Abstract:
A method for acquiring transient electromagnetic survey signals includes applying a transient electric current to an electromagnetic transmitter disposed above a portion of the Earth&#39;s subsurface to be surveyed. Electromagnetic signals are detected at spaced apart locations above the portion of the subsurface in response to an electromagnetic field induced in the Earth&#39;s subsurface by the applying transient current. Electromagnetic signals are detected at least one position proximate a position of the electromagnetic transmitter such that the subsurface transient response is substantially always identifiable therefrom.

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 surveying of formations in the Earth&#39;s subsurface. More particularly, the invention relates to methods for acquiring data suitable for attenuating certain types of noise from controlled source electromagnetic survey data. 
         [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 an electric current or a magnetic field into the Earth, when such surveys are conducted on land, or imparting the same into the water column (or on the 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, on the sea floor or at a selected depth in the water. 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 or within the water column in marine surveys) with the subsurface Earth formations. 
         [0007]    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. A 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 100 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. 
         [0008]    Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying. In transient controlled source electromagnetic surveying, electric current can be 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 off, and induced voltages are measured, typically with respect to time over a selected time interval, using electrodes disposed on the water bottom 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. t-CSEM surveying techniques are described, for example, in Strack, K.-M. (1992),  Exploration with deep transient electromagnetics , Elsevier, 373 pp. (reprinted 1999). 
         [0009]    A source of noise in controlled source electromagnetic surveying is naturally occurring electromagnetic fields called magnetotelluric fields. Magnetotelluric fields are believed to result from interaction of electromagnetic activity in the ionosphere with the electrically conducting formations in the Earth&#39;s subsurface. Correlated noise, especially magnetotelluric noise, is a particular issue in transient electromagnetic data. Magnetotelluric noise appears in such data at about 1 Hz uppermost frequency and increases in amplitude approximately as the inverse of the frequency. 1 Hz and below is the frequency band of interest of much transient controlled source electromagnetic survey data. The bandwidth of the impulse response of transient electromagnetic survey data generally decreases in frequency with respect to the depth in the subsurface of target rock formations and as the overburden (materials above the target) become more electrically conductive. In shallow water (approx 100 m) marine electromagnetic survey data, for example, the water has almost no attenuating effect on the magnetotelluric fields. This is in contrast to water of 2 km depth or more where the magnetotelluric field noise at the sea floor is greatly attenuated by the layer of conductive sea water. 
         [0010]    It is known in the art that the magnetotelluric field noise, specifically, the induced electric field therefrom, is substantially coherent over quite large distances, as shown in noise records from survey data recorded in the North Sea. See, for example, Wright, D and Ziolkowski, A., 2007 , Suppression of noise in multi transient EM data , Expanded Abstracts, SEG San Antonio Annual Meeting. Techniques for attenuating correlated noise are known in the art. Some of such techniques may make use of recorded transient electromagnetic signals having suitably high signal to noise ratio to be able to identify those portions of signals recorded at each of a plurality of sensors that are attributable to the transient response so as to be able to identify and attenuate the correlated noise response. 
         [0011]    It is desirable to have a method for acquiring electromagnetic signals in which signal response from at least one receiver or detector has sufficient signal to noise ratio to be able to perform correlated noise attenuation methods. 
       SUMMARY OF THE INVENTION 
       [0012]    A method for acquiring transient electromagnetic survey signals according to one aspect of the invention includes applying a transient electric current to an electromagnetic transmitter disposed above a portion of the Earth&#39;s subsurface to be surveyed. Electromagnetic signals are detected at spaced apart locations above the subsurface portion in response to an electromagnetic field induced in the Earth&#39;s subsurface by the applying transient current. Electromagnetic signals are detected at least one position proximate a position of the electromagnetic transmitter such that the subsurface transient response is substantially always identifiable therefrom. 
         [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 schematically an example of marine electromagnetic surveying. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  shows examples of marine electromagnetic survey systems that may acquire transient controlled source electromagnetic survey signals according to the invention. The system may include a survey vessel  10  that moves along the surface  12 A of a body of water  12  such as a lake or the ocean. The vessel  10  may include thereon equipment, referred to for convenience as a “recording system” and shown generally at  14 , for generating electromagnetic signals to be imparted into formations  24  below the bottom of the water  12  and for recording responses therefrom detected by various electromagnetic receivers (explained below). The recording system  14  may include (none shown separately for clarity of the illustration): navigation devices to determine the geodetic position of the vessel  10 ; for determining geodetic position and/or heading of one or more electromagnetic transmitters and receivers (described below); devices for imparting electric current to the transmitter(s); and data storage equipment for recording signals detected by the one or more receivers. 
         [0016]    The electromagnetic transmitter in the present example may be a bipole electrode, shown at  16 A,  16 B disposed along a cable  16  towed by the vessel  10 . In other examples, the transmitter cable  16  may be disposed on the water bottom. At selected times, the recording system  14  may pass electric current through the electrodes  16 A,  16 B. The current is preferably configured so that its passage through the transmitter (electrodes  16 A,  16 B) induces transient electromagnetic fields in the formations  24 . Examples of such current include switched direct current, wherein the current may be switched on, switched off, reversed polarity, or an extended set of switching events such as a pseudo random binary sequence (“PRBS”). In the present example, the vessel  10  may tow one or more receiver cables  18  having thereon receivers. In one example, each of the receivers can each be a bipole electrode  18 A,  18 B, and a plurality of such bipole electrode receivers are typically disposed at spaced apart positions along the receiver cable  18 . The bipole electrodes  18 A,  18 B will have voltages imparted across them related to, among other things, the amplitude of the electric field component of the electromagnetic field emanating from the formations  24 . The recording system  14  on the vessel  10  may include, as explained above, devices for recording signals generated by the receivers (electrodes  18 A,  18 B). The recording of each receiver&#39;s response is typically indexed with respect to a reference time such as a current switching event in the transmitter current. A sensor  17  such as a magnetic field sensor (magnetometer) or current meter may be disposed proximate the transmitter, for example at a selected position in the transmitter cable  16 , and such sensor may be used to measure a parameter related to the amount of current flowing through the transmitter. The measurements from such sensor may be used in processing the receiver signals as explained below. 
         [0017]    In the present example, in substitution of or in addition to the receiver cable  18  towed by the vessel  10 , a water bottom cable  20  may be disposed along the bottom of the water  12 , and may include a plurality of receivers such as bipole electrodes  20 A,  20 B similar in configuration to the electrodes  18 A,  18 B on the towed receiver cable  18 . The electrodes  20 A,  20 B may be in signal communication with a second vessel (not shown) or with a recording buoy  22  or similar device either near the water surface  12 A or on the water bottom that may record signals detected by the electrodes  20 A,  20 B. 
         [0018]    In a method according to the invention, electromagnetic signals may also be measured using what may be referred to as a “near field” receiver. Examples of such near field receivers may include a bipole electrode  17 A,  17 B disposed near or at an end of the transmitter cable  16 . Another example may include a bipole electrode disposed on the water bottom receiver cable  20 , for example, as shown at  17 E and  17 F. Another example is a bipole electrode disposed on a receiver cable  20 C deployed on the water bottom and coupled to a second recording buoy  22 A. Any or all of such near field receivers may be used in any particular implementation. It is contemplated that at any time a distance between the transmitter (electrode pair  16 A,  16 B) and any one or more of the near field receivers will not exceed an amount such as will ensure sufficient signal to noise ratio to perform correlated noise attenuation techniques known in the art. While the distance between the transmitter and one or more of the near field receivers in any example will depend on factors such as the depth of the body of water, the depth of target formations in the subsurface and the distance between the transmitter and the other electromagnetic receivers, a distance typically at least three times and not exceeding about 10 times the distance between the current electrodes  16 A,  16 B in the transmitter is believed to provide adequate signal to noise for most purposes. In some examples the distance between the transmitter and the one or more near field receivers is at most about 2000 meters. Any time dependence of the distance between the transmitter and a particular near field receiver is a result of movement of the transmitter through the water as the vessel moves, as may be inferred from the above description. 
         [0019]    The implementation in which the near field receiver is disposed at the end of the transmitter cable  16 , for example, receiver electrode pair  17 A and  17 B has the advantage of providing substantially constant distance between the transmitter and the near field receiver. As will be appreciated by those skilled in the art, passage of high transient current along the transmitter cable  16  may result in induction noise in electrical conductors used to communicate signals from the near field receiver. In such event, it may be advantageous to provide a recording device  17 G to record signals from the near field receiver  17 A,  17 B on the transmitter cable  16 . Such recording device  17 G is preferably disposed opposite to the end from which current is applied to the transmitter. Such arrangement may substantially avoid or reduce electromagnetic induction effects on the near field receiver from within the transmitter cable  16  itself. 
         [0020]    In another example as mentioned above, a separate near field receiver may be deployed on the water bottom using the second receiver cable  20 C coupled to the associated second recording buoy  22 A. The second receiver cable  20 C may include as the near field receiver an electrode pair  17 C,  17 D at a selected position along the second receiver cable  20 C. In such example, the electrode pair  17 C,  17 D may be disposed such that during the survey the distance between the near field receiver (electrodes  17 C and  17 D) and the transmitter (electrodes  16 A,  16 B) typically does not exceed about 10 times the distance between the transmitter electrodes  16 A,  16 B. 
         [0021]    Irrespective of the type of near field receiver used in any example, it is contemplated that the distance between the near field receiver and the transmitter will be kept within such amount so as to be able to substantially always identify the subsurface transient response in the near field receiver response. 
         [0022]    It will be appreciated by those skilled in the art that the invention is not limited in scope to the transmitter and receiver arrangements shown in  FIG. 1 . Other examples may use, in substitution of or in addition to the bipole electrodes shown in  FIG. 1 , wire coils or wire loops for the transmitter to impart a time varying electromagnetic field into the formations  24 . The receiver cables  18 ,  20  may include other sensing devices, such as magnetometers or wire loops or coils to detect the magnetic field component of the induced electromagnetic field from the formation  24 . 
         [0023]    Methods according to the invention may provide controlled source electromagnetic survey measurements that can be processed to have reduced effect of correlated noise, such as magnetotelluric noise. 
         [0024]    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.