Crosswell Tomography for Investigating Subsurface Structure

Technical feature and potential

Abstract

Crosswell seismic tomography is a new geophysical technique for investigating a subsurface structure. This method is characterized by the geometry that both a seismic source and a receiver are located in boreholes. Energy level of a downhole seismic source is relatively small in order to avoid the damage to well-bores. This restricts the distance between a source well and a receiver well to several hundred meters. However, observed seismic waves contain much higher frequency components than those of surface seismic method. This infers that the crosswell method has a potential to delineate detailed inter-well structure.
The analysis of crosswell seismic tomography is to reconstruct inter-well velocity structure from traveltimes and ray paths of observed direct waves. Because ray paths of seismic waves depend on the velocity structure which would be the solution, the analysis is equivalent with solving non-linear equations as an inverse problem. The velocity structure is generally reconstructed by iteratively renewing a velocity model so that the traveltimes calculated from the velocity model converge to observed traveltimes.
The crosswell seismic tomography has been applied to resource exploration and environmental studies through the research for investigating the relationship between seismic velocity and physical properties of rocks. The effect of EOR (Enhanced Oil Recovery) processes can be identified by the decrease of seismic velocity. Such a velocity change can be detected by comparing the results of crosswell experiments conducted in several stages of the EOR processes
The crosswell tomography can construct detailed subsurface images in the volcanic area where surface seismic method is not available. This means that the crosswell method will be the best seismic approach to identify fracture zones which forms a geothermal reservoir in vol-canic rocks. In addition, the detection of fracture zones is extremely important to investigate environmental problem of underground waste disposal.
Crosswell seismic tomography still has several research problems that should be overcome. Nevertheless, in the future the crosswell seismic method has the possibility to become the best seismic approach to investigate detailed subsurface structure and physical properties of rocks.