Martin Widmaier

AVO correction for scalar waves in the case of a thinly layered reflector overburden

The reflection response of a seismic target is significantly affected by a thinly layered overburden, which creates velocity anisotropy and a transmission loss by scattering attenuation. These effects must be taken into account when imaging a target reflector and when inverting reflection coefficients. Describing scalar wave (i.e., acoustic wave or SH‐wave) propagation through a stack of thin layers by equivalent‐medium theory provides a simple generalized O’Doherty‐Anstey formula. This formulation is defined by a few statistical parameters that depend on the 1-D random fluctuations of the reflector overburden. The formula has been combined with well‐known target‐oriented and amplitude‐preserving migration/inversion algorithms and amplitude variation with offset (AVO) analysis procedures. The application of these combined procedures is demonstrated for SH‐waves in an elastic thinly‐layered medium. These techniques offer a suitable tool to compensate for the thin‐layer influence on traveltimes and amplitud...

Multi-Azimuth towed streamer 3D Seismic in the Nile Delta, Egypt

A thin but complex layer of partially eroded anhydrite and other facies lie at a depth of around 3km across large areas of the Nile Delta in the Mediterranean. Wavefield distortion, attenuation and the generation of complex multiple diffraction noise cause the quality of the underlying seismic image to be highly variable. (Figure 1) In this paper we describe the problem and then demonstrate how multi-azimuth seismic is able to improve the PreMessinian image.

Surface Related Multiple Suppression in Dual- sensor Towed Streamer Data

Conventional surface related multiple prediction for towed streamer configuration is flawed by the sea surface level variation and sea surface reflection coefficient fluctuation. Knowledge of the sea surface and reflection coefficient allows approximate correction of the prediction errors. Based on a novel dual-sensor towed streamer acquisition we propose a multiple prediction approach from the down-going vertical velocity field and up-going pressure field. This approach handles the obliquity factor and sea surface variations implicitly and may reduce bad-weather-related acquisition downtime.

Using virtual reality for quality control and interpretation of 4-D reservoir characterization and AVO analysis

Development of stereographic visualization has opened the third and fourth dimensions in seismic analysis and interpretation. The ability to look, simultaneously, at several data volumes and attributes in a true 3-D sense opens the door to visualization systems applied to regional AVO analysis and volume-based repeatability analysis, as part of the 4-D processing flow.

The use of multi‐azimuth streamer acquisition for attenuation of diffracted multiples

This paper discusses the nature and severity of multiple diffraction energy that is commonly seen on deep-water marine seismic data. Analysis and modeling show the nature of the problem and illustrate why this type of noise is difficult to remove using today’s standard acquisition and processing techniques. An alternative high -fold and multiazimuth streamer acquisition technique is proposed and field-tested in the Norwegian Sea. The basic idea is simple; that improved sampling of azimuth and offset allows the CMP stacking process to better attenuate noise. Constructive stacking of primary signal is improved through higher CMP fold. The concept, the acquisition configuration and processing results from a 2001 acquisition test will be presented.

Surface‐related multiple suppression in dual‐sensor towed‐streamer data

SUMMARY Conventional surface related multiple prediction for towed streamer configuration is flawed by the sea surface level variation and sea surface reflection coefficient fluctuation. Knowledge of the sea surface and reflection coefficient allows approximate correction of the prediction errors. Based on a novel dual-sensor towed streamer acquisition we propose a multiple prediction approach from the down-going vertical velocity field and up-going pressure field. This approach handles the obliquity factor and sea surface variations implicitly and may reduce bad-weather-related acquisition downtime.

A strategy for optimal marine 4D acquisition

Repeating source-receiver azimuths can be an important aspect of 4D acquisition. Seismic repeatability will decrease with an increase of source-receiver azimuth differences between base and monitor surveys. This paper discusses a marine acquisition strategy with respect to the optimal preservation of source-receiver azimuths in the presence of feathering. We show that repeating shot positions is favorable for azimuth preservation in 4D acquisition in combination with overlap configurations (additional outer streamers). With a dense streamer separation source-receiver azimuths can be repeated very accurately by using this strategy. In a base survey, overlap configurations allow the vessel to follow the survey’s preplot sail lines with significantly reduced crossline deviations. A well-conditioned base survey simplifies and optimizes the repetition of vessel/source positions in a future monitor survey.

Increased streamer depth for dual-sensor acquisition - Challenges and solutions

The towing depth applicable to dual sensor streamer acquisition has hitherto been limited by operational challenges associated with maintaining the fronts of the streamers at a deeper tow position, which creates additional drag, and noise recorded by the vertical particle velocity sensor. These restrictions have limited 3-D acquisition to a maximum towing depth of 20 m whilst 25 m towing depth is routinely used for 2-D acquisition. In July 2013, a field trial was performed with a slanted streamer, from 15m depth at the front to 30m at larger offsets. Since the front of the streamer is deployed at a depth routinely used for dual sensor streamer acquisition, such a slanted streamer profile is no more operationally difficult to achieve and has comparable noise performance to a horizontal streamer. Wavefield separation can be performed for arbitrary streamer profiles and the up-going wavefield output at a horizontal datum, thereby presenting no additional difficulties for subsequent processing steps. The benefit of deploying a substantial proportion of the streamer at greater depth is increased low frequency signal-to-noise ratio (less than 16 Hz). This uplift was demonstrated by comparing the data acquired using a slanted streamer profile to that obtained using a horizontal streamer.

Case studies in quality control for 4-D seismic processing

Summary Quality control in 4D seismic projects is vital due to the small magnitudes of the production-related change s that are being sought. Comprehensive quantitative comparisons between base and monitor data are frequently required, as differences in amplitude, frequency, and phase are indicators of non-repeatability. The non-repeatability that is a function of acquis ition and processing effects has to be removed during the processing sequence to increase the resolution of the production related changes. One very efficient 4D quality control strategy is to calculate relevant QC attribute volumes as part of the processing flow and then to utilize the capabilities of a visualization system for interactive and simultaneous analysis of the 4D seismic and QC attribute volumes. We present a number of case studies showing the importance and advantages of such a methodology.

Multiple suppression and sub basalt depth imaging of OBS data

Summary Seismic imaging beneath high velocity basalt layers is still a challenge in the seismic industry. The penetration of seismic energy is significantly reduced by high top basalt reflectivity and scattering losses. Deeper targets are also poorly imaged due to strong interfering multiples. Based on a study with synthetic data, the main objective of this work is to find an appropriate processing and depth imaging strategy for OBS sub basalt data. The basic steps turned out to be: (1) wave equation based multiple elimination, and (2) velocity model determination and prestack depth migration. The suggested multiple suppression methodology is composed of: a) wave field separation, b) extrapolation of the up going pressure wave field to the surface, and c) SRME application to OBS data. The methodology has been tested on synthetic and real data. The improvement of the sub basalt depth imaging is demonstrated.