John Sherwood

Depth migration before stack

When seismic data are migrated using operators derived from the scalar wave equation, an assumption is normally made that the seismic velocity in the propagating medium is locally laterally invariant. This simplifying assumption causes reflectors to be imaged incorrectly when lateral velocity gradients exist, irrespective of the degree of accuracy to which the subsurface velocity structure is known. A finite‐difference method has been implemented for migration of unstacked data in the presence of lateral velocity gradients, where the operation of wave field extrapolation is done in increments of depth rather than time. Performing this depth migration on unstacked data results in the imaging of reflectors on the zero‐offset trace, whereupon a zero‐offset section becomes a fully imaged‐in‐depth seismic section. Such a section, in addition to being a correctly migrated depth section, shows the same order of signal amplitude enhancement as in a normal stacking process.

3D beam prestack depth migration with examples from around the world

In 1999, we specialized in 2D seismic depth processing. It was necessary to progress into 3D depth migration, but it seemed unrealistic to utilize Kirchhoff or wave-equation methods and be able to compete with large companies, their compute resources, and their economies of scale. Consequently, it was decided to attempt the commercial development of an alternative approach that would hopefully provide similar or improved technical quality, but with capital outlay and economics that were more appropriate to a small company. A very straightforward approach referred to as beam prestack depth migration (BPSDM) was adopted; its origins dated to work performed in the 1930s by Frank Rieber. The anticipated merits of BPSDM were overall simplicity, economy, flexibility, and future development possibilities. The unknowns were the migration accuracy and quality achievable. BPSDM was essentially developed independently, but aspects of it obviously are related to much recently published work on other beam migrations a...

Identification of lithology in the Gulf of Mexico

In a small town outside of Houston, a local rancher was overheard saying, “Do you have any 3-D seismic across your place? You ought to get some, because it tells you exactly what’s down there and where to drill.” Yes, the transfer of technology has been accelerated by new electronic media such as the Internet, but is it possible that it bypassed the geophysicists?

Depth sections and interval velocities from surface seismic data

First of all, what is the nature of the problem to be addressed here? Conventional seismic processing consists of unstacked reflection seismic data as input and, typically, a CDP stacked section in time as output. Except for extremely simple situations, a time migrated section will also be formed. The problem is that the time structure on that final section does not necessarily represent the true depth structure in the earth. This is particularly true when strong lateral velocity variations are present.

Hybrid tomography based on beam migration

We propose a method of tomography that utilizes the time residual and other attributes associated with each wavelet in beam migration. This time residual is referred to as 3D residual normal moveout (3DRNMO) and is related to error in the velocity model. The spatial dip components of the wavelet with respect to time enable very efficient ray tracing. The proposed method possesses the features of both postmigration tomography and stereotomography. Furthermore, not only reflections but also diffractions and turning waves are included in the tomography. The method is validated using a TTI synthetic data set from a 3D syncline model. The method is also applied to fine tune the velocity model for a marine field data set.

Analysis of a channel-fan complex in San Joaquin Basin, California, with aid of an interactive workstation and lithologic modeling

As part of a basinwide evaluation of a stratigraphic potential, they performed a stratigraphic analysis of a channel fan complex in the San Joaquin basin in California. Integration of biostratigraphic information and seismic data helped define sequence boundaries, which formed the basis of more detailed correlation of the seismic data. Well-log data were then brought into play to determine the depositional environment within the channel fan complex. Within each of the paleogeographic sections of the complex, seismic facies patterns could be recognized, and with the aid of an interactive workstation, characteristic seismic attributes (amplitude, phase, and frequency) of the important units within the vertical sequence of the channel fan were examined at each well. With displays and maps of these attributes, they could then project the depositional environment away from the borehole. Lateral predictions of environment then followed, with control provided by seismic lithological modeling, a method for estimating details of thin bedding. Finally, the seismic attributes and displays of amplitude-preserved data provided information for refining the model of the channel fan complex.