Advanced Seismic Data Acquisition and processing

Seismic data is one of the main sources of information on the subsurface. We not only need to obtain the structure that could contain hydrocarbons, but also the rock properties so we can decide on whether we are dealing with reservoir rocks (sandstone, carbonates, even shales), sealing rocks (shales, salt) or source rocks (shales, coals). To obtain the best image of the subsurface we first need optimum acquisition. Optimum means fit for purpose. There are several criteria that need to be satisfied. An acquisition principle that should be adhered to as much as possible is symmetric sampling, which means equal shot and receiver spacing and equal in-line and crossline distances (for a 3D). The spacings should be such that no spatial aliasing of the data occurs. Surface and subsurface diagrams are useful to see what CMP spacing and offsets in each CMP gather result from the surface geometry. The data recorded is the ground motion which gives a continuous (analogue) signal in time which needs to be digitized for the processing. This needs to be done so that nor temporal aliasing occurs. Hence, the complete wave-field which arrives at the surface must be faithfully represented by the discrete/digital data.

Although all the information is present in the shot or field records, processing is needed to make them accessible for interpretation. In interpretation, we try to obtain a true image of the “geology” of the subsurface. Processing can be divided into signal processing and wave-propagation based processing steps. Signal processing steps are, for example, static corrections, removal of shot-generated noise by velocity filtering, shortening of the wavelet by de-convolution, NMO correction, etc. The wave-propagation part consists of migration or imaging. For wave propagation we need, in principle, to use equations describing full elastic wave propagation in an inhomogeneous, anisotropic, visco-elastic earth (as that is what really happens in the subsurface). However, this would lead to complicated and computer intensive processing algorithms. So, we usually simplify our description of the wave propagation to acoustic wave equation, which describes only a single P-wave reflection per reflection ray-path and ignores S-waves. This will provide us, for example, with migration algorithms/operators (for time- as well as depth migration). It can give a migration output that may show the errors of ignoring anisotropy, attenuation, wave conversions, etc. An improvement is to use the two-way wave equation. This is implemented in so-called Reverse Time Migration (RTM). Even better to apply Full Waveform Inversion (FWI).

Despite the use of this acoustic approximation in our processing, amplitudes can still often be used to determine pore-fluids and pre-stack migrated data, using AVA analysis for deriving shear wave properties. But note that if we model, as in inversion, a geophysical quantity related to amplitudes, such as the reflection coefficient, we need to include densities and shear conversions at interfaces.

All these relevant topics will be extensively discussed during the course and applied in exercises.