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Geotech spotlight: Future probe

A Japanese probe that was developed in the 1980s, and used on many high-profile structures, has now been updated to a digital version.

Japanese geotechnical research firm OYO Corporation developed the PS-suspension probe, which was first used successfully for micro-zonation and geotechnical studies in Japan in the early 1980s.

It was later introduced into the US and employed on high-profile projects, including the Golden Gate Bridge, the San Francisco-Oakland Bay Bridge and most of the other toll bridges in California, as well as on San Francisco airport.

The probe provides high-resolution shear-wave and compressional velocity data in rock and soils at depths up to 500m from measurements within a single borehole. In many cases, a PS-suspension log is technically superior and is more cost-effective than a down-hole or cross-hole seismic survey.

Further development

In 2007, OYO transferred the PS-suspension technology to Robertson Geologging, a subsidiary company, for further development and updating. The resulting new product, a digital version of the probe, has been recently deployed on offshore projects ranging from shallow oil and gas in Russia, offshore wind farms in Germany and nuclear sites in the UK and elsewhere.

The PS-suspension method needs only a single uncased borehole, which means surveys can be performed immediately after drilling. The tool does not require clamping to the borehole wall and can be moved rapidly between stations, minimising survey time and the risk of hole collapse.

The tool is well suited for surveys from offshore platforms, boats and barges, where access to the ground surface is inconvenient or impossible.

The probe operating software includes the facility to stack multiple shots and to apply digital filtering to the data as in normal seismic data acquisition. Further options include calculation of elasticity moduli and Poisson’s ratio when density information is available from other logs or core samples.

The PS-suspension probe operates best in open water-filled boreholes between 100mm and 200mm in diameter.

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