Andreas Laake

Surface waves: use them then lose them. Surface-wave analysis, inversion and attenuation in land reflection seismic surveying

While in other domains of applied geophysics the surface-wave is considered a source of information for near-surface characterization, in the seismic industry the so-called ground roll has been traditionally regarded only as coherent noise to be filtered out as soon as possible. This difference of perspective is mainly due to the limitations of conventional land acquisition. The Rayleigh waves, which constitute a large part of the recorded energy, can be acquired properly, analysed and inverted to characterize the near-surface with a surprisingly high resolution, even in large 3D surveys, with point receiver acquisition. Surface waves can play a new role: they contribute to a better near-surface characterization for the perturbation correction and can be used for velocity modelling and geological modelling. Their proper identification enables alternative filtering strategies. Surface waves are not coherent noise but a signal that can be lifted from the seismic record and exploited in a variety of well-established geophysical solutions. In this paper we discuss a workflow for the analysis, inversion and attenuation of surface waves with 3D land data, showing examples from a land 3D survey in Egypt.

Surface-consistent deconvolution using reciprocity and waveform inversion

Source and receiver responses must be equalized when their behavior or coupling changes with location within a given survey. Existing surface-consistent deconvolution techniques that account for these effects assume that common-midpoint (CMP) gathering is valid — the seismic trace is decomposed into a source function, a receiver response, a normal-incidence reflectivity term, and an offsetrelated component that is laterally shift invariant. As a result, the performance of existing surface-consistent deconvolution techniques is best when applied to primary reflection data only, since the offset dependency of ground roll and multiples varies laterally in media with lateral variations. We have developed an alternative method for surfaceconsistent deconvolution that is applicable to the entire seismic trace and is therefore essentially a raw-data preprocessing step. The method is based on reciprocity of the medium response. Assuming that conditions for applicability of reciprocity are met, we can attribute differences between normal and reciprocal recordings to the source and receiver perturbations. Contrary to existing surfaceconsistent deconvolution methods, this approach uses the full description of the wavefield and is therefore ideally suited for prestack processing. We have applied this technique to single-sensor data acquired in Manistee County, Michigan. At this site, nearsurface conditions vary, and this significantly affects data quality. The application of the new deconvolution procedure substantially improves S/N ratio on both prestack and poststack data, and these results compare favorably to those obtained using existing surface-consistent deconvolution techniques, since they require subjective data scaling to obtain acceptable results. The obtained source corrections are correlated to changes in near-surface conditions — in this case, to changes in water-saturation levels. We do not observe such a correlation for the receiver corrections, which vary rapidly along the spread. Finally, the receiver response does not agree with the generally accepted damped harmonic oscillator model. For frequencies below 100 Hz, the retrieved receiver variations are larger than predicted by this model, and we cannot explain the receiver response using a single resonant frequency for the geophone-ground coupling.

Integration of surface/subsurface techniques reveals faults in Gulf of Suez oilfields

ABSTRACT Reservoir mapping in the Gulf of Suez petroleum system is challenging because rift-parallel and cross-rift faults disrupted the sediments, leaving the reservoirs confined to stratigraphic, structural, and combined traps. We have developed a technique to address this challenge that integrates fault outcrop mapping using satellite image interpretation, seismic near-surface characterization techniques such as Rayleigh wave velocity mapping and ray parameter interferometry, as well as ant tracking of faults and geobody delineation on a prestack time-migrated (PSTM) cube. The technique uses a combination of geographic information system (GIS) and geological modelling software such as Petrel for surface/subsurface integration. The joint analysis of Rayleigh wave data with satellite imagery provides a near-surface structural geological model. The acquisition, processing, and interpretation of point-receiver seismic data enables the interpretation of near-surface geological structures. Detailed shallow structural geology can be imaged in the near surface, a data regime that is conventionally masked by the acquisition noise from the seismic acquisition. The shallow geological model comprises shallow lithological horizons as well as fault zones, the mapping of which may assist with the mitigation of shallow drilling risks. The integration of surface and subsurface structural mapping provides a tectonic framework for the delineation of reservoirs in the rift-faulted environment of the Gulf of Suez.

Applications of satellite imagery to seismic survey design

Satellite imagery provides high-resolution information about the earth surface that can improve logistical, data quality, and safety aspects of surface seismic survey design.

Estimation of static corrections from geologic and remote-sensing data

Statics fluctuations in surface seismic data result from variations in the near surface, and, therefore, it is important to understand the effect that topography and lithology have on the propagation of seismic waves in the near surface. The approach presented here introduces geomorphologic near-surface characterization from remote-sensing data and shows how topographic and lithologic classification allow the generation of a 3D near-surface geologic model. Using standardized elastic properties for the rock types identified in the classification step, the 3D geologic model can be converted into a 3D elastic model. Statics are estimated assuming vertical propagation of the seismic waves through the layers of the elastic model.

Source and receiver amplitude equalization using reciprocity — application to land seismic data

Source and receiver amplitude equalization is necessary when their behavior changes with location within a given survey. Preferably, these corrections are performed in the early stages of processing. However, existing techniques which account for these effects, such as surface-consistent deconvolution, are applicable to primary reflection data only. Therefore, these techniques require prior processing. We developed an alternative method to compensate for source and receiver perturbations which has the advantage of being purely a preprocessing step. It is applicable to the whole seismic trace, and no assumptions are imposed on the subsurface. The method is based on reciprocity of the medium response. As a result of reciprocity, differences between normal and reciprocal recordings can be attributed to the source and receiver perturbations. We applied this technique to single-sensor data acquired in Manistee County, Michigan. At this site, near-surface conditions vary, and this significantly affects the data quality. The application of the equalization procedure led to a significant improvement in signal-to-noise ratio, on both prestack and poststack data.

Meeting the challenge of Mesozoic exploration

The Western Desert of Egypt challenges the exploration for oil and gas both from the reservoir as well as from the surface. The reservoir rocks, Jurassic sandstones and Cretaceous sandstones and carbonates, are often intensely faulted and fractured as a result of the long tectonic history of the area.

Surface Seismic Techniques for the Recovery of Heavy Oil

This paper shows the results from a number of experiments conducted using single-sensor and single-vibrator techniques to image heavy-oil sands at a depth of 300 m to 600 m in an existing oilfield in the Partitioned Neutral Zone between Kuwait and the Kingdom of Saudi Arabia. The aim of the experiments was to investigate whether surface seismic techniques can obtain the bandwidth required to be able to image the thick heavy-oil layers in the near surface in the presence of high-amplitude coherent noise that masks most of the reflected energy from the target. Further complications are introduced by the presence of high acoustic impedance layers above and below the heavy oil deposits and the presence of high ambient noise levels from the oilfield infrastructure. An 800-m x 800-m 3D seismic grid was acquired over an area that will be the site of pilot long-term steam-flood in 200X. This heavy oil (18 API) reservoir currently produces 78,000 barrels of oil per day. For efficient steam injection, knowledge of the heavy oil reservoir is essential.