3-D Physical Modeling of a Complex Salt Canopy
Authors: Wiley R.W. , Sekharan K.K. 
Published: October 2008

Recent drilling has confirmed both significant reservoir potential and the presence of commercial hydrocarbons below salt structures in the Gulf of Mexico, Obtaining definitive seismic images with standard processing schemes beneath these salt structures is very difficult if not impossible. Because of the complicated seismic behavior of these structures, full volume 3-D prestack depth migration is required l. Unfortunately, carrying out the multitude of calculations needed to create a proper image requires the largest and fastest supercomputers and rather complex numerical algorithms. Furthermore, developing and testing the imaging algorithms is quite involved and requires appropriate test data sets,

To better understand the problems and issues of sub salt imaging, Marathon Oil Company and Louisiana Land and Exploration Company contracted with the University of Houston's Allied Geophysical Laboratories (AGL) (o construct a "salt canopy" physical model. The model is patterned after the SEG/EAEG Salt Model and is made from synthetic materials. his a full three-dimensional model with an irregularly shaped, lateral salt structure embedded in five distinct ‘sedimentary’ layers.

The model was used to acquire a multi-offset 3-D marine style survey. These data are being used to address problems of sub salt imaging. In addition to standard processing techniques, we will be investigating algorithms for multiple removal and prestack depth migration.

Introduction

Recent exploration and subsequent drilling in the Gulf of Mexico has demonstrated significant reservoir potential below salt structures. Conventional seismic methods fail to image beneath salt structures because of the complex nature of the salt body and the high velocity contrast at the salt-sediment interface. Imaging beneath salt requires full volume 3-D prestack depth migration (Ratcliff et al 1992). The development of these algorithms is involved and requires appropriate test data.

Numerical modeling has frequently been used to provide data for test algorithms and to provide insights into seismic wave propagation. The SEC research committee developed a salt model which used to obtain synthetic seismic data, Finite difference modeling was used to compute the seismic response of this model. These data can be used to provide insight into sub At the same time, Marathon Oil Company and Louisiana Land and Exploration proposed a physical model for the acquisition of a conventional marine seismic data set. These data would provide a test data set for standard processing algorithms and for most techniques being developed. It was felt that the physical model could be produced and the data acquired at a fraction of the cost of the numerical model data set. The model would be fully 3-D, support shear waves, and data acquisition would closely correspond to current techniques.

Model Design

The basic design of the physical model came from the SEG /EAEG numerical model. A cross-section of the physical model is shown in Figure 1. The sediments in the SEG /EAEG model contain curved interfaces and a vertical velocity gradient. The physical model has flat horizons and the constant velocity layers. Also the physical model does not have the shale sheath, the sub salt lenses or the over-pressure region.