Constant Offset Sections Research Articles

Plane-wave depth migration

We present fast and efficient plane-wave migration methods for densely sampled seismic data in both the source and receiver domains. The methods are based on slant stacking over both shot and receiver positions (or offsets) for all the recorded data. If the data-acquisition geometry permits, both inline and crossline source and receiver positions can be incorporated into a multidimensional phase-velocity space, which is regular even for randomly positioned input data. By noting the maximum time dips present in the shot and receiver gathers and constant-offset sections, the number of plane waves required can be estimated, and this generally results in a reduction of the data volume used for migration. The required traveltime computations for depth imaging are independent for each particular plane-wave component. It thus can be used for either the source or the receiver plane waves during extrapolation in phase space, reducing considerably the computational burden. Since only vertical delay times are required, many traveltime techniques can be employed, and the problems with multipathing and first arrivals are either reduced or eliminated. Further, the plane-wave integrals can be pruned to concentrate the image on selected targets. In this way, the computation time can be further reduced, and the technique lends itself naturally to a velocity-modeling scheme where, for example, horizontal and then steeply dipping events are gradually introduced into the velocity analysis. The migration method also lends itself to imaging in anisotropic media because phase space is the natural domain for such an analysis.

Prestack migration concepts for AVO measurements on horizontal and dipping layers

Amplitude versus offset (AVO) processing is typically based on a model of horizontal reflectors and common midpoint (CMP) gathers. Seismic prestack migration on constant offset sections has also been used to improve the signal to noise ratio (SNR) of the AVO effect on the migrated CMP gathers that are referred to as common reflection point (CRP) gathers. The use of prestack migration in AVO measurements is aided by the scatterpoint concept that assumes reflectors can be composed of many scatterpoints (or reflecting elements) that are aligned along the reflector. The energy scattered by each point reconstructs to match the specula reflected energy. The energy from each scatterpoint forms a surface in the prestack volume that is often referred to as Cheops pyramid. The location of specula energy from a scatterpoint is identified by the tangential area between the reflected specula energy and the surface of Cheops pyramid. In addition, the incident and reflection angles from a scatterpoint may be superimposed on Cheops pyramid to identify any smearing that may occur. These principles apply to both horizontal and dipping layering. Conventional constant offset prestack migration can map horizontal and dipping specula energy to corresponding CRP gathers for AVO analysis. This method, however, requires an accurate velocity model before AVO measurements can be obtained. The equivalent offset method (EOM) of prestack migration forms prestack migration gathers that are referred to as common scatterpoint (CSP) gathers with no time shifting of the input data. This process uses minimal velocity information to rapidly form CSP gathers that that are also well suited for AVO analysis. After the gathers are formed, accurate velocities can be estimated and AVO measurements made.

Application of high resolution seismic reflection techniques for fracture mapping in crystalline rocks

Surface reflection seismic techniques have the capability of mapping subsurface geological features without disturbing the rock mass. They also have an added capability of penetrating to a much deeper depth than any other geophysical technique, including the ground probing radar. However, the successful application of reflection seismic techniques in crystalline rocks has in general been more difficult than in sedimentary basins, because of the irregular geometry and low acoustic impedance contrasts across geological boundaries. In this paper, we describe the imaging of fracture zones in crystalline rocks. Effective data processing, carefully modified from the conventional approaches, was applied on two high-resolution field data previously collected by different contractors. The strategy included enhancement of the signal hidden under the large-amplitude scattering noise, through pre- and post-stack processing such as shot f-k filtering, residual statics and careful muting after NMO correction. Two sets of low S/N test data from Canada and Sweden are analyzed in this research. The reflected energy in these data sets appeared to be more closely related to fracturing than to lithologic boundaries. The major fracture zones at shallow depth have been mapped with the desired resolution and can be correlated to the available well-log and seismic crosshole tomographic data. Once the surface waves were removed, shallow reflectors in the fracture zones could be identified and analyzed on the field records. Focusing analysis of the seismic image was performed in the constant-offset section to investigate the trends of major fracture zones. The complex attributes were also analyzed to determine whether they could be applied to the shallow fracture zones. Instantaneous frequency plots outline the intense fracturing zone and instantaneous phase plots identify the major and minor fractures, and other coherent events with different dip attitudes which interfere with each other.

Acoustics on a Real Scale Model: Application to Fractured Media

Fluid migration depends on the properties of the geological medium. It increases in fractured media. A better understanding of fractured media can be obtained by the full waveform acoustic technique only if the accuracy and the efficiency of the acoustic method can be correctly calibrated on a known medium. For this purpose, a real scale model made of concrete was built. The first part of the paper describes the real scale model, with its advantages and pitfalls. It illustrates the benefit of using data obtained on the model for tool development. The second part shows acoustic data recorded in the real scale model which represents a fractured medium. The open fracture is perpendicular to the borehole axis. The acoustic know-how acquired from the model has been used to develop an acoustic dipmeter method. This method is briefly described. Two field examples of full waveform acoustic data recorded in fractured media are shown in the third part. The first example shows open and cemented fractures observed on a constant offset section recorded in a vertical well. The second example shows both dipping reflectors and fractures observed in a highly deviated well. In both examples, the fractures and reflectors are confirmed by a core analysis.

Complex trace analysis of DTAGS data recorded at the Blake Escarpment face

Lateral inhomogeneity of the subsurface beyond the vertical/near vertical face of the Blake Escarpments is investigated using data acquired in 1989 by the Navy Research Laboratory’s (NRL’s) Deep Towed Acoustics/Geophysics System (DTAGS). These data are ideal for the application of Hilbert transform-based analysis to characterize the face and the shallow interior at the Blake Escarpment. This study presents local characterization of lateral inhomogeneity in terms of reflection strength, instantaneous phase, and instantaneous frequency. Those attributes are studied using both constant offset sections and individual shot gathers, and reveal interesting geological features and their related Fresnel effects. [Work supported by the Office of Naval Technology.]

DMO and NMO as applied in seismic data processing

DMO (dip-moveout) is recognized as the technique to image the steep dip events which are indiscriminately smeared when the data are processed with the conventional seismic imaging method, which includes NMO, CMP stacking and zero offset migration. From these published papers, we may find many ways to do DMO, such as prestack partial migration, DMO by Fourier transform, offset continuation, single channel DMO ... etc, and the improvements of these techniques are widely recognized. But every technique is available only in two-dimensional case. To extend DMO to 3D (three dimensional) case, those techniques operating on constant offset section are generally available only when the feathering angle can be constrained within a very small value, but this condition is really hard to be achieved when the sea current is rough. In this paper the author is going to extend the Rocca's smile operator into 3D case, so that an easy way of doing DMO will be available to any 3D data. A further consideration is also given to mixing the NMO with the DMO in this paper. The reason for doing this is not only to simplfy the process and to save computer time, but also to search for more accurate velocity information for the NMO correction when the dip angle is large. This technique will be very effective compared to sole DMO, when the velocity changes very rapidly with depth.

Source directivity and its effects on resolution and signature deconvolution

When a source pattern is directional, particularly in the 'in-line' direction, serious losses of high-frequency data can occur in the recorded signal at large offsets. Furthermore, since source directivity increases with frequency, signature deconvolution becomes less and less effective as one moves up the frequency band. In this artiele a controlled marine field experiment is described where the same line was shot with two source arrays, the only variable being the in-line dimension of the source pattern. Though the source was waterguns, the author believes that either sleeve airguns or marine vibrators would also have the required bandwidth to demonstrate the effect illustrated. Raw field records from the two source arrays, which have overall lengths of 30 and 10 m, are compared after filtering with a narrow bandpass filter. Thereafter constant offset sections with signature deconvolution and predictive deconvolution applied are compared for the same offsets.

Directional deconvolution of the seismic source signature combined with prestack migration

Modern marine seismic source arrays are highly directional. The directivity of the far-field source signature is ignored in conventional one-dimensional (lD) deconvolution. For a single seismic reflection profile it is assumed that all arrivals arise from raypaths lying in the in-line plane, and so some kind of two-dimensional (2D) deconvolution operator is required to remove the directivity of the source signature. This may be achieved in combination with prestack migration. Conceptually, the prestack migration is done by taking each data sample on a constant-offset section and smearing it along a locus on the migrated section. This locus is an ellipse for constant velocity. There is a one-for-one correspondence between points on the migration ellipse and the take-off angle of the waveform radiated from the source. So the constant-offset section may be deconvolved before migration for the source waveforms radiated in different directions, and the output values distributed at the appropriate points on the migration ellipses. Suitable weighting factors may be chosen for the migration process, and a spectral modification factor should be applied. Directional deconvolution of the seismie source signature could also be implemented in combination with dip moveout (prestack partial migration of constant offset sections to zero offset) and could be extended to three-dimensional (3D) data and to incorporate receiver directivity. We illustrate the technique using 2D data from a physical modelling system.