Abstract

In the last decade a new acquisition scheme, called "global offset", has been tested and successfully applied for optimising seismic imaging. It allows to record high-fold seismic data with a wide range of offsets with limited additional costs. The first advantage is the possibility to record a redundant data set that can be used for stable tomographic inversions. Moreover highly energetic wide-angle reflections can be included in the pre-stack depth migration process. In this paper we show, using real cases, how tomography and pre-stack depth migration of global offset data can help in solving critical problems of the time-depth processing, improving seismic imaging and interpretation.

Finally we discuss the importance of including also DC (direct current) and electromagnetic (EM) methods for improving the velocity fields when seismic is not sufficient for an appropriate imaging. The key idea is that non seismic applications, such as continuous profiling magneto-telluric and geo-electric methods, can be complementary to seismic.

During the last ten years we optimised the techniques for acquiring and processing simultaneously seismic, DC and EM data in the appropriate way in order to integrate them quantitatively. This integration is performed through joint and/or cooperative inversion.

Real cases of this approach are discussed.

Introduction

In complex geological settings one of the first requirements is imaging accurately the shallow sedimentary sequence. In fact near surface geological complexity can cause significant distortions of the seismic events below.

Due to low signal-to-noise ratio, sometimes, the shallow velocity structures cannot be modelled properly by standard velocity analysis. The consequence can be a velocity field that is inadequate for static corrections and misleading for stack and migration purposes.

In this framework it is well known that transmission tomography can produce a reliable velocity field through first arrivals inversion.

That velocity model can be used, at the same time, for processing purposes (for example as a laterally variable replacement velocity field to be used for static corrections) and as a guide for interpretation (in terms of velocity structures).

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