Abstract:
A method is provided for analyzing data from an electromagnetic survey of a region so as to indicate the presence of a hydrocarbon reservoir. The survey provides vertical magnetic dipole data and electric dipole data, or provides measurement data from which these dipole data may be determined. The amplitude of the vertical magnetic dipole data is determined, optionally after normalising the data with reference date relating to the same region. Similarly, the amplitude of the electric dipole data is determined, optionally after similarly being normalised. The amplitudes are then compared to provide an indication of the likelihood of the presence of hydrocarbons in the region.

Description:
PRIORITY CLAIM 
     The present application is a National Phase entry of PCT Application No. PCT/EP2008/050200, filed Jan. 9, 2008, which claims priority from Great Britain Application Number 0700281.0, filed Jan. 9, 2007, the disclosures of which are hereby incorporated by reference herein in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a method of and an apparatus for analyzing data from an electromagnetic survey. Such methods and apparatuses may be used, for example, for land or marine electromagnetic surveying for locating hydrocarbon reservoirs. 
     BACKGROUND ART 
     Throughout the specification, the terms “source” and “transmitter” are used interchangeably, as are the terms “receiver,” “sensor” and “detector”. 
     Use of marine controlled source electromagnetic (mCSEM) surveying, also referred to as sea bed logging (SBL), for mapping hydrocarbons over shallow prospects in deep water is disclosed in patent specification number GB 01/00419. Further examples of this technique are disclosed by Eidesmo T, S Ellingsurd, L M MacGregor, S Constable, M C Sinha, S Johansen, F N Kong, and H Westerdahl 2002 Sea Bed logging (SBL), “A new method for remote and direct identification of hydrocarbon filled layers in deepwater areas”: First Break 20 144-152, and Ellingsrud S, T Eidesmo, S Johansen, M C Sinha, L M MacGregor, and S Constable 2002 “Remote sensing of hydrocarbon layers by sea bed logging (SBL); Results from a cruise offshore Angola”: The Leading Edge 21 972-982. 
     GB 2 390 904 discloses an electromagnetic surveying technique using a vertical electric dipole and a vertical magnetic dipole or naturally occurring electromagnetic fields. 
     Another known type of electromagnetic surveying technique is known as the multi-transient electromagnetic (MTEM) method and an example of this is disclosed in U.S. Pat. No. 6,914,433. 
     The mCSEM/SBL technique is based on the fact that hydrocarbons in a subsurface are significantly more resistive to electromagnetic waves than non-hydrocarbon-bearing layers, such as shale or sandstone containing saltwater. The resistivity of shale is in the range of 0.5-3 Ωm and that of water-filled sandstone is around 1 Ωm, whereas that of hydrocarbon-filled sandstone is in the range 5-200 Ωm. Hydrocarbons may therefore be detected by transmitting electromagnetic fields into the subsurface and recording the returning signal at a range of distances or “offsets” from a source. Such an electromagnetic surveying technique is sensitive to the types of fluid in the rock. 
     A mCSEM/SBL survey typically emits electromagnetic signals close to the sea bed from a powerful electric source. This is generally a horizontal electric dipole (HED) transmitter driven by a low frequency alternating current (AC) of quasi-square waveform. The first several harmonics may be processed so as to increase the bandwidth. 
     In a typical mCSEM/SBL survey, one or more receivers are deployed along a line on the sea bed or across a section of the sea bed using high-symmetry or irregular receiver grids. Each of the receivers includes an instrument package, a detector, a flotation device and a ballast weight. The detector may comprise, for example, a three component electric dipole detector set and a three component magnetic dipolar detector set. In each of these sets, two dipole detectors are oriented in the horizontal plane as an orthogonal pair and the third dipole detector is oriented in the vertical direction. 
     Once the survey has been completed and the data collected, the receivers may be retrieved. A telemetric signal from the survey ship prompts each receiver to detach itself from its ballast weight and float to the surface by means of the flotation device, which typically comprises a top-mounted buoyancy system. The receiver position is monitored by a hydro acoustic tracking system. Once the raw data have been downloaded to a computer, they are collated and processed for subsequent data interpretation. The end product is typically an electromagnetic map of the surveyed area, in which hydrocarbon-bearing layers can be distinguished from other layers. 
     The three major pathways for propagation of an mCSEM/SBL signal are through the sea water, through the subsurface, and through the air. The direct field through the sea water is the signal which is transmitted directly from, for example, an electric dipole source to a receiver. This field dominates in amplitude at short source-receiver separations or offsets but is strongly damped at larger offsets due to a combination of geometrical spreading associated with the source dipole geometry and skin-effect-related exponential attenuation. 
     The signal that travels partly through air is called the source-induced “airwave”. The airwave is dominated by the signal component that diffuses upwards from the source to the sea surface, then propagates through the air at the speed of light with no attenuation, before diffusing back down through the seawater column to the sea bottom where it is picked up by the receivers. 
     The subsurface structures are, in general, more resistive than the sea water. As a result, skin depths in the subsurface are larger than those in sea water so that the electromagnetic fields propagating in the subsurface before returning to the seabed at intermediate to long offsets are less attenuated than the direct field. 
     A hydrocarbon-filled reservoir has relatively high resistivity compared with shales and water-filled sandstones of the subsurface. The field of main interest for hydrocarbon mapping is related to the energy propagating downwards from the source into the subsurface and then interacting with the resistive reservoir before returning upwards at intermediate to large offsets. Thus, the electric fields at the receivers should be larger in magnitude over resistive subsurface structures such as hydrocarbon reservoirs than the more-attenuated background electromagnetic fields caused by host sediments. This is related to the lower attenuation experienced by the component of the electromagnetic signal that travels along the higher resistivity hydrocarbon filled reservoir. Thus, when an electromagnetic field propagates over a long distance in hydrocarbon reservoirs, the amplitude of the detected signals dominate those signals which have propagated in the water-bearing sediments. This “enhancement” in electric field amplitude at long source-receiver separations (compared to the depth of the reservoir) allows hydrocarbon reservoirs to be detected. 
     It is known, however, that an increase in electromagnetic field amplitude need not solely be related to the presence of hydrocarbons. Also, local large-scale resistive bodies other than hydrocarbon reservoirs beneath the seabed can significantly affect the electromagnetic fields due to longer skin depths with increasing resistivity. Increasing resistivity structures with depth are a feature of some submarine sedimentary basins and are known to arise due to the progressive explusion of conductive pore fluids with increasing depth by rising overburden pressure. Accordingly, in order to determine reliably whether an enhancement in electric field amplitude is caused by a subsurface hydrocarbon reservoir or whether it is caused by local large-scale resistive structures, independent information about the large background structures in the survey area is needed. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a method as defined in the appended claim  1 . 
     Other aspects and embodiments of the invention are defined in the other appended claims. 
     It is thus possible to provide an improved technique for analyzing data from an electromagnetic survey so as to provide improved detection of hydrocarbons. This technique may be applied to electromagnetic survey data in respect of any type of surveyed region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention will be further described, by way of example, with reference to the accompanying drawing, which is a diagram illustrating schematically an example of an electromagnetic surveying technique. 
         FIG. 1  illustrates a typical example of an electromagnetic surveying technique. Although such a technique may be employed for land or marine CSEM/SBL surveying or land or marine MTEM surveying, the illustrated example is for Mcsem/SBL surveying. 
     
    
    
     DETAILED DESCRIPTION 
     A survey ship  1  is shown towing a controlled continuous signal source  2 , for example of the type used in frequency-domain marine electromagnetic surveying. However, the source used in electromagnetic surveying may be of any electric or magnetic type and a transient signature source may, for example, be used in multi-transient land or marine electromagnetic surveying. The ship  1  is shown at the surface  3  of the sea and the source  2  is towed through the water column  4  between the sea surface  3  and the sea bed  5  above a region being surveyed. However, the source  2  need not be deployed in the water column  4  or on the sea bed  5 . For example, the source  2  may be deployed anywhere in the subsurface below the sea bed  5 . In the case of a land survey, for example, the or each source may be disposed in a borehole or well. The source  2  is typically a horizontal electric dipole (HED) source but other sources which may be used comprise vertical electric dipole (VED) sources, horizontal magnetic dipole (HMD) sources, vertical magnetic dipole (VMD) sources, or combinations of such sources. 
     Purely by way of example, the illustrated subsurface rock formation includes a hydrocarbon reservoir  9  above a layer  10  containing water. As described hereinafter, the data obtained from the illustrated surveying arrangement may be processed to provide detection of the hydrocarbon reservoir  9 . 
     Receivers  6 - 8 , only three of which are illustrated in the drawing, are disposed in any desired arrangement on the sea bed  5 . For example, a line survey geometry may be used with the receivers being regularly spaced along a straight line (in plan view) on the sea bed  5 . Each of the receivers comprises an instrument packet and an antenna. The antenna may comprise two mutually orthogonal horizontal electric dipole detectors, a vertical electric dipole detector, two mutually orthogonal horizontal magnetic dipole detectors, and a vertical magnetic dipole detector. In other examples, the antenna comprises: two mutually orthogonal horizontal electric dipole detectors and a vertical magnetic dipole detector; a vertical electric dipole detector and a vertical magnetic dipole detector; two mutually orthogonal horizontal magnetic dipole detectors and a vertical magnetic dipole detector. Any current data acquisition system can be used to collect the specific data. 
     In electromagnetic data analysis for hydrocarbon-filled reservoirs, it is common practice to compare data acquired above the reservoir identified potentially to contain hydrocarbons with reference data which are not sensitive to the postulated hydrocarbon reservoir, but otherwise specific to the area being surveyed. This data analysis step is called normalisation. The reference data can be obtained in several ways. One way is to acquire mCSEM/SBL reference data in the vicinity but outside the location of the postulated reservoir, which can be mapped for instance from seismic data. Another way is to provide reference data by magnetotelluric surveying, or from direct geophysical measurement. Another way is to simulate reference data in a computer using a non-hydrocarbon rock formation model with a body of water above specific to all the survey area. The normalisation step can be applied as a function of survey position to any of the collected electric and magnetic dipole response data. 
     Current electromagnetic data analysis and interpretation is based on the amplitude and phase information contained in the electric dipole response data. If there is no marked enhancement in the normalised electric dipole amplitude above a reservoir that has been identified potentially to contain hydrocarbons compared with the normalised electric dipole amplitude outside the postulated reservoir, the expert geophysicist interprets this information as indicating a low probability of finding hydrocarbons in the reservoir. On the other hand, a marked enhancement in the normalised electric dipole amplitude above the postulated reservoir compared to the normalised electric dipole amplitude outside the reservoir is interpreted by the expert geophysicist to represent a direct hydrocarbon indicator. However, in practice the enhanced response in the electric dipole measurement need not necessarily indicate a subsurface hydrocarbon reservoir. The response can also be caused by geological features relating to local larger-scale resistive structures, to layers with increasing resistivity with depth, or to layers with resistivity anisotropy. Thus, the single analysis of the horizontal electric dipole response or the vertical electric dipole response has an inherent ambiguity. 
     The ambiguity is resolved when the amplitude information contained in the vertical component of the magnetic field measured by the vertical magnetic dipole sensor is combined or compared with the amplitude information from the electric dipole measurement. From Maxwell&#39;s equations, it follows that the vertical magnetic dipole responds insignificantly to hydrocarbon-presence in the sub-surface due to its insensitivity to galvanically coupled modes induced by the horizontal electric dipole transmitter. On the other hand, the vertical magnetic dipole is sensitive to most inductively coupled modes and so detects the larger scale-resistive structures. Therefore, a combined, concurrent analysis of the individual electric dipole and vertical magnetic dipole components unequivocally solves the ambiguity as to whether an enhanced in electric dipole response is related to resistive hydrocarbon-filled reservoirs or to local large-scale resistive non-hydrocarbon-filled structures. An enhancement electric dipole response measured together with an unchanged, small vertical magnetic dipole response is indicative of the likely presence of a subsurface hydrocarbon-filled reservoir. The variation in amplitude enhancement as a function of source-receiver separation provides information on the depth and extent of the buried hydrocarbon reservoir. Enhanced magnitude responses of both the electric dipole and vertical magnetic dipole components, on the other hand, indicate local large-scale resistive subsurface bodies that are not necessarily hydrocarbon-filled. 
     For shallow-water electromagnetic surveying where the airwave is known to have a degrading effect on the horizontal electric dipole measurements, but not on the vertical electric dipole measurements, the combined, concurrent analysis may be based on the analysis of the vertical electric dipole measurement together with the vertical magnetic dipole measurement. Alternatively, the airwave can be attenuated or eliminated from the field recordings by up/down decomposition as described in GB2385923 and WO 2005/0906021 or by TM mode decomposition as described in our copending application(s) based on British patent application no. 0617517.8. 
     In mCSEM/SBL surveying it is common to introduce cylindrical (polar) coordinates to describe the relative source-receiver positions. When the center of the source is at the orgin, the position of the receiver can be described by distance r and azimuth Ø. For the horizontal electric dipole transmitter, the angle Ø is measured clockwise from a line passing through, and running parallel to, the horizontal electric dipole transmitter axis. A receiver placed with azimuthal angle Ø=0 degree is referred to as being in an inline position. A receiver with an azimuthal angle of Ø=90 degrees is referred to as being in a broadside position. In cylindrical coordinates the electromagnetic fields comprise radial and azimuthal components of the electric field and the magnetic field measured at the detector. The present method of analysis is equally valid when the radial electric dipole or radial magnetic dipole components are combined with the vertical magnetic dipole measurement. 
     Although a vertically oriented electric dipole antenna may be used to provide the vertical electric dipole data, other techniques may be used. For example, Maxwell&#39;s equations state the vertical component of the electric field, E 3  is related to the two horizontal magnetic field gradients H 1,2  and H 2,1  as:
 
 E   3 =( i ω{tilde over (ε)}) −1 ( H   1,2   −H   2,1 )   (1)
 
where
     {tilde over (ε)} is the complex permittivity;   

     
       
         
           
             
               
                 H 
                 
                   1 
                   , 
                   2 
                 
               
               = 
               
                 
                   ∂ 
                   
                     H 
                     1 
                   
                 
                 
                   ∂ 
                   
                     x 
                     1 
                   
                 
               
             
             ; 
             and 
           
         
       
       
         
           
             
               H 
               
                 2 
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             = 
             
               
                 
                   ∂ 
                   
                     H 
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                   ∂ 
                   
                     x 
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               . 
             
           
         
       
     
     The gradients of the magnetic fields may be measured by the use of electromagnetic gradiometers. As an alternative, the gradients may be approximated, for example by dividing the difference between magnetic field measurements at different locations by the distance between the locations. Thus, processing may be based on data obtained solely by measuring the gradients of the two horizontal magnetic field components along with the vertical magnetic component. 
     As another alternative, processing may use measurement of the vertical electric dipole data H 3  and measurements (or calculations) of the gradients E 1,2 , E 2,1  of the two horizontal electric fields. Maxwell&#39;s equations state that the vertical component of the magnetic field, H 3 , is related to the two horizontal electric field gradients as
 
 H   3 =−( i ωμ) −1 ( E   1,2   −E   2,1 )   (2)
 
where:
     i is √{square root over (−1)};   ω is the angular frequency;   μ is the permeability;   

     
       
         
           
             
               
                 E 
                 
                   2 
                   , 
                   1 
                 
               
               = 
               
                 
                   ∂ 
                   
                     E 
                     2 
                   
                 
                 
                   ∂ 
                   
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                     1 
                   
                 
               
             
             ; 
             and 
           
         
       
       
         
           
             
               E 
               
                 1 
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                     x 
                     2 
                   
                 
               
               . 
             
           
         
       
     
     The vertical component of the magnetic field may thus be estimated from the measurements of the horizontal gradients of the electric field. 
     The data can be further analysed using standard techniques, for example, electromagnetic inversion or migration, to produce subsurface resistivity maps of the area being surveyed. 
     The present technique is equally applicable to the surveying of freshwater, for example large lakes or estuaries, so that reference to seabed, water-column etc should not be regarded as limiting and should be interpreted as covering lakebed, riverbed etc.