Abstract:
A marine electromagnetic sensor system includes a sensor cable having at least one electromagnetic sensor thereon. A first calibration electrode is disposed on the cable on one side of the sensor. A second calibration electrode is disposed on the cable on an opposite side of the sensor. A calibration power supply is coupled to the first and second calibration electrodes. A measuring circuit is coupled to the sensor. A method for calibrating marine electromagnetic survey electrodes includes imparting electric current between calibration electrodes disposed at spaced apart locations on a cable deployed in the water. Voltages impressed across a pair of electrodes disposed on the cable between the calibration electrodes are detected in response to the current. A change in sensitivity of the at least one pair of electrodes is determined using the detected voltages.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates generally to the field of electromagnetic survey apparatus for subsurface exploration in the Earth. More particularly, the invention relates to structures for detector electrodes and arrays thereof for detection of electromagnetic fields resulting from electromagnetic energy imparted into the Earth. 
         [0005]    2. Background Art 
         [0006]    Electromagnetic surveying is used for, among other purposes, determining the presence of hydrocarbon bearing structures in the Earth&#39;s subsurface. Electromagnetic surveying includes what are called “controlled source” survey techniques. Controlled source electromagnetic surveying techniques include imparting a time varying electric current or a time varying magnetic field into the Earth, when such surveys are conducted on land, or imparting such fields into sediments below the water bottom (sea floor) when such surveys are conducted in a marine environment. The techniques include measuring voltages and/or magnetic fields induced in electrodes, antennas and/or magnetometers disposed at the Earth&#39;s surface, in the water or on the water bottom. The voltages and/or magnetic fields are induced by interaction of the electromagnetic field caused by the electric current and/or magnetic field imparted into the Earth&#39;s subsurface (through the water bottom in marine surveys) with subsurface Earth formations. 
         [0007]    One type of marine controlled source electromagnetic surveying known in the art includes imparting alternating electric current into the sediments below the water bottom by applying current from a source, usually disposed on a survey vessel, to a bipole electrode towed by the survey vessel. The bipole electrode is typically an insulated electrical cable having two electrodes thereon at a selected spacing, sometimes 300 to 1000 meters or more. The alternating current has one or more selected frequencies, typically within a range of about 0.1 to 10 Hz. A plurality of detector electrodes is disposed on the water bottom at spaced apart locations, and the detector electrodes are connected to devices that record the voltages induced across various pairs of such electrodes. Such surveying is known as frequency domain controlled source electromagnetic surveying. Frequency domain EM survey techniques are described, for example in U.S. Pat. No. 7,026,819 issued to Eidesmo et al. 
         [0008]    Another technique for controlled source electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying. In transient controlled source electromagnetic surveying, electric current is imparted into the Earth&#39;s subsurface using electrodes on a cable similar to those explained above as used for frequency domain surveying. The electric current may be direct current (DC). At a selected time or times, the electric current is switched, and induced voltages are measured, typically with respect to time over a selected time interval, using electrodes disposed on the water bottom or towed in the water as previously explained with reference to frequency domain surveying. Structure and composition of the Earth&#39;s subsurface are inferred by the time distribution of the induced voltages. Transient electromagnetic surveying techniques are described, for example, in U.S. Pat. No. 6,914,433 issued to Wright et al. 
         [0009]    Some types of marine electromagnetic survey systems use pairs of electrodes disposed at spaced apart locations along one or more streamer cables for detecting electric field components of electromagnetic fields in the subsurface. One type of electrode for use in a towed marine streamer is described in U.S. Pat. No. 7,446,535 issued to Tenghamn et al. and assigned to the assignee of the present invention. The electrode pairs are coupled across input terminals or respective preamplifiers, the output of which is communicated to signal processing and recording devices. Another example electrode-type electromagnetic receiver system is disclosed in U.S. Patent Application Publication No. 2009/001987 filed by Davidsson, the underlying patent application for which is assigned to the assignee of the present invention. The system disclosed in the &#39;987 publication includes a plurality of sensor modules each having a single electrode thereon, and a common reference electrode extending between all of the sensor modules. Voltages impressed between the common reference electrode and each of the sensor module electrodes are measured in response to the electric and/or magnetic fields imparted into the subsurface formations below the water bottom. 
         [0010]    In marine electromagnetic survey systems such as the foregoing that use electrodes to detect electric field components of electromagnetic fields from the subsurface, it is desirable to be able to determine the response characteristics of the electrodes over time, as such response characteristics may change. It is particularly desirable to be able to characterize electrode response while the sensing cable is still deployed in the water. It is also desirable to be able to monitor motion of electromagnetic sensor cable in the water in order to be able to determine components of the detected and recorded signals that result from electromagnetic induction caused by motion of the sensor cable in the water. 
       SUMMARY OF THE INVENTION 
       [0011]    A marine electromagnetic sensor system according to one aspect of the invention includes an electromagnetic sensor cable having at least one electromagnetic field sensor thereon. A first calibration source electrode is disposed on the cable on one side of the at least one sensor. A second calibration source electrode is disposed on the cable on an opposite side of the at least one sensor. A calibration power supply is functionally coupled to the first and second calibration source electrodes. A measuring circuit is functionally coupled to the at least one sensor. 
         [0012]    A method for calibrating marine electromagnetic surveying electrodes according to another aspect of the invention includes imparting an alternating electric current between calibration electrodes disposed at spaced apart locations on a cable deployed in a body of water. Voltages impressed across at least one pair of electrodes disposed between the calibration electrodes are detected in response to the alternating current. A change in sensitivity of the at least one pair of electrodes is determined using the detected voltages. 
         [0013]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows one example of a cable-type electromagnetic receiver system according to the invention. 
           [0015]      FIG. 2  shows more detail of one example of a sensor module in the cable system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    An example marine electromagnetic survey system is shown generally in  FIG. 1 . The electromagnetic survey system can include a sensor cable  10  having thereon at spaced apart positions a plurality of sensor modules  12 . The sensor modules  12  will be explained in more detail with reference to  FIG. 2 . The sensor cable  10  is shown being towed by a survey vessel  18  moving on the surface of a body of water  22  such as a lake or the ocean. Towing the sensor cable  10  is only one possible implementation of a sensor cable. It is within the scope of the present invention for the sensor cable  10  to be deployed on the water bottom  23 . 
         [0017]    The vessel  18  may include thereon equipment, shown generally at  20  and referred to for convenience as a “recording system” that may include devices (none shown separately) for navigation, for energizing electrodes or antennas for imparting an electromagnetic field into the formations below the water bottom  23 , and for recording and processing signals generated by the various sensor modules  12  on the sensor cable  10 . 
         [0018]    The electromagnetic survey system shown includes an electromagnetic field transmitter, which may be a pair of electrodes  16  disposed at spaced apart positions along an electrically insulated source cable  14 . The source cable  14  may be towed by the survey vessel  18  or by a different vessel (not shown). The source cable  14  alternatively may be deployed on the water bottom  23 . The electrodes  16  may be energized at selected times by an electrical current source (not shown separately) in the recording system  20  or within other equipment (not shown). The electrical current source applies electrical current across the electrodes  16  to induce an electromagnetic field in the formations below the water bottom  23 . The electrical current may be monochromatic or may have a plurality of discrete frequencies for frequency domain electromagnetic surveying, or may include forms of switched current for transient electromagnetic surveying, such as switching direct current on or off, reversing its polarity of using a coded switching sequence such as a pseudorandom binary sequence. 
         [0019]    The configuration shown in  FIG. 1  induces a horizontal dipole electric field in the subsurface when the electrodes  16  are energized by electric current source. It is entirely within the scope of the present invention to induce vertical dipole electric fields in the subsurface, as well as to induce vertical and/or horizontal dipole magnetic fields in the subsurface using alternative transmitter structures such as wire loops or solenoid coils. Accordingly, the polarization direction of and the type of electromagnetic field induced is not intended to limit the scope of the present invention. Further, the invention is applicable to use with both frequency domain (continuous wave) and transient induced electromagnetic fields, as suggested above. See, for example, U.S. Patent Application Publication No. 2006/0186887 filed by Strack et al. for examples of all of the foregoing techniques for inducing an electromagnetic field in the subsurface. 
         [0020]    One example of a sensor cable  10  and one of the sensor modules  12  is shown in more detail in  FIG. 2 . The sensor cable  10  may be made from helically wound, electrically conductive armor wires  10 A, such as may be made from steel or other high strength material. Preferably the armor wires  10 A are covered on their exterior with an electrically insulating jacket (not shown). In one example, to be explained in more detail below the cable  10  may include one or more insulated electrical conductors and one or more optical fibers inside the armor wires  10 A. 
         [0021]    The sensor cable  10  in the present example may be divided into segments, each of which terminates with a combination mechanical/electrical/optical connector  25  (“cable connector”) coupled to the longitudinal ends of each cable segment. The cable connector  25  may be any type known in the art to make electrical and optical connection, and to transfer axial loading to a mating connector  27 . In the present example such mating connector  27  can be mounted in each longitudinal end of one of the sensor modules  12 . The connectors  25 ,  27  resist entry of fluid under pressure when the connectors  25 ,  27  are coupled to each other. 
         [0022]    The sensor module housing  24  is preferably pressure resistant and defines a sealed interior chamber  26  therein. The housing  24  may be made from electrically non-conductive, high strength material such as glass fiber reinforced plastic, and should have a wall thickness selected to resist crushing at the maximum expected hydrostatic pressure expected to be exerted on the housing  24 . The mating connectors  27  may be arranged in the longitudinal ends of the housing  24  as shown in  FIG. 2  such that axial loading along the cable  10  is transferred through the housing  24  by the coupled cable connectors  25  and mating connectors  27 . Thus, the sensor cable  10  may be assembled from a plurality of connector-terminated segments each coupled to a corresponding mating connector on a sensor module housing  24 . Alternatively, the cable  10  may include armor wires  10 A extending substantially continuously from end to end, and the sensor modules  12  may be affixed to the exterior of the armor wires  10 A. 
         [0023]    A measuring electrode  28  may be disposed on the outer surface of the housing  24 , and may be made, for example, from lead, gold, graphite or other corrosion resistant, electrically conductive, low electrode potential material. Electrical connection between the measuring electrode  28  and measuring circuits  34  disposed inside the chamber  26  in the housing  24  may be made through a pressure sealed, electrical feed through bulkhead  30  disposed through the wall of the housing  24  and exposed at one end to the interior of the chamber  26 . One such feed through bulkhead is sold under model designation BMS by Kemlon Products, 1424 N. Main Street, Pearland, Tex. 77581. 
         [0024]    The measuring circuits  34  may be powered by a battery  36  disposed inside the chamber  26  in the housing  24 . Battery power may be preferable to supplying power from the recording system ( 20  in  FIG. 1 ) over insulated electrical conductors in the sensor cable  10  so as to reduce the possibility of any electromagnetic fields resulting from current flowing along the cable  10  from interfering with the electromagnetic survey measurements made in the various sensor modules  12 . 
         [0025]    The cable  10  may include one or more optical fibers  38  for conducting command signals, such as from the recording unit ( 20  in  FIG. 1 ) to the circuits  34  in the various sensor modules  12 , and for conducting signal telemetry from the modules  12  to the recording unit ( 20  in  FIG. 1 ) or to a separate data storage device (not shown). An insulated electrical conductor  32  forming part of the cable ( 10  in  FIG. 2 ) may pass through the chamber  26  in the housing  24  such that electrical continuity in such conductor  32  is maintained along substantially the entire length of the cable  10 . Optical telemetry may be preferable to electrical telemetry for the same reason as using batteries for powering the circuits  34 , namely, to reduce the incidence of electromagnetic fields caused by electrical current moving along the cable  10 . The insulated electrical conductor  32  in the present example serves as a common potential reference between all of the sensor modules  12 . 
         [0026]    Returning to  FIG. 1 , at respective longitudinal ends of the sensor cable  10 , each of a pair of calibration source electrodes may be disposed. The calibration source electrodes are shown at  13 A and  13 B. In the present example, the calibration source electrodes  13 A,  13 B are disposed on the sensor cable  10  so as to include all the sensor modules  12  between them. In other examples, additional pairs of such calibration source electrodes (not shown) may be disposed on the sensor cable  10  so as to include between the electrodes in each such calibration source electrode pair a selected number of the modules  12 . 
         [0027]    The recording system  20  in the present example may include therein a calibration power supply  15 . The recording system  20  can be configured to operate the calibration power supply  15  at selected times during operation of the acquisition system. The calibration power supply  15  may be configured to generate, for example, sinusoidal alternating current at one or more selected frequencies, preferably within a range of 10 to 1000 Hz. The calibration power supply output is applied across the calibration source electrodes  13 A,  13 B. The voltage output of the calibration power supply  15  may be set so that the amount of current imparted across the electrodes  13 A,  13 B is on the order of a few to several tens of milliamperes. The configuration shown in  FIG. 1  is such that the current density will be highest along a straight line (along the sensor cable  10 ) between the calibration source electrodes  13 A,  13 B. The current density falls off rapidly out from the sensor cable  10 , and the current applied by the calibration power supply  15  to the electrodes  13 A,  13 B is such that only a substantially undetectable electromagnetic field therefrom affects the subsurface formations below the water bottom  23 . The frequency range of the calibration power supply  15  and the amount of current applied thereby may be considered to be a compromise between the frequency response range of electrodes ( 28  in  FIG. 2 ) on the sensor modules  12 , the electromagnetic field attenuation with respect to distance from the sensor cable  10  and the skin effect. 
         [0028]    In operating the system shown in  FIG. 1 , when the sensor cable  10  is first deployed in the water, the calibration power supply  15  may be actuated, and voltage impressed across the reference electrode and the electrodes in each sensor module  12  may be measured. The foregoing measurements may be used as a reference for subsequent measurements of the same parameters. During operation of the system shown in  FIG. 1 , at selected times after the initial deployment of the sensor cable  10 , the calibration power supply  15  may be again actuated and the impressed voltages may be measured. Variations in the measured voltages at certain electrodes may correspond to changes in sensitivity of such electrodes (e.g.,  28  in  FIG. 2 ). Measurements made by such electrodes during operation of the transmitter (e.g., by imparting current across electrodes  16 ) may be scaled to cancel any changes in sensitivity of the various electrodes determined during operation of the calibration power supply  15 . 
         [0029]    It is also possible using measurements made during operation of the calibration power supply  15  to estimate motion of the sensor cable through the water for determining an amount of the total signal measured during transmitter operation that results from electromagnetic induction caused by such motion. Motion induced signals may be present in sidebands of the current used to energize the calibration source electrodes, and, for example, measurement of the amplitude of the sideband energy may be used to infer motion of the sensor cable. 
         [0030]    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.