Optimizing The Design Of Vertical Seismic Profiling (VSP) Surveys For Imaging Of Geothermal Reservoirs

Abstract

Seismic techniques are widely used in the oil and gas industry to image the subsurface but their use in geothermal exploration has been comparatively quite restricted. In recent years there has been a resurgence of interest in geothermal energy and so it is timely to investigate how seismic techniques need to be adapted and applied to geothermal sites, given the different nature of the targets and host rock. The primary focus of geothermal seismic imaging is to map dipping faults and fracture zones that control the permeability and the fluid flow. However, in hardrock basement the detailed seismic imaging of subsurface structures such as fracture zones is challenging for a number of reasons (E.G., salisbury et al., 2003): (1) The large impedance contrast between the overlying sediments and the crystalline basement, (2) Generally low signal-to-noise ratios due to weaker impedance contrasts between lithological units in the target crystalline rocks, (3) Reflectors are either small or laterally discontinuous due to the complex morphology, lithology and deformation, (4) High velocities in crystalline basement that result in a loss of resolution due to the relatively longer wavelengths, (5) The often steeply dipping structures which are hard to image compared to usual sedimentary structures, especially for surface seismic where large offsets are required. Vertical seismic profiling (Vsp) Has a favourable geometry to map gently to steeply dipping interfaces (E.G., cosma et al., 2003). Additionally, due to shorter travel-times compared to surface seismic surveys, and the fact that the waves only make one pass through the absorbing overburden, the signal amplitudes experience less attenuation and hence higher frequencies are preserved. The goal of this study was to optimize the survey design of vertical seismic profiling (Vsp) Surveying for imaging gently to steeply dipping fracture zones in the basement and to optimize the combination of migrated images for imaging a fracture zone of a certain dip.