High-frequency restoration of surface seismic data

Often we come across examples in which the initial processing of a 3D seismic volume results in interpretations that are geologically suspect—e.g., cases involving complex faulted patterns or subtle stratigraphic plays. Similarly, postmortem analysis may cite small fault displacements or obscure seismic data as reasons for dry wells. In such cases, the usual practice is to create a new version of the 3D volume with some target-oriented processing to improve imaging in the zone of interest that will, in turn, lead to more accurate interpretation. This helps in some cases, but in others some questions remain unresolved. In the latter, more often than not, more accurate stratigraphic interpretation is needed but the available bandwidth of the data is inadequate to image or resolve the thickness of many thin targets seen in wells. This can be addressed by having data of reasonable quality and augmenting it by some frequency restoration procedure that improves the vertical resolution. Frequency restoration is necessary because seismic waves propagating in the subsurface are attenuated and this phenomenon is frequency dependent—higher frequencies are absorbed more rapidly than lower frequencies. Consequently, the highest frequency recovered on most seismic data is usually about 80 Hz. This article describes a new method for restoring high frequencies within the seismic bandwidth that is based on the frequency decay experienced at different VSP depth levels in a well. It is well known that VSP data contain higher frequencies than surface seismic data because the energy recorded by VSP traverses the unconsolidated weathering zone just once. Figure 1, which shows sectional amplitude spectra computed for a profile through spatially coincident 3D seismic and 3D VSP volumes, confirms this observation. The frequency content of the surface seismic data extends to 60–65 Hz but it reaches 90 Hz in the 3D VSP data. Figure 1. Sectional …