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Crosshole Surveys

A consultant called us in to help with foundation design assessment. One of the requirements was to know the Compression and Shear wave velocity to derive rock strength. This is a bulk modulus that helps give a better feel for the foundation response to seismic loading.

The survey location was on the site of the main foundation piers.

There’s some trade knowledge to get good results. These are type of equipment to use, survey design and practices to use when processing data. The survey must include run BH deviation surveys, if you don’t the velocity can be incorrect from the effect of the BH deviation.
 

This is a plot of the S wave velocity with depth. There’s a sharp boundary at 20m, this is the base of hard volcanic rock and below this is weathered sandstone. This result was important as it was in the compression footprint of the foundations. However it’s clear at 30m plus the weathering of the sandstones is less. In this case Cross-hole seismic successfully imaged the foundations saved about $2,000,000 and has helped guide the construction progress.   

1. locating weaker zones to help with geotechnical design
2. mapping mineral contacts
3. imaging voids.

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Using Acoustic Televiewer on projects

Downhole geophysics is a great way to get the real data about targets. There are a variety of sensors we can run down hole including sonic density and temperature.This example to uses an acoustic scanner to give a hole side wall image. Using software tools we can convert this image into Information about the bedding and structure (faults and joint systems).These are all oriented correctly to magnetic north.The tool also corrects for hole off angle issues (i.e. deviation).

Some real data,

This is presented as a synthetic image that looks like a core of the rock in the borehole. In effect it’s an image of the sonic velocity of a thin veneer of the material in the borehole walls.This can show defects, faults joint and bedding structure. The white lines on the images are the picks of defects in the wall side.

This second image is another data set.The “sine waves” are the structures we can see in the bore walls

Here the data shows subdivisions (actually bedding) and the joints and defects show up more within specific intervals.In fact here the more interesting sections are
less weathered sandstones and the less interesting mudstones.

Give us a call and discuss your particular requirements.We are can customise data collection and digital data output formats to suit your specific needs.

SSI use a variety of geophysical methods, Radar, EM, Electrical Resistivity Imaging, Cross-hole surveys

Grant at SSI or email us at

info@ss-i.biz for more information

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Unit 17 , 3 Hamley Rd

Mt Kuring Gai NSW 2800 Australia1800-359-905

New Zealand

3061b Gt North Rd

New Lynn Auckland NZ

Phone NZ : +64-9-826-0700

Fax: +64-9-826-0900

Singapore

Teban Garden Cres

Phone +65 9389-2432

E-mail: info@ss-i.biz

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