Guralp Systems CMG-3T - Manual (Page 18)

CMG-3T
By following these guidelines, you will ensure that your seismic
installation is ready to produce the highest quality data.
2.5 Installing in post-holes
The 3T is suitable for installation in post-holes. In soft subsoil, a hole
two to four metres deep and 20cm wide can be conveniently excavated
using a tractor-mounted or hand-operated post-hole auger. To
minimize surface effects, you should ensure that the hole is one metre
deeper than the length of the instrument or, preferably, somewhat
more.
Since the hole has no lining, it may occasionally flood. However, most
soil types are sufficiently permeable to allow water to soak away,
leaving the packing material moist.
A slim-profile 3T instrument is available with vertically stacked
sensors and built-in inclinometer, specifically designed for post-hole
installations. Digitizers or other recording equipment can be placed
either within the hole or in a separate enclosure; alternatively, the
Güralp DM24 is available in modular form, allowing you to attach the
digitizer directly to the instrument.
To install a 3T in a post-hole:
1. Clean the post-hole, making sure there is no loose material
around the mouth of the hole or on its base.
2. Prepare the instrument package, making sure the inclinometer
is visible, and attach it to a winch or hoist by clamping a light
steel cable to the centre of the handle so that the package hangs
vertically. Connect the signal cable to the instrument.
3. Add packing material to the hole to about 15cm depth. Fine
18 Issue N