RU2423728C2 - Method and device for processing electromagnetic exploration data - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: disclosed is an electromagnetic exploration method and a device for processing electromagnetic exploration data associated with a geological environment covered with water, and obtained by at least one electromagnetic field receiver in form of response to at least one electromagnetic field source. The method comprises steps for providing electromagnetic exploration data and eliminating the atmospheric wave component from the electromagnetic exploration data, where said component contains a first component propagating without reflection from at least one source to at least one receiver, and at least one second component, the propagation path of which, from at least one source to at least one receiver, has at least one vertical area near at least one of the said sources and near at least one receiver, and which contains at least one reflection from at least one of the water surfaces and the boundary surface between the region and the water. The second and third aspects of the invention propose use of a method of processing data on hydrocarbon reserves during well drilling, as well as use of the method of processing data on hydrocarbon reserves during hydrocarbon extraction. The fourth aspect of the invention discloses a device for detecting electromagnetic exploration data, which is adapted to implement the method in accordance with the first aspect of the invention.

EFFECT: method which allows considerable weakening or elimination of atmospheric wave effects from electromagnetic data and increases reliability of detecting or controlling hydrocarbon reserves.

15 cl, 13 dwg

Description

The present device relates to a method and apparatus for processing electromagnetic intelligence data, for example, obtained by marine electromagnetic sounding methods with a controlled field source.

Marine electromagnetic field sensing with a controlled field source (mCSEM) is a method by which hydrocarbon reserves in offshore fields or hydrocarbon reserves under inland waters, such as lakes, can be detected. The method, also known as seabed logging (SBL) (Eidesmo et al., 2002; Ellingsrud et al., 2002), uses a horizontal electric dipole (HED) antenna as its source of alternating current (AC), usually in frequency range from 0.01 Hz to 10 Hz. A source in the form of a HED (HED source) is towed at an altitude of about 20-40 m above the seafloor, while a group of stationary receivers located on the seabed registers the resulting electromagnetic (EM) field. The basic principle used in the mCSEM / SBL exploration (seabed logging using marine electromagnetic sounding with a controlled field source) is that reservoir reservoirs saturated with hydrocarbons usually have 5-100 times higher resistance than the host deposits. This reservoir will direct EM energy over long distances with low attenuation. If hydrocarbons are in the geological environment, the electric fields at the receivers located at large intervals between the source and the receiver will have a higher amplitude than the more weakened background electromagnetic fields caused by the host deposits.

During mCSEM / SBL reconnaissance in shallow water, the air layer is known to create the problem of the atmospheric-wave component (radio wave interference propagating in the air) generated by the source. As shown in figure 1 of the accompanying drawings, the atmospheric wave component is mainly due to the signal component, which scatters upward from the source 1 to the sea surface 3 and then spreads through the air at the speed of light without attenuation, until it scatters back down through the thickness of 2 sea water depth z _b to the seabed 4, where the aforementioned component is received by the receivers 5. The atmospheric-wave component is not a particular problem for a specialist in marine electromagnetic exploration in deep water in a consequence of the attenuation of the signal in two sections of the path. Unfortunately, at a shallow depth of the sea compared to a given depth and at low frequencies, the atmospheric wave signal can dominate at source-receiver distances in the range from intermediate to large, so the signal from the geological environment, possibly containing valuable information about the high-resistivity layer The hydrocarbon reservoir is difficult to recognize.

The electromagnetic field emitted by the HED source can be considered as consisting of two different modes: one component of the transverse electric (TE) mode and one component of the transverse magnetic (TM) mode. The response of sea water (and the geological environment) to the source signal, in general, varies greatly for the components of the TE and TM modes. It is known that the atmospheric-wave component is created mainly by the component of the TE mode of the source, since the component of the TE mode is effectively inductively transmitted through the interface between sea water and air. On the contrary, the component of the TM mode, as is known, is less efficiently transmitted across the interface between sea water and air and therefore makes an insignificant contribution to the atmospheric-wave component at finite distances, the source-receiver recorded during the mCSEM / SBL surveys.

The contribution of the atmospheric wave was studied by Chave and Cox (1982) using a numerical model study for reconnaissance using the mCSEM method with a HED source. The mentioned authors indicated that the influence of the atmospheric-wave component is important at large receiver-source distances, low frequencies, or relatively shallow sea depths. However, the authors mentioned did not offer any method for calculating the influence of the atmospheric wave, except for numerical simulation.

Let the z axis have a positive downward direction, and let z _s and z _r denote the depths of the source and receiver, respectively. From the work of Wait (Wait) (1961); Banos (Banos) (1966); Bannister (1984) and King et al. (1992) are well aware that the atmospheric-wave component shown in Figure 1, at a distance r of the receiver-source in the radial direction in the problem for the water half-space, has asymptotic expression in the spatial domain

In this expression, ρ denotes the dipole moment, ϕ denotes the azimuthal angle, k = ( i ωμ ₀ σ ₁ ) ^1/2 means the complex wave number at low frequencies for sea water with specific electrical conductivity σ ₁ , k ₀ = ω (μ ₀ ε ₀ ) ^1/2 ≈ 0 means the wave number in air, ω means the circular frequency, μ ₀ means the vacuum magnetic permeability, and ε ₀ means the dielectric constant in vacuum.

Equation 1 was recently used by Constable and Weiss (2006) to demonstrate the behavior of an atmospheric wave. Constable and Weiss (2006) noted that Equation 1 describes the propagation (including attenuation) of a signal on the source side vertically upward from the source through the water column to the sea surface. This propagation of the signal upwards creates an atmospheric-wave component on the sea surface that propagates horizontally through the air. An atmospheric wave scatters signals into the water column, propagating vertically downward (including attenuation) through the water column to the receiver.

In accordance with a first aspect of the invention, there is provided a method for processing electromagnetic reconnaissance data related to a geological environment covered by water and obtained by at least one electromagnetic field receiver in the form of a response to at least one electromagnetic field source, the method contains the step of providing electromagnetic intelligence data and the step of excluding from the electromagnetic intelligence data the contribution of the atmospheric wave containing the first component propagating without reflection from, at least one source to at least one receiver, and at least one second component, the propagation path of which from at least one source to at least one receiver contains at least one vertical section near at least one of the at least one source and near at least one receiver and which contains at least one reflection from at least one of the water surface and the interface between the region and water.

The contribution of the atmospheric wave can be excluded by subtraction.

At least one second component may contain many components having propagation paths near at least one source with a different number of reflections. The contribution of the atmospheric wave can be proportional to the following value:

(exp (ikz _s )) (l + Rexp (2ik (z _b -z _s ))) / (l-Rexp (2ikz _b )),

означает комплексное волновое число для воды с удельной электрической проводимостью σ₁, ω означает круговую частоту, μ₀ означает магнитную проницаемость вакуума, z_b означает глубину моря, z_s означает глубину источника, R=

означает коэффициент отражения на границе раздела между областью и водой и σ₂ означает удельную электрическую проводимость области на границе раздела.Where

means the complex wave number for water with electrical conductivity σ ₁ , ω means the circular frequency, μ ₀ means the magnetic permeability of the vacuum, z _b means the depth of the sea, z _s means the depth of the source, R =

means the reflection coefficient at the interface between the region and the water and σ ₂ means the electrical conductivity of the region at the interface.

At least one second component may comprise a plurality of components having propagation paths near at least one receiver with a different number of reflections. The contribution of the atmospheric wave can be proportional to the following value:

(exp (ikz _r )) (l + Rexp (2ik (z _b -z _r ))) / (l-Rexp (2ikz _b )),

означает комплексное волновое число для воды с удельной электрической проводимостью σ₁, ω означает круговую частоту, μ₀ означает магнитную проницаемость вакуума, z_b означает глубину моря, z_r означает глубину приемника, R=

означает коэффициент отражения на границе раздела между областью и водой и σ₂ означает удельную электрическую проводимость области на границе раздела.Where

means the complex wave number for water with electrical conductivity σ ₁ , ω means the circular frequency, μ ₀ means the magnetic permeability of the vacuum, z _b means the depth of the sea, z _r means the depth of the receiver, R =

means the reflection coefficient at the interface between the region and the water and σ ₂ means the electrical conductivity of the region at the interface.

Let the vector x = (x, y, z) denote a rectangular coordinate system. The contribution of the atmospheric wave may be proportional to the following quantity

,

,

with reflection function - reverb on the receiver side in the form

and the reflection function - reverb on the source side in the form

,

,

where ρ means the dipole moment of the source, ϕ is the azimuthal angle of the receiver relative to the source, 1 / r ³ takes into account the geometric discrepancy corresponding to the HED source at position x _s , F ( x _s ) means a function that takes into account the propagation of the field created by the atmospheric wave source up from source to the sea surface for example

,

,

F (x _r ) means a function that takes into account the propagation of the field created by the source of the atmospheric wave down from the sea surface to the receiver in the position with the coordinate x _r , for example,

,

,

R _r and R _s mean reflection coefficients on the side of the receiver and source, respectively. The electrical conductivity σ _{2 of} the seabed can be obtained in several ways, for example, by inverse transformation of EM data, or calculated from the data in the form -iμ ₀ ω (H _j / E _i ) ² , where H _j and E _i represent the orthogonal components of the horizontal electric and magnetic fields induced by natural primary sources.

The electromagnetic data are electromagnetic intelligence data with a controlled field source.

At least one source may comprise a horizontal electric dipole.

The method may include the step of analyzing processed hydrocarbon reserves data.

In accordance with a second aspect of the invention, there is provided a drilling method comprising performing the above method and a drilling control step in accordance with an analysis result.

In accordance with a third aspect of the invention, there is provided a production method comprising performing the above method and a hydrocarbon production control step in accordance with the analysis result.

In accordance with a fourth aspect of the invention, there is provided an apparatus configured to perform a method in accordance with a first aspect of the invention.

In accordance with a fifth aspect of the invention, there is provided a computer program configured to control a computer to execute a method in accordance with the first aspect of the invention.

In accordance with a sixth aspect of the invention, there is provided a computer-readable medium comprising a program in accordance with the fifth aspect of the invention.

In accordance with a seventh aspect of the invention, there is provided a program transfer in accordance with a fifth aspect of the invention.

According to an eighth aspect of the invention, there is provided a computer comprising a program or programmed by a program in accordance with the fifth aspect of the invention.

Therefore, it is possible to provide a method that allows significant attenuation or exclusion of atmospheric wave effects from electromagnetic data, for example, obtained by mCSEM methods. Thus, it is possible to increase the reliability of the detection or control of hydrocarbon reserves.

The following is a further description of the invention with reference, for example, to the accompanying drawings, in which:

figure 1 is a schematic section depicting a mCSEM data acquisition system and an atmospheric wave;

figures 2 (a) -2 (f) are diagrams depicting reflections and field reverberations from a HED source generating an atmospheric wave;

figures 3 (a) -3 (c) are diagrams depicting a downward field from an atmospheric wave near a receiver with reflections and reverberations;

figures 4 (a) -4 (c) are diagrams depicting three models used in the analysis of atmospheric waves;

figures 5 and 6 are graphs depicting the amplitude in volts per meter and phase in radians for the radial component of electric fields depending on the distance (distance) of the source-receiver in kilometers at different depths of the source and receiver for the model of the water half-space;

figures 7-9 are graphs similar to figures 5 and 6 for a model of a limited water column;

figure 10 is a graph similar to figures 5 and 6, for models with and without hydrocarbon reservoir;

figure 11 is a graph of the normalized amplitude of the radial electric fields depending on the distance of the source-receiver in kilometers;

figure 12 is a graph similar to figures 5 and 6, for models with and without reservoir reservoir, after subtracting the effect of the atmospheric wave; and

figure 13 is a graph similar to figure 11, after subtracting the effect of the atmospheric wave.

The following description provides a generalization for the atmospheric wave component when the water layer has a finite thickness z _b . In particular, an asymptotic expansion is proposed in the spatial domain of the atmospheric-wave expression, which excludes the influence of the seabed.

It is generally accepted that a TE mode signal propagating vertically upward from a source, in addition to creating an atmospheric wave component on the sea surface, generates a reverberation series of signals propagating between the sea surface and the seabed. Each time a signal in a reverberation series reaches a surface, a new atmospheric-wave component is excited. Likewise, a TE mode signal vertically descending from a source to the seabed generates a reverberation series in which each signal excites an atmospheric wave component on the sea surface. In addition, on the receiver side, the initially downward signal will reverb between the seabed and the sea surface.

The formula for the generalized atmospheric wave is derived for a special case in which the electrical conductivity is constant and the depth of the seabed is the same on the sides of the source and receiver. It will be apparent to those skilled in the art of modeling that these two assumptions may be less rigorous and lead to previously presented results. The atmospheric wave created by the source changes as a result of reflections from the seabed and reverberations in the water column on the side of the source and the side of the receiver. Then, the generalized atmospheric-wave response of the water layer with varying thickness is numerically compared with the response obtained on the basis of a complete simulation of Maxwell's equations. At large source-receiver distances, when the atmospheric wave has a predominant influence in the water layer due to the HED source, the generalized asymptotic modeling of the atmospheric wave is a very good approximation of the exact atmospheric wave. A numerical example is presented in which the atmospheric wave has a predominant effect on the measured amplitude of the electric field, so that it is impossible to detect the resistive layer below the seabed. However, after modeling and subtracting the atmospheric wave from the electric field, it is possible to construct a map of the directly resistive layer from the data.

Figures 2 (a) -2 (f) show how the atmospheric wave created by the source changes as a result of reflection from the seabed and the associated reverberations. The seabed 4 at a depth of z _b has a specific electrical conductivity σ ₂ . Figure 2 (a) shows the signal from source 1, scattering up to the sea surface 3 at a depth of z = 0. The signal on the sea surface 3 is asymptotically represented by the expression

exp ( ikz _s ).

Figure 2 (d) shows that the signal from source 1, scattered down to the seafloor 4, is reflected in the form of a signal propagating upwards, which forms the atmospheric-wave component on the sea surface 3. The mentioned signal in the water column is asymptotically represented as

exp (ikz _s ) Rexp [2ik (z _b -z _s )],

означает коэффициент отражения морского дна для вертикально распространяющейся TE-моды плоской волны и z_s означает глубину источника. На фигурах 2(b) и 2(c) показано, что восходящий сигнал источника, показанный на фигуре 2(a), может однократно и дважды реверберировать перед тем, как формируется атмосферная волна. В математическом выражении слагаемые на морской поверхности соответственно равны:where R =

denotes the reflection coefficient of the seabed for a vertically propagating TE mode of a plane wave, and z _s denotes the depth of the source. Figures 2 (b) and 2 (c) show that the upstream source signal shown in Figure 2 (a) can reverb once and twice before an atmospheric wave is generated. In mathematical terms, the terms on the sea surface are respectively equal:

exp (ikz _s ) Rexp (2ikz _b )

and

exp (ikz _s ) R ² exp (4ikz _b ).

Similarly, Figures 2 (e) and 2 (f) show that the initially downstream source signal shown in Figure 2 (b) can reverb once and twice before an atmospheric wave is generated. The terms are respectively equal to:

exp (ikz _s ) Rexp [2ik (z _b - z _s )] Rexp (2ikz _b )

and

exp (ikz _s ) Rexp [2ik (z _b - z _s )] R ² exp (4ikz _b ).

The mentioned process continues, in principle, an infinite number of times, so that the total signal that comes to the sea surface and forms an atmospheric wave asymptotically approaches:

exp (ikz _s ) {1 + Rexp [2ik (z _b - z _s )]} [1 + Rexp (2ikz _b ) + R ² exp (4ikz _b ) + ...] = exp (ikz _s ) S, (2)

Where

is a filter that reflects a modification of the atmospheric wave on the source side as a result of the influence of the seabed.

Similarly, Figures 3 (a) -3 (c) show how an atmospheric wave is modified on the receiver side as a result of reflection from the seabed and the associated reverberations. Figure 3 (a) shows that an atmospheric wave arrives at receiver 5 at a depth of z _r , then is reflected from the seabed 4 and returns up to the receiver. The asymptotic description of the process has the form:

exp (ikz _r ) [1 + Rexp [2ik (z _b - z _r )]]

In figures 3 (b) and 3 (c) shows the account of one and two reverberations in the water column, having the following mathematical expression:

exp (ikz _r ) [1 + Rexp [2ik (z _b - z _r )]] Rexp (2ikz _b )

and

exp (ikz _r ) [1 + Rexp [2ik (z _b - z _r )]] R ² exp (4ikz _b )

respectively. Given the infinite number of reverbs, the series of reflections and reverbs on the receiver side takes the form

exp (ikz _r ) R, (4)

Where

is a filter that reflects a modification of the atmospheric wave on the receiver side as a result of the influence of the seabed.

After that, the description of the atmospheric wave containing reverberation on the sides of the source and receiver has the form

. (6)

. (6)

Equation 6 is based on the assumption that a HED source in the water column will generate a series of reflections and reverberations of vertically propagating modes, and each component of the TE mode on the sea surface 3 will asymptotically form atmospheric wave components. At the receiver side, atmospheric wave components are reflected and reverberated similarly to those at the source side.

The following is a numerical verification of equation 6.

First, the validity of Equation 1 will be demonstrated and the atmospheric-wave response calculated for a model of marine aquatic half-space adjacent to air (Figure 4 (a)). Secondly, equation 6 will be checked by taking into account the influence of the finiteness of the water layer (Figure 4 (b)). Verification is provided by comparing the responses represented by equations 1 and 6 with a complete numerical simulation of the Maxwell equations for multilayer media, as proposed by Loseth (2000). For all models, a radially oriented HED source was used with a frequency of 0.25 Hz and a single dipole moment. The electrical conductivity of sea water is σ ₁ = 3.33 S / m. The receivers are located on the seabed along a line in the same plane with the source.

The figure 5 presents graphs of the dependences of the amplitude and phase of the radial component of the electric field on the distance of the source-receiver for the model of the aquatic half-space. The depths of the source and receivers are respectively z _s = 5 m and z _r = 10 m below the interface. The dashed curve represents the atmospheric wave component modeled in accordance with Equation 1, and the dotted curve represents the response obtained by fully simulating EM (electromagnetic field). The solid curve represents the amplitude of the difference between the signals. Figure 6 presents a comparison of the simulation in the case when the receivers are moved to a depth of z _r = 500 m below the sea surface. The depth of the source in this case is 25 m above the level of the receivers. In general, when the atmospheric wave component is the prevailing mode, Equation 1 describes the atmospheric wave for the model of the water half-space with sufficient accuracy. In figure 5, the atmospheric-wave expression in equation 1 is calculated for a relatively small receiver depth of 10 m. For receiver-source distances greater than 1-2 km, a good approximation of the results obtained by the numerical method takes place. When the atmospheric-wave expression is calculated for the 500th receiver depths (see Figure 6), Equation 1 demonstrates satisfactory agreement with the responses obtained by a complete numerical simulation of the electric field for source-receiver distances greater than 3-4 km. In this case, the direct field is likely to have a stronger effect than the atmospheric-wave component at source-receiver distances less than about 3 km.

Below is explained the model shown in figure 4 (b), with a finite thickness of water. The depths of the source, receiver and seabed were changed using three options: z _s = 5 m, z _r = z _b = 10 m; z _s = 75 m, z _r = z _b = 100 m; and z _s = 475 m, z _r = z _b = 500 m. The specific electrical conductivity of the seabed is σ ₂ = 1 S / m. In figures 7-9 shows graphs of the dependence of the amplitude and phase of the radial component of the electric field on the distance of the source-receiver for the three above cases. Dotted and dotted lines compare the simulation results in accordance with the generalized atmospheric wave according to equation 6 with full numerical simulation. For comparison, the atmospheric-wave response for the aquatic half-space is also presented in accordance with equation 1 (dashed line). The presence of the seabed significantly affects the atmospheric wave response. The solid curve shows the amplitude of the difference between the total electric field and the generalized atmospheric wave. The generalized expression for an atmospheric wave in Equation 6 very well approximates the atmospheric-wave response obtained by full numerical simulation. The mentioned result can be observed when the dotted and dotted-dotted curves are superimposed at source-receiver distances of more than 4-5 km, at which the atmospheric wave begins to prevail in the simulated response when the sea depths are 10 m and 100 m, respectively. When the sea depth is 500 m, the atmospheric-wave component absolutely prevails at source-receiver distances of more than 6-7 km. At shorter source-receiver distances, a direct field from the source and a transverse field along the seabed prevail. The phases obtained from equations 1 and 6 are almost equal for all three depths of the sea, which corresponds to almost instantaneous propagation of the atmospheric wave. It should be noted that the difference between the amplitude of the generalized atmospheric wave (dashed-dotted curve) and the amplitude of the airborne atmospheric wave for the aquatic half-space (dashed curve) becomes smaller with increasing sea depths, which indicates a decrease in the effect of reverberations in the water layer in deep water areas.

The problem of the atmospheric wave in the analysis and interpretation of CSEM marine intelligence data can be illustrated using the simple 1-dimensional model in Figure 4 (c). The model consists of five layers from top to bottom: a non-conductive air layer, a layer of seawater of a 100-m thickness, a sediment layer of a 2000-m thickness with a specific electrical conductivity of 1 S / m, a resistive layer of a 100-m thickness (0.02 S / m), which can be a reservoir-saturated hydrocarbon reservoir and sedimentary half-spaces with a specific electrical conductivity of 1 S / m. The model in figure 4 (c) is normalized to the reference model in figure 4 (b) to determine if there is a response enhancement due to the presence of the resistive layer. The figure 10 presents graphs of the dependence of the amplitude and phase of the radial component of the electric field on the source-receiver distance for a model with a reservoir layer (dotted curve) and a reference model without a reservoir (solid curve), together with a generalized atmospheric wave (dotted dotted curve ) The depth of the water layer is small, so the atmospheric wave prevails in the received signal at source-receiver distances of more than about 3 km. This can be seen from both the amplitude and phase curves. The phase is constant at source-receiver distances of more than approximately 4 km, which indicates the prevalence of the atmospheric wave in the results of field measurements. As a result, it is difficult to distinguish between a model with a reservoir and a model without a reservoir at all source-receiver distances. Since the amplitude of the electric field varies widely with the distance of the source-receiver, it is useful to consider the normalized amplitude of the electric field. The curve in figure 11 represents the normalized amplitude of the electric field, which is close to unity for all source-receiver distances. Therefore, the geophysical information interpreter cannot reliably determine whether a resistive hydrocarbon-rich layer is present in the geological environment. The mentioned example clearly demonstrates the problem of the atmospheric wave in shallow water.

Various data processing schemes have been proposed to extract the atmospheric wave effect from the results of field measurements using the mCSEM / SBL method. In the simplest scheme, you can include the air layer in the process of interpretation and inverse transformation. The effect of the atmospheric wave can also be suppressed by selecting the frequency at which the minimum contribution of the atmospheric wave is provided at a given depth. It is also possible to decompose the electromagnetic (EM) signal into the upstream and downstream components, where the upstream component contains information about the geological environment, while the atmospheric-wave mode is contained in the downstream component (Amundsen et al., 2006). Another suppression method is to model the effect of the atmospheric wave in the water layer and then subtract it from the collected data (Lu et al., 2005).

In the case where it is possible to determine the electrical conductivity of the seabed, the above method described in this application can be used to model and subtract the effect of an atmospheric wave from a registered electric field, as proposed by Lu et al. (2005), to improve reservoir response. The solid curves in Fig. 12 show the dependence of the amplitude and phase of the electric field on the source-receiver distance for the above-described five-layer model with a reservoir after modeling in accordance with Equation 6 and subtracting the effect of the atmospheric wave. The amplitude and phase of the reference electric field, obtained by subtracting the effect of the atmospheric wave from the electric field data obtained on the basis of the model without a reservoir, are shown by dashed curves. Significant spacing of the curves is observed in the range of 4-10 km of source-receiver distances, which indicates a strong signal from the resistive layer located at a 2-km depth under the seabed. The lack of diversity between the data plots for the reservoir model and for the reference model at small source-receiver distances is associated with a low attenuation of the forward field and the transverse field along the seabed, since the two signals prevail in the field measurements at source-receiver distances less than 3 km. The foregoing is further illustrated in FIG. 13, which shows a normalized amplitude characteristic of an electric field when an atmospheric wave is subtracted. After this, the influence of the reservoir on the characteristic becomes noticeable.

The same principle described above can be applied for modeling and subtracting an atmospheric wave from a magnetic field. The method remains the same and leads only to a slight change in the equation for modeling the atmospheric wave.

Simulation of the atmospheric wave for an electric field is given by equation (6). Simulation of an atmospheric wave for a magnetic field is carried out directly using the same principle. The total atmospheric-wave response at the location x _{r of the} receiver for the source with coordinate x _s has the form

(7)

(7)

Where

and

is a function of reflections - reverberations on the receiver side for a magnetic field.

Instead of using the electrical conductivity σ _{2 of} the seabed, there is another possibility, namely the use of the apparent electrical conductivity σ _a , which can be calculated by the magnetotelluric (MT) method. During the MT method, the orthogonal components of the horizontal electric and magnetic fields generated by the natural primary sources are measured simultaneously as a function of frequency. The apparent electrical conductivity as a function of frequency is determined by the expression

σ _a = -iμω (H _j / E _i ) ² (8)

The above method of eliminating the contribution of the atmospheric wave can be applied to electromagnetic reconnaissance data obtained, for example, by mCSEM methods. For example, many receivers are located on the seabed above the area to be explored. At least one source, for example a horizontal electric dipole, is towed in water above the receiver, with simultaneous excitation, and the results of measurements made by the receivers are stored in memory for subsequent processing.

The data processing comprises or includes the step of eliminating or at least attenuating the contribution of the atmospheric wave recorded by each receiver with respect to each source. An optional preliminary step contains the above rationing. Then, the contribution of the atmospheric wave is determined in accordance with equation 6 and, if desired, in accordance with equation 7. In particular, the contribution of the atmospheric wave is at least excluded by subtracting the contribution found in accordance with the above equations. The processed data can then be further processed and analyzed to provide information on any hydrocarbon fields or reservoirs in the area of interest. In the corresponding case, the above-described measurements by the MT method can be used to determine the apparent electrical conductivity in accordance with Equation 8 in order to determine the reflection coefficient at the lower interface of the water column. The above definition can be used to determine the contribution of the atmospheric wave.

The above method can be applied in conditions where the contribution of the atmospheric wave is unclear. For example, the above method can be applied when the water column is relatively small compared to the frequency of the electromagnetic waves used in the measurement. Then, the information obtained on hydrocarbon reserves can be used for various purposes, depending on the application. For example, processed data can be used to identify new hydrocarbon reservoirs and to estimate the amount of hydrocarbons present in such reservoirs along with their locations. Then you can control the drilling of wells and direct their drilling to produce or optimize hydrocarbon production. For known reservoirs, the amount of hydrocarbons remaining during production can be determined and the result of the determination used to control production, for example, to optimize drainage of the reservoir.

In practice, the data processing methods described above are performed by appropriately programmed computers. It is possible to use a computer of a standard type, for example of the type that is usually used to process hydrocarbon production data, and the processing methods can be encoded as suitable application programs for controlling said computers to execute data processing methods.

Information sources

Amundsen, L., L. Loseth, R. Mittet, S. Ellingsrud, and B. Ursin, 2006, Decomposition of electromagnetic fields into upgoing and downgoing components: Geophysics, 71 , G211-G223.

Bannister, PR 1984, New simplified formulas for elf subsurface-to-subsurface propagation: IEEE Journal of Oceanic Engineering, OE-9, 154-163.

Banos, A., 1966, Dipole radiation in the presence of conducting half-space: Pergamon Press.

Chave, AD and CS Cox, 1982, Controlled electromagnetic sources for measuring electrical conductivity beneath the oceans, 1: Forward problem and model study: Journal of Geophysical Research, 87 , 5327-5388.

Constable, S. and CJ Weiss, 2006, Mapping thin resistors and hydrocarbons with marine em methods: Insights from 1d modeling: Geophysics, 71 , G43-G51.

Eidesmo, T., S. Ellingsrud, LM MacGregor, S. Constable, MC Sinha, S. Johnsen, FN 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.

Ellingsrud, S., T. Eidesmo, S. Johansen, MC Sinha, LM 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.

King, R. W. P., M. Owens, and T. T. Wu, 1992, Lateral electromagnetic waves: Springer-Verlag.

Loseth, L., 2000, Electromagnetic waves in layered media: Master's thesis, The Norwegian University of Science and Technology.

Lu, X., L. J. Srnka, and J. J. Carazzone, 2005, Method for removing air wave effect from offshore frequency domain controlled-source electromagnetic data: International patent application WO 2005/010560 A1.

Wait, J. R., 1961, The electromagnetic fields of a horizontal dipole in the presence of a conducting half-space: Canadian Journal of Physics, 39, 1017-1027.

Claims (15)

1. The method of electromagnetic exploration using data related to the area of the geological environment covered with water and obtained by at least one receiver of the electromagnetic field in the form of a response to at least one source of the electromagnetic field, the method comprising the steps of in that they provide electromagnetic intelligence data and exclude from the electromagnetic intelligence data the contribution of the atmospheric wave containing the first component propagating without reflection from at least one source k, at least one receiver, and at least one second component, the propagation path of which from at least one source to at least one receiver contains at least one vertical section near at least of one of at least one source and near at least one receiver, and which contains at least one reflection from at least one of the water surface and the interface between the region and the water . 2. The method according to claim 1, in which the contribution of the atmospheric wave is excluded by subtraction. 3. The method according to claim 1, in which at least one second component contains many components having propagation paths near at least one source with a different number of reflections. 4. The method according to claim 3, in which the contribution of the atmospheric wave is proportional to the following value:

Where

means the complex wave number for water with electrical conductivity σ ₁ , ω means the circular frequency, μ ₀ means the magnetic permeability of the vacuum, z _b means the depth of the sea, z _s means the depth of the source,

means the reflection coefficient at the interface between the region and the water, and σ ₂ means the electrical conductivity of the region at the interface. 5. The method according to claim 1, in which at least one second component comprises a plurality of components having propagation paths near at least one receiver with a different number of reflections. 6. The method according to claim 5, in which the contribution of the atmospheric wave is proportional to the following value:

Where

means the complex wave number for water with electrical conductivity σ ₁ , ω means the circular frequency, μ ₀ means the magnetic permeability of the vacuum, z _b means the depth of the sea, z _r means the depth of the source,

means the reflection coefficient at the interface between the region and the water, and σ ₂ means the electrical conductivity of the region at the interface. 7. The method according to claim 5, in which the contribution of the atmospheric wave for the electric field component is proportional to the following value:

with reflection function - reverb on the receiver side in the form

and the reflection function - reverb on the source side in the form

where p means the dipole moment of the source, ϕ the azimuthal angle of the receiver relative to the source, 1 / r3 takes into account the geometric discrepancy corresponding to the HED source in position x _s , F (x _s ) means a function that takes into account the field propagation of the atmospheric wave created by the source upward from the source to sea surface

,

means a function that takes into account the propagation of the field created by the source of the atmospheric wave down from the sea surface to the receiver in the position with the coordinate x _r ,

,

and

mean reflection coefficients on the side of the receiver and source, respectively. 8. The method according to claim 5, in which the contribution of the atmospheric wave for the magnetic field component is proportional to the following value:

with reflection function - reverb on the receiver side in the form

and the reflection function - reverb on the source side in the form

where p means the dipole moment of the source, ϕ the azimuthal angle of the receiver relative to the source, 1 / r3 takes into account the geometric discrepancy corresponding to the HED source in position x _s , F (x _s ) means a function that takes into account the field propagation of the atmospheric wave created by the source upward from the source to sea surface

,
F (x _r ) means a function that takes into account the propagation of the field created by the source of the atmospheric wave down from the sea surface to the receiver in the position with the coordinate x _r ,

,
R _r and R _s mean reflection coefficients on the side of the receiver and source, respectively. 9. The method according to any one of claims 4 and 6-8, wherein the electrical conductivity σ _{2 is} obtained in the form -iµ ₀ ω (H _j / E _i ) 2, where H _j and E _i represent the orthogonal components of the horizontal electric and magnetic fields induced by natural primary sources. 10. The method according to claim 1, in which the electromagnetic data are electromagnetic intelligence data with a controlled field source. 11. The method according to claim 1, in which at least one source contains a horizontal electric dipole. 12. The method according to claim 1, comprising the step of analyzing the processed hydrocarbon reserves data. 13. The use of a method for processing hydrocarbon reserves data as described in claim 12 when drilling wells. 14. The use in hydrocarbon production of a method of processing data on hydrocarbon reserves according to item 12. 15. A device for detecting electromagnetic intelligence data, configured to perform the method according to claims 1-12.

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