Abstract

The imaging of steep salt boundaries has received much attention with the advent of improved wider azimuth acquisition designs and advanced imaging techniques such as reverse time migration (RTM), for example. However, despite these advancements in capability, there are cases in which the salt boundary is either poorly illuminated or completely absent in the migrated image. To provide a solution to this problem, we have developed two RTM methods for imaging salt boundaries, which use transmitted wavefields. In the first technique, downgoing waves, typically recorded in walkaway vertical seismic profile surveys, are used to image the salt flank via the generation of aplanatic isochrones. This image can be generated in the absence of an explicit interpretation of the salt flank using dual migration velocity models, as demonstrated on a 3D walkaway field data set from the Gulf of Mexico. In the second technique, we extend the basic theory to include imaging of upgoing source wavefields, which are transmitted at the base salt from below, as acquired by a surface acquisition geometry. This technique has similarities to the prism-imaging method, yet it uses transmitted instead of reflected waves at the salt boundary. Downgoing and upgoing methods are shown to satisfactorily generate an image of the salt flank; however, transmission imaging can create artifacts if reflection arrivals are included in the migration or the acquisition geometry is limited in extent. Increased wavelet stretch is also observed due to the higher transmission coefficient. An important benefit of these methods is that transmission imaging produces an opposite depth shift to errors in the velocity model compared with imaging of reflections. When combined with conventional seismic reflection surveys, this behavior can be used to provide a constraint on the accuracy of the salt and/or subsalt velocities.

Highlights

  • The imaging of steeply dipping or vertical salt flanks has traditionally been a challenging problem for the oil and gas industry

  • We follow the general workflow published by McMechan et al (1988) but we present a mathematical solution to solve the salt imaging problem using reverse time imaging of a downgoing wavefield and demonstrate this on synthetic and a 3D walkaway vertical seismic profile (VSP) field data set from the Gulf of Mexico

  • The VSP geometry to test is indicated by the red source to receiver shown in the figure

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Summary

Introduction

The imaging of steeply dipping or vertical salt flanks has traditionally been a challenging problem for the oil and gas industry. In the case of the steeply dipping salt that surrounds the Mars Basin in the Gulf of Mexico, narrow and wide tow streamer data sets, as well as ocean bottom seismometer (OBS) acquisition, have failed to adequately illuminate the overhanging sides of the salt on either side of the basin (Figure 1) In this example, the 3D structure of the salt is complex, and the basin narrows to the west, and it is bounded by salt on three sides. The authors migrate a traditional salt proximity survey from a single source location using their hybrid method incorporating ray tracing and the use of a one-way finite-difference migration In their workflow, the ray tracing is applied to propagate the source wavefield from the surface down to the receiver for a VSP acquisition geometry. The use of a one-way wave-equation migration operator to back propagate the receiver wavefield provides the benefit of being able to image multiple wavefront arrivals; the dip coverage from the finite-difference implementation is limited and cannot image horizontally propagating waves (Etgen et al, 2009; Liu et al, 2011; Diaz and Sava, 2016)

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