Richard E. Duren

Sensitivity of the dispersion correction to Q error

A controlled‐phase acquisition and processing methodology for our company has been described by Trantham (1994). He pointed out that it is careful attention to wavelet phase that leads to improved well ties and a more geologically accurate seismic image. In addition, we prefer zero‐phase wavelets on our seismic sections. For a given amplitude spectrum they have the simplest shape and the highest peak; further, the peak occurs at the reflection time of the event. This alignment is important since the seismic wavelet generally broadens with increasing depth with a zero‐phase wavelet remaining symmetrical about the event time. Our experience has been that a true zero‐phase section can be tied over the entire length of a synthetic trace without having to slide the synthetic trace to tie different time zones.

A comparison of streamer and OBC seismic data at Beryl Alpha field, U. K. North Sea

Compressional P-wave ocean-bottom-cable (OBC) seismic data from the Beryl Alpha field in the U. K. North Sea provide a superior image of the subsurface compared to heritage streamer seismic data. To determine the reason for the superiority of OBC data, the results of a detailed comparison of these OBC and streamer data sets are compared. The streamer and OBC data sets are reprocessed using a strategy that attempts to isolate the roles of processing, fold, azimuth, PZ combination, and hydrophone and geophone data have on the improved OBC image. The vertical component of the geophone (OBC Z) provides the major contribution to the improved OBC image. The imaged OBC Z datacontain fewer multiples and have a higher signal-to-noise ratio than the streamer. The OBC data have a lower level of multiple contamination because of the contribution from the OBC Z component, together with an effective suppression of receiver-side water-column reverberations as a result of the combination of the OBC hydrophone and geophon...

Effect of Noise Suppression on Quality of 2C OBC Image

Two component ocean bottom cable (2C OBC) data are often affected by substantial noise. The noise is probably caused by shear wave energy registered on the vertical geophone. It exhibits random properties in common shot domain but coherent properties in common receiver domain. This phenomenon is observed in most OBC surveys worldwide, and the noise level can be very substantial. Typically, there is a substantial move out difference between the noise and the signal, i.e., reflected PP waves, allowing use of velocity filtering for noise suppression. Velocity of sound in water can be used as a quite universal parameter for normal moveout (NMO) correction before the velocity filtering. The NMO correction substantially simplifies design of the velocity filter. Efficiency of the proposed approach is illustrated using real 2C OBC data.

General Fourier solution and causality in attenuating media

It has been previously reported that a general electric field solution and its initial condition, E/sub g/(z,t) and E/sub g/(z,0), respectively, are not causal when formed by a superposition of time-harmonic waves in an attenuating medium. However, this is not the case. Further, the relationship between attenuation and phase velocity as well as their dependence on frequency arise simply from the form chosen for the time harmonic particular solutions. Even though causality is not introduced during the solution to the wave equation, the general solution can subsequently be shown to be a time convolution of a causal boundary condition (time history of the electric field as it crosses the z=0 plane, E/sub g/(0,t)), and the mediums impulse response g(z,t), which can be shown to be causal. Hence, the general solution is also causal. The initial condition occurs at the instant, t=0, when the electric field arrives at the z=0 plane, and it has been previously reported that the initial condition depends on the boundary condition for times after the initial time thereby violating causality. A re-examination shows that the initial condition does not depend on times after the initial time. Hence, the initial condition obeys causality, and it can also be shown to be properly determined (E/sub g/(z,0)=0 for z>0) even when the boundary condition is not zero. It has also been reported that limiting expressions for the boundary and initial conditions, E/sub g/(0,t/spl rarr/0) and E/sub g/(z/spl rarr/0,0), respectively, are not equal. However, a re-examination reveals that E/sub g/(0,t/spl rarr/0)=E/sub g/(z/spl rarr/0,0). >

Removal of water‐bottom multiples and peg‐leg multiples via wavefield reconstruction

Wavefield reconstruction is a technique that reconstructs the primary and multiple wavefields from undersampled data. It has been found to be particularly effective for attenuation of high amplitude water-bottom or peg-leg multiples which obscure primary reflections of interest. Wavefield reconstruction is deterministic and robust in the presence of high amplitude coherent noise. In practice, the multiple wavefield is reconstructed (using the recorded data) and subtracted from the recorded data. Next the primary wavefield is reconstructed from these almost multiple-free input data. This primary reconstruction further attenuates the contribution of multiples and other unwanted wavefields. In addition, wavefield reconstruction properly handles dipping reflectors, curved reflectors, and diffractions.

Using the Seismic Range Equation (SRE) For AVO Analysis to Detect Low Gas Saturation

An application of the Seismic Range Equation to AVO modelinu will be shown. With this methodology, and for this difficult geologic case, gas can be distinguished from water, but low gas saturation cannot be distinguished from economic saturations of gas. We infer low gas saturation because the well logs show no indication of gas. The water sand is incapable of generating the observed seismic amplitudes. The low gas saturation is indistinguishable from economic concentrations using AVO because the presence of small amounts of gas has the same effect on a rocks compressional velocity as economic quantities of gas (Domenico, 1976).

Theory of marine source arrays

I have modified the time-harmonic directivity definition for the seismic case. The resulting definition, appropriate for non time-harmonic sources, is the ratio of the radiated energy density per unit solid angle in a particular direction divided by the average radiated energy density per unit sol id angle. This definition allows directivity to be expressed explicity in terms of the individual frequency spectra and geometry. It can be used to ill assist those attempting to reduce shot-generated noise and 12) allow direct estimation of the directivity improvement between competing source arroy designs.