Schlumberger

Technical Paper: Diving-wave refraction tomography and reflection tomography for velocity model building

Society: SEG
Paper Number: 2370225
Presentation Date: 2006
 

Abstract

Trinidad and Azerbaijan offshore areas are strongly affected by shallow gas anomalies which greatly attenuate seismic signals. Building velocity models in such areas with shallow water depths and gas can be a difficult task. Here we present two alternative ways to build reliable velocity models in the presence of shallow gas; one that is suitable to very shallow (<100m) and poor data quality areas and the other for deeper water depths. In the first instance, we make use of Diving-Wave refraction tomography method to build shallow velocity models offshore Trinidad and Azerbaijan. Previous use of this method has been limited to processing seismic data to produce a shallow velocity model to determine static corrections in time processing. Our success is in using the velocity model derived from Diving-Wave tomography as a starting model for reflection tomography in depth processing. We show that Diving-Wave method is a robust technique that produces reliable near surface models in the presence of gas and in areas with low signal to noise ratio. In the second case, we show that where data has reasonable offset to work with, reflection tomography can produce fairly accurate and high fidelity velocity models that can be further improved with iterative migration velocity analysis. As a result, depending on available data quality, either Diving-Wave derived shallow velocity model or reflection tomography derived model can be used to improve the ultimate product from iterative pre-stack depth migration and reflection tomography.

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