1. Field of the Disclosure
This disclosure relates to a method of geophysical prospecting which improves the subsurface area imaged by Vertical Seismic Profiling (VSP) data. Specifically, the disclosure uses surface multiples, which have previously been regarded as noise, to provide a migrated image of a larger portion of the subsurface.
2. Description of the Related Art
In surface seismic exploration, energy imparted into the earth by a seismic source reflects and scatters from subsurface geophysical features and is recorded by a multiplicity of surface positioned receivers. This process is repeated numerous times, using source and receiver configurations which may either form a line (2-D acquisition) or cover an area (3-D acquisition). The acquired data are processed to produce an image of the reflectors in the subsurface using a procedure known as migration. The produced image is then used in prospect evaluation and development. Prospect evaluation and development specifically includes using the determined geometry of subsurface traps to establish the volume of recoverable reserves, and the drilling of additional exploration, evaluation, and development wells based on the image.
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution, and coherence due to the long and complicated travel paths between source, reflector, and receiver.
In vertical seismic profiling, seismic signals are recorded using detectors in a wellbore to record the energy generated by the activation of seismic sources at the surface. This results in the geometry such as that shown in FIG. 1 and is discussed further below. It is well recognized that traditional migration of upgoing primary reflections produces a VSP data image of only a very narrow conical zone around a borehole with the cone tip centered at the shallowest receiver in a borehole. This is schematically illustrated in FIG. 1 wherein exemplary reflected 129a, 129b, and 129c are shown with an imaging zone denoted by 131.
The present disclosure is directed towards a method and system for increasing the coverage obtained in VSP data acquisition.