In areas with favourable rock properties it is possible to detect hydrocarbons directly by using standard 3-D seismic data. Amplitude interpretation is then very effective in reducing risk when selecting exploration and production drilling locations. Not all areas have such favourable rock physics, but it is always useful to understand what seismic amplitudes may be telling us about hydrocarbon presence or reservoir quality. As well as amplitudes on migrated stacked data, it is often useful to look at pre-stack data and the way that amplitude varies with source–receiver offset (AVO). The first step is to use well log data to predict how seismic response will change with different reservoir fluid fill (gas or oil or brine), with changing reservoir porosity, and with changing reservoir thickness. Then we can use this understanding to interpret observed changes in seismic amplitude or other measures of the size and shape of individual seismic loops. In principle this process can also be used to interpret amplitudes on 2-D seismic data, but as we saw in chapter 1 the power of 3-D seismic lies in the ability to make maps based on very densely sampled data, allowing us to see systematic amplitude changes that are only just above the noise level.

Basic rock properties

We shall consider in detail only isotropic rocks, where the seismic velocities are independent of the direction of propagation through the rock. In practice, many rocks are anisotropic.