Fracture imaging and saline tracer detection by crosshole borehole radar data migration

摘要

Because the resolution of tomography is not high, crosshole tomography is suitable for imaging large weakly inhomogeneous bodies, but not suitable for bodies of small size, for example, thin aperture fractures. We try to use crosshole migration method to obtain underground small aperture fractures between two boreholes. Since fractures are filled with water, although the apertures of fractures are small, the electromagnetic property of fractures is very different from bedrocks. Field experiments show that reflection and diffraction from water filled fractures can be received by radar sondes. Therefore, it is possible to get images of fractures by crosshole migration. We apply the crosshole migration technique to crosshole radar data sets in frequency and time domain collected both by our research group and by USGS in Mirror Lake area, USA. By comparing the migration result of crosshole data recorded in frequency domain with USGS's results of crosshole tomography and directional single- hole measurements, we find that the migration result is consistent with USGS's results very well. Two known fracture zones are correctly imaged. The migrated section of background data illustrates two fracture zones. The difference images between migrated profile from USGS's background data and these from USGS's crosshole time-lapse data indicate that the transportation of saline tracer inside a transmissive zone can be known during the water and tracer injection and pumping. This result is also consistent with USGS's time-lapse difference-tomography results. Moreover, the detail structures inside the fracture zones could be seen from the migration profiles and the difference sections. However, differences between two results exist. This may indicate the limitation of tomography: the resolution and sensitivity of tomography are not high enough.