The invention generally relates to electromagnetically detecting thin resistive bodies in shallow water and terrestrial environments.
One technique to locate an oil reservoir beneath the sea floor is to measure electromagnetic fields that are produced by a controlled electromagnetic source. More specifically, in a technique called controlled source electromagnetic (CSEM) surveying, an electrical dipole (i.e., a controlled electromagnetic source) may be towed by a surface vessel a short distance above the sea floor. Measurements of the resulting electric and/or magnetic fields are then measured using receivers, which may be deployed, for example, on the sea floor. Ideally, the presence of a thin resistive layer, such as an oil reservoir, affects the measured electric and magnetic fields in a way that can be detected from the measured data.
CSEM surveying typically is limited to deep water, as a phenomenon called an “air wave effect” (as referred to in the literature) currently limits the use of CSEM surveying in shallow water environments. More specifically, the electromagnetic fields that are produced by the electric dipole interact with the air-sea interface to generate electromagnetic energy, or “air waves,” which diffuse from the sea surface down to the receiver. For shallow water, the air waves dominate the measured electromagnetic data so that the presence of a thin resistive body may not be readily discernible from the data. Similar challenges limit the application of CSEM surveying to terrestrial environments.
Thus, there is a continuing need for better ways to process data that is generated by controlled source electromagnetic surveying in shallow water and terrestrial environments.
In an embodiment of the invention, a technique includes performing first electromagnetic field measurements to obtain a first set of data and performing second electromagnetic field measurements to obtain a second set of data. The first set of data is relatively sensitive to an effect caused by an air layer boundary and is relatively insensitive to the presence of a resistive body. The second set of data is relatively sensitive to the effect and is relatively sensitive to the presence of the resistive body. The technique includes combining the first and second sets of data to generate a third set of data, which is relatively insensitive to the effect and is relatively sensitive to the presence of the resistive body.
Advantages and other features of the invention will become apparent from the following drawing, description and claims.
In accordance with some embodiments of the invention, a system 20 that is depicted in
A potential challenge associated with CSEM surveying is that relatively shallow water may produce an air wave effect due to a boundary between the sea surface 38 and the air. A similar effect exists when the CSEM survey is conducted in a terrestrial environment. To desensitize the measured data to the air wave effect, in accordance with some embodiments of the invention described herein, two sets of measurements are collected using different survey configurations. These measurements are then combined to produce a set of measurement data, which is relatively insensitive to the air wave effect.
More specifically, referring to
Because both sets of measurements are sensitive to the air wave effect, the first and second sets of data may be combined (block 46) in a way that effectively cancels out the air wave effect. This combination, described further below, generates a third set of data, which is insensitive to the air wave effect and is sensitive to resistive bodies. Thus, measurement-based data is created, which has an enhanced sensitivity to the resistive bodies.
Referring back to
E,H≅P0+P1+L, Eq. 1
where “P0” represents a direct wave response at the receiver, which is produced by the electric dipole 24. It is assumed that the direct wave is produced in a medium of uniform conductivity, corresponding to that of the earth. Here, the sea water has a uniform conductivity of “σsw,” and “P1” represents a modified image term, which is conceptually generated by a second electric dipole source also located in a earth of uniform conductivity but is positioned a distance approximately equal to 2z above the true electric dipole source 24. The term “L” in Eq. 1 represents the air effect, or the lateral electromagnetic field, which is attenuated as it travels straight upward from the source to the air-sea water interface, travels laterally along the interface with the amplitude decreasing only through geometrical spreading and then is attenuated as it travels down from the air-sea water interface to the receiver 30. Thus, the component “L” in Eq. 1 represents the air wave effect, which may dominate and thus obscure, the measured electric or magnetic fields, if not for the techniques that are discussed herein.
For purposes of removing the air wave effect from the measured data (or at least substantially diminishing the impact of the air wave effect), in accordance with some embodiments of the invention, two sets of electromagnetic field measurements that derived with different polarizations are combined. More specifically, in accordance with some embodiments of the invention, two CSEM surveys are performed using two different survey configurations: an inline survey configuration in which the electric dipole 24 is in line with the receiver 30 (and other such receivers 30); and a broadside, or crossline, survey configuration in which the electric dipole 24 is orthogonal with respect to the orientation of the receiver 30 (and other such receivers 30). As described further below, the inline survey configuration is relatively sensitive both to resistive bodies and to the air wave effect, while the crossline survey configuration is also relatively sensitive to the air wave effect but is relatively insensitive to resistive bodies. Therefore, in accordance with some embodiments of the invention, the electric fields (or alternatively, the magnetic fields, if magnetic field measurements are used) that are measured via the crossline configuration are subtracted from the electric fields that are measured via the inline configuration to derive electric fields that are relatively more sensitive to the resistive bodies and relatively less sensitive to the air wave effect (as compared to the electric fields derived from the inline or crossline configuration only).
Turning now to the more specific details, the inline survey configuration measures a lateral wave Lil, which has the following form:
where “m” represents the moment of the source, which for an electric dipole is given as the applied current times the length of the source; and “ksw” represents the propagation constant or wave number for sea water, as set forth below:
ksw=√{square root over (2πif μσsw)} Eq. 3
In Eq. 3, the notation “i” represents the square root of negative one (√{square root over (−1)}), “f” represents the frequency of operation and “μ” represents the magnetic permeability of the medium, which is assumed to be that of free space (μ=μ0=4π×107H/m).
The crossline configuration measures a lateral wave Lcl, which may be expressed as follows:
It is noted that Eqs. 2, 3 and 4 depict the lateral waves Lil and Lcl when electric fields are measured. However, magnetic fields may also be measured and processed in a similar manner, in accordance with other embodiments of the invention.
As can be seen from comparing Eq. 2 to 4, the crossline lateral wave Lcl is twice as large as the inline lateral wave Lil. This result, coupled with the fact that the inline configuration is sensitive to thin resistive bodies while the crossline configuration is not, gives rise to the result that the reservoir response may be enhanced by multiplying the measured inline response by two and subtracting off the measured broadside response. Thus, the resultant electromagnetic field, called “EDIFF” below, may be derived as follows:
EDIFF=2EIL−ECL, Eq. 5
where “EIL” represents the measured inline electric field, and “ECL” represents the measured crossline electric field. Therefore, the computed EDIFF electromagnetic field magnitude may be used as an indicator for resistive bodies, such as oil reservoirs, gas reservoirs and high quality fresh water aquifers, as just a few examples.
Referring to
The technique 100 is relatively more accurate if the two data sets with different polarizations are obtained such that the measurements occupy the same position simultaneously. However, if the two data sets are collected at different times, then the technique 100 may be susceptible to positioning errors. Orientation uncertainties in both the source and the receiver may introduce additional error.
One technique for reducing the errors attributable to positioning and rotation is to calculate the impedance, which is defined as the ratio between orthogonal components of the measured electric and magnetic fields. Assuming the orientations for the y and x axes, which are depicted in
Zil=Ex/Hy, Eq. 6
where “Ex” represents the component of the electric field, which was measured in the x direction in the inline survey configuration; and “Hy” represents the magnitude of the magnetic field in the y direction, which was obtained in the inline survey configuration.
A crossline impedance Zcl may be described as follows:
Zcl=Ey/Hx, Eq. 7
wherein “Ey” represents the y component of the electric field measured in the crossline survey configuration; and “Hx” represents the x component of the magnetic field measured in the crossline survey configuration.
Positioning errors may further be minimized by rotating the electric and magnetic field data prior to calculation of the impedance to the minimum and maximum of the polarization ellipses.
After the Zil and Zcl impedances are calculated, a difference impedance (called “ZDIFF”) may be computed as follows:
ZDIFF=Zil−Zcl, Eq. 8
Note that in the impedance calculation, the factor of two is dropped out due to the normalization effect of the impedance. The ZDIFF impedance may thus be used as an indicator of a resistive body.
Referring to
For terrestrial cases in which the both the source and receivers are on the land surface, z is equal to h is equal to zero (see
In the terrestrial case, although the signal reduction is entirely due to geometrical falloff, the same factor of two differences between the two polarizations exists. Thus, the differencing methods that are described above also may be used for land-based measurements.
A simulation was conducted in a simulated marine environments for purposes of demonstrating the techniques described herein. In particular, a marine model, the specifics which are described in the table of
In the simulation, the data was simulated for a horizontal electrical dipole source of unit moment aligned in the x and y directions operating at a frequency of 0.1 Hz, located 15 meters above the seabed. Measurements of the horizontal electric and magnetic fields were taken every 500 meters along the x axis from 500 meters to 15,000 meters relative to the source.
The crossline results show no discernable difference between the background and reservoir models; and the inline results demonstrate only a small difference that would be difficult to detect in field data. However, the computed differences for the two models show large variations between each other. In addition, these differences are well above a theoretical noise level 215 and thus, should be measurable.
Referring to
Other embodiments are within the scope of the appended claims. For example, in accordance with some embodiments of the invention, the above-described survey data processing techniques (such as the techniques 40, 100 and 120, which are depicted in
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
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