Reflection Tomography and Velocity Model Building in an Area Characterized by Shallow Gas
Authors: Sernpere Janet E. , Hardy Pierre B. 
Published: October 2008

Abstract
This paper discusses prestack reflection tomography as a tool for building accurate velocity models to be used during prestack depth migration of seismic data. The example presented here focuses on high resolution 2-D shallow marine data characterized by the presence of shallow gas pockets. The area was expected to exhibit strong lateral velocity gradients that caused severe degradation of the data below the anomalies. Velocity modeling was performed in an iterative fashion starting with three key horizons. An inversion process was used to simultaneously solve for depth and velocity using traveltime picks on 12 offsets for the three initial horizons. After each tomography iteration, a prestack depth migration was performed to assess progress and guide the picking of new reflectors. The final velocity model contained seven horizons and was used in a Kirchhoff prestack depth migration. Substantial improvement was noted when comparing the depth image to the image obtained by time migration. In comparison to other velocity model building techniques, reflection tomography is a successful, primary, velocity analysis tool that is well suited to constraining shallow lateral velocity variations. It is a global technique that delivers an optimum solution for a complete set of horizons rather than a disconnected series of velocity analyses. The traveltime inversion algorithm is very robust, with the ability to cope with the intermittent and even conflicting horizon segment picks that often characterize low signal-to-noise reflection events on common-offset planes in complex areas.

Introduction
It is well accepted that prestack depth migration is more accurate than time migration or post-stack depth migration in areas of sharp lateral velocity variations or structural complexity. However, the validity of the prestack depth migration is strongly related to the quality of the velocity model used in migration1. In turn, velocity model building is generally the most time-consuming step leading to a prestack depth migration. It is thus cost effective to have an efficient way of developing an accurate velocity model. Most of the techniques for building velocity models from prestack seismic data revolve around finding the model in which the simulation of forward or reverse propagation is consistent with the data. Migration velocity analysis methods2–5 look for the model that optimally prestack-depth-migrates (reverse propagates) the data. Reflection tomography6,7 finds the model that best explains the arrival times observed on the input traces. The two approaches are theoretically equivalent. In practice, they differ in effectiveness on particular types of data. While reflection tomography is well suited for simple structural problems, migration velocity analysis is more effective in cases of complex geology. In this paper we discuss a specific implementation of reflection tomography that has been designed to be effective in all cases. It operates in the pre- or postmigrated time domain, according to which method is best suited to the data. As an example of the technique, we focus on a 2-D seismic line collected in the Gulf of Mexico. The area was expected to show lateral velocity variations due to the presence of shallow gas. These velocity anomalies caused degradation of the time data, making interpretation difficult.