Warren S. Ross

Field tests of electroseismic hydrocarbon detection

Geophysicists, looking for new exploration tools, have studied the coupling between seismic and electromagnetic waves in the near-surface since the 1930s. Our research explores the possibility that electromagnetic-to-seismic (ES) conversion is useful at greater depths. Field tests of ES conversion over gas sands and carbonate oil reservoirs succeeded in delineating known hydrocarbon accumulations from depths up to 1500 m . This is the first observation of electromagnetic-to-seismic coupling from surface electrodes and geophones. Electrodes at the earth’s surface generate electric fields at the target and digital accelerometers detect the returning seismic wave. Conversion at depth is confirmed with hydrophones placed in wells. The gas sands yielded a linear ES response, as expected for electrokinetic energy conversion, and in qualitative agreement with numerical simulations. The carbonate oil reservoirs generate nonlinear conversions; a qualitatively new observation and a new probe of rock properties. The...

The velocity-depth ambiguity in seismic traveltime data

An observed disturbance in seismic traveltimes to a reflector can be caused either by an anomalous velocity zone between the surface and the reflector or by a structural variation in the reflector itself. This velocity‐depth ambiguity is formulated in terms of linear estimation theory. Such a formulation allows integration of various published results on velocity‐depth ambiguity and suggests improved methods of stabilizing the solution of a depth‐conversion problem. By solving a relatively simple problem that is amenable to analysis—a single reflector beneath an overburden with a variable velocity—the following conclusions can be drawn: 1) The velocity‐depth ambiguity is caused by traveltime errors and can be quantitatively related to those errors by closed‐form expressions if the velocities do not vary laterally (or vary very slowly). Among other things, those expressions show that for small spread lengths (shorter than haft the depth) the errors in velocity and depth are inversely proportional to the sq...

Adaptive subtraction using complex-valued curvelet transforms

We propose a complex-valued curvelet transform-based (CCT-based) algorithm that adaptively subtracts from seismic data those noises for which an approximate template is available. The CCT decomposes a geophysical data set in terms of small reflection pieces, with each piece having a different characteristic frequency, location, and dip. One can precisely change the amplitude and shift the location of each seismic reflection piece in a template by controlling the amplitude and phase of the templates CCT coefficients. Based on these insights, our approach uses the phase and amplitude of the datas and templates CCT coefficients to correct misalignment and amplitude errors in the noise template, thereby matching the adapted template with the actual noise in the seismic data, reflection event-by-event. We also extend our approach to subtract noises that require several templates to be approximated. By itself, the method can only correct small misalignment errors ( ±5 ms in 50-Hz data) in the template; it re...

Traveltime prediction and suppression of 3-D multiples

We analyze the characteristics and suppression of multiples generated in three dimensions. We derive expressions for the traveltime of peg‐leg multiples generated by two dipping planes. Using this theory, we demonstrate that multiples generated in three dimensions may have traveltimes substantially different from 2-D multiples. For a case studied of 3° cross‐line dip, second‐order water‐bottom and peg‐leg multiples arrive on the order of 10 ms earlier than they would in two dimensions. For fifth‐order multiples, this difference is as high as 65 ms. These 2-D/3-D time differences grow to many hundreds of milliseconds for 10° of cross‐line dip. We confirm the presence of the cross‐line dip effect in field data containing salt‐related multiples. Using dips estimated from a 3-D interpretation of a top‐salt horizon, we further demonstrate that the theory can accurately predict traveltimes for such multiples. Finally, based on the theory, we present an algorithm for suppressing 3-D multiples when cross‐line dip...

A practical procedure for application of 3D SRME to conventional marine data

Complex 3D multiples are a pervasive problem affecting data sets from a variety of geographic areas. 3D surface-related multiple elimination (SRME) is a powerful method capable of attenuating a large class of even the most complex 3D multiples. While SRME does not make any assumptions about the subsurface and is a data-driven procedure, its 3D implementation requires dense sampling of the wavefield on the surface, amounting to the deployment of a shot and a receiver at every surface location. Typical marine acquisition geometries deliver much sparser surface coverage, which results in severe shot- and receiver-domain aliasing in the crossline direction.

3-D Mitigation of Surface-wave Noise In Spatially Inhomogeneous Media

A method is developed for 3-D mitigation of surface waves in spatially inhomogeneous media. Mitigation is enabled by modifying conventional phase-matched filtering techniques to dynamically update the phase-velocity estimates based on shot-receiver locations of the record being processed. This update is achieved by first estimating local phase velocities over the survey area, and then by path-averaging the local phase velocities for each shot-receiver pair. It is shown that surface wave mitigation using the new method is superior to conventional methods when there is spatial variation in medium properties.

Characterization of spatially varying surface waves in a land seismic survey

Summary We present several methods for analyzing surface waves in a highly sampled 3-C, 3D survey, and report the most important characteristics derived from those analyses. In particular, we demonstrate the spatial variability of surfacewave velocities and polarization properties. We also show that surface-wave velocities are correlated with other seismic and nonseismic properties of the near surface, such as shear-wave statics and surface texture derived from satellite imagery.

Field Tests of Electroseismic Hydrocarbon Detection

Geophysicists, looking for new exploration tools, have studied the coupling between seismic and electromagnetic waves in the near-surface since the 1930s. Our research explores the possibility that electromagnetic-to-seismic ES conversion is useful at greater depths. Field tests of ES conversion over gas sands and carbonate oil reservoirs succeeded in delineating known hydrocarbon accumulations from depths up to 1500 m. This is the first observation of electromagnetic-to-seismic coupling from surface electrodes and geophones. Electrodes at the earth’s surface generate electric fields at the target and digital accelerometers detect the returning seismic wave. Conversion at depth is confirmed with hydrophones placed in wells. The gas sands yielded a linear ES response, as expected for electrokinetic energy conversion, and in qualitative agreement with numerical simulations. The carbonate oil reservoirs generate nonlinear conversions; a qualitatively new observation and a new probe of rock properties. The hard-rock results suggest applications in lithologies where seismic hydrocarbon indicators are weak. With greater effort, deeper penetration should be possible.

Constrained Polarization Filtering For Surface-wave Mitigation

We present a new method of surface-wave mitigation using polarization filtering. The method evolves from the polarization-analysis technique developed by Diallo et al., (2006) and introduces new constraints to effectively detect and mitigate surface waves without damaging the signal. Straightforward application of polarization filtering without these constraints results in ineffective filtering or damage to the signal, due to the complexity of surface-wave wavetrains. We illustrate the performance of the method with examples from multicomponent seismic data, and demonstrate the superiority of the filtering compared to the unconstrained approach.

Effect of a Nonwhite Reflection Series Spectrum On Pulse Estimation

where S,(f) is the magnitude squared of the seismic pulse’s Fourier transform, and S,(f) is the reflection series power spectrum. A common approach to wavelet extraction is to measure the smoothed periodogram of seismic data averaged over a number of adjacent traces, and to equate the resulting spectral estimate with the spectrum of the pulse. The validity of this approach depends upon satisfaction of two conditions: (1) a good quality estimate of the seismic spectrum can be obtained; and (2) the reflection series spectrum is white. The purpose of the work reported here was to determine whether these conditions are in fact satisfied in typical geologic contexts. From the theory of the smoothed periodogram, it is shown that the first of the above assumptions is severely strained in many situations where this spectral estimator is used. Using a data base of 21 wells, it is also shown that the second assumption is violated quite often. We have found that the combined effect of estimator variance and spectrum nonwhiteness degrades the pulse estimate enough to produce distortions in one-dimensional inverse models computed with the pulse.