beautypg.com

2 installing a sensor using sand backfill – Guralp Systems CMG-3TB User Manual

Page 31

background image

Operator's guide

3.2 Installing a sensor using sand backfill

Dry sand backfill is a convenient and effective way of installing a
borehole or posthole sensor in a time-stable environment. The

presence of sand not only fixes the sensor in place at the bottom of the
hole, but also reduces noise due to air convection.

The ideal type of sand to use is the fine, kiln-dried sand used for

children's play sandpits. This is readily available in airtight bags, is
thoroughly washed and clean, and will contain little sediment. (When

dried out after wetting, sand containing foreign matter may solidify
and “concrete” the sensor in position.) This sand is suitable for use in

both dry and damp boreholes.

In the procedure outlined below, the sensor rests on a pad of sand
around 300mm thick. This pad will absorb any residual moisture at

the bottom of the borehole, and ensure that the surroundings of the
instrument are kept dry.

After positioning the sensor, more sand is added to fill the space

between it and the borehole casing, holding it firmly in place. The
sand should reach within 30mm of the top of the instrument, but

should not cover it. This way, the instrument can be more easily
recovered when it requires maintenance or replacement. This is

particularly important if the borehole is not completely dry, since
moist sand does not flow well.

The following photographs show the steps involved in backfilling with

sand:

November 2006

31

This manual is related to the following products: