3D Marine Data Acquisition and Processing over ACTI/EAGE Physical Model Using Vertical Hydrophone Cables
Authors: Sekharan K.K. , Guimareas M.G. , Jackson R.A. , Ebrom D.A. , Krall P. , Sukup D. 
Published: October 2008

Imaging geological structures beneath salt is a great challenge. Since the presence of commercially viable hydrocarbon deposits has been confirmed below salt structures, especially in the Gulf of Mexico, imaging subsalt structures has added significance. However, massive amounts of three dimensional marine data have to be acquired and processed to image these complex subsalt structures.

To understand the problems and issues in acquiring and processing three dimensional offshore marine data, a physical model (funded by Marathon Oil Co. and Louisiana Land and Exploration) was constructed at the Allied Geophysical Laboratories of the University of Houston. The model is imilar to the SEG/EAGE numerical salt model consisting of a complex salt block with a canopy, toe faults, and a rugose surface at the top. There is a three dimensional layer underneath the salt. The salt is surrounded by severalsedimentary layers.

A conventional 3D marine data set (1) consisting of nearly 20 million traces was acquired (funded by DOE and 11 companies) over this model. Several groups are processing this data set separately through pre-stack depth migration.

A different approach to conventional marine acquisition is the use of vertical cables anchored at the bottom of the ocean, and a boat pulling just the source. This method acquires a significantly different data set. The acquisition of a 3D data set over the SEG/EAGE physical model using vertical cable simulation is in progress. This project will be a direct comparison of conventional versus vertical cable acquisition and processing of marine 3D data. The results will be presented.

Introduction

Imaging geological structures beneath salt is a great challenge. Since the presence of commercially viable hydrocarbon deposits has been confirmed below salt structures, especially in the Gulf of Mexico, imaging subsalt structures has added significance. However, massive amounts of three dimensional marine data have to be acquired and processed to image these complex subsalt structures.

In conventional 3D marine data acquisition several closely spaced 20 lines are acquired. The boat has to pull long receiver cables and hence travel a long distance beyond the area of coverage. The data are of narrow azimuth and subject to the surface conditions of the water. Vertical cable acquisition has many advantages over the conventional method. In the acquisition phase, a considerable savings of time and money arises since smaller boats could pull the source, eliminating the large distances required for turns. Coverage is better since the boats can be closer to any obstructions in the survey area. Vertical cables anchored at the ocean bottom surface is less susceptible to weather conditions at the water surface. Wide azimuth data is obtained (2), and 3D pre-stack migration is very cost effective.

The vertical cable acquisition method is a patented method for acquiring and processing pre-stack 3D marine seismic data (3). It is a less mature technique and, therefore, a large number of problems remain to be solved as the technique is developed. The physical model project presented in this paper provides an opportunity to understand not only the merits of this technique but also the problems associated with it.