Abstract:
An electromagnetic survey acquisition system includes a sensor cable and a source cable, each deployable in a body of water, and a recording system. The sensor cable includes an electromagnetic sensor thereon. The source cable includes an electromagnetic antenna thereon. The recording system includes a source current generator, a current sensor, and an acquisition controller. The source current generator powers the source cable to emit an electromagnetic field from the antenna. The current sensor is coupled to the source current generator. The acquisition controller interrogates the electromagnetic sensor and the current sensor at selected times in a synchronized fashion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     The invention relates generally to the field of marine electromagnetic surveying. More particularly, the invention relates to towed streamer-type marine electromagnetic surveying and acquisition systems having reduced signal sensitivity to vessel and streamer motion in a body of water. 
     Marine electromagnetic surveying includes acquisition of electromagnetic signals from formations below the bottom of a body of water using electromagnetic sensor streamers that may be towed by a vessel in the body of water. An electromagnetic energy source may also be towed by the same vessel or by a different vessel. 
     U.S. Pat. No. 7,671,598 issued to Ronaess et al. describes a system and method for reducing induction noise in a marine electromagnetic survey system resulting from motion of the various sensor streamer components in the water. High quality electromagnetic data acquisition using towed streamer(s) and a towed electromagnetic energy source may require determining noise that may be induced in other components of the acquisition system, such as the tow vessel and/or the electromagnetic energy source as they move along the body of water. 
     Thus there exists a need for a marine electromagnetic survey system and method that can provide reduced vessel and source motion-induced noise in the acquired signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example embodiment of an acquisition system for marine electromagnetic surveying. 
         FIG. 2  shows an example embodiment of electromagnetic signal generation and recording devices that may form part of the recording system explained with reference to  FIG. 1 . 
         FIG. 3  shows another example embodiment of electromagnetic signal generation and recording devices. 
         FIG. 4  shows another example embodiment of electromagnetic signal generation and recording devices. 
         FIG. 5  shows an example embodiment of a sensor cable. 
         FIG. 6  shows an example embodiment of a signal processing and configuration module 
     
    
    
     DETAILED DESCRIPTION 
     An example embodiment of an acquisition system for marine electromagnetic surveying is shown schematically in  FIG. 1 . A survey vessel  10  moves along the surface of a body of water  11  such as a lake or an ocean. The survey vessel  10  may include equipment, shown generally at  12  and referred to for convenience as a “recording system” that may comprise devices (none shown separately in  FIG. 1 ) for applying electric current to an antenna such as source electrodes  18  and/or other devices disposed on or along a source cable  14 , which may be towed by the survey vessel  10 . The recording system  12  may also include equipment (not shown separately in  FIG. 1 ) for navigating the survey vessel  10 , for determining the geodetic position of the survey vessel  10  and of components towed by the survey vessel  10  in the body of water  11 , and devices for recording signals detected by one or more sensors on one or more sensor cables  16 . As shown in  FIG. 1 , the sensor cable(s)  16  may also be towed by the survey vessel  10 . In other embodiments, sensor cable(s)  16  may be towed by another survey vessel (not shown). 
     The source cable  14  in the present example may include an electromagnetic antenna consisting of two source electrodes  18  disposed at spaced apart positions along the source cable  14 . At selected times certain of the equipment (not shown separately in  FIG. 1 ) in the recording system  12  applies electric current across the source electrodes  18 . The time varying components of such electric current produce an electromagnetic field that propagates through the body water  11  and into the formations  30  below the water bottom  19 . The particular type of current conducted across the source electrodes  18  may be various forms of switched direct current, such used in transient controlled source electromagnetic surveying or types of current used in frequency domain electromagnetic surveying. Non-limiting examples of switched direct current for transient controlled source electromagnetic surveying include switching the current on, switching the current off, reversing current polarity and selected switching sequences such as pseudo-random binary sequences. It is within the scope of the invention, therefore, to perform either or both frequency domain and transient controlled source electromagnetic surveying. It should also be understood that the arrangement of the source electrodes  18  shown in  FIG. 1 , referred to as a horizontal electric dipole antenna, is not the only type of electromagnetic source antenna that may be used with the invention. The source cable  14  may also include, in addition to or in substitution of the horizontal electric dipole source antenna shown in  FIG. 1 , any one or more of a vertical electric dipole antenna, and horizontal or vertical magnetic dipole antenna (current loop). Accordingly, the electromagnetic field source antenna configuration shown in  FIG. 1  is not intended to limit the scope of the present invention. 
     In the example embodiment of  FIG. 1 , the survey vessel  10  may also tow more than one sensor cable  16 . A single sensor cable is shown in  FIG. 1  only for clarity of the illustration and is not a limit on the scope of the invention. The sensor cable  16  may include at least one electromagnetic sensor  20 , and preferably a plurality of such electromagnetic sensors disposed at spaced apart positions along the length of the sensor cable  16 . Each of the one or more electromagnetic sensors  20  may measure a parameter related to the electromagnetic field resulting from interaction of the electromagnetic field induced by the source (e.g., source electrodes  18 ) with the subsurface formations  30  below the water bottom  19 . In the present example, the electromagnetic sensors  20  may be pairs of receiver electrodes disposed at spaced apart positions along the sensor cable  16 . An electric field component of the electromagnetic field resulting from interaction of the induced electromagnetic field with the formations  30  below the water bottom  19  can induce voltages across each of the pairs of receiver electrodes, and such voltages may be detected by any form of voltage measuring circuit (not shown in the present figure) known in the art. Such voltage measuring circuits (not shown) may be disposed in the sensor cable  16  and/or in the recording system  12 . 
     Another example of an electromagnetic sensor that may be used in other examples is a single axis or multi-axis magnetometer, such as a flux gate magnetometer. 
     It should be understood that the example electromagnetic survey system of  FIG. 1  including only one sensor cable  16  is shown to illustrate how to make and use a sensor cable according to various aspects of the invention. A sensor cable according to the various aspects of the invention may be used with acquisition systems that include a plurality of laterally spaced apart sensors cables towed by the survey vessel  10  and/or by another vessel in a selected configuration to provide “in line” and “cross line” electromagnetic and/or seismic signals. Accordingly, the number of sensor cables and their particular geometric configuration in the water  11  are not limits on the scope of the present invention. 
     If electrode pairs are used as the electromagnetic sensors  20 , such electrode pairs may measure voltages induced by the electromagnetic field generated as a result of the interaction of the induced electromagnetic field with the formations  30  below the water bottom  19 . It will be appreciated by those skilled in the art that motion of the survey vessel  10  and motion of the sensor cable(s)  16  through the water  11  may not be uniform. Such non-uniform motion may result from currents in the water and acceleration of the survey vessel  10  (change in velocity or direction) transferred to the sensor cable  16  through towing equipment used to connect the sensor cable  16  to the survey vessel  10 . Such non-uniform motion of the sensor cable  16  through the Earth&#39;s magnetic field can induce voltages along electrical conductors (not shown) in the sensor cable  16  as well as in the electromagnetic sensors  20 . The motion-induced voltages may be calculated or estimated if the motion of sensor cable  16  proximate the electromagnetic sensors  20  is known. In the present example, motion sensors  25  may be disposed at selected positions along the sensor cable  16 . In the example of  FIG. 1 , the motion sensors  25  are each shown as located proximate to one of the electromagnetic sensors  20 . The example number of motion sensors  25  and their placement as shown in  FIG. 1  are not intended to limit the number of motion sensors or their particular geometric configuration that may be used in other examples of an electromagnetic sensor cable according to the invention. The signals measured by the motion sensors  25  may be detected and processed by certain equipment (not shown in  FIG. 1 ) in the recording system  12 , and may be used to estimate magnitude of induced voltages resulting from motion of parts of the sensor cable  16  with respect to the Earth&#39;s magnetic field. Such estimates may be used in processing measurements made from the electromagnetic sensors  20  in the sensor cable  16  to reduce the effects of the motion induced voltages on the measurements made by the electromagnetic sensors  20 . An example embodiment for using the signals detected by the motion sensors  25  to correct measurements made by the electromagnetic sensors  20  is described in U.S. Pat. No. 7,671,598 issued to Ronaess et al. and incorporated herein by reference. 
     In some embodiments, similar motion sensors may be included on board the survey vessel  10 , for example, in or proximate the recording system  12 . A schematic diagram showing an example embodiment of an acquisition system including such additional sensors may be better understood with reference to  FIG. 2 . The term “acquisition system” as used herein is intended to mean all the components typically used to generate, detect and record electromagnetic survey signals. The acquisition system thus may include the recording system  12 , the source cable  14  and at least one sensor cable  16 . 
     The recording system  12  may include a source current generator  48  that may provide one or more types of alternating current or switched direct current as previously explained. A “signature”, or amplitude with respect to time of the current provided by the source current generator  48  may be measured by one or more current sensors  40 ,  42  coupled to an output of the source current generator  48 . The current sensors  40 ,  42  may generate an electrical and/or optical signal corresponding to the current magnitude with respect to time. The recording system  12  may also include a data acquisition controller  50 , which may be, for example, a built-to-purpose microcomputer or suitably programmed general purpose microcomputer. The acquisition controller  50  may obtain an absolute time reference signal from, for example, a geodetic position signal receiver  52 , such as a GNSS signal receiver. The absolute time reference signal may be used to synchronize signal detection and recording functions, so that signal amplitude of the output of all the sensors in the acquisition system may be recorded with respect to an identical time reference. 
     The acquisition controller  50  may send an interrogation command signal over a command signal line  54 , which may be, for example, one or more insulated electrical conductors, or one or more optical fibers. The interrogation command signal may interact with each of a plurality of acquisition nodes  60  in the sensor cable  16 . The acquisition nodes  60  may include circuitry, for example, for detection of voltages impressed across pairs of electrodes or other electromagnetic sensor(s) (e.g.,  20  in  FIG. 1 ) and for detection of signals from the motion sensors ( 25  in  FIG. 1 ). The signals detected by each sensor may be converted by circuits (not shown separately) associated with each acquisition node  60  into a form suitable for transmission to the recording system  12  over a signal return line  55  ( FIG. 3 ). Example circuitry that may be used to convert the detected voltages and motion signals into suitable form for communication, such as optical signals, is described in the Ronaess et al. &#39;598 patent. Conversion of the foregoing detected signals into electrical telemetry signals is also within the scope of the present invention. In the example embodiment in  FIG. 2 , the command signal line  54  may extend the entire length of the sensor cable  16  and may return to the survey vessel ( 10  in  FIG. 1 ) and to acquisition controller  50  using the signal return line  56 . The signal return line  56  may extend to the one or more sensors disposed on the survey vessel ( 10  in  FIG. 1 ) such as the current sensors  40 ,  42 , and motion sensors  44 ,  46 . The motion sensors  44 ,  46  may be similar in configuration to the motion sensors ( 25  in  FIG. 1 ) in the sensor cable  16 . The signals emitted by each of the on board current sensors  40 ,  42 , and motion sensors  44 ,  46  may be converted to a form, such as optical signals, suitable for transmission to the acquisition controller  50  using the signal return line  56 . Signals from the motion sensors  44 ,  46  on board the survey vessel ( 10  in  FIG. 1 ) may be used substantially as explained in the Ronaess et al. &#39;598 patent to further reduce motion induced noise in the signals generated by the electromagnetic sensors ( 20  in  FIG. 1 ) in the sensor cable ( 16  in  FIG. 1 ). 
     The acquisition controller  50  may also generate control signals for operation of the source current generator  48 . Current from the source current generator  48  may be conducted to the source electrodes  18  in the source cable  14 . Signals detected by the various sensors may thereby be precisely synchronized. For example, by having the acquisition controller  50  operate both signal generation through control of the source current generator  48  and detection of signals from the various sensors and acquisition nodes by sending an interrogation signal to each acquisition node  60  and on board current sensors  40 ,  42 , the source current may be synchronized with signals from the acquisition nodes. Acquisition controller  50  may also detect signals from motion sensors  44 ,  46 , allowing the signals detected by the motion sensors to be precisely synchronized with the source current. In other example embodiments, the acquisition controller  50  may only provide control signals to synchronously interrogate the on board current sensors  40 ,  42 , and motion sensors  44 ,  46  and each acquisition node  60 . 
     The example embodiment shown in  FIG. 2  provides that all the sensors in the acquisition system may be interrogated using a single command signal line  54  with an associated signal return line  56  in communication with all the sensors and acquisition nodes  60 . In another example embodiment, shown in  FIG. 3 , a separate command signal line  54  and signal return line  55  may be used to obtain signals from the sensor cable  16 . The on board current sensors  40 ,  42 , and motion sensors  44 ,  46  may have separate provision for signal communication with the acquisition controller  50 . Because acquisition of all signals may be performed by the same acquisition controller  50 , the acquisition may be synchronized as accurately or more accurately as in the example embodiment shown in  FIG. 2 . Other components of the acquisition system shown in  FIG. 3  may be substantially the same as those of the example embodiment of the acquisition system shown in  FIG. 2 . 
     A particular embodiment of equipment in the recording system  12  that may provide reduced noise in the detected signals from each of the sensors as well as reduced cross-coupling between sensor signals and between the senor signals and the current imparted to the source cable ( 14  in  FIG. 1 ) may be better understood with reference to  FIG. 4 . The acquisition controller  50  may be operated from its own power supply. In some embodiments, the power supply for acquisition controller  50  may consist of one or more storage batteries  50 C, rechargeable by a battery charger  50 D, a DC to AC converter  50 B coupled to the storage batteries  50 C and a power unit  50 A coupled to the output of the DC to AC converter  50 B. The power unit  50 A may provide electric power to operate the acquisition controller  50  and the circuits (not shown) in the sensor cable ( 16  in  FIG. 1 ). Acquisition command signals from the acquisition controller  50  may be conducted in electrical form to an optical signal transceiver  57 . The optical signal transceiver  57  may include its own separate power supply  53  so that any variations in the power used to operate the acquisition controller  50  will not affect the signals generated by the optical signal transceiver  57 . As explained with reference to  FIG. 2 , command signals from the acquisition controller  50  may be conducted to the sensor cable ( 16  in  FIG. 1 ) using a command signal line  54 . In the present embodiment, the command signal line may be one or more optical fibers. Signals returned from the various acquisition nodes ( 60  in  FIG. 2 ) and on board current sensors (e.g.,  40 ,  42  in  FIG. 2 ) may be conducted to the acquisition controller  50  using a signal return line  56 . 
     It will be appreciated by those skilled in the art that the sensor cable ( 16  in  FIG. 1 ) may be deployed into the water using a winch or similar spooling device. Typically a winch or similar spooling device that extends and retracts cables having insulated electrical conductors and/or optical fibers will include a slip ring set  17 A or similar device enabling relative rotation between the winch reel and rotationally fixed connections to the electrical conductors and/or optical fibers on the cable. In the present embodiment, the sensor cable ( 16  in  FIG. 1 ) may use optical fibers, e.g., command signal line  54  and signal return line  56 , for communication of command signals and sensor signals, and may use insulated electrical conductors  20 A to transmit electrical power to the circuits (not show separately in  FIG. 4 ) in the various components of the sensor cable ( 16  in  FIG. 2 ). In the present embodiment, the winch may include optical to electrical converters  17 B on both the rotating part of winch and on the rotationally fixed part, and in signal communication with the command signal line  54  and the signal return line  56 . The optical to electrical converters  17 B may enable signals to be transmitted in electrical form through the slip rings  17 A, while being transmitted in optical form along the sensor cable ( 16  in  FIG. 2 ) and in the recording system  12 . Other embodiments may use optical slip rings for transmission of optical signals between the rotating and fixed parts of the winch 
     In the present example embodiment, the source current generator  48  and associated current sensors  40 ,  42  may include their own separate power supply  48 A. Such separate power supply  48 A may be used to isolate the large current generated by the source current generator  48 , so that the large current from the source current generator  48  may only minimally affect the other components of the recording system  12 . Output of the source current generator  48  may be applied to the source cable  14  as explained with reference to  FIG. 1 . 
     An example of a sensor cable  16  is shown schematically in  FIG. 5 . The sensor cable  16  may include a lead in  17  or similar device to enable towing by the survey vessel ( 10  in  FIG. 1 ). The sensor cable  16  may be assembled from sensor segments  16 A coupled end to end in pairs. Each pair of sensor segments  16 A may be coupled to a succeeding pair of sensor segments  16 A through a signal processing and configuration module  16 B. The modules  16 B may each contain one or more of the acquisition nodes ( 60  in  FIG. 3 ). The sensor segments  16 A may include suitable electromagnetic sensors  20  as explained with reference to  FIG. 1 . The motion sensors are omitted from  FIG. 5  for clarity of the illustration. 
     An example embodiment of one of the signal processing and configuration modules  16 B (hereinafter “module”) is shown in cut away view in  FIG. 6 . The module  16 B may be enclosed in a pressure resistant housing  131  such as may be made from high strength plastic or non-magnetic steel alloy. The housing  131  may be substantially cylindrically shaped, and may include electrical/mechanical terminations  130  configured to couple to the terminations  130 B on a sensor segment  16 A. The example embodiment shown in  FIG. 6  may provide for such connection between cable segments by including a flange  134  on the exterior of the housing  131  which engages a mating flange (not shown) in a connecting sleeve  133 . The connecting sleeve  133  may be substantially cylindrical in shape, and when moved along the exterior of the housing  131  may engage o-rings  135  or similar seal elements positioned longitudinally on either side of an opening  132  in the wall of the housing  131 . Thus, with the sleeve  133  removed, the opening  132  is accessible. With the sleeve  133  in the connected position, for example, by engaging internal threads  133 A on the end of the sleeve  133  with mating threads  130 A on the adjacent termination  130 B of sensor segment  16 A, the interior of the housing  131  is sealed from water intrusion by the sleeve  133 . 
     The interior of the housing  131  may include circuits for selective electrical interconnection of the various sensor segments  16 A and connection of sensors on the sensor segments  16 A to voltage or current measuring circuitry. In the present embodiment, each of the electrical connections  146  at each end of the housing  130  may be electrically connected, such as by twisted pairs of wires to a corresponding electrical contact on a configuration plug receptacle  136 . Other electrical contacts on the configuration plug receptacle  136  may connect to the input terminals of one or more low noise amplifier/digitizer (LNA/ADC) combinations  137 . Output of the combination(s)  137  may be coupled to an electrical to optical signal converter (EOC)  138  and thence to the one or more optical fibers  54  for communication of digitized voltage signals along the sensor cable and if required to the recording system ( 12  in  FIG. 1 ). Power for the LNA/ADC  137  and EOC  138  may be provided by a battery (not shown) inside the module  16 B. Such battery may be rechargeable while the sensor cable is deployed by using a module charging circuit such as one described in U.S. Pat. No. 7,602,191 issued to Davidsson, incorporated herein by reference. 
     The particular sensors on any sensor segment  16 A located either toward the forward or aft end of the sensor cable ( 16  in  FIG. 5 ) with respect to the module  16 B that are connected to through wires or to the input of the LNA/ADC combination  137  may be selected prior to deployment of the sensor cable by inserting a suitably wired configuration plug  136 A into the receptacle  136 . Thus, in combination, suitable numbers of sensor segments  16 A, and suitably configured modules  16 B may provide the system user with a large number of options as to electrode spacing and offset while making only two basic sensor cable components. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.