This invention relates generally to the field of geophysical prospecting, and more particularly to controlled source electromagnetic (“CSEM”) prospecting, including field delineation. Specifically, the invention is a method for correcting CSEM data for phase and amplitude errors.
Controlled-source electromagnetic surveys are an important geophysical tool for evaluating the presence of hydrocarbon-bearing strata within the earth. CSEM surveys typically record the electromagnetic signal induced in the earth by a source (transmitter) and measured at one or more receivers. The behavior of this signal as a function of transmitter location, frequency, and separation (offset) between transmitter and receiver can be diagnostic of rock properties associated with the presence or absence of hydrocarbons. Specifically, CSEM measurements are used to determine the spatially-varying resistivity of the subsurface.
In the marine environment, CSEM data (“MCSEM” data) are typically acquired by towing an electric bipole transmitting antenna 11 among a number of receivers 12 positioned on the seafloor 13 (
MCSEM data collected in deep water are typically interpreted in the temporal frequency domain, each signal representing the response of the earth to electromagnetic energy at that temporal frequency. In raw data, the strength of each frequency component varies depending on how much energy the transmitter broadcasts and on the receiver sensitivity at that frequency. These effects are typically removed from the data prior to interpretation.
In practice, the receiver data are converted to temporal frequency by dividing (or “binning”) the recorded time-domain data into time intervals equal to the transmitter waveform period (
The transmitter signal may be a more complex waveform than that depicted in
MCSEM receivers (
Clearly, other configurations are possible, such as connecting several receivers in a towed array (see, for example, U.S. Pat. No. 4,617,518 to Srnka). The receiver depicted in
In general, the received signals are made up of components both in-phase and out-of-phase with the transmitter signal. The signals are therefore conveniently represented as complex numbers in either rectangular (real-imaginary) or polar (amplitude-phase) form. As shown in
Every CSEM signal frequency, ω, measured in radians per second is associated with a signal period, T=2π/ω, measured in seconds. Any phase value, φ, or phase shift, Δφ, is associated with an equivalent time shift, Δt, by the formula
Δφ=2π(Δt/T).
While phase is customarily measured as an angle between 0 and 2π radians, it can be equivalently thought of as a time between 0 and T seconds.
While the amplitude and phase of MCSEM data can provide valuable constraints on the present or absence of hydrocarbons, each can be difficult to measure accurately in practice because of factors such as the following:
Additionally, phase errors can arise because transmitter and receiver signals are recorded separately using different time bases (clocks) that must themselves be synchronized against a common GPS time base.
The problem of phase and amplitude errors has been recognized in published literature. Approaches to address the problem include:
Experience shows, however, that compensating clock drift, adjusting for transmitter variations, and applying receiver calibrations can leave residual phase and amplitude errors in the data—as judged by the inability to match the corrected data to synthetics from a realistic earth resistivity model. Furthermore, the combination of hardware and software needed to monitor the injected transmitter current is both costly and subject to breakdown as it must make real-time measurements (without interruption) while being dragged through the deep ocean. What is needed is a method that can be practiced as long as the transmitter continues to operate correctly, even if a monitoring system has failed. It should be noted, however, that the above-described techniques of compensating receiver clocks for drift, adjusting for transmitter variations, and applying receiver calibrations can be individually or collectively practiced with the present invention.
Application of timing corrections, measured transmitter current data, and receiver response functions improves the correspondence between real and simulated data significantly. Nevertheless phase differences of 5-10 degrees usually exist even after these corrections. Possible reasons for these are transmitter and/or receiver clock drift, instrument calibration issues, and localized resistivity anomalies in the near-surface.
These remaining errors in the data prevent an interpreter from developing a geoelectric model of the earth that will explain the data at all frequencies and all offsets. The ability to match field data to simulated data is critical in MCSEM interpretation, because the interpreter generally uses this method to infer the presence or absence of hydrocarbon reservoirs in the subsurface.
Additionally, these remaining errors can lead to errors in estimates of the subsurface resistivity when CSEM data are used for inversion or imaging. Inversion is an iterative method for determining the resistivity of the subsurface from CSEM data measured at the earth's surface or seafloor. See, for example, D. L. Alumbaugh and G. A. Newman, “3-D massively parallel electromagnetic inversion—Part II, Analysis of a cross well experiment,” Geophysical J, Int. 128, 355-363 (1997). The result of inversion is a geo-electric model of the subsurface obtained by automatically updating a starting model of the earth resistivity to minimize the mismatch between measured and simulated data. Data errors could prevent the inversion process from converging to a reliable image of the subsurface.
Methods for correcting seismic data for amplitude and phase errors are not directly applicable to the CSEM problem because, at their core, all seismic methods estimate phase or timing errors from differences in arrival times or amplitudes of distinct seismic pulses. Distinctly arriving pulses do not generally appear in CSEM data, which is acquired at much lower frequencies, significant less bandwidth, and longer wavelengths than seismic data.
Correction methods for well log data are generally specific to measurements made in well bores and cannot be readily adapted to CSEM surveys. For example, sonic traveltime measurements in well bores are often corrected for tool tilt by averaging the traveltimes determined from transmitters above and below the receivers (R. E. Sheriff, Encyclopedic Dictionary of Applied Geophysics, Society of Exploration Geophysicists, Fourth edition, p. 325 (2002)). Density logs compensate for borehole irregularities and the effect of a mudcake on the borehole wall by contrasting the number of backscattered gamma rays measured by two detectors at different distances from the source (Sheriff, loc. cit, p. 83). Array-type induction logging tools compensate for formation invasion by drilling fluids by combining measurements made at different source-receiver spacings to respond preferentially to the resistivity at different distances from the center of the well bore (Sheriff, loc. cit., p. 22).
In one embodiment, the invention is a method for estimating error in measured data from a controlled-source electromagnetic survey of a subsurface region, comprising: (a) selecting data from the electric or magnetic field survey data, said selected data corresponding to a plurality of different offsets (source-receiver spacings) and to a selected single frequency in the source's signal's frequency spectrum; (b) generating a geo-electrical model of the subsurface region, said model including at least resistivity or conductivity as a function of position in the region; (c) solving electromagnetic field equations to simulate the survey's electromagnetic responses for said selected frequency and for source and receiver locations corresponding to said selected data, using the geo-electrical model of the subsurface region and survey source parameters; (d) selecting a near-zero offset range; and (e) estimating data error at all offsets by comparing measured survey data to simulated data at least one offset within said selected range of near-zero offsets, said estimated error providing a means for correcting measured data.
The present invention and its advantages will be better understood by referring to the following detailed description and the attached drawings in which:
The invention will be described in connection with its preferred embodiments. However, to the extent that the following detailed description is specific to a particular embodiment or a particular use of the invention, this is intended to be illustrative only, and is not to be construed as limiting the scope of the invention. On the contrary, it is intended to cover all alternatives, modifications and equivalents that may be included within the spirit and scope of the invention, as defined by the appended claims.
The present invention mitigates phase and/or amplitude errors in MCSEM surveys by exploiting the fact that marine electromagnetic data generated by the source near the receiver are dominated by the water conductivity and influenced to a lesser extent by the conductivity of the shallow sediments. The conductivity of seawater is usually in the neighborhood of 3.3 S/m and can often be measured directly while the conductivity of the shallow sediments can be adjusted to best match the data across multiple frequencies. Moreover, when the electric bipole transmitter approaches zero offset from the receiver, the electric field parallel to the transmitter bipole should approach zero phase relative to the transmitter and approach infinite amplitude. This provides a means to determine the required amplitude and phase correction using numerical modeling on a computer, since an earth model with the correct water conductivity and approximately correct sediment conductivity will give rise to synthetic data with the correct amplitude and phase near the transmitter. The present invention provides a method to correct field MCSEM data based on 3D modeling. After correction, the data become consistent with underlying physics of electromagnetic fields which is, in turn, used to simulate data from the geo-electric model.
The basic steps of one embodiment of the present inventive method for correcting the phase of MCSEM data are shown in the flowchart of
Phase of field data+Phase Shift=Phase of simulated data
The corrected phase of the field data is:
Corrected phase of field data=Phase of field data+Phase Shift
The phase shift computed over this selected interval is applied to all of the data collected by this receiver at this frequency during this tow line, including data outside the selected interval. Then the process of correction (steps 74-76) may be repeated for all selected frequencies and receivers (step 77), which amounts to cycling through steps 74-77 until the data are all corrected. The result is that the phase of the corrected field data matches the phase of the simulated data at small transmitter-receiver offset.
In a more general formulation of the invention, step 74 may be performed by any method of comparing measured survey data to simulated data; i.e., although plotting is a very useful way to compare, the data do not necessarily have to be plotted together, or plotted at all, to make the comparison. Moreover, the data compared may alternatively represent a single source location and different receiver locations. In other words, in these embodiments, the method looks at different offsets (transmitter-receiver spacing) with either transmitter (i.e., source) location or receiver location fixed. In yet other embodiments, the different offsets may be obtained from different source and receiver locations, i.e., neither source location nor receiver location is held fixed. An example of the latter type of embodiment might be N different source locations paired up with N different receiver locations in such a way that all offsets are different. The situation of a fixed transmitter with many receivers or, alternatively, a fixed receiver and many transmitters are the most computationally efficient.
A similar process can be applied to the amplitude of field data if necessary. In this case, however, a multiplicative scale factor is computed to adjust the measured data:
Corrected amplitude of field data=Amplitude of field data*Amplitude scale factor
If it is not otherwise known from direct measurement or from geologic knowledge of the area, the shallow sediment conductivity can be determined by selecting a conductivity which gives the best match between field and synthetic data at near offsets over a range of angular frequencies.
When a transmitter is located very close to the receiver, the electronics in the receiver may become saturated by very big electromagnetic field. As a rough general rule, offsets in the range of 0.5 to 2.0 km. are most useful for developing phase and amplitude corrections by means of this invention. Stated more generally, a useful offset range may often be up to about 1.5 km beyond the saturation zone on either side of that zone, but excluding the saturation zone. By restricting the analysis to these small offsets, the computed phase shift or amplitude scale factor will depend primarily on the seawater and shallow sediment conductivities and be relatively insensitive to the deeper details of the geo-electric model. The method is not limited to either a static or moving electric dipole transmitter. Other possible transmitters include, but not limited to: static or moving electric pole, magnetic dipole, electric loop, and arrays of all the above-listed transmitters. By way of example,
The person skilled in the art of electromagnetic data processing and interpretation will readily recognize variations and embellishments of the basic method exemplified by
Variation No. 5 above warrants additional explanation. The requirement that measured data have phase and amplitude adjusted to be consistent with simulated data at near offsets is valid whether or not the tow line passes directly over the receiver. However, for tow lines well away from a receiver, near offsets may not be available. As a result, it may be preferable to phase- and amplitude-correct those gathers based on receiver-specific and towline-specific corrections generated from other, near-offset data.
Experts in CSEM data analysis will recognize that ambiguous situations can arise for exceptional combinations of phase, frequency, and receiver orientation. For example, a transmitted square wave contains the frequencies 1/T, 3/T, 5/T, etc. where T is the waveform period. If T=4 seconds for example, then a timing error of 2 seconds will result in phase shifts of exactly 180° for every frequency. Experts will further recognize that phase shifts of 180° (or −1) are equivalent to 180° changes in the orientation of the horizontal receiver antennae. Experience has shown that it is sometimes necessary to compare phase corrections for multiple tow lines over the same receiver in order to unambiguously separate the effects of receiver orientation from phase error.
CSEM receivers such as that shown in
The foregoing application is directed to particular embodiments of the present invention for the purpose of illustrating it. It will be apparent, however, to one skilled in the art, that many modifications and variations to the embodiments described herein are possible. For example, the invention is described in terms of marine CSEM survey data, but the method can be applied to data collected on land or using airborne technique. Also, the geo-electrical model and the simulation of electromagnetic responses may be either one, two or three-dimensional, including the so-called two and one-half dimensional approximation in which the model is assumed to be unchanging along just one direction. The amplitude and phase correction of the present invention is preferably performed at least partly on a computer, i.e., computer-implemented embodiments of the present inventive method are preferred, but not essential. All such modifications and variations are intended to be within the scope of the present invention, as defined in the appended claims.
This application claims the benefit of U.S. Provisional Application No. 60/732,336 filed on Nov. 1, 2005.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US2006/037386 | 9/25/2006 | WO | 00 | 4/21/2008 |
Number | Date | Country | |
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60732336 | Nov 2005 | US |