Measuring instruments | Pasisrl

BOREHOLE SEISMICS

These prospections involve the use of a surface source and a borehole geophone ( downhole surveys) or a source and a geophone arranged in two separate holes (crosshole surveys). Through the use of triaxial borehole geophones, both P and S waves are usually detected.

In this type of seismic prospecting, the source and/or geophones are located in a specially prepared hole in the ground. To guarantee high quality data, these holes must comply with the standards indicated in the relevant ASTM standards. Among the most common methods are downhole testing (ASTM 7000-08), where the source is on the surface while the sensor is instead made up of a 3D hole geophone (P & S waves) or a hydrophone chain (only P waves in a water-filled hole or drilling fluid). Downhole testing is aimed at determining the profiles of the seismic compression, P, and shear waves, S, with depth. It consists in producing a disturbance on the ground surface by a mechanical source and in measuring the arrival time of the direct waves, P and S, at the various depths within the appropriately prepared hole. 

Although far more expensive than superficial methods like MASW and Re.Mi., this technique is used not for the calculation, but for the measurement of the Vs30; this usually happens in case there is already a perforation with a good cementation and/ or in case there is not enough space to realize a  geophone spreading on the ground surface. 

Crosshole seismic testing (ASTM D4428) instead involves the measurement of seismic wave velocities between two probing holes, one for the shot (normally carried out with a bore or explosive energiser) and the other for measurement (with a three-dimensional anchored geophone). For each acquisition, the depth of the shot and of the measurement in the two holes must be the same. In this case, therefore, two coated and well-cemented boreholes must be available, of which the mutual distance to all measurement dimensions is known. Crosshole investigations are also critical for obtaining high-quality seismic tomography profiles.