Two Dimensional Seismic Modeling for the Identification of Free Gas in Marine Sediments

Abstract

Seismic modeling is basically a construction of geologic computer model, which
simulating the seismic wave propagation responses of the earth. The synthetic seismic traces generated are then compared with the real seismic data. If the geological models are known with sufficient accuracy, such synthetic responses are used to validate the choice of acquisition and processing parameters, and to verify interpretation decisions. The physical properties of earth materials are not uniform because subsurface variations occur in lithology, porosity, mineralogy, density, permeability, and pore fluids. To understand wave propagation in these materials, simplified mathematical models are usually constructed.The goals of this study is to identify the presence of free gas by performing seismic modeling. This paper investigates the relative abundance of free gas associated with the BSR by modeling the reflection coefficient, or amplitude changes. The models are based on multichannel seismic profiles, and seismic velocity data.
Because the strength of the reflected signal is proportional to the change in elastic impedance (product of velocity and density), the top of free gas sediment produces a strong reflection amplitude (bright spot). Furthermore, the reflection coefficient for the top of free gas was estimated and evaluated for elastic isotropic situations. The presence of gas produces a significant reduction of seismic wave velocity in the reservoir rocks, relative to the water-saturated condition. The low velocity zone can be directly related to the derived interval velocity from RMS, and/or stacking velocity when layer boundary is flat and constant velocity is assumed based on Dix’s equation, and thus the separated reflections signals can be used to image the top and the bottom of free-gas bearing zones.