Richard S. Lu

The construction of subsurface illumination and amplitude maps via ray tracing

Editors note: This article was selected as the Best Poster Paper at SEGs 1999 Annual Meeting. Due to its quantitative nature, the 1999 Best Student Poster Paper will not be published in TLE but in a future issue of Gℯℴ𝓅𝒽𝓎𝓈𝒾𝒸𝓈, without undergoing peer-review. The goal of a seismic survey is to illuminate subsurface geologic formations that may hold hydrocarbon accumulations. Conventional seismic survey design relies on the assumption that uniform midpoint coverage will lead to uniform illumination in the subsurface as long as each midpoint is hit by a sufficient range of offsets. In areas of complex velocity structure, severe wavefield distortions lead to irregular subsurface illumination patterns, even if surface midpoint maps show a uniform distribution. A more appropriate approach is to design seismic surveys to ensure illumination of key subsurface horizons. The difference between midpoint coverages and subsurface illumination patterns is particularly large in salt-prone areas (Muerdter et al., 1997). Due to severe wave distortion through complex, high-velocity salt bodies, conventional design methods that result in relatively uniform surface coverage (Figure 1) generate uneven amplitudes and shadow zones on subsalt horizons, an effect that is shown clearly by ray-trace modeling of an entire seismic survey (Figure 2). Figure 1. Total hits in each surface bin resulting from a 3-D seismic survey collected along east-west lines. Distances are in kilofeet. Figure 2. Amplitude of a subsurface …

Identifying, removing, and imaging P-S conversions at salt-sediment interfaces

The large velocity contrast between salt and the surrounding sediments generates strong conversions between P‐ and S‐wave energy. The resulting converted events can be noise on P‐wave migrated images and should be identified and removed to facilitate interpretation. On the other hand, they can also be used to image a salt body and its adjacent sediments when the P‐wave image is inadequate. The converted waves with smaller reflection and transmission angles and much larger critical angles generate substantially different illumination than does the P‐wave.In areas where time migration is valid, the ratio between salt thickness in time and the time interval between the P‐wave and the converted‐wave salt base on a time‐migrated image is about 2.6 or 1.3, depending upon whether the seismic wave propagates along one or both of the downgoing and upcoming raypaths in salt as the S‐wave, respectively. These ratios can be used together with forward seismic modeling and 2D prestack depth migration to identify the co...

Differential reduction of magnetic anomalies to the pole on a massively parallel computer

A finite‐impulse‐response filter was implemented on a computer with massively parallel processors to reduce a magnetic anomaly map to the magnetic pole, allowing each grid node to have a different inclination and declination (differential reduction to the pole, DRTP). The dramatic speed improvement of such an implementation for the filter design and application via space‐domain convolution makes DRTP a practical tool for hydrocarbon and mineral exploration.Application of this tool to magnetic anomalies in east China reveals that the northward shift in position of the anomaly maximum generated by DRTP is 6 km for anomalies with dominant wavelengths of approximately 25 km in the northernmost part of the study area. The shift increases as the anomaly wavelength increases. Shifts for all anomaly wavelengths are even larger in the southern part of the study area, where the magnetic inclination is lower. The shift in position of the anomaly maximum for anomalies of wavelengths 25 km in the northernmost area pro...

Finite‐impulse‐response reduction‐to‐the‐pole filter

Convolving a finite-impulse-response (FIR) filter with a magnetic anomaly map produces a reduction-to-the-pole (RTP) that is superior to that of the conventional Fourier-transform approach. The conventional approach, in which the maps Fourier transform is multiplied by the frequency response of the RTP filter, is flawed by not accounting properly for the dimensions of the respective Fourier transforms. The resultant wraparound effect of circular convolution degrades the RTP map. The FIR filter, combined with linear convolution and appropriate choices for dimensions of data and filter, eliminates the wraparound effect, minimizes contamination of the result by noise, and improves stability. These properties are illustrated by a synthetic example and by application to an actual data set.

Compensation of the gas-cap attenuation with viscoacoustic wave-equation migration: a case study from west Africa

Summary The amplitude and frequency loss caused by attenuation through the gas cap of a reservoir (Res A) in offshore west Africa has been recovered using viscoacoustic waveequation migration (Q-migration) that enhances the seismic resolution beneath the gas cap. Amplitude and frequency attributes below the gas cap match those outside of the gas gap in the Q-migrated volumes. The Q-migration results are being used for well planning and to create a geologic model for reservoir characterization and simulation.