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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.
In this case Cross-hole seismic successfully imaged the
foundations saved about $2,000,000 and has helped guide the
construction progress.
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.
Cross-Bore Seismic has many applications which include,
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
Call
Grant at SSI
or
email
us at
info@ss-i.biz
for more information
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Australia
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
Web
http://www.subsurfaceimaging.net
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