Diving‐wave refraction tomography and reflection tomography for velocity model building

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.

Use of refraction, reflection, and wave-equation-based tomography for imaging beneath shallow gas: A Trinidad field data example

“…The present study is an extension of that methodology, which includes applying wave-equation-based tomography. Tanis et al ͑2006b͒ suggest that reflection tomography alone does not effectively update the very shallow part ͑as many as 250 depth samples͒ of the velocity model. This part of the model was generated by WesternGeco using refraction ͑diving-wave͒ tomography.…”

“…We have very limited information to build a near-surface velocity model using ray-based reflection tomography alone. In a viable methodology presented by Tanis et al ͑2006b͒, the very shallow part of the velocity model is built using divingwave refraction tomography, and the deeper part of the velocity model is then built using iterative reflection tomography and prestack depth migration ͑PSDM͒. Application of targeted deconvolution shows promise in handling multiples ͑Lancaster, 2005͒.…”

Estimation of the Near-surface Q Distribution from Pre-stack Surface Seismic Data

“…First-arrival travel time tomography has been successfully applied to both synthetic and real data (Tanis et al, 2006) to build nearsurface velocity models. Near-surface Q models can thus be obtained by performing a kinematic two-point ray tracing of the first arrivals (using an efficient Eikonal solver) within the near-surface velocity model and then solving the linear system (3).…”

Maximizing the value of 3D seismic vintage through state of art land depth imaging and quantitative quality controls - A case study at Yucal Placer field, Venezuela

“…Various approaches have been tested to produce deep targets depth imaging from true surface topography using the most accurate near surface velocity information (Rajasekaran & McMehan 1995;Hu & Kim 2002,). Several authors have also proposed static tomography as an efficient way to incorporate accurate near surface velocity heterogeneities to enhance depth imaging results of deep targets (Zhang et al 2005, Tanis at al 2006, Han et al 2014, Ji et. al 2015.…”