Crosshole - seismic tomography between boreholes

The crosshole geophysical technique allows the velocities of the primary and secondary waves, P and S, to be obtained along direct paths between two points. Each point located at the same depth inside a different borehole. In one of the borehole the energy source (emitter borehole) is located and in the other the geophone (receiver borehole).

As in the downhole, the velocities Vp and Vs are recorded, the corresponding dynamic elastic modulus can be calculated. This method also serves to detect the existence of zones with "velocity anomalies" between boreholes.
These tests are to be carried out according to the D4428 ASTM standard, with distances between the "emitter pborehole" and the "receiver borehole" of 3 to 10 m (one hole or several) and with the energy source (emitter) and triaxial geophone (Receiver) at the same height.

The crosshole test or seismic tomography reconstructs an image of the internal structure of the subsoil, from the travel times of the seismic waves. They propagate inside a section of land between two geotechnical surveys.
The energy source, as in other seismic geophysical techniques, may be an explosive, a sparker, a pneumatic hammer, etc; which is introduced in the emission survey. While the reception borehole and/or surface record seismic waves generated; this record can be made by hydrophones and / or geophones (generally referred to as "captors"). Both the energy source and the "captors" have to be spaced on a regular basis, the most advisable situation being the use of equal spacing in both.

The methodology of work presents the following sequence:

1. The seismic waves generated by the energy source, at a certain depth, are recorded throughout the entire survey. And if possible on the surface between the boreholes.
   See next image, where a schematic with some paths is represented
2. The power source is placed in the next position and the operation is repeated until reaching the maximum depth

It is very important that the distance between the boreholes is not excessive. Since the energy of the sources is limited, and therefore, the quality of the signal decreases with increasing the separation between the boreholes. Also, the method's resolving power decreases with distance, since there must always be a relationship between the size of the "anomaly" and the section of ground between the reception and emission boreholes.

The conditions for testing of seismic tomography are:

1. The boreholes, of emission and reception, must be tubed with PVC and filled with water
2. The spatial borehole's position must be known (deviation and inclination), with an accuracy of +/- 5 cm in X, Y, Z; throughout the entire survey, because with such precision should know the coordinates of the energy source and of the "captors"
3. The probes must reach a depth that exceeds by 20-30% the depth to which the objective of the study is placed