Ron Chambers

Migration velocity analysis by wave-field extrapolation

Velocity information is essential to both common midpoint (CMP) stacking and migration. CMP stacking provides the basis for conventional velocity estimation techniques in that, for a number of trial velocities, the stack response of a CMP gather is computed and displayed in the form of a velocity table. An alternative approach to velocity estimation makes use of the basic ingredients of migration—downward extrapolation and imaging of seismic wave fields. The procedure involves migration of a CMP gather with a number of trial velocities and collection of the zero‐offset information, again in the form of a velocity table. Operating on a CMP gather, the migration‐based approach produces results similar to those of the conventional method. Analyses of synthetic CMP gathers using both methods show essentially equivalent treatments of seismic signal, and similar dependence of accuracy and resolving power on recording geometry. We have extended the migration‐based approach to include more than one CMP gather in ...

Coherent noise in marine seismic data

Three mechanisms that might produce noise patterns on stacked sections are examined: direct and trapped waves that propagate outward from the seismic source, cable motion caused by the tugging action of the boat and tail buoy, and scattered energy from irregularities in the water bottom and sub-bottom. Different noise patterns can be observed on shot profiles and common-midpoint (CMP) gathers; these patterns can be diagnostic of the dominant mechanism in a set of data. Field data suggest that the dominant noise is from waves scattered within the shallow sub-bottom. This type of noise is enhanced by CMP stacking. Moreover, it can be as strong as surface wave noise on stacked land seismic data as well. Moveout filtering is best for suppressing the noise while preserving signal. Since the scattered noise does not exhibit a linear moveout pattern on CMP-sorted gathers, moveout filtering must be applied either to traces within shot records and common-receiver gathers or to stacked traces. Our data example demonstrates that although it is more costly, moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data-dependent processing steps of predictive deconvolution and velocity analysis.--Modified journal abstract.

Simultaneous estimation of residual static and crossdip corrections

Seismic surveys on land are frequently conducted along nonlinear survey lines. Familiar examples include crooked lines controlled by existing road networks or by surface typography, lines that are otherwise linear but along which shotpoints occasionally must be offset laterally, and intentionally designed three‐dimensional (3-D) or broad‐line surveys. Departures from linear profiles introduce an element of complexity—crossdip—into the problem of estimating residual near‐surface reflection static time corrections (statics). Crossdip is the component of dip normal to the local profile direction. We have incorporated the effect of crossdip into the system of simultaneous equations that model residual static anomalies. The observed traveltimes of all reflections selected for analysis are represented as linear combinations of source and receiver static anomalies, structural shapes, residual normal moveouts, and crossdip terms. The static time components are taken to be surface‐consistent and independent of ref...

A Comparison of Methods For Combining Dual-sensor Ocean-bottom Cable Traces

Five methods of combining ocean-bottom hydrophone and geophone traces were tested using a simple model that clearly illustrated their relative abilities to attenuate watercolumn reverberations. The model consisted of a 26.6 meter water layer and a single deep reflector at 1.0 seconds. It contained a hydrophone and geophone trace corresponding to each water-bottom reflection coefficient value ranging from -0.8 to 0.8 in steps of 0.1. The water layer generated source and receiver location reverberations with a period of 36 milliseconds.

Enhancements to prestack frequency‐wavenumber (f-k) migration

Prestack frequency‐wavenumber (f-k) migration is a particularly efficient method of doing both full prestack time migration and migration velocity analysis. Conventional implementations of the method, however, can encounter several drawbacks: (1) poor resolution and spatial aliasing noise caused by insufficient sampling in the offset dimension, (2) poor definition of steep events caused by insufficient sampling in the velocity dimension, and (3) inadequate handling of ray bending for steep events. All three of these problems can be mitigated with modifications to the prestack f-k algorithm. The application of linear moveout (LMO) in the offset dimension prior to migration reduces event moveout and hence increases the bandwidth of non‐spatially aliased signals. To reduce problems of interpolation for steep events, the number of constant‐velocity migrations can be economically increased by performing residual poststack migrations. Finally, migration with a dip‐dependent imaging velocity addresses the issue ...

Fundamentals of 3-D migration, Part 1

This is a tutorial on 3-D migration of seismic data. Subsurface geological features of interest in hydrocarbon exploration are three dimensional in nature. Examples are salt diapirs, overthrust and folded belts, major unconformities, and deltaic sands. The 2-D seismic section is a cross‐section of the 3-D seismic wave field. It contains signals from all directions, including out‐of‐plane of the profile (sideswipe energy). This sideswipe energy can only be handled by 3-D migration of 3-D seismic data.

A successful 3‐D seismic survey in the “no‐data zone,” offshore Mississippi delta: Survey design and refraction static correction processing

This is a success story of survey design and refraction static correction processing of a large 3D seismic survey in the South Pass area of the Mississippi delta. In this transition zone, subaqueous mudflow gullies and lobes of the delta, in various states of consolidation and gas saturation, are strong absorbers of seismic energy. Seismic waves penetrating the mud are severely restricted in bandwidth and variously delayed by changes in mud velocity and thickness. Using a delay-time refraction static correction method, the authors find compensation for the various delays, i.e., static corrections, commonly vary 150 ms over a short distance. Application of the static corrections markedly improves the seismic stack volume. This paper shows that intelligent survey design and delay-time refraction static correction processing economically eliminate the historic no data status of this area.

Imaging Beneath Complex Structure: A Case History: ABSTRACT

Migration is recognized as the essential step in converting seismic data into a representation of the earths subsurface structure. Ironically, conventional migration often fails where migration is needed most--when the data are recorded over complex structures. Processing field data shot in Central America and synthetic data derived for that section, demonstrates that time migration actually degrades the image of the deep structure that lies below a complicated overburden. In the Central American example, velocities increase nearly two-fold across an arched and thrust-faulted interface. Wave-front distortion introduced by this feature gives rise to distorted reflections from depth. Even with interval velocity known perfectly, no velocity is proper for time migrating the data here; time migration is the wrong process because it does not honor Snells law. Depth migration of the stacked data, on the other hand, produces a reasonable image of the deeper section. The depth migration, however, leaves artifacts that could be attributed to problems that are common in structurally complicated areas: (1) departures of the stacked section from the ideal, a zero-offset section, (2) incorrect specification of velocities, and (3) loss of energy transmitted through t e complex zone. For such an inhomogeneous velocity structure, shortcomings in CDP stacking are related directly to highly nonhyperbolic moveout. As with migration velocity, no proper stacking velocity can be developed for these data, even from the known interval-velocity model. Proper treatment of nonzero-offset reflection data could be accomplished by depth migration before stacking. Simple ray-theoretical correction of the complex moveouts, however, can produce a stack that is similar to the desired zero-offset section. Overall, the choice of velocity model most strongly influences the results of depth migration. Processing the data with a range of plausible velocity models, however, leads to an important conclusion: although the velocities can never be known exactly, depth migration is essential for clarifying structure beneath complex overburden. End_of_Article - Last_Page 1339------------