Patent Publication Number: US-8990019-B2

Title: Methods and apparatus for rapid determination of target depth and transverse resistance

Description:
BACKGROUND 
     Electromagnetic geophysical surveying of the Earth&#39;s subsurface involves imparting an electric field or a magnetic field into subsurface Earth formations, such formations being below the sea floor in marine surveys, and measuring electric and/or magnetic fields by measuring voltage differences induced in electrodes, antennas and/or interrogating magnetometers disposed at the Earth&#39;s surface, or on or above the sea floor. The electric and/or magnetic fields are induced in response to the electric field and/or magnetic field imparted into the Earth&#39;s subsurface, and inferences about the spatial distribution of conductivity of the Earth&#39;s subsurface are made from recordings of the induced electric and/or magnetic fields. 
     Often, electromagnetic surveying includes imparting a substantially continuous, time varying electromagnetic field into the subsurface formations by passing time varying electric current through a transmitter antenna. The alternating current may have one or more selected discrete frequencies. Such surveying is known as frequency domain surveying. Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient electromagnetic surveying. Therein, direct current (DC) electric current passes through a transmitter at the Earth&#39;s surface (or near the sea floor). At a selected time, the electric current is switched off, switched on, or has its polarity changed, and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth&#39;s surface or water surface. Alternative switching techniques are possible. 
     The above methods have been adapted for use in marine environments. Cable-based sensors have been devised for detecting electric and/or magnetic field signals resulting from imparting electric and/or magnetic fields into formations below the bottom of a body of water. Systems with towed electromagnetic receivers have also been devised. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example marine electromagnetic survey system which may be used to acquire electromagnetic data in accordance with an embodiment of the invention. 
         FIG. 2  shows a top view of a set of survey lines over a region with a resistive anomaly in accordance with an embodiment of the invention. 
         FIG. 3  is a cross-sectional diagram showing the EM surveying geometry at one common midpoint in accordance with an embodiment of the invention. 
         FIG. 4  is a flow chart of a method of real-time target depth and transverse resistance estimation in accordance with an embodiment of the invention. 
         FIG. 5  shows a top view of an actual sea area including approximate outlines of resistive regions under the sea floor in accordance with an embodiment of the invention. 
         FIG. 6  shows a one-dimensional resistivity background model determined in accordance with an embodiment of the invention. 
         FIG. 7A  shows a computed dipole depth along the survey line in accordance with an embodiment of the invention. 
         FIG. 7B  shows transverse resistance of the anomaly regions along the survey line as computed in accordance with an embodiment of the invention. 
         FIG. 8  is a high-level diagram showing an example computer apparatus in accordance with an embodiment of the invention. 
     
    
    
     Note that the figures provided herewith are not necessarily to scale. They are provided for purposes of illustration to ease in the understanding of the presently-disclosed invention. 
     DETAILED DESCRIPTION 
     In electromagnetic surveying techniques, the resistivity of the sub-bottom structure may be determined from the time distribution of the induced voltages and/or magnetic fields. Conventional techniques, such as traditional two-and-a-half dimensional (2.5D) or three dimensional (3D) inversion, may be applied to obtain a resistivity map of the survey area. However, such conventional techniques for determining sub-bottom resistivity information from electromagnetic (EM) data are highly tedious and time consuming. After acquiring the EM data, it often takes many days to obtain a reliable resistivity map of the survey area. 
     It is, therefore, highly desirable to provide a more efficient EM data analysis technique to reduce the time lag between acquiring the EM data and obtaining useful sub-bottom resistivity information. The present disclosure provides an innovative technique for efficiently determining target depth and transverse resistance from data acquired by a towed EM system. Using this technique and currently-available computing apparatus, the target depth and transverse resistance may be determined in real time during the EM data acquisition from a survey area. 
       FIG. 1  shows an example marine electromagnetic survey system which may be used to acquire EM data in accordance with an embodiment of the invention. As shown, a survey vessel  10  may move along the surface of a body of water  11 , such as a lake, sea, or the ocean. The vessel  10  may include equipment, shown generally at  12  and referred to for convenience as a “recording system” that includes devices for applying electric current to an antenna such as source electrodes  18  and/or other devices disposed on or along a source cable  14  towed by the vessel  10 . The recording system  12  may also include equipment for navigating the vessel  10 , for determining the geodetic position of the vessel  10  and of components towed by the vessel  10  in the water  11 , and for recording signals detected by one or more sensors on a sensor cable  16 . As shown, the sensor cable  16  may also be towed by the vessel  10 . In other embodiments, there may be multiple sensor cables  16 , and the sensor cable(s)  16  may be towed by a second vessel (not shown). 
     The source cable  14  in the present example may include an antenna consisting of multiple (two are shown) source electrodes  18  disposed at spaced apart positions along the source cable  14 . At selected times, certain of the equipment in the recording system  12  may conduct electric current across the source electrodes  18 . The time varying component of such electric current produces an electromagnetic field that propagates through the water  11  and into the formations below the water bottom  19 . The particular type of electrical current conducted across the source electrodes  18  may be a single or multiple discrete frequency alternating current as is used in frequency domain electromagnetic surveying, or various forms of switched direct current, as is used in transient electromagnetic surveying. 
     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 transmitter 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 transmitter antenna shown in the figure, any one or more of a vertical electric dipole antenna, and horizontal or vertical magnetic dipole antenna (current loop). The horizontal dipole(s) may also be aligned broadside (perpendicular to the direction of towing). 
     In the illustrated example, the vessel  10  may also tow at least one sensor cable  16 . The sensor cable  16  may include a plurality of electromagnetic sensors  20  at spaced apart positions along the sensor cable  16 . Each of the electromagnetic sensors may measure a parameter related to the electromagnetic field resulting from interaction of the electromagnetic field induced by the transmitter (e.g., source electrodes  18 ) with the subsurface formations below the water bottom  19 . In the present example, the electromagnetic sensors may be a pair 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 below the water bottom  19  may induce voltages across each of the pairs of receiver electrodes, and such voltages may be detected by a voltage measuring circuit. Such voltage measuring circuits 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 embodiments is a single axis or multi-axis magnetometer, such as a flux gate magnetometer. 
     The sensor cable  16  in some examples may also include seismic sensors, such as hydrophones and/or geophones, shown generally at  22 , disposed at spaced apart locations along the sensor cable  16 . For such examples where the sensor cable  16  includes seismic sensors, the survey vessel  10  or another vessel may tow a seismic energy source  24 , such as an air gun or array of air guns. The seismic energy source  24  may be actuated at selected times by certain equipment in the recording system  12  and signals detected by the seismic sensors  22  may be recorded by a signal recording device in the recording system  12 . During survey operations, seismic signals may be acquired substantially contemporaneously with electromagnetic signals detected by the electromagnetic sensor  20  or may be acquired at other times. 
     It should be understood that the example system in the figure 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. Such a sensor cable may be used in 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. 
       FIG. 2  is a top view of an example region of interest of a body of water with a sub-bottom resistive anomaly  202  (within the outline shown). A survey vessel  204  may travel along survey lines  206  which cross the region of interest. The survey vessel  204  may be configured to perform a towed EM survey. In one embodiment, the survey vessel  204  may be configured as shown in  FIG. 1  and described above. Further depicted in  FIG. 2  is a set of common midpoints (cmps)  208  along the survey lines  206 . 
     The EM surveying geometry at a cmp  208  is shown in the cross-sectional diagram in  FIG. 3 . Note that the diagram in  FIG. 3  is for illustrative purposes and is not necessarily to scale. 
     In  FIG. 3 , the sea (or lake or ocean) surface  302  and the sea (or lake or ocean) floor  304  are depicted as horizontal lines. There is, of course, air  301  above the sea surface  302 , and water  303  between the sea surface  302  and the sea floor  304 . The x-axis is defined as a horizontal line along a survey line  206  on the sea surface  302 , and the z-axis is defined as a vertical line going through the cmp  208 . 
     As further shown, there are spaced apart source points r″ below the x-axis at a depth z source  in the water  303 . In addition, there are spaced apart receiver points r below the x-axis at a depth z receiver  in the water  303 . For example, the depth z source  may be 10 meters deep, and the depth z receiver  may be within a range of 8 to 100 meters deep. Other depths may be used for one or more of the source and receiver points, depending on the implementation of the EM survey. Each source point r″ may represent an EM source location, such as a location of a source electrode  18  as described above. Each receiver point r may represent the location of an EM receiver device, such as the location of an electromagnetic sensor  20 . 
     In this example, the cross section of a resistive anomaly (also referred to as the resistive region, anomaly region, or target region)  306  within background sediment  310  beneath the sea floor  304  is depicted (within the outline shown). In accordance with an embodiment of the invention, a sensitive zone  308  is defined as a rectangular box within the resistive anomaly  306  is used as a simplified approximation of the resistive anomaly  306 . 
       FIG. 4  is a flow chart of a method  400  of real-time target depth and transverse resistance estimation in accordance with an embodiment of the invention. The method  400  includes acquiring  402  EM receiver data from at least one common midpoint (cmp). The geometry for the EM data acquisition from the cmp may be as described above in relation to  FIG. 3 . 
     Determining the Resistivity Background Model 
     To analyze the data efficiently, according to an embodiment of the invention, a determination  404  may be made as to a one-dimensional resistivity background model. The one-dimensional resistivity background model refers to a simplified model for the resistivity of the water  303  above the sea floor  304  and the background sediment  310  surrounding the targeted resistivity anomaly  306  (or surrounding the sensitive zone  308  in the simplified model of the anomaly). 
     The background model is one-dimensional in that the resistivity varies as a function of depth z while not varying laterally in x and y within the air  301 , water  303 , or background sediment  310 . In one embodiment, the one-dimensional resistivity background model may assume a first resistivity level for the water  303  above the sea floor  304 , a second resistivity level for the horizontal resistivity of the background sediment  310 , and a third resistivity level for the vertical resistivity of the background sediment  310 . A specific example of such a one-dimensional resistivity background model is described below in relation to  FIG. 6 . 
     In accordance with an embodiment of the invention, the one-dimensional resistivity background model may be determined  404  by a non-linear minimization procedure. In one implementation, the objective function to be minimized may be given by: 
                     F     1   ⁢           ⁢   D   ⁢           ⁢   bg   ⁢           ⁢   inversion       =       ∑     l   =   1     N     ⁢           ⁢                E   l   bg     -     E   l   mea              E   l   mea                 2               (   1   )               
where E l   bg  is the calculated response field with a 1D background model, and E l   mea  is the measured response field. The objective function in Equation (1) may be summed over both several frequencies and offsets at a cmp outside the high resistive region. A gradient-based interior point procedure may be used to minimize the objective function of Equation (1). In other words, the objective function in Equation (1) includes a difference between a calculated electric field response based on the one-dimensional resistivity background model and a measured electric field response for a cmp that is outside (i.e. not above) the sub-bottom resistive region. This background inversion procedure results advantageously in a stable and unique solution of the background resistivity profile.
 
Determining the Dipole
 
     Using the one-dimensional resistivity background model, a determination  406  may then be made of the depth and strength of the dipole of the targeted anomaly region  306 . This determination  406  may be based upon the following formulation. 
     The frequency-dependent response field E(r,ω) may be formulated as the sum of a background response field E bg (r,ω) and the change in the background response field due to the resistive region (anomaly). This is shown in Equation (2): 
                     E   ⁡     (     r   ,   ω     )       =         E   bg     ⁡     (     r   ,   ω     )       +       ∫   anomaly     ⁢           G   e     ⁡     (     r   ,     r   ′     ,   ω     )       ·     (       (       σ   ⁡     (     r   ′     )       -       σ   bg     ⁡     (     r   ′     )         )     ·     E   ⁡     (       r   ′     ,   ω     )         )       ⁢           ⁢     ⅆ     V   ⁡     (     r   ′     )                       (   2   )               
where G e (r,r′,ω) is the Green&#39;s function of the background. The electric conductivity in the anomaly is σ(r′) and in the background σ bg (r′). Hence, the conductivity change is given within Equation (2). In accordance with an embodiment of the invention, the towed EM surveying system only measures the x-component (in-line) of the electric field response which approximately reduces the expression in Equation (2) to:
 
     
       
         
           
             
               
                 
                   
                     
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                               G 
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                               - 
                               
                                 
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     A vertical cross section at an arbitrary cmp may be as shown in the example depicted in  FIG. 3  and described above. In accordance with  FIG. 3 , the sensitive zone  308  of the resistive anomaly  306  for the data from this cmp may be approximated as a box of size (Δx,Δy,Δz). Applicant has determined numerically that the Green&#39;s function and the electric field E x (r′,ω) in Equation (3) are fairly constant within the box of the sensitive zone  308  provided that the sensitive zone  308  is sufficiently deep underneath the sea bottom (mud line)  304 . Hence, given a sensitive zone  308  sufficiently deep below the sea bottom  304 , the following expression has been determined to be a reasonably good approximation for the electric field of the frequency response:
 
 E   x ( r ,ω)≈ E   x   bg ( r ,ω)+ E   x   anomaly ( r ,ω)  (4)
 
E x   anomaly (r,ω) in Equation (4) is the anomaly response field and represents the change in the response field (compared to the background response field E x   bg (r,ω)) which due to the anomaly region embedded in the background material.
 
     In accordance with an embodiment of the invention, the anomaly response field E x   anomaly (r,ω) may be approximated by the response field E x   Idl (r,ω) due to an electric dipole at r c ′, where r c ′ is a location along the z-axis at a depth at the center of the vertical extent of the sensitive zone  308 . Hence, the change in the response field that is due to the resistive anomaly may be approximated as follows:
 
 E   x   anomaly ( r ,ω)≈ E   x   Idl ( r ,ω)=(− G   exx ( r,r′   c ,ω)arg( E   x ( r′   c ,ω)))Idl( r′   c ,ω)  (5)
 
where the dipole strength Idl(r′ c ,ω) is given by:
 
Idl( r′   c ,ω)=| E   x ( r′   c ,ω)|(σ h   bg ( r′   c )−σ h ( r′   c ))Δ xΔyΔz   (6)
 
As shown above, the dipole strength Idl(r′ c ,ω) is the product of the box volume ΔxΔyΔz of the sensitive zone  308 , the magnitude of the x-component of the response field due to the dipole at r c ′, and the change in horizontal conductivity from the sensitive zone  308  to the background sediment  310 . The dipole strength Idl(r′ c ,ω) is positive for resistive anomalies (with lower conductivity than the background).
 
     In accordance with an embodiment of the invention, the depth and strength of the dipole may be determined  406  by a non-linear minimization procedure. In one implementation, the objective function to be minimized may be given by: 
                     F     dipole   ⁢           ⁢   inversion       =       ∑     l   =   1     N     ⁢           ⁢       (                E   l   bg     +     E   l   dip            -          E   l   mea                   E   l   mea            )     2               (   7   )               
where E l   bg  is the calculated response field with a 1D background model, E l   dip  is the calculated response field due to the dipole, and the measured response field is denoted as E l   mea . For this dipole inversion procedure, the lowest sensitive frequency may be used for a sensitive set of offsets at every cmp, and a gradient-based interior point procedure may be used to minimize the objective function of Equation (7).
 
Computing the Transverse Resistance of the Anomaly
 
     After determining  406  the dipole depth and strength, the transverse resistance of the anomaly may be computed  408  based on the following formulation. From Ohm&#39;s law and the fact that the electric current density is approximately unchanged when flowing from the background material into the anomaly material results in: 
                            E   x     ⁡     (       r   c   ′     ,   ω     )            ≈           σ   h   bg     ⁡     (     r   c   ′     )           σ   h     ⁡     (     r   c   ′     )         ⁢            E   x   bg     ⁡     (       r   c   ′     ,   ω     )                      (   8   )               
Equation (8) gives a linear relation between the strength of the background response field (the response field without the anomaly) and the strength of the response field with the anomaly present. The resistivity in the sensitive zone  308  may then be written as:
 
     
       
         
           
             
               
                 
                   
                     
                       ρ 
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     Applicant has determined that the second term in the numerator may be neglected for typical values of background and anomaly conductivities. Hence, the anomaly transverse resistance ρ h (r′ c )Δz, defined in this approximation as the resistivity of the anomaly material ρ h (r′ c ) multiplied by the vertical extent (height) Δz of the sensitive zone  308 , may be approximated as: 
                         ρ   h     ⁡     (     r   c   ′     )       ⁢   Δ   ⁢           ⁢   z     ≈         Idl   ⁡     (       r   c   ′     ,   ω     )                  E   x   bg     ⁢     (       r   c   ′     ,   ω     )            ⁢   Δ   ⁢           ⁢   x   ⁢           ⁢   Δ   ⁢           ⁢   y       ⁢       (       ρ   h   bg     ⁡     (     r   c   ′     )       )     2               (   10   )               
Equation (10) indicates that the anomaly transverse resistance ρ h (r′ c )Δz may be estimated from the dipole strength Idl(r′ c ,ω), the background response field E x   bg (r′ c ,ω), the background resistivity ρ h   bg (r′ c ), and the horizontal extent ΔxΔy of the sensitive zone  308  for each cmp.
 
     In accordance with an embodiment of the invention, the anomaly transverse resistance (i.e. the transverse resistance of the anomaly) may be computed  408  using the expression of Equation (10). The factors on the right-hand side of Equation (10) may be determined as described above. 
     Example Application to High Resistive Reservoir 
     A real world example application of the method  400  of  FIG. 4  is now described in relation to  FIGS. 5 ,  6 ,  7 A and  7 B.  FIG. 5 , which is based on real electromagnetic survey data, shows a top view of an actual sea area including approximate outlines of three sub-bottom anomaly regions (resistive regions)  502  under the sea floor. These anomaly regions  502  have resistivities that are substantially higher than the surrounding background sediment. An example of an actual survey line  504  going across the anomaly regions  502  is also depicted. 
     A vessel traveled along the survey line  504  and used EM surveying apparatus (such as the example apparatus described above in relation to  FIG. 1 ) to acquire  402  EM survey data at periodically spaced apart cmps. As indicated by the arrow, the survey line  504  may be traveled across from left to right in the drawing (i.e. from west to east). As seen in  FIG. 5 , there are three line segments  506 ,  508 , and  510  of the survey line  504  which are above one of the anomaly regions  502 . 
       FIG. 6  is a graph of an example one-dimensional resistivity background model determined  404  in accordance with an embodiment of the invention. The background model in  FIG. 6  may be used, for example, for the real-time estimation of the depths and transverse resistances of the anomaly regions  502  as the survey vessel travels along the survey line  504 . 
     In  FIG. 6 , the vertical axis of the graph shows the depth in meters (m), and the horizontal axis shows the resistivity in Ohm meters (Ohm m). Both the vertical and the horizontal resistivities are graphed. Assuming that the depth of the sea floor is many times greater than the distance between the EM source and EM receiver, the vertical resistivity is a good approximation for the longitudinal resistivity, and the horizontal resistivity is a good approximation for the transverse resistivity. 
     In this example, the sea floor  304  is approximately 330 meters beneath the sea surface  302 . The vertical and horizontal resistivities are set to be a same low resistivity level  602  of approximately 0.25 Ohm meters in the sea water  303  above the sea floor  304 . 
     Beneath the sea floor  304 , in the background sediment  310 , the vertical and horizontal resistivities are set to have different resistivities. As shown, the vertical resistivity level  604  of the background sediment  310  is determined to be approximately 3.1 Ohm meters, and the horizontal resistivity level  606  of the background sediment  310  is determined to be approximately 1.8 Ohm meters. The resistivity ratio may be defined as the vertical resistivity divided by the horizontal resistivity. Hence, in this example, the resistivity ratio for the water  303  may be determined to be one, while the resistivity ration for the background sediment  310  may be determined to be about 1.7. 
       FIG. 7A  shows the dipole depth along the example survey line  504  as determined  406  in accordance with an embodiment of the invention. The vertical axis of the graph shows the depth in meters, and the horizontal axis shows the cmp number. EM data was acquired at 102 cmps during the survey along the example survey line  504 . Interpolation may be used to estimate the dipole depth for points in between cmps. 
     The graph shown in  FIG. 7A  shows three “peaks” in the estimated dipole depth. Each peak corresponds to one of the three line segments ( 506 ,  508 , and  510 ) of the survey line  504  which are above one of the anomaly (resistive) regions  502 . As seen by the estimated dipole depth of the peaks, the anomaly regions are indicated to have a center depth of roughly 1,400 meters (m). 
       FIG. 7B  shows transverse resistance of the anomaly regions along the survey line as computed in accordance with an embodiment of the invention. The vertical axis of the graph shows the anomaly transverse resistance in 10 4  Ohm m 2 , and the horizontal axis shows the cmp number. Interpolation may be used to estimate the anomaly transverse resistance for points in between cmps. 
     The graph shown in  FIG. 7B  shows three “peaks” in the anomaly transverse resistance. Again, each peak corresponds to one of the three line segments ( 506 ,  508 , and  510 ) of the survey line  504  which are above one of the anomaly (resistive) regions  502 . As seen by the estimated transverse resistance of the peaks, the anomaly transverse resistance is highest for the middle segment  508 , lowest for the left segment  506 , and in between for the right segment  510 . Assuming a same high-resistivity substance throughout the anomaly regions  502 , it may be inferred from this result that the high-resistivity anomaly is thickest under the middle segment  508 , least thick under the left segment  506 , and of in between thickness under the right segment  510 . This is because, per the left side of Equation (10), the anomaly transverse resistance is proportional to the vertical extent (i.e. the height) Δz of the sensitive zone  308 . 
     Advantageously, the computations to obtain the results shown in  FIGS. 7A  and  7 B may be performed very rapidly. An implementation using a laptop computer (with a dual core processor and 6 gigabytes of RAM) performed the computations for all 102 cmps in about 20 seconds. Hence, the computations for each cmp were performed in only about 0.2 seconds. The rapid speed of these computations enable the target depth and transverse resistance to be estimated in real time as the EM survey data is acquired by the vessel. Such real-time computations of target depth and transverse resistance enable adjustments to be made dynamically to the survey plan while the survey is being performed. For example, the survey plan may be changed to explore in further detail a target region that appears to have a particularly promising subsurface formation based on the real-time depth and resistance information. 
       FIG. 8  is a high-level diagram of an example computer apparatus  800  in accordance with an embodiment of the invention. The computer apparatus  800  may be configured with executable instructions so as to perform the data processing methods described herein. This figure shows just one example of a computer which may be used to perform the data processing methods described herein. Many other types of computers may also be employed, such as multi-processor computers, server computers, cloud computing via a computer network, and so forth. 
     The computer apparatus  800  may include a processor  801 , such as those from the Intel Corporation of Santa Clara, Calif., for example. The computer apparatus  800  may have one or more buses  803  communicatively interconnecting its various components. The computer apparatus  800  may include one or more user input devices  802  (e.g., keyboard, mouse), one or more data storage devices  806  (e.g., hard drive, optical disk, USB memory), a display monitor  804  (e.g., LCD, flat panel monitor, CRT), a computer network interface  805  (e.g., network adapter, modem), and a main memory  810  (e.g., RAM). 
     In the example shown in this figure, the main memory  810  includes executable code  812  and data  814 . The executable code  812  may comprise computer-readable program code (i.e., software) components which may be loaded from the data storage device  806  to the main memory  810  for execution by the processor  801 . In particular, the executable code  812  may be configured to perform the data processing methods described herein. 
     In conclusion, the present disclosure provides a fast and efficient technique for estimating the depth and transverse resistance of resistive anomaly regions below the sea floor. The technique may use real-time processed frequency responses from at least one cmp. A background resistivity model is determined. An inversion procedure is used to determine the depth and transverse resistance of a resistive region based, at least in part, on the change in the frequency response when moving from one cmp that is above background sediment to another cmp that is above the resistive region. A map of the survey area showing the depth and transverse resistance (or vertical extent) may then be generated by mapping the results for an array of cmps. 
     In an application of particular interest, the resistivity of a sub-bottom resistive region (i.e. the anomaly region) may be substantially higher than a resistivity of the background material. Such a sub-bottom resistive region may be indicative of an oil and/or gas deposit underneath the sea floor. 
     In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.