DOWNHOLE SEIMIC TEST

The downhole seismic test method makes direct measurements of compression (P) or shear (S) wave velocities, or both, in a borehole passed through soil or rock.

In this survey technique, a seismic source is placed near a borehole on the surface, and two tri-axial geophones are also placed at certain selected depths within the borehole.

A seismic source is used when generating a seismic wave train at the ground surface offset region, horizontally from the top of a cased borehole. In detecting the arrival of the seismic wave train, downhole receivers are implemented. The downhole receiver(s) that detects the wave may be positioned at selected test depths. This is usually done with a three-component geophone. The seismic source is linked, as well as the data recording system, responsible for recording the response of the downhole receiver(s), thus measuring the wave train travel time between the source and receiver(s). Hence, the measurements of the arrival times (time taken to travel from the source to sensor) of the generated waves (P and S waves) are then made. One major difference between refraction and downhole is that downhole looks at direct ray paths, and as a result of this, it does not require increasing velocity with depth. If there is any concern about a near-surface velocity inversion, a downhole survey can make clarifications on this and help constrain the results of our refraction survey.

• Liquefaction analysis

• Calculation of building design parameters

• Seismic site characterization as per IBC/Eurocode 8

• Estimate geotechnical parameters & Vs30