Abstract
Seismic reflection methods are widely used for the detection of hydrocarbons in subsurface structures up to several kilometers depth. However, since most data are acquired at or close to the Earth's surface, it is essential to understand the influence of the near-surface on the acquired data in order that its
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effects are not interpreted as pertaining to the reservoir. The near-surface effect on seismic data has two main origins: (i) near-surface wave propagation and (ii) wavefield acquisition. Wavefield acquisition comprises both wavefield excitation and wavefield measurement, i.e. source and receiver effects.
Since both P- and S-wave velocities are observed to vary rapidly close to the Earth's surface, wave propagation in the near-surface is often very complex. In order to improve our understanding of near-surface wave propagation, we investigated and developed methods to determine near-surface P- and S-wave velocities. For this purpose, we used dense recording geometries including buried geophones, since these material properties cannot be resolved with a conventional acquisition geometry using geophones only at the free surface. One of the methods developed is based on the estimation and inversion of the propagator matrix, and therefore referred to as propagator inversion. The propagator inversion was applied to data which we acquired in Zeist, The Netherlands. We obtained a low near-surface P velocity, namely 270 ± 15 m/s, which is well below the sound velocity in air, and 150 ± 9 m/s for the S velocity. The buried geophone was located at approximately 1.0 m depth, thus the obtained velocities are only representative of the top meter of the near-surface.
The second part of this thesis is devoted to the influence of wavefield acquisition. Corrections for source and receiver perturbations are necessary when their behaviour changes within a given survey, and should be performed in the early stages of processing. However, existing techniques, such as surface-consistent deconvolution, require prior processing before even they can be applied. We developed an alternative approach to compensate for source and receiver amplitude perturbations which has the advantage of being purely a raw data preprocessing step. It is applicable to the whole seismic trace, and does not impose additional assumptions on the subsurface. The approach is based on reciprocity of the medium response. This implies that differences between normal and reciprocal traces can be attributed to the source and receiver perturbations. We successfully demonstrated the procedure to compensate for these perturbations on both synthetic and field data. The field data were acquired in Manistee County, Michigan (courtesy of WesternGeco). Along the acquisition line, near-surface conditions change from moist-to-wet sediments to dry sands. The obtained source corrections are strongly correlated to these changing near-surface conditions, whereas the receiver corrections vary more strongly from geophone to geophone.
Compensation of the recorded data for the source and receiver perturbations resulted in a significant improvement of the signal-to-noise ratio, both on prestack and poststack data. Finally, the receiver response we found did not agree with the generally accepted damped harmonic oscillator model, implying that this model need to be revised.
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