Downhole Seismic Technique:

In downhole seismic surveys a triaxial geophone transducer is incrementally lowered down a borehole while seismic waves are generated from either the surface or another borehole. Based on the nature of the seismic waves when they reach the geophone, information on stratigraphy and seismic response can be deduced.

Downhole seismic velocity data are utilized in a variety of geotechnical site investigations, including earthquake site
response modeling, foundation design for dynamic (vibratory) loads, estimating

pile depth, and rock quality assessment. Downhole seismic data provides detailed information on stratigraphy and the engineering properties of subsurface soils and rock that is not available from surface seismic surveys and thus are often used for the enhancement and quality control of surface seismic interpretations.

A downhole seismic survey (also called a borehole velocity survey) is conducted by measuring the time for seismic waves generated by an impulsive source at the surface to travel to a sensor located at a sequence of depths in the borehole. The sensor consists of three geophones arranged in an X-Y-Z pattern. Two orthogonal horizontal geophones are used to detect shear-wave (S-wave) arrivals and a vertical geophone is used to detect compression-wave (P-wave) arrivals. At each measurement level, the sensor assembly is locked to the borehole wall using a clamping mechanism so that the geophones will couple with the seismic signals propagating in the earth.

Northland Geophysical maintains a state-of-the-art BHG-2 three-component slimhole transducer manufactured by Geostuff Instruments. This transducer can be used in boreholes as small as 2 inches in diameter, thereby significantly reducing the costs of seismic borehole preparation. The BHG-2 employs a mechanical clamping “spring arm” to provide positive coupling of the geophone assembly with the borehole casing and the surrounding formation.

NGA and Northland Geophysical have developed specialized software which aids in the identification and timing of shear-wave arrivals. This software computes the vector sum of the two orthogonal horizontal geophone signals and determines the orientation of maximum shear-wave particle motion. The software then mathematically rotates the signal into this orientation and plots the “optimized” shear-wave trace at each measurement level. In general, this optimized or projected trace presents a more consistent waveform than either of the two horizontal geophones taken separately. In the field, a vertical stack of the optimized traces on a portable computer is used for preliminary analysis of the downhole data. This on-site analysis allows us to add supplemental data at intermediate levels where appropriate to refine the velocity profile.

Over our twenty years of experience our geophysical team has identified several key factors in obtaining high quality downhole seismic data. These include: proper borehole preparation to assure good coupling between the geophone and the surrounding soils or rock formation; recognition and enhancement of the shear-wave and/or compression-wave arrival, and the suppression of spurious (e.g., tube-wave) arrivals; consistent timing of the wave-form arrivals; selection of a depth sampling interval appropriate for resolving the transitions of interest. Knowledge of these key factors enables us to provide detailed, high quality seismic velocity profiles which correlate well with other geological and geotechnical parameters. In sum NGA has the equipment, in house software, and experience to create exceptional interpretation using the downhole seismic technique.