Seismic traveltime data can be described in terms of instantaneous slope (dT/dX) and intercept time (τ). With this parameterization, the most general form of the traveltime equation is developed and found to be valid for reflections and refractions, all source‐receiver offsets, and all commonly used experimental geometries. For common source‐receiver experiments, the instantaneous slope of the traveltime data represents the true horizontal ray parameter p, and the intercept time is the sum along the ray of thickness‐vertical slowness products referenced to the fixed surface station. For common midpoint (CMP) experiments, we show that the instantaneous slope is the average slowness of upgoing and downgoing rays at the surface, and the intercept time is again the sum of thickness‐vertical slowness products, but referenced to the common midpoint. This inherent averaging immediately explains the superiority of the CMP experiment in the presence of dip. In addition to providing a general traveltime equation, w...

The traveltime equation, tau‐p mapping, and inversion of common midpoint data

Using an elementary theory of migration one can consider a reflecting horizon as a continuum of scattering centres for seismic waves. Reflections arising at interfaces can thus be looked upon as the sum of energy scattered by interface points. The energy from one point is distributed among signals upon its reflection time surface. This surface is usually well approximated by a hyperboloid in the vicinity of its apex. Migration aims at focusing the scattered energy of each depth point into an image point upon the reflection time surface. To ensure a complete migration the image must be vertical above the depth point. This is difficult to achieve for subsurface interfaces which fall below laterally in-homogeneous velocity media. Migration is hence frequently performed for these interfaces as well by the Kirchhoff summation method which systematically sums signals into the apex of the approximation hyperboloid even though the Kirchhoff integral is in this case not strictly valid. For a multilayered subsurface isovelocity layer model with interfaces of a generally curved nature this can only provide a complete migration for the uppermost interface. Still there are various advantages gained by having a process which sums signals consistently into the minimum of the reflection time surface. The position of the time surface minimum is the place where a ray from the depth point emerges vertically to the surface. The Kirchhoff migration, if applied to media with laterally inhomogeneous velocity, must necessarily be followed by a further time-to-depth migration if the true depth structure is to be recovered. Primary normal reflections and their respective migrated reflections have a complementary relationship to each other. Normal reflections relate to rays normal to the reflector and migrated reflections relate to rays normal to the free surface. Ray modeling is performed to indicate a new approach for simulating seismic reflections. Commonly occuring situations are investigated from which lessons can be learned which are of immediate value for those concerned with interpreting time migrated reflections. The concept of the ‘image ray’ is introduced.

Time migration—some ray theoretical aspects*

A system and method is provided for analyzing and imaging three-dimensional volume data sets. In one embodiment of the invention, a ribbon section is produced which may include a plurality of planes projected from a polyline. The polyline may include one or more line segments preferably formed within a plane. The projected planes intersect the three-dimensional volume data set and the data located at the intersection may be selectively viewed. The polyline may be edited or varied by editing or varying the control points which define the polyline. In another embodiment of the present invention, a method is provided for quickly tracking a physical phenomena represented within the three-dimensional volume data set. A plurality of planes may be successively displayed in the three-dimensional volume data set from which points are digitized related to the structure of interest to create a spline curve on each plane. The area between the spline curves is interpolated to produce a surface representative of the structure of interest, which may for example be a fault plane described by the three-dimensional volume data set. In this manner, the user can more easily and effectively visualize and interpret the features and physical parameters that are inherent in the three-dimensional volume data set.

System and method for analyzing and imaging three-dimensional volume data sets

Seismic prospecting for oil and gas has undergone a digital revolution during the past decade. Most stages of the exploration process have been affected: the acquisition of data, the reduction of this data in preparation for signal processing, the design of digital filters to detect primary echoes (reflections) from buried interfaces, and the development of technology to extract from these detected signals information on the geometry and physical properties of the subsurface. The seismic reflection is genenlly weak, and it must be strengthened by the use of signal summing (stacking) procedures. The determination of depths to a target horizon requires knowledge of the propagational velocities of seismic stress waves, and a wealth of technology has evolved for this purpose. More recently, it has been possible to relate signal amplitude to the physical properties of the medium traversed and, in particular, to make inferences about the oil and gas content of the buried rocks. Much of the exploration effort occurs in offshore areas, where reverberations in the water layer mask reflections from below. The method of predictive deconvolution has been most effective in its ability to attenuate these reverberations, making it possible to detect reflections from structures at depth. Seismic signal processing is neither pure science nor pure art, and offers a continuing challenge to the practitioners of both cultures.

Seismic signal processing

We describe a new method of automatic normal moveout (NMO) correction and velocity analysis that combines a feedforward neural network (FNN) with a simulated annealing technique known as very fast simulated annealing (VFSA). The task of the FNN is to map common midpoint (CMP) gathers at control locations along a 2-D seismic line into seismic velocities within predefined velocity search limits. The network is trained while the velocity analysis is performed at the selected control locations. The method minimizes a cost function defined in terms of the NMO-corrected data. Network weights are updated at each iteration of the optimization process using VFSA. Once the control CMP gathers have been properly NMO corrected, the derived weights are used to interpolate results at the intermediate CMP locations. In practical situations in which lateral velocity variations are expected, the method is applied in spatial data windows, each window being defined by a separate FNN. The method is illustrated with synthetic data and a real marine data set from the Carolina Trough area.

Automatic NMO correction and velocity estimation by a feedforward neural network

Piece-wise local linearity of the subsurface reflectors and uniqueness of primary reflection travel path between particular source-receiver pairs are two of the fundamental assumptions of multiple ground coverage reflection seismic techniques as currently employed. Some recent developments in seismic exploration which spring from the progress made in velocity determination techniques violate these fundamental assumptions with potentially serious consequences such as spurious geometries after migration and unrealistic interval velocities.In this paper we apply analytic techniques and computer simulation to linear and nonlinear subsurface models in order to obtain better definitions of the limitations of the reflection seismic method. The resulting lessons are of immediate practical value in seismic interpretation and clarify a variety of commonly occurring but puzzling situations such as phantom faults, incomplete contacts and segmented reflectors at unconformities, and apparent reversal of dip.In its basic approach this work has much in common with the generation and study of synthetic seismograms. Examination of the groundwork of the seismic method provides a firm base for extending its use.

Limitations of the reflection seismic method; lessons from computer simulations

This invention consists of producing a two-dimensional display from exploration seismic data designed to indicate zones of anomalous low velocity in the subsurface. Such zones may be indicative of porosity and the possible occurrence of hydrocarbons. They will be localized in terms of position along the seismic profile and in approximate zone of reflection arrival time. Date used in making the display are derived from both the CDP stacked seismic profile and corresponding velocity analyses used also to stack the data itself. Stacking velocity curves are plotted according to CDP location for each reflector designated by an interpreter to be of interest. These curves are overlain in pairs using calibration calculations and empirical criteria. Calibration helps smooth "noisy" values and compensates for velocity variations resulting from changes in separation or dip of the two reflectors from which the overlain velocity curves derive. In addition, it is presumed that the inverval bracketed by each reflector pair taken in turn has uniform or only regional lateral variations of interval velocity - no local lateral variation is assumed. "Cross overs" or convergences of the paired curves thus indicate zones of either high or low interval velocity of local nature. The display itself consists of the velocity curves for all possible reflector pairs and vertical bands of color which are assigned to identify the reflection interval of particular low velocity zones which are shown consistently by all reflectors taken in pairs which bracket such zones. Horizontal position is of course indicated by the CDP location.

Seismic method for identifying low velocity subsurface zones

During the last three years the discovery of the world’s second largest natural gas field at Groningen in the Netherlands has touched off in the North Sea one of the greatest competitive offshore geophysical operations in history. Before 1962, only minor amounts of geophysical work had been done there. Severe weather conditions were expected, but seismic operations have not been as much affected by weather as was originally anticipated. However, strong currents, making cable location uncertain, hampered reflection stacking and refraction operations. Location by radio was also a serious problem due to lack of sufficient available frequencies for a lane identification system. Seismic work was carried out safely in areas mapped as mine fields. Few problems with the fishing industry have so far been encountered. As a preliminary, the geophysics of the Groningen area are discussed. A gravity compilation of the North Sea shows that there are three major basins—the Northwest German Zechstein Basin, the British N...

Geophysical operations in the North Sea

Paleo Seismic And Color Acoustic Impedance Sections: Applications of Downward Continuation In Structural And Stratigraphic Context

A new seismic technique, the DIVA (Differential Interformational Velocity Analysis) (trademark, Copyright Zenith Exploration Co., patents pending), which makes use of ultra refined seismic velocity analysis has been developed to identify and localize low velocity anomalies in the subsurface. Reservoir quality porous rock formations will always be distinguished by reduced seismic velocities whether or not hydrocarbons are present. In favorable settings, however, low velocities correlate well with hydrocarbon reservoirs. Where gas is present, the velocity reductions can be spectacular making the DIVA display an indicator with as much visual impact as the "bright spot" offshore. Color acoustic impedance sections further corroborate the reality of DIVA detection anomalies and assist in localizing them in reflection time. In addition, the color display, which in fact represents the essential information content of the seismic amplitudes, though very imprecise, is a vital monitor of the detailed changes in lithology. Fourteen wells to date have tested the concept ranging in depths from 7,00 to 15,000 ft (2,133 to 4,572 m) and targeting both carbonate and sandstone reservoirs. Several of these results are reviewed to illustrate the power of the approach. The technology has not only been proven by the drill but has also initiated new and exciting exploration plays which can not even be detected with usual seismic approaches. An east Texas reef bank sequence illustrates such a circumstance. End_of_Article - Last_Page 442------------

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Precision Measurements of Interval Velocity Differences from Seismic Data: ABSTRACT

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