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 (FIG. 1). The transmitter antenna is typically towed a few tens of meters above the seafloor. The receivers have multiple sensors designed to record one or more different vector components of the electric and/or magnetic fields. Alternative configurations include stationary transmitters on the seafloor or in the water column as well as magnetic transmitter antennae. The transmitting and receiving systems typically operate independently (without any connection), so that receiver data must be synchronized with shipboard measurements of transmitter position by comparing clock times on the receivers to time from a shipboard or GPS (Global Positioning System) standard.
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. FIGS. 2A and 2B depict raw receiver data 21 together with (in FIG. 2B) the transmitter waveform 22 that gave rise to it. FIG. 2A shows examples of received CSEM signals on a time scale of several hours, while FIG. 2B shows the same received signal on a much shorter time scale 23, comparable to the period, T, of the transmitter waveform. Typical values for T are between 4 and 64 seconds. The transmitter waveform is depicted as a dashed line overlaying the receiver waveform. (The transmitter waveform is shown for reference only: the vertical scale applies only to the receiver signal.)
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 (FIG. 3A) and determining the spectrum (FIG. 3B) within each bin (x1, x2, x3) by standard methods based on the Fourier Transform. The phases of the spectral components are not shown. With each bin is associated a time, typically the Julian date at the center of the bin. Since the transmitter location is known as a function of time, these bins may be interchangeably labeled in several different ways including: by Julian date of the bin center; by transmitter position; by the signed offset distance between source and receiver; or by the cumulative distance traveled by the transmitter relative to some starting point.
The transmitter signal may be a more complex waveform than that depicted in FIGS. 2B and 3A.
MCSEM receivers (FIG. 4) typically include:                a power system, e.g. batteries (inside data logger and pressure case 40);        one or more electric-field (E) or magnetic-field (B) antennae (bipoles 41 receive + and −Ex fields, dipoles 42 + and −Ey, coils 43 for Bx and coils 44 for By);        other measuring devices, such as a compass and thermometer (not shown);        electronics packages that begin sensing, digitizing, and storing these measurements at a pre-programmed time (inside case 40);        a means to extract data from the receiver to a shipboard computer after the receiver returns to the surface (not shown);        a weight (e.g., concrete anchor 49) sufficient to cause the receiver to fall to the seafloor; a mechanism 45 to release the receiver from its weight up receiving (acoustic release and navigation unit 46) an acoustic signal from a surface vessel (14 in FIG. 1);        glass flotation spheres 47;        strayline float 48; and        various (not shown) hooks, flags, strobe lights, and radio beacons to simplify deployment and recovery of the receiver from a ship at the surface.        
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 FIG. 4 is a 4-component (Ex, Ey, Bx, and By) seafloor CSEM receiver. The devices can be configured to record different field types, including vertical electric (Ez) and magnetic (Bz) fields.
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 FIGS. 5 and 6, both the phase and amplitude of MCSEM data can be indicative of resistive (and potentially hydrocarbon-bearing) strata. Both the phase and amplitude must be accurately determined in order to distinguish signal characteristics associated with hydrocarbons from the much larger portion of the signal that is associated with other geologic features of the subsurface. FIG. 5 shows a cross-section view of a typical MCSEM survey. The signal measured in a receiver 12 has contributions from many different paths through the subsurface, including paths associated with resistive (potentially hydrocarbon-bearing) strata such as 51. FIG. 6A shows Electric-field amplitude and FIG. 6B shows the corresponding phase responses that might result from the MCSEM measurements depicted in FIG. 5. The dashed curves show signals in the absence of the resistive unit 51. Signals in the presence of the resistive unit (solid curves) show a larger amplitude, as current is forced back toward the surface, and a delayed phase, due to the longer wavelengths of electromagnetic waves in the resistive unit.
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 formulaΔφ=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:                the transmitter current waveform must be accurately measured and reported from several hundreds or thousands of meters below the surface;        the responses of the receiver amplifiers must by accurately known at the frequencies where data are measured;        the receiver antennae (particularly the magnetic antennae) generally have a frequency-dependent response, and the response of the combined antennae-amplifier circuit can differ from the combined responses of the components;        small changes in the earth's resistivity close to the receiver may alter the electric and magnetic field values; and,        the chemical interaction of the transmitting antenna electrodes with conductive and corroding seawater is not completely understood and may cause effects such as increased electrode resistance with usage.        
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:                Employing high-precision and temperature-compensated clocks in the seafloor receivers. Direct measurement of the drift (time error) of these clocks relative to a time reference (such as GPS) at the start and end of the survey allows the user to stretch or compress measured data to an estimate of the reference time (S. C. Constable, et al., “Marine magnetotellurics for petroleum exploration Part 1: A sea-floor equipment system,” Geophysics 63, 816-825 (1998)).        Mounting an independent receiver to the transmitter to monitor the transmitter current that is actually injected into the water (L. M. MacGregor, et al., “The RAMESSES experiment—III. Controlled—source electromagnetic sounding of the Reykjanes Ridge at 57° 45′ N,”Geophys. J Int. 135, 773-789 (1998)). As before, the receiver data are corrected for the measured transmitter behavior.        Taking laboratory measurements of the response of the receiver's amplifier-antenna system (calibrating the receiver amplitude and phase versus frequency) and compensating field CSEM data for these values (S. Ellingsrud, et al., “Remote sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola,” The Leading Edge 21, 972-982 (2002)).        
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).
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