Abstract:
The invention discloses a receiver section towed by an airborne electromagnetic survey system, comprising a plurality of receivers, each receiver comprising at least one receiver coil. The invention further discloses an airborne electromagnetic survey system, comprising: (a) a transmitter section for generating a primary electromagnetic field that induces a secondary electromagnetic field; and (b) a receiver section for detecting the secondary electromagnetic field, wherein the receiver section comprises a plurality of receivers, each receiver further comprising at least one receiver coil; and a tow assembly for an airborne electromagnetic surveying system, comprising a means for suspending a receiver section from an aircraft, the receiver section comprising a plurality of receivers each receiver comprising at least one receiver coil.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to airborne electromagnetic (EM) surveying systems and methods, and particularly to airborne EM systems comprising a plurality of receivers, each receiver comprising at least one receiver coil, for measuring magnetic fields or EM gradients. 
       BACKGROUND OF THE INVENTION 
       [0002]    An airborne EM survey system generally includes a transmitter for generating a primary electromagnetic field that induces eddy currents in the earth. These eddy currents generate a secondary electromagnetic field or ground response. A receiver of the EM system then measures the response of the ground. The currents induced in the ground are a function of conductivity. By processing and interpreting the received signals, it is possible to estimate the distribution of conductivity in the subsurface. 
         [0003]    EM measurements can be made in either frequency domain or time domain. In a frequency domain EM system, the transmitter generates a sinusoidal electromagnetic field at one or more frequencies. The amplitude and phase of the secondary field relative to the primary field are indicative of the subsurface conductivity. In a time domain EM system, transient pulses are applied to the transmitter during an ON-period to generate a primary electromagnetic field that induces a decaying secondary electromagnetic field. The receiver measures the amplitude and decay characteristics of the secondary field. 
         [0004]    An airborne EM system&#39;s signal-to-noise ratio (SNR) is an important indication of the effective depth of exploration of the EM system and its ability to recognize and measure a potential target. Various systems and methods for improving SNR have been known in the art. For example, increasing the distance between the transmitter and receiver may reduce system noise thereby improving the SNR. In time domain systems, increasing the size of the transmitter loop may help increase SNR. However, these conventional improvements are transmitter dependent and usually result in increased overall system size and complexity. 
         [0005]    Canadian Patent Application No. 2,748,278 proposes a passive geological surveying system using audio frequency magnetic (AFMAG) technology. The proposed system has a first aircraft towed receiver coil assembly and a second ground-based or airborne receiver coil assembly, wherein the differences in the audio magnetic field measured at the first and second receivers are used to interpret the location of the underground conductors. While this proposed system does not involve an active transmitter, it requires that the two receivers be sufficiently spaced apart, resulting in overall increased system size and complexity in computing the differences between the measurements at different locations. 
         [0006]    Therefore, there remains a need for a system that provides improvements to the SNR and/or target discrimination performance of an EM system, independent of the transmitter, and without significantly increasing the overall size and complexity of the system. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention improves the overall target discrimination performance and/or reduces the system operation noises of an EM system by providing an airborne EM system having two or more receivers. 
         [0008]    In accordance with one aspect of the present invention, there is provided an airborne electromagnetic survey system, comprising:
       (a) a transmitter section for generating a primary electromagnetic field that induces a secondary electromagnetic field; and   (b) a receiver section for detecting the secondary electromagnetic field, wherein the receiver section comprises a plurality of receivers, each receiver further comprising at least one receiver coil.       
 
         [0011]    In accordance with another aspect of the present invention, there is provided a receiver section towed by an airborne electromagnetic survey system, comprising a plurality of receivers, each receiver comprising at least one receiver coil. 
         [0012]    In accordance with a further aspect of the present invention, there is provided a tow assembly for an airborne electromagnetic surveying system, comprising means for suspending a receiver section from an aircraft, the receiver section comprising a plurality of receivers each receiver comprising at least one receiver coil; and means for suspending a transmitter section from the aircraft. 
         [0013]    Other features and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings, which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will now be described by way of reference to the drawings, in which: 
           [0015]      FIG. 1  is a schematic view of a receiver section according to an embodiment of the airborne EM system; 
           [0016]      FIG. 2  is a perspective view of a receiver section according to an embodiment of the airborne EM system; 
           [0017]      FIG. 3  is a perspective top view of a receiver section according to an embodiment of the airborne EM system; 
           [0018]      FIG. 4  is a perspective side view of a receiver section according to an embodiment of the airborne EM system; 
           [0019]      FIG. 5  is a perspective side view of a receiver comprising one or more receiver coils; 
           [0020]      FIG. 6  is a schematic view of a single receiver section tow assembly comprising multiple receivers according to an embodiment of the airborne EM system; 
           [0021]      FIG. 7  is a schematic view of a single receiver section tow assembly comprising multiple receivers attached serially along the tow assembly according to an embodiment of the airborne EM system; 
           [0022]      FIG. 8  is a schematic view of multiple receiver section tow assemblies each comprising a single receiver according to an embodiment of the airborne EM system; 
           [0023]      FIG. 9  is a schematic view of multiple receiver section tow assemblies each comprising multiple receivers according to an embodiment of the airborne EM system; 
           [0024]      FIG. 10  is a schematic view of multiple receiver section tow assemblies each comprising multiple receivers attached serially along the tow assembly according to an embodiment of the airborne EM system; 
           [0025]      FIG. 11  is a schematic perspective view of an illustrative embodiment of the airborne EM system in an airborne position flying at surveying speeds. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    An aircraft towed EM survey system generally comprises a tow assembly further comprising a transmitter section and a receiver section. 
         [0027]    The aircraft can be maimed or unmanned power driven fixed-wing aeroplane, helicopter, airship or any other flying machine, as a person skilled in the art would appreciate. 
         [0028]    Referring to  FIG. 1 , one embodiment of the receiver section  10  described herein comprises a plurality of receivers  20  and a receiver support structure  12  for mounting the receivers. Each receiver  20  is positioned in proximity to at least one neighbouring receiver  20 . 
         [0029]    In one embodiment, the receivers  20  are positioned in proximity to each other, and may be disposed in any orientation relative to each other. Each receiver  20  comprises at least one receiver coil. The receiver  20  may independently detect the secondary electromagnetic fields. 
         [0030]    The receiver section  10  or the receiver support structure thereof may include a receiver housing  14  for enclosing at least one of the receivers  20 . The receiver housing  14  isolates the receivers  20  from external forces and noises, and keeps at least some receivers  20  close to each other. 
         [0031]    In one embodiment, the receiver housing  14  can be a “bird” structure, which is an aerodynamic support structure that houses the EM receivers or sensors and other electronics. 
         [0032]    However, such an enclosure structure is not required in all situations. In some embodiments, the at least one receiver  20  can be supported in its own housing or protective enclosure. 
         [0033]    Multiple receivers can take many forms, for example, in-plane multiple receivers as illustrated in  FIG. 2  or platform multiple receivers as shown in  FIG. 3 . A person skilled in the art would appreciate that multiple receivers of any shape or size which is suitable may be used. 
         [0034]    Referring to  FIG. 2 , and according to one embodiment of the airborne EM system described herein, the receiver section  10  comprises a plurality of receivers  20  supported by a substantially polygonal receiver support structure  12 , wherein the receivers  20  are generally located in close proximity to their neighbouring receivers  20  and are disposed along a circumference of the receiver support structure  12 . While a polygonal receiver support structure  12  is depicted in  FIG. 3 , a person skilled in the art would understand that a receiver support structure of any shape or size which is suitable may be used. Furthermore, the receivers  20  need not be identical and may be disposed in any orientation relative to each other. 
         [0035]    Preferably, the receiver support structure  12  is modular and comprises serially connected tubular sections which can internally house the at least one receiver  20  at one or more locations. Alternatively, the receivers  20  can be supported on the receiver support structure  12  in any suitable manner. 
         [0036]    The receiver support structure  12  may be constructed to form a support for the receivers  20  so that the configured distance between the receivers  20  and their relative orientations can be maintained substantially unchanged to provide stability of the receiver section  10 . It is to be understood that rigid, non-rigid, semi-rigid or flexible receiver support structure  12  can be used depending on the requirements for a particular survey flight. 
         [0037]    The embodiment illustrated in  FIG. 2  may further include at least one receiver  20  positioned along a central axis that is substantially perpendicular to the plane defined by the receiver support structure  12 , and being coupled to the receiver support frame  12  by a plurality of cross support means  24  such as cross ropes or cross bars or rods. At least one central receiver  20  may be disposed in a co-planar fashion with the receiver support frame  12 , or may be concentric or co-axial with the receiver support frame  12 . For example, the at least one central receiver  20  may be positioned above or below the plane as defined by the receiver support structure  12 , or at the center of the receiver support structure  12 . 
         [0038]    Referring to  FIGS. 3 and 4 , and according to a further embodiment of the airborne EM system described herein, the receiver section  10  comprises a receiver support structure  12  having a plurality of mounting locations in proximity to each other to mount a plurality of receivers  20 . At least one of the receivers  20  can be housed in an enclosure of any suitable configuration, size and shape. The receiver support structure  12  can be constructed to provide support for the receivers  20  and may include hollow portions or apertures to reduce the weight of receiver support structure  12 . A person skilled in the art would understand that a rigid, semi-rigid or flexible receiver support structure  12  or enclosure can be used. 
         [0039]    The receiver  20  comprises one or more receiver coils  22  as shown in  FIG. 5 . In the present disclosure, the non-limiting term “receiver coil” refers to a broad range of means for sensing electromagnetic fields, including various wires, induction magnetometers, and any associated electronics or circuitries for the proper functioning of the receiver coil. 
         [0040]    The receiver coils  22  may be identical to each other, or may comprise coils of various sizes, shapes, materials or other physical characteristics. 
         [0041]    Each receiver coil array may have three coplanar coils  22  that are partially overlapping with each other. It should also be understood that the coils in a receiver coil array need not to be coplanar in all circumstances. In other words, the coils  22  can be disposed in separate planes or surfaces while overlapping with each other. 
         [0042]    The receiver configuration described above, namely substantially juxtaposing a plurality of receivers or configuring each receiver to be positioned in close proximity to at least one neighbouring receiver, allows an increased amount of electromagnetic flux to pass through the receiver section, and therefore improves the overall signal sensitivity of the receiver section. In addition, grouping multiple receiver coils into an array provides an increased effective area of a receiver for secondary electromagnetic flux to pass through, and therefore improves the overall signal sensitivity of the receiver. Furthermore, configuring a receiver such that all of its receiver coils are located in close proximity to each other allows an increased amount of electromagnetic flux to pass through the receiver, thereby improving the overall signal sensitivity of the receiver and the SNR of the EM system. 
         [0043]    In practice, one issue arising from using such a configuration is that placing receivers or receiver coils one next to another may create mutual inductance between the coils. Mutual inductance increases instrumentation noise or system noise and may significantly limit the receiver&#39;s ability to respond to the secondary electromagnetic fields and the receiver&#39;s response bandwidth. 
         [0044]    The present invention has discovered that the overall mutual inductance of coils somehow decreases significantly when multiple receivers, each having at least one receiver coil, are positioned close to their neighbouring receivers within a range of spacing that is dependent on various attributes of the coils. Based on this, the undesirable mutual inductance between the coils is minimized in the receiver section  10  described herein by maintaining close spacing between the receiver coils or between the receivers. 
         [0045]    As described above, receiver section  10  comprises multiple receivers  20  wherein each receiver has one or more receiver coils  22 . This receiver configuration provides some advantages over the current practice in the art. 
         [0046]    One advantage is that the receiver configuration described herein can be implemented independent from the transmitter. In other words, improvement to the SNR of the EM system can be realized without increasing the transmitter size or modifying the distance or configuration relationship between the receiver section and the transmitter section. 
         [0047]    Advantageously, keeping multiple receivers in proximity to each other minimizes or reduces the motion or vibration of a receiver relative to another, therefore improving the overall stability of the receiver section. 
         [0048]    Furthermore, maintaining the receivers in close spatial proximity may reduce deviation between the secondary response measurements taken at neighbouring receiver locations, thus significantly simplifies the computation involved in determining the EM gradients of the earth response. For example, when the neighbouring receivers and respective receiver coils are configured to have substantially the same orientation, faster EM gradient computation can be achieved. 
         [0049]    As depicted in  FIG. 6 , to maintain physical proximity between the receivers  20 , a receiver section  10  may comprise multiple receivers  20  towed by a common receiver tow assembly  13 . The multiple receivers  20  are co-located in a single receiver section  10 . 
         [0050]    In one embodiment as shown in  FIG. 7 , the receivers  20  are serially connected within a common tow assembly  13  or a portion thereof and the receivers  20  are positioned in relative remoteness to each other. Each receiver  20  comprises at least one receiver coil. The receiver  20  may independently detect the secondary electromagnetic fields. 
         [0051]    While a transmitter section is not shown in these Figures, a person skilled in the art would understand that the transmitter section can be configured in any suitable manner that is known in the art, for example, it can be towed below, concentric, or above the receiver section  10 , or mounted to the aircraft. 
         [0052]    In some embodiments, flexible means, such as tow ropes, tension cables can be used to tow the receiver section  10 . Alternatively, the receiver section  10  can be towed by more rigid means such as connecting rods, bars, struts or other similar structures. Any other rigid, non-rigid, semi-rigid or flexible connections can also be used to provide the spacing or association between the receiver section and the rest of the airborne EM system. 
         [0053]    Referring to  FIGS. 8 ,  9  and  10 , multiple receiver tow assemblies  13  may be used to tow the receiver section  10  having multiple receivers  20 . Each tow assembly  13  may support at least one receiver  20 . The multiple receiver tow assemblies  13  may have the same or different lengths. As such, the towed receivers  20  may operate at the same or different altitude during flight. In  FIG. 8 , each receiver  20  is towed separately and supported by its receiver support structure or protective housing structure. 
         [0054]    In  FIG. 9 , each tow assembly  13  tows multiple receivers  20  which share a common protective housing structure. 
         [0055]    In  FIG. 10 , each tow assembly  13  is towing multiple receivers  20  which are serially coupled, or attached to the tow assembly  13 . 
         [0056]    The receivers  20  can be positioned in parallel tow assemblies or support structures so that they are substantially located in close proximity to each other or to their neighbouring receivers during flight. Therefore, by simply grouping the existing receivers in accordance with the spacing that is appropriate for the receivers and coils in use, SNR improvement can be realized. 
         [0057]    Furthermore, by locating multiple receivers  20  at a relatively remote distance and known position with respect to neighboring receivers  20 , the shape of the secondary field can be measured at each individual transmitter location. Measurement of the field shape at each transmitter location is a technique used in terrestrial or marine geophysical exploration but has heretofore not been possible with airborne systems. The shape of the secondary field provides additional discrimination in the form of diagnostic information about the shape, orientation or position of a geological target. 
         [0058]    Additionally, data measured with multiple receivers  20  at a relatively remote distance and known position with respect to neighboring receivers  20  may be manipulated in processing to focus or orient the sensitivity of the receiver section  10 . This focussing reduces the spatial resolution of the system and improves target discrimination. 
         [0059]    When multiple receiver tow assemblies  13  are used, it may be desirable to reduce relative motion and vibration between the receivers  20  supported on different tow assemblies  13 . Accordingly, the receiver section  10  may further comprise means for connecting the tow assemblies  13  and relevant structures and means for stabilizing the receivers disposed in respective tow assembly. The connecting and stabilizing structures can be constructed to form rigid, non-rigid, semi-rigid or flexible coupling or connections. 
         [0060]    For example, when multiple receivers are towed in series by a tow assembly  13 , the tow assembly  13  may be configured to comprise a main tow rope, a split rope portion where the main rope is splitting into at least two ropes coupled to stabilizing structures therebetween. 
         [0061]    Still further, for example, two or more spaced apart stabilizer bars can be coupled to the split ropes to form a stable and flexible support suitable for housing a receiver  20 . A receiver  20  as described in the present disclosure, with or without protective cover, can be securely mounted to the above stable and flexible support structures. The tow assembly  13  may further comprise a converging portion where the at least two split ropes merge into another tow rope. The merged tow rope can be used as a connection rope to a transmitter section  4 , another receiver  20 , or any other component of the EM system described herein. 
         [0062]    Stabilizer structures can also be used to provide stable and flexible coupling between multiple receiver tow assemblies  13 . For example, when multiple receivers  20  are towed in parallel in at least two tow assemblies  13 , the tow assemblies  13  may be coupled to stabilizing structures therebetween as described above. Such stabilizing structures can be deployed at multiple positions along the lengths of the tow assemblies  13  to provide the overall stability of the receiver section  10 . 
         [0063]    In some embodiments, the parallel tow assemblies  13  are connected to each other by rigid connecting means such as rods, joints, bars or the like. However, as a person skilled in the art would appreciate, various rigid, non-rigid, semi-rigid or flexible supporting means and configurations are equally applicable in such an embodiment. 
         [0064]    A person skilled in the art would appreciate that the receiver section  10  described herein can be scaled according to the surveying task at hand. For example, a large size aircraft or helicopter may readily carry more than one receiver sections as described in the present disclosure. 
         [0065]    The receiver section  10  described in the present disclosure can be used in a variety of airborne EM systems include time domain and frequency domain systems. The transmitter section  4  and receiver section  10  described herein may cooperate with each other during flight in any suitable configuration. For example, they can be configured to cooperate in a spaced apart relationship. Depending on the surveying tasks, system load capacity, and the availability of operating space, they can also be deployed in relatively close proximity, or substantially co-located or supported by common supporting means. Regardless of their relative position and configuration, various supporting means can be used to support the transmitter and receiver sections, including flexible, semi-rigid, or rigid support frames. 
         [0066]    In one embodiment, the transmitter and receiver sections are disposed in a common tow assembly  13  and are operative at different altitudes relative to the ground. Preferably, the transmitter section  4  comprises a transmitter loop  24  that has a substantially circular or polygonal periphery. The receiver section  10  may be positioned above or below the transmitter section  4 , as illustrated in  FIG. 11 , and is offset from the center of the transmitter section. However, it is to be understood that the spacing between the transmitter section  4  and the receiver section  10  can also be maintained by a concentric configuration, or in a coplanar fashion. For example, in one embodiment, the receiver section  10  is positioned above the transmitter section  4  and in between the aircraft and the transmitter section  4 . 
         [0067]    In one embodiment, the receiver section  10  is located at the center of the transmitter section  4 . Preferably, the centrally located receiver section  10  is coupled to the transmitter section  4  using cross support structures such as cross ropes  44 , as shown in  FIG. 11 . A transmitter driver  40  can be co-located with the receiver section  10  or can be placed at a different location. In case the receiver section  10  and the transmitter driver  40  are both located at the center of the transmitter loop, it is preferable that a transmitter platform rope is used to support the transmitter driver  40  from the tow assembly  2  to maintain the planar stability of the transmitter loop  24 . 
         [0068]    In some embodiments, flexible means, such as tow ropes, tension cables can be used to connect or interconnect the transmitter section  4  and the receiver section  10 . Alternatively, the transmitter section is connected or coupled to the receiver section by rigid means such as connecting rods, bars, struts or other similar structures. Any other rigid, semi-rigid or flexible connections can also be used to provide the spacing or association between the transmitter section and the receiver section. 
         [0069]    Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments and modifications are possible. Therefore, the scope of the appended claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.