems for the geophysical survey using an electromagnetic method functioning in impulsive mode. It also relates to the transported system fabrication processes of TDEM type and to the use of these systems in geophysical surveys by air, overland or seaway. Furthermore, it also relates to a new mining and/or environmental detection method using at least one transported system of the present invention.

BACKGROUND The geologic and environmental investigations at large scale generally start by airborne geophysical surveys. In the mining exploration domain, the magnetic and electromagnetic methods are the most commonly used. The airborne electromagnetic methods (AEM) are historically well developed in Canada because of the high resistivity of the Canadian ground and due to the fact that this ground is thus easily penetrable by electromagnetic fields. The AEM methods have been used since 1948 and the number of these methods has increased considerably during the last decades.

Three types of electromagnetic systems have been developed during this period: the EM-VLF, systems of electromagnetic type in the "Frequency Domain" (FDEM) and the systems of electromagnetic type in the "Time Domain" (TDEM).

The EM-VLF surveys use the electromagnetic fields emitted by the military radioelectric transmitter waves in a frequency domain included between 20 and 25 kHz, as a primary source. Each emitter diffuses an intense vertical dipole which excites the perpendicular conductors in the direction of the propagation. Currents thus generated, which induce secondary magnetic fields, which modify the total magnetic field measured. EM-VLF methods provide limited information and are usually used jointly with other types of airborne detection methods.

FDEM surveys use as active source an alternative current which induces Foucault currents in all the conductors present in the ground. EM secondary fields generated by these currents are weaker by order of sizes than those of the primary field which give them birth. They are, for this reason, difficult to extract from the total EM field measured.

This difficulty to separate the primary and secondary fields is the main reason of the success of the TDEM surveys which measure the response of the ground when the emission is neutralized. The emitter generates a strong current which is abruptly stopped and the Foucault currents generate in turns a secondary magnetic field that is registered during the absence of the primary field. The "On-Time" period, during the impulse is immediately followed by an "Off-Time" period during which a series of "windows'" is registered. The following patents and patent applications relate to TDEM methods and notably to the neutralization of the primary electromagnetic field:

WO 2010/022515 entitled "Bucking coil and B-field measurement system and apparatus for time domain electromagnetic measurements" describes, according to one example embodiment, a geophysical survey transported system (TDEM) for producing a B-field measurement, including: a transmitter coil; a bucking coil positioned in a substantially concentric and coplanar orientation relative to the transmitter coil; a receiver coil positioned in a substantially concentric and coplanar orientation relative to the bucking coil; an electrical current source connected to the transmitter coil and bucking coil for applying periodic current thereto; and a data collection system configured to receive a magnetic field time-derivative signal dB/dt from the receiver coil and integrate the magnetic field time-derivative signal dB/dt to generate a magnetic B-field measurement, the transmitter coil, bucking coil and receiver coil being positioned relative to each other such that, at the location of the receiver coil, a magnetic field generated by the bucking coil has cancelling effect on a primary magnetic field generated.

U.S. patent application No. US 2003/0169045 entitled "method and apparatus for a rigidly joined together and floating bucking and receiver coil assembly for use in airborne electromagnetic survey systems" describes a housing and bucking coil receiving coil system for a helicopter towed concentric coil electromagnetic survey system that reduces micro phonic and primary field noise. The device includes isolation housing, a bucking coil and receiving coil assembly with structural members to rigidly join the two coils together and a suspension system to suspend the joined bucking and receiving coils, in a floating manner, by bungee cords or similar non-metallic vibration dampening devices. A housing with dimensions large enough to enclose the suspended bucking and receiving coil assembly that is lined with acoustic and other vibration dampening material. A method for suspending the joined bucking and receiving coil assembly that isolates the assembly from vibration at the same time keeps the coil assembly from twisting and turning in angular planes from the plane of the transmitter, wherein the acceptable minor motions the coil assembly will be allowed to make by the suspension system are up-down, back-forward and left-right motions.

US patent No. 7,646,201 describes an airborne electromagnetic survey system for conducting geological mapping. A transmitter closed loop structure is used in the system and is designed for connection to a towing airborne vehicle. The transmitter loop structure includes a plurality of interconnected loop segments, and transmitting means are fitted to at least one of the loop segments for generating and transmitting an earthbound primary electromagnetic field effective for geological surveying. Sensing means are fitted to the loop segments for receiving and sensing a vertical component of a secondary resulting electromagnetic field which arises from an interaction of the primary electromagnetic field with ground bodies that are traversed by the sensing means, while simultaneously nulling the primary electromagnetic field. Helical sensing means are positioned in close proximity to the transmitting means to receive and sense a horizontal electromagnetic field contained in the secondary resulting field, while simultaneously nulling the primary electromagnetic field.

U.S. Patent No. 5,557,206 discloses an apparatus and method for creating a magnetic cavity in a region about the centerpoint of two concentric, magnetic field-generating electrical wire coils. The outer of the two coils generates a strong primary magnetic field that may induce a relatively weak magnetic field in a remote conductive material, such as subterranean mineral deposits. The inner of two coils, generates a secondary magnetic field having a smaller amplitude and an opposite polarity from the primary field. Various parameters of the apparatus are calculated so that the two oppositely polarized magnetic fields mutually cancel each other in a specified region inward of the two coils about their centerpoint, creating the magnetic cavity. A magnetic sensor can then be isolated within the magnetic cavity for detecting the weak induced magnetic fields in the remote conductive material without interference from the nearby primary and secondary magnetic fields.

The Time Domain Electromagnetic systems (TDEM) include at least the following component: an impulse source, a transmitter coil, one or several receiver loop, an amplification circuit, a digital circuit (AID converter) and a micro-computer for control, stocking and data processing.

The electromagnetic field that comes back to the ground is very weak in comparison to the excitation field (more than 1 million times weaker). In general the readings are taken at a certain time after the end of the (Off-Time) impulse (more than 10 μβ) so as to wait for the desaturation of the acquisition chain. This has the effect of losing a significant quantity of electromagnetic information that would be related to the very conductor grounds. This approach does not allow obtaining measurements during the (On-Time) excitation impulse.

Therefore, there was a need for a geophysical transported survey system using an electromagnetic method functioning in impulsive mode and free of at least one disadvantage of the prior art systems. More particularly, there was a need for a geophysical survey transported system using an electromagnetic method and presenting at least one of the following advantages: a neutralization of the primary field;

a high sensibility even when using weak currents;

an absolute sensibility, higher than one of the actual systems, so as to be able to reduce in a significant way the power required to the measurement system operation;

- a great rigidity; and

a great capacity allowing notably the use nearby an aircraft.

There was also a need for a fabrication and/or assembly process for transported system of the invention and free of at least one of the disadvantages of the prior art systems and presenting at least one of the following advantages: - a facility to assemble in factory; and

a facility to assemble and to disassemble on site.

There was also a need for a new mining and/or environmental detection method using at least on airborne system of the invention, and this method being free of at least one of the drawbacks of prior art and presenting at least one of the following advantages: - a facility of implementation;

a high sensibility; and

a rapidity of implementations.

SUMMARY

The present concept is based on an electromagnetic approach functioning in impulsive mode. The system is composed of a receiver loop and several receiver loops. The set of loops may be notably hung under, in front or on the sides of an aircraft. The particularity of the system is that the receiver loops are configured in such a way that they are slightly or not influenced by the primary field induced of the receiver loop. It is, therefore, possible to obtain measurements during the length of the (On-Time) impulse without using the bucking coil and more so being able to obtain measurements when the inductor current is back to 0 ampere (Off-Time).

The configuration of the receiver loops is such that it greatly facilitates the assembly of the set of components and that it assures a very good rigidity between the transmitter loop and receiver loops. This has the advantage to reduce the noise induced by geometric deformation of the entire probe.

A first object of the present invention is constituted by a geophysical survey transported system of TDEM type, characterized in that such system includes at least one inductor element emitting a primary magnetic field induced during the measurement period and at least one receiver element receiving the secondary magnetic field generated in return by the ground exposed to the primary field induced during the measurement period, the systems being configured in such a way that the receiver element is slightly sensitive, preferably almost not sensitive, and most preferably insensitive to the primary field induced.

Advantageously, in these systems the receiver element, due to the spatial positioning of the receiver element in relation with the inductor element, is slightly sensitive, preferably almost not sensitive, and most preferably insensitive to the primary field induced. These transported systems possess, advantageously, the ability to neutralize the primary field induced, the neutralization being realized by the summation of the electromagnetic fluxes entering and exiting the system.

According to a preferred embodiment of the invention, neutralization is realized by the summation of the electromagnetic fluxes entering and exiting on both sides of the at least one inductor element.

Advantageously, these transported systems are configured in a way that the neutralization of the entering and exiting fluxes occurs through the surfaces of the at least one receiver element which is positioned on both sides of the at least one inductor element. A preferred family of systems of the invention is constituted by the transported systems capable to neutralize at least 90%, preferably is capable to neutralize approximately 100% of the primary field induced, and most preferably is capable to neutralize 100% of the primary field induced.

These transported systems allow to realized the (On-Time) measurements during the excitation period of the inductor element by an induction current which generates the primary field, in addition to be able to realize measurements when the induction current is returned at 0 ampere (Off-Time).

The transported systems of the invention present preferably at least one of the following features: a pulse source of a current which is for example about 800 amperes for an field induced of about 90 000 A/m², and which supplies the at least one inductor element;

an inductor element including one or several transmitter coil(s) transforming pulse current in a front of electromagnetic waves of the field which is of a transitory type, preferably the length of the front of electromagnetic field being programmable, advantageously of some milliseconds to more than 10 milliseconds;

a receiver element including one or several receiver coil(s) to capture the secondary electromagnetic field coming back from the ground following the excitation of the ground by the primary field induced;

an amplification circuit of the fields present in the system, notably of the secondary fields, the amplification circuit being preferably of a very low level of noise type and his gain varying advantageously dynamically, preferably from +6 to +40 dB by the receiver loop; and a digital circuit AID converter converting the returned signal, induced at the level of the receiver loops by the secondary magnetic field, into numerical data that may advantageously be treated and stocked by a micro-computer.

Advantageously, these transported systems are configured in order to minimise any alteration of the measurement sensibility. More particularly, they are configured in a manner that no error or no alteration of the measurement sensibility of the system will occur during the utilisation of the system. According to a preferred embodiment, the transported systems of the invention include at least one inductor element include n induction loop(s), n being a whole number superior or equal to 1, preferably n being superior or equal to 2, mounted in parallel, preferably each of the induction loops being fed from an independent injector of current.

Advantageously, each of the injectors of current possesses the same current features, in particular the same type of current produced and the same current intensity.

The transported systems of the invention that include n (preferably 2) induction loops fed by n (preferably 2) independent injectors of current, each injector of current injecting the same current in each of the n (n is preferably 2) induction loops, are of particular interest.

The transported systems configured to limit the inverse voltages at the clips of the inductor element to high values while allowing the return to 0 ampere of the current circulating in the loop, are also of a particular interest.

In the transported systems of the invention, the induced voltage (Vind) at the clips of the induction loops has a fixed value allowing to neutralize the current circulating in the induction loop in a short lap of time, which is for example inferior or equal to 30 microseconds, preferably inferior to 20 microseconds, for a voltage inferior or equal to 1 kilovolt; the induced voltage following the following equation: Vind= Ldl/dt.

According to a preferred embodiment, the systems include two independent induction loops coupled to two injectors in a manner to generate an inverse voltage which is preferably about 1 kilovolt, allowing the neutralization, preferably in less than 30 μβ, of the current circulating in the induction loop.

Preferably, the physical configuration of the receiver loops present in the system is such that the configuration facilitates the mounting of the overall components of the system.

These transported systems are such that the configuration of the receiver loops insures the, preferably very good, stability of the relative positioning between the transmitter loop and the receiver loops.

Advantageously, the stability of the relative positioning between the transmitter loop and the receiver loops is due to the proximity of the loops, without creating a significant amplitude of movement.

The transported systems of the invention are characterized by an important reduction of the noise mechanically generated by the physical deformations of the structure of the system, the deformations taking place when using the system for the survey measurement.

According to an embodiment of a particular interest, the transported systems include an even or an odd number of segments which are preferably about identical, preferably the system is constituted by at least four segments, advantageously of at least six segments, more preferably of at least eight linear segments (notably in the case of a kit transportable by airborne means) and even more preferably of least ten segments, the segments being linked together, two by two, by a joining element insuring the attachment of the two adjacent segments. Advantageously, the systems are constituted by several interconnected segments, each including at least one part of the inductor element and at least one receiver element.

Advantageously, each of the segments is built from, preferably from three, tubes secured by panels, preferably secured by three panels which are advantageously perforated panels.

Preferably, the tubes, which are preferably made of carbon fibre, are maintained together with the help of panels that are preferably made of a composite material. Advantageously, in these systems, two of the three panels contain a groove intended to host a receiver loop.

Preferably, two of the three panels include an oval form groove whose length is advantageously about 4 meters by 0.4 meter in width.

According to a preferred embodiment, the third one of the three panels includes a support wherein the transmitter coil is positioned, this support is preferably of a square form and the transmitter coil is maintained in place in this support with the help of a removable upholding device.

The configuration of the transported systems is such that the length of the receiver loop is suited to the specifications of the system.

The specifications of the system are function of the intensity of the wished magnetic field and/or function of the spatial positioning of the system relative to the vehicle insuring the motion of the system when using the system for the survey measurement. As a matter of illustration, the transported systems intended to be transported by helicopter, including two receiver loops having a length of about 4 meters and a width which is preferably of about 0.4 meter.

Advantageously, in each of the grooves are inserted several spring coils made of a conducting wire which is preferably a copper wire, the coils, with such a configuration, serving for the reception of the secondary electromagnetic field coming from the ground.

Preferably, wherein the coil present in the panels placed horizontally serves to measure the component Z of the electromagnetic field while the panels wherein are positioned the receiver loops are placed in the vertical plan serve to measure the components X and Y of the electromagnetic field.

A preferential family is constituted by transported systems including n' receiver loops, n' being a whole number superior or equal to 1, preferably n' is the whole number 16.

The transported systems of the invention wherein each of the receiver loops is placed relative to the transmitter loop in such a way that the voltage induced at the level of these receiver loops is almost neutralized during the excitation period of the inductor element are particularly interesting (see Figure 2).

Advantageously, in these systems, the cable of the transmitter loop is, in each segment, positioned relative to the at least one receiver loop, in such a way that the resulting of the flux passing through the receiver loops is neutralized or almost neutralize. Preferably, wherein the transmitter loop is a cable positioned in such a way to form an isosceles triangle with the centers of the receiver loops.

Preferably, the receiver sets placed in the same plan than the transmitter loop (Z plane) are connected to an analogical summation circuit in such a manner to provide a signal that integrates the totality of the field on 360°. More preferably, each receiver loop includes an amplifier allowing electrical isolating one from each other.

Advantageously, the systems include n (preferably n= 8) segments, the signals of the n (preferably of the 8) amplifiers present are added with the help of a summation circuit. Therefore, these systems allows to obtain the same sensibility as obtain with a system including only one 8 times bigger receiver loop.

In the systems of the invention, contrary to the resonance frequency of a single big receiver loop, the resonance frequency of the receiver loops stay high.

Preferably, the vertical receiver loops are grouped two by two, preferably with the help of an analogical summation circuit. According to a preferred embodiment of the invention, the transported systems include 8 segments and wherein the loops 1 & 8 and 4 & 5 measure the Y+ and Y- components of the electromagnetic field while the loops 2 & 3 and 6 & 7 measure the X- and X+ components of the electromagnetic field. The transported systems additionally include a device allowing hanging the system under or in front or at the back or over a vehicle in motion, the vehicle being preferably of an airborne vehicle type that is preferably a plane or a helicopter.

The transported systems are of type: autonomous, including at least one current generator and at least one emitter configured to transmit the results of the conducted measurements to a measurement analysis center, the center being positioned outside the system; and

semi-autonomous, including at least one physical connection with a current generator located outside the system and/or at least one connection with a device receiving the results of the detection measurements conducted with the system and/or at least one emitter configured to transmit the results of the measurements conducted by the system.

Advantageously, the vehicle is chosen in the group constituted by the vehicles directly moving at the contact of the ground, the vehicles moving in the sky and the vehicles moving on and/or under water.

A second object of the invention is constituted by the fabrication process of a system as claimed, wherein the constitutive elements of the system are assembled according to the known methods of assembly.

Advantageously, the known methods of assembly are chosen in the group constituted by: riveting, detent, bonding, welding and screwing. A third object of the present invention is constituted by mining and/or environmental detection method, consisting in hanging a system, which system is defined in the or which system is fabricated according to one of the fabrication processes described in the second object of the invention, under or at the front of an airborne vehicle or of an automobile vehicle or of a boat or of an amphibian vehicle.

Advantageously, wherein the system is hanged by means of a cable and/or by means of a rigid rod (device of "stinger" type), preferably, the distance between the carrier and the working system is maintained between 1 and 5 meters.

According to an advantageous embodiment of the methods of the invention, wherein the system is fixed at the front of the vehicle and its plan makes, with the horizontal of the location, an angle advantageously included between 0 and 30 degrees.

Preferably, the system after being connected to a power supply is used in a manner to generate a complete electromagnetic acquisition cycle.

Advantageously, the complete electromagnetic acquisition cycle is the following:

- the micro-computer generates a signal intended to synchronise the entire electronic system, preferably this signal takes the form of a synchronisation slot, which the recurrence, as well as the time at the "0" and "1" values, are variable, advantageously, the recurrence of the synchronisation slot varies from 25 to 125 Hz while the duration of the synchronisation signal, at the value "1", varies advantageously from 1 to 4 milliseconds; this signal serves to synchronise the whole electronic system; - the passage of the value "1" of the synchronisation signal activates the source of current and starts the acquisition of the electromagnetic components and of the value of the excitation current, this part of the cycle corresponds to the "On time";

- the passage of the value "0" of the synchronisation signal cuts the source of the current and continues the acquisition of the electromagnetic components and of the value of the excitation current, this part of the cycle corresponds to the "Off-Time";

- throughout the "Off-Time", the micro-computer evaluates the amplitude of each of the electromagnetic signals coming from each components of the electromagnetic field; when, preferably at the precise moment wherein, each of the Z, X+, X-, Y+, Y- components find themselves under a certain level of amplitude, advantageously when the amplitude level is about 100 millivolts, the micro-computer increases the gain, advantageously the increase of the gain is in the range of +40 dB, in order to increase the sensibility for weak amplitude signals; and

- numerical data are stocked in real time in the memory of a set of calculation and/or of a set of data conservation, preferably stocking of data is done on a microcomputer.

Advantageously, the same acquisition cycle repeats itself as long as the electromagnetic collection takes place (also called complete cartographic measurements).

More details about these aspects as well as about the other aspects of the proposed concept will be apparent in the light of the detailed description that follows and of the appended figures. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a top view of an example of an airborne detection system according to the invention, of octagonal type, and wherein eight segments are assembled.

Figure 2 is a side view the structure of one of the eight segments of the airborne system shown in Figure 1.

Figure 3 is a side view the structure shown in Figure 2.

Figure 4 is a slanted isometric the structure shown in Figure 2.

Figure 5 is a slanted isometric the detail of the structure shown in Figure 2 as well as its joint with the adjacent segment. Figure 6 is a schematic way in top view the example of Figure 1 and with the electronic elements incorporated in the system.

Figure 7 is an example of a recording of the signals obtained during the (On-Time) impulse by using the system shown in Figure 1 and wherein the empty signal (in the absence of a conductor in proximity) was subtracted of the produced response by a metal piece used as a test (in the example it is firstly a metal panel placed near the detection systems and then a stainless steel 316, and after an aluminum panel); the responses thus obtained represent the induced voltage at the clips of the receiver by Foucault currents induced in the various metal panels.

Figure 8 is an example of a recording of the signal obtained with the help of the detection system shown in Figure 1, immediately after the end (Off-Time) impulse, without contribution of any metal piece at proximity of the detection system; it is here the typical response of a TDEM system in Off-Time.

Figure 9 is an example of an electronic circuit associated to the detection system shown in Figures 1 to 6. Figure 10 is a side view and according to a vertical segment, the coil of a segment of the system shown in Figure 1.

DETAILED DESCRIPTION

The following examples are given only as a matter of illustration and would not be interpreted as constituting any limitation of the present invention. Preliminary definitions:

System: in the broadest meaning of the invention a system is a set of physical elements including at least one inductor element susceptible of generating a primary magnetic field when it is excited by a current and at least one receiver element susceptible of capturing the secondary magnetic field generated in return by the prospected ground as well as a holding structure in which are positioned at least one inductor element and at least one receiver element.

Segment: in the broadest meaning of the invention, is one of the units that constitute the system and including a holding structure and at least one part of an inductor element and at least one receiver element, these units can be interconnected two by two. The system (S) of measurements represented on Figure 1 includes eight linear segments (1) to (8) linked together with the help of joint pieces ( ) to (8'). The rigidity of the entire system is insured by 8 stays numbered from (1 ") to (8"). Each of the segments is constructed from three tubes, identified by numbers (1 1), (12) and (13), apparent notably on Figures 2, 3, 4 and 5. According to an advantageous embodiment of the invention these tubes are made of carbon fibre. These tubes are maintained together with the help of three perforated panels (14), (15) and (16), the panel (14), bearing an inductor loop number (17), is according to an advantageous embodiment made of composite material. Two of these panels (15) and (16) include a groove identified respectively by the numbers (18) and (19) for the panel (16) and identified by the numbers (20) and (21) for the panel (15), having within the framework of the example the form of a loop (corresponding approximately to the system (S) loop of hexagonal form) that being about 3.7 meter long by about 0.2 meter wide. In each of these grooves are winded several cooper wire coil as shown in Figure 10. These coils serve to the reception of the electromagnetic field coming from the ground. The panel (15), placed horizontally, serves to measure the component Z of the electromagnetic field while the panel (16), placed in the vertical plan, serves to measure components X and Y of the electromagnetic field.

The probe includes (16) receiver loops, that being two receiver loops per segment numbered (1) to (8) on Figure 1. The copper wire coil forming receiver loops find themselves in the groove identified by numbers (18) to (19) for each panel identified by number (16) and in the groove identified by numbers (20) and (21) of the panel identified by number (15). Each of these loops is placed relative to the receiver loop in such a way that the induced voltage at the level of the receiver loops is almost neutralized (see Figure 7).

Figure 8 illustrates an example of the Off-Time signal. The amplitude of this signal is superior to 9 volts, which represents almost the maximum dynamic of the system. This figure, therefore, does not show the insensibility of the loop at the primary field. Figure 7 illustrates an example of the signal during the On-Time which amplitudes are inferior to 300 millivolts, therefore, very weak taking into account that these measurements are done during the impulse. It is, therefore, this figure that puts into evidence the insensibility of the receiver loops at the primary field. These loops, that are in the same plan as the receiver loops identified, by (23) of Figure 5 are connected in such a way to provide a signal which integrates the totality of the field on 360°. Each of the receiver loops is connected to an amplifier shown in Figure 9 and identified by numbers (31) to (38) allowing electrical isolating one from each other. Then following, the amplifiers (35) (36) (37) and (38) are added with the help of a summation circuit shown in Figure 9 by number (39). In proceeding in such a way, it is possible to obtain the same sensibility as one loop 8 times bigger, which resonance frequency stays high, contrary to what would give one only big loop.

As described earlier on Figure 6, the system includes eight linear segments which each of the segments is described in detail in Figure 5. Each of the linear segments includes a vertical receiver loop as described in Figure 5 by number (22). As shown in Figure 6, the vertical loops located in segments 1 & 8 and 4 & 5 measure the components Y+ and Y- of the electromagnetic field while those located on segments 2 & 3 and 6 & 7 measure the components X- and X+ of the electromagnetic field.

Advantages brought by this technology This new technology includes two improvements. The first improvement is relative to the use of a power source including two independent current exits as shown in Figure 9 by number (40) serving to supply the excitation loop and the second improvement is relative to the positioning of the receivers relatively to the receiver loop as shown in Figure 5.

First improvement: For such a system, it is very important to be able to neutralize the current circulating in the excitation loop (17) in the shortest time possible. Since the excitation loop is inductive, longer is the time to zero out the circulating current in the excitation loop is short, more the voltage at the clips of this loop will be high. This voltage finds itself when at the clips of the power source (40). The electronic components of the power source (40), being able at the same time to support the high current values and the high voltage values, are very difficult to obtain on the market and additionally being more expensive. The approach that was retained for this system consists, therefore, to use two excitation loops in parallel identified by numbers (41) and (42) in Figure 9. Each of the loops is fed from an injector of independent current. This same excitation loop is also shown in Figure 5 by number (17) and is made of two aluminum or copper conductors. These two conductors correspond to number (41) and (42) of Figures 9 and 10. The system in its configuration shown in Figure 10 includes also conductor (51), in the case where it is useful to use a third excitation loop in order to increase the magnetic moment of the system.

Since the induced voltage follows the following equation: Vind= Ldl/dt wherein L represents the inductance; dl represents the variation of voltage I (included between I max and 1=0 ampere); and dt represents the period of time required for current neutralization.

As the inductance value L varies with the square number of coils which it includes, the fact that there are two timeless coils reduces from a factor 4 the inductance value L, therefore from a factor 4 the value of the induced voltage required to neutralize the current in a given time in the case where one only excitation loop would be used.

In the case where the two excitation loops are used at the same time, the induced voltage at the clips of the loops will be twice of the induced voltage when one only excitation loop is used since the induced magnetic field is the sum of the produced field for each of the excitation loops.

In the present case, to be able to neutralize the current in less than 30 microseconds using an excitation loops with two coils, the voltage of discharge of the loop (kick-back) must be of 2 kilovolts while that the fact of using two independent loops with only one coil coupled with two injectors generate a voltage discharge of 1 kilovolt. Second improvement:

The mounting of this system consists roughly to link the eight segments (1) to (8) together with the help of joint pieces ( ) to (8'), to link the stays (1 ") to (8") and to place the wire serving to the induction of the primary field in the space (20) intended to this purpose. Each of the 8 segments of this system integrates 2 receiver loops numbers (22) and (23). The receiver loops are located in the grooves (18-19) and (20-21). The positioning of the receiver loops are fabricated in factory and never need to be adjusted on site. The positioning of the location (20) of the excitation loop is done in such a way that it does not induce or slightly induce the signal at the level of the receiver loops. There is than no delicate adjustment to do on site, which simplifies the mounting and come with a very important reduction of mounting time of the system (S).

In laboratory, it was possible to attenuate the signal of the inductor loop by a factor of 60 dB (factor of 1000 in voltage). It seems reasonable to expect to obtain an attenuation of dB (factor of 100) of the signal in the inductor loop in real use condition on site. The fact to place the receiver loops, relative to the inductor loop in such a way that there is no or slightly induced voltage, results in at least four important advantages:

1) There is never saturation of the amplifiers in the receiver loops. This allows starting acquisition signals very early after the end of the impulse because it is not necessary to wait for the recovering of the amplifiers and the low pass filters (43 to 50). It is, therefore, typically possible to start measurements some microseconds after the end of the impulse.

2) It is possible to take measurements during the impulse period (On-Time) as demonstrated on the graph of Figure 7.

3) This approach allows having a reading surface more important than with traditional approach. The sensibility of the receiver type is proportional to the surface, according to the following equation: V=NAdB/dt wherein N is the number of coils of the receiver loops, A its surface and dB/dt represents the variation ratio of the magnetic field in function of time. In the case of this approach, the increase of the surface of the receiver coils has no effect on the insensibility of the receiver loops relative to the primary field created by the excitation loop. It is totally the contrary with the traditional systems that are not able to increase to much the surface of the receiver loops because it has for effect to increase the electromagnetic coupling between the excitation loop and the receiver loop.

In most of the other systems, in order that the induced voltage during the excitation impulse is not too important and that the systems may start the acquisition as soon as possible after the end of the impulse, it is necessary to use a receiver loop which diameter is about 10 times weaker than the excitation loop. For example, for an excitation loop of 11 m, the receiver loop has a diameter of 1.1 m and the receiver surface is then of 0.95 m². In the case of this example of the embodiment of the invention, each receiver loop has a surface of about 0.62 m². As the measurement of the component Z of the magnetic field obtained with the help of eight units, this gives a measurement surface of about 5 m². To obtain an equal sensibility with other systems, the receiver loops of the system of the present invention require to have 5 times less coils, which reduces the inductance value from a factor 25, which increase the resonance frequency of the receivers wherein the bandwidth is as much increased.

4) Being very slightly sensitive to the inductor field, the system does not have to correct eventual measurement errors of the components X, Y and Z of the electromagnetic field, errors caused by the residual current in the inductor loop, with all proportions kept, is very long to neutralize. The graph of Figure 8 presents in black the current variation and in red the measurements of the electromagnetic field. It is easy to note that the current takes more than 40 microseconds to stabilise at value of 0 ampere while the electromagnetic reading is stable after less than 5 microseconds.

In order to be able to do measurements during the impulse period and very early at the end of this impulse period, as of today, several methods have been the subject of patent applications or patents. The method that seems the most commonly known consists to use a supplemental coil in addition to the excitation coil, this coil, smaller than the excitation one, is place near the receiver coil. The diameters and the number of coils must be well adjusted in order to neutralize the excitation field. This approach habitually entitled Bucking Coil finds itself in the patent documents WO 2010/022515 Al, US-5,557,206 and CA-2,420,806. This approach has the following drawbacks: the set of coils (minimum of 3, may also be of 5) must be mechanically very stable one relative to the others. The slightest variation of positioning between these coils causes important parasite signals. A second approach of the prior art consists in using two receiver coils for the component Z of the magnetic field. These two receiver loops are connected in order to subtract the signal from the inductor loop. In principle, if correctly done, i.e. having the right diameters at the level of the three coils (receivers 2 and inductor 1), a good concentricity and a good ratio for the coils between the two receiver coils, it is then possible to neutralize the inductor field. The response of the receiver for the component Z of the field is then the difference between the amplitude obtained at the level of each receivers. This means that this approach integrates the field included in the space separating the two receivers.

Briefly stated, this approach as the following drawbacks:

• Since the receivers are loops which the diameter is near the maximal diameter of the set, they must be winded on their support after the mounting of the set is completed. The mechanic support is generally made of more than 10 meters of diameters to be assembled to be able then to wind the two receivers in phase opposition and to wind the inductor coil.

• The Rogowsky receivers must be winded around the inductor loop. These receivers are than really winded on tubes in which passes the inductor loop; this approach complicates mounting of the inductor loop and more so the measurement surface of this receiver type is generally weak.

• This approach necessitates to wind and to unwind at least three loops when assembling and disassembling the system, which complicates assembling/disassembling and increases the time of realization. The conception of these systems of the invention is function of the survey's objectives: in minerals explorations ("EM prospecting"), the principal need is to locate massive deposit of sulphites or metals. The response by means of conductors is 10 to 100 times superior to the background level. Depending upon the overcharge conduction, the investigation depths may attain up to 600 meters, in environmental investigations ("EM sounding"), there is a need to restraint the contrasts of conductivity associated with the lithology and with the hydrothermal alterations. The level of sensibility of these applications is about 50 times superior to the background level of noise. The investigation depths of these applications are between 0 to 250 meters. Notably based on the usage of rigid coupling of a pair of emitters-receivers, the systems proposed (entitled NovaTEM™) is conceived to allow a large penetration for TDEM survey and a large resolution for TDEM probes.

NovaTEM™ may be compared to existing devices for depth penetration and/or for resolution. The major difference when compared to these systems is the weak level of new noise resulting from the construction rigidity of the pair emitters-receivers

In brief this prior art approach as the following drawbacks:

• Since the receivers are loops which the diameter is near the maximal diameter of the set, they must be winded on their support after the mounting of the set is completed. The mechanic support is generally made of more than 10 meters of diameters to be assembled to be able then to wind the two receivers in phase opposition and the inductor coil. • The Rogowsky receiver must be winded around the inductor loop. These receivers are than very likely winded on the tubes in which passes the inductor loop; this approach complicates mounting of the inductor loop and more so the measurement surface of this receiver type is generally weak.

• This approach necessitate to wind and unwind at least three loops when assembling and disassembling the system, which complicates assembling/disassembling and increase the time of realization.

While the invention has been described in detail with respect to specific embodiments thereof, it will be apparent to one skilled in the art that modifications and changes can be therein without departing from the spirit and scope thereof.

Claims

CLAIMS:

1. A geophysical survey transported system of TDEM type, characterized in that the system includes at least one inductor element emitting a primary magnetic field induced during a measurement period and at least one receiver element receiving a secondary magnetic field generated in return by the ground exposed to the primary magnetic field induced during the measurement period, the system being configured in such a way that the receiver element is slightly sensitive, preferably almost not sensitive, and most preferably insensitive to the primary field induced.

2. The system as defined in claim 1, characterized in that the receiver element, due to the spatial positioning of the receiver element in relation with the inductor element, is slightly sensitive, preferably almost not sensitive, and most preferably insensitive to the primary field induced.

3. The system as defined in claim 1 or 2, characterized in that the system is able to neutralize the primary field induced, the neutralization being realized by the summation of the electromagnetic fluxes entering and exiting the system.

4. The system as defined in claim 3, characterized in that the neutralization is realized by the summation of the electromagnetic fluxes entering and exiting on both sides of the at least one inductor element.

5. The system as defined in claim 4, characterized in that the neutralization of the entering and exiting fluxes occurs through the surfaces of the at least one receiver element which is positioned on both sides of the at least one inductor element.

The system as defined in any one of claims 1 to 5, characterized in that the system is capable to neutralize at least 90%, preferably is capable to neutralize approximately 100% of the primary field induced, and most preferably is capable to neutralize 100% of the primary field induced.

The system as defined in any one of claims 1 to 6, characterized in that it allows to realized the (On-Time) measurements during the excitation period of the inductor element by an induction current which generates the primary field, in addition to be able to realize measurements when the induction current is returned at 0 ampere (Off-Time).

The system as defined in any one of claims 1 to 7, including of at least on the following elements:

a pulse source of a current which is for example about 800 amperes for an induced field of about 90 000 A/m², and which supplies the at least one inductor element;

a receiver element including one or several receiver coils to capture the secondary electromagnetic field coming back from the ground following the excitation of the ground by the primary field induced;

an amplification circuit of the fields present in the system, notably of the secondary fields, the amplification circuit being preferably of a very low level of noise type and his gain varying advantageously dynamically, preferably from +6 to +40 dB by receiver loop; and a digital circuit AID converter converting the returned signal, induced at the level of the receiver loops by the secondary magnetic field, into numerical data that may advantageously be treated and stocked by a micro-computer.

9. The system as defined in claim 8, characterized in that is it configured in order to minimise any alteration of the measurement sensibility of the system.

10. The system as defined in any one of claims 1 to 9, characterized in that it is configured in a manner that no error or no alteration of the measurement sensibility of the system will occur during the utilisation of the system.

11. The system as defined in any one of claims 1 to 10, wherein the at least one inductor element include n induction loop(s), n being a whole number superior or equal to 1, preferably n being superior or equal to 2, mounted in parallel, preferably each of the induction loops being fed from an independent injector of current.

12. The system as defined in claim 11, wherein each of the injectors of current possesses the same current features, in particular the same type of current produced and the same current intensity.

13. The system as defined in claim 12, including two induction loops fed by two independent injectors of current, each injector of current injecting the same current in each of the two induction loops.

14. The system as defined in any one of claims 11 to 13, characterized in that it is configured to limit the inverse voltages at the clips of the inductor element to high values while allowing the return to 0 ampere of the current circulating in the loop.

15. The system as defined in any one of claims 1 to 14, wherein the induced voltage (Vind) at the clips of the induction loops has a fixed value allowing to neutralize the current circulating in the induction loop in a short lap of time, which is for example inferior or equal to 30 microseconds, preferably inferior to 20 microseconds, for a voltage inferior or equal to 1 kilovolt; the induced voltage following the following equation: Vind= Ldl/dt.

16. The system as defined in claim 15, including two independent induction loops coupled to two injectors in a manner to generate an inverse voltage which is preferably about 1 Kilovolt, allowing the neutralization, preferably in less than 30 μβ, of the current circulating in the induction loop.

17. The system as defined in any one of claims 8 to 16, characterized in that the physical configuration of the receiver loops present in the system is such that the configuration facilitates the mounting of the overall components of the system.

18. The system as defined in any one of claims 8 to 15, wherein the configuration of the receiver loops insures the, preferably very good, stability of the relative positioning between the transmitter loop and the receiver loops.

19. The system as defined in claim 18, wherein the stability of the relative positioning between the transmitter loop and the receiver loops is due to the proximity of the loops, without creating a significant amplitude of movement.

20. The system as defined in any one of claims 1 to 19, characterized by an important reduction of the noise mechanically generated by the physical deformations of the structure of the system, the deformations taking place when using the system for the survey measurement.

21. The system as defined in any one of claims 1 to 20, characterized in that it includes an even or an odd number of segments which are preferably about identical, preferably the system is constituted by at least four segments, advantageously of at least six segments, more preferably of at least eight linear segments (notably in the case of a kit transportable by airborne means) and even more preferably of least ten segments, the segments being linked together, two by two, by a joining element insuring the attachment of the two adjacent segments.

22. The system as defined in claim 21, characterized in that each segments includes at least one part of the inductor element and at least one receiver element.

23. The system as defined in claim 22, characterized in that each of the segments is built from, preferably from three, tubes secured by panels, preferably secured by three panels which are advantageously perforated panels.

24. The system as defined in claim 23, characterized in that the tubes, which are preferably made of carbon fibre, are maintained together with the help of panels that are preferably made of a composite material.

25. The system as defined in claim 23 or 24, characterized in that each of two of the three panels contains a groove intended to host a receiver loop.

26. The system as defined in claim 25, characterized in that two of the three panels include an oval form groove whose length is advantageously about 4 meters by 0.4 meter in width.

The system as defined in claim 23, characterized in that one of the panels, preferably the third one of the three panels, includes a support wherein the transmitter coil is positioned, this support is preferably of a square form and the transmitter coil is maintained in place in this support with the help of a removable upholding device.

The system as defined in any one of claims 8 to 27, characterized in that the length of the receiver loop is suited to the specifications of the system.

The system as defined in claim 28, characterized in that the specifications of the system are function of the intensity of the wished magnetic field and/or function of the spatial positioning of the system relative to the vehicle insuring the motion of the system when using the system for the survey measurement.

The system as defined in claim 25, intended to be transported by helicopter, including two receiver loops having a length of about 4 meters and a width which is preferably of about 0.4 meter.

The system as defined in any one of claims 25 to 30, characterized in that, in each of the grooves, are inserted several spring coils made of a conducting wire which is preferably a copper wire, the coils, with such a configuration, serving for the reception of the secondary electromagnetic field coming from the ground.

The system as defined in any one of claims 23 to 31, characterized in that the coil present in the horizontally placed panels serves to measure the component Z of the electromagnetic field while the panels, wherein are positioned the receiver loops, that serve to measure the components X and Y of the electromagnetic field, are placed in the vertical plan.

33. The system as defined in claim 15, including n' receiver loops, n' being a whole number superior or equal to 1, preferably n' is the whole number 16.

34. The system as defined in any one of claims 1 to 29, characterized in that each of the receiver loops is placed relative to the transmitter loop in such a way that the voltage induced at the level of these receiver loops is almost neutralized during the excitation period of the inductor element.

35. The system as defined in claim 34, characterized in that the cable of the transmitter loop is, in each segment, positioned relative to the at least one receiver loop, in such a way that the resulting of the flux passing through the receiver loops is neutralized or is almost neutralized.

36. The system as defined in claim 35, characterized in that the transmitter loop is a cable positioned in such a way to form an isosceles triangle with the centers of the receiver loops.

37. The system as defined in any one of claims 17 to 36, characterized in that the, preferably eight, receiver sets placed in the same plan than the transmitter loop (Z plane) are connected to an analogical summation circuit in such a manner to provide a signal that integrates the totality of the field on 360°.

38. The system as defined in any one of claims 1 to 37, characterized in that each receiver loop include an amplifier allowing electrical isolating one from each other.

39. The system as defined in any one of claims 1 to 38, characterized in that, in the case where the system include n (preferably n= 8) segments, the signals of the n (preferably of the 8) amplifiers present are added with the help of a summation circuit.

40. The system as defined in claim 39, characterized in that the system allows to obtain the same sensibility as obtained with a system including only one eight times bigger receiver loop.

41. The system as defined in claim 35, characterized in that, contrary to the resonance frequency of a single big receiver loop, the resonance frequency of the receiver loops stays high.

42. The system as defined in any one of claims 17 to 36, characterized in that the vertical receiver loops are grouped two by two, preferably with the help of an analogical summation circuit.

43. The system as defined in claim 42, including eight segments and wherein the loops 1 & 8 and 4 & 5 measure the Y+ and Y- components of the electromagnetic field while the loops 2 & 3 and 6 & 7 measure the X- et X+ components of the electromagnetic field.

44. The system as defined in any one of claims 1 to 43, additionally including a device allowing hanging the system under or in front or at the back or over a vehicle in motion, the vehicle being preferably of an airborne vehicle type that is preferably a plane or a helicopter.

45. The system as defined in any one of claims 1 to 44, of autonomous type, including at least one current generator and at least one emitter configured to transmit the results of the conducted measurements to a measurement analysis center, the center being positioned outside the system.

The system as defined in any one of claims 1 to 45, of semi-autonomous type, including at least one physical connection with a current generator located outside the system and/or at least one connection with a device receiving the results of the detection measurements conducted with the system and/or at least one emitter configured to transmit the results of the measurements conducted by the system.

The system as defined in any one of claims 1 to 46, characterized in that the vehicle is chosen in the group constituted by the vehicles directly moving at the contact of the ground, the vehicles moving in the sky and the vehicles moving on and/or under water.

A fabrication process of the system as defined in any one of claims 1 to 47, characterized in that the constitutive elements of the system are assembled according to the known methods of assembly.

The fabrication process as defined in claim 48, characterized in that the known methods of assembly are chosen in the group constituted by: riveting, detent, bonding, welding and screwing.

A mining and/or environmental detection method, characterized in that the system as defined in any one of claims 1 to 47 is hanged under or at the front of an airborne vehicle or of an automobile vehicle or of a boat or of an amphibian vehicle.

The mining and/or detection method as defined in claim 50, characterized in that the system is hanged by means of a cable and/or by means of a rigid rod.

52. The mining and/or detection method as defined in claim 51, characterized in that the distance between the carrier and the working system is maintained between 1 and 5 meters.

53. The mining and/or detection method as defined in any one of claims 50 to 52, characterized in that the system is fixed at the front of the vehicle and its plan makes, with the horizontal of the location, an angle advantageously included between 0 and 30 degrees.

54. The mining and/or detection method as defined in any one of claims 50 to 53, characterized in that the system after being connected to a power supply is used in a manner to generate a complete electromagnetic acquisition cycle.

55. The mining and/or detection method as defined in claim 54, characterized in that the complete electromagnetic acquisition cycle is the following:

- the micro-computer generates a signal intended to synchronise the entire electronic system, preferably this signal takes the form of a synchronisation slot, which the recurrence, as well as the time at the "0" and " 1" values, are variable, advantageously, the recurrence of the synchronisation slot varies from 25 to 125 Hz while the duration of the synchronisation signal, at the value "1", varies advantageously from 1 to 4 milliseconds; this signal serves to synchronise the whole electronic system ;

- the passage of the value " 1 " of the synchronisation signal activates the source of current and starts the acquisition of the electromagnetic components and of the value of the excitation current, this part of the cycle corresponds to the "On-Time"; - the passage of the value "0" of the synchronisation signal cuts the source of the current and continues the acquisition of the electromagnetic components and of the value of the excitation current, this part of the cycle corresponds to the "Off-Time";

- throughout the "Off-Time", the micro-computer evaluates the amplitude of each of the electromagnetic signals coming from each components of the electromagnetic field; when, preferably at the precise moment wherein, each of the components Z, X+, X-, Y+, Y- find themselves under a certain level of amplitude, advantageously when the amplitude level is about 100 millivolts, the micro-computer increases the gain, advantageously the increase of the gain is in the range of +40 dB, in order to increase the sensibility for weak amplitude signals; and

- numerical data are stocked in real time in the memory of a set of calculation and/or of a set of data conservation, preferably stocking of data is done on a micro-computer.

The mining and/or detection method as defined in claim 50 or 55, characterized in that the same acquisition cycle repeats itself as long as the electromagnetic collection takes place (also called complete cartographic measurements).