Skip to main content
Contact Us

Downhole Geophysics

The downhole or borehole test is a seismic technique for the determination of the physical-mechanical properties of the soil and rock; In particular, it allows to determine the speed of propagation of seismic waves of volume, compression and shear at different depths, measuring the time that these waves take in the journey between the source and the receiver, and to derive the parameters of the soils and rock crossed.

Downhole Seismic: Cross-Hole Seismic Tomography (CHST) and Surface-To-Borehole Tomography (STBT): High-frequency acoustic pulses are generated at predetermined source locations (S) in the source borehole (SB) or surface. The amplitude and arrival time of direct arrivals (and others) is recorded at predetermined receiver locations in the receiver borehole (RB). The recorded travel time–amplitude data are statistically analysed and used to generate a velocity–attenuation cross-sectional model of the area between the source and receiver boreholes.

Modified from “Introduction to Geotechnical Geophysics – N. Anderson and N. Croxton, Circular – Number E-C130, October 2008”.

For both systems a seismic receiver, which consists of a hydrophones streamer, is lowered inside a water-filled borehole. The seismic source will be lowered in a second borehole for CHST and generated on the surface at specific intervals for STBT. Seismic energy source for CHST can be an air-pressure-gun or down-hole sparker. For STBT, a sledgehammer, WDS system, “Betsy” gun or explosive charges can be used.

Sample of STBT (Ch. -30m to 0m), and STBT and CHST (Ch. 0m to 15m). The investigation delineated top of rock, which correlated very well with boreholes and identified “soft spots” (soft sediments, infilled voids etc.) missed by the boreholes.

What are the most common uses of Borehole/Downhole Seismic?

Depth to rock and soil profile
Rock and soil characterisation
Geological structures/features detection/delineation, e.g., faulting and fault zone, dykes
Subsurface assessment of riverbank for abutments
Vs30 (the time-averaged shear-wave velocity (Vs) to a depth of 30 meters, is a key index adopted by the earthquake engineering community to account for seismic site conditions)

Reach Out and Let’s Discuss Your Project

No matter how simple or complex is your project, we can assist in many ways…a technical explanation? A practical field implementation? A budget discussion? Or a full project planning and execution? Just ask away.
Contact Us