Downhole Seismic Testing
This technique measures vertical changes in seismic velocity by placing a source at the top of a borehole and measuring travel-times at multiple intervals in the borehole, usually with a 3-component geophone.
Cross hole Seismic Testing & Cross hole seismic tomography
Cross hole seismic testing measures the velocity of seismic waves between boreholes. There are two types of cross hole approaches. The conventional approach involves lowering a 3-component borehole geophone down one hole while lowering a source down an adjacent hole(s), firing the source at some prescribed depth interval. The source and geophone are always at the same elevation, and the energy from each shot is measured at a single depth in each receiver hole. The traveltimes are then converted to velocities by dividing them into the distance between the holes. The cross hole seismic testing can be realized in a couple of boreholes or into three holes (source in a hole and two receivers in the other holes). In conventional cross hole surveying there is only a single raypath considered for each shot
In seismic tomography multiple raypaths are measured for each shot. This is accomplished by using a multiple sensor receiver array. The resulting tomogram not only delineates vertical, but also horizontal variations in velocity. This is the main advantage of cross hole tomography over convention cross hole seismic.