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Creating low noise environments for borehole seismometers

These guidelines are intended to help you make design decisions about your borehole installation, and give some points to bear in mind during construction. We hope that they will help you get the best performance from your Güralp Systems borehole instruments.

The major factors to consider when designing a borehole installation are the same as those for a vault, namely location, coupling and stability.

Borehole types

The main difference between boreholes is the type of casing:

In some cases, instruments are installed in boreholes which already contain seismic equipment, for example in cases where the existing instrument has been concreted in to the bottom of the borehole, or where a custom hole lock has been fitted. We can supply modified sensors and hole lock mechanisms, and advise on installation techniques for this kind of situation. Contact us for more details.

Critical specifications

For a borehole to be useful for installing seismic instrumentation, it should meet the following requirements:


Boreholes are normally drilled with a rotary drill. A drill type should be chosen which is capable of performing both drilling and finishing. The uncased hole should have a diameter between 21 and 26 cm.

Whilst drilling is taking place, you should ensure the borehole is not drifting from vertical by continuous measurement. In addition, a complete driller’s log should be kept, showing the geological conditions the borehole passes through. Samples of drill cuttings should be taken at intervals of around 3 m and stored.

The casing string should have around 17 cm inner diameter. The same weight and grade of casing should be used at all points if possible. Casing joints may be any length, but must be straight and free of irregularities to prevent the sensor or cables snagging during installation. You should record the number and depth of any joints or collars in the casing.

At the bottom of the borehole, the casing string should be supported 1 m above the floor before cement is added. A mixture of cement, water, and salt is recommended to ensure the cement expands to fill any gaps. Cement should be pumped upwards from the bottom, around the outside of the casing to within 1 m of the surface. When enough cement has been added, a plug should seal the bottom of the borehole and prevent back-flow. The cement should be allowed to cure for 24 hours.

After curing, inspect the casing and cement. If the level of cement has dropped from the surface, more cement can be added from the top. Scrape and brush the casing, and empty the borehole of any loose material.

If a test cylinder 4 m long and 16 cm in diameter can be passed freely along the whole length of the borehole, it should be straight and wide enough for the hole lock and seismometer. We also recommend that you check the borehole for leaks by filling with pressurised water. Finally, cap the borehole and cover with thermally-insulating material to minimise convection noise.

At the surface, you should install a suitable small vault around the borehole head for housing the breakout box and other equipment. This vault can be covered with a concrete pad and hatch, with cables running in conduit to power supplies and communications equipment as required. It should also be checked for leaks, or include a pump system to prevent water from getting in.


Preparing for installation

Your borehole is now ready to be equipped with seismic equipment to your requirements. Güralp Systems provide borehole versions of all their sensor types:

The down-hole environment is also ideal for the sensitive analogue-to-digital converters of our digitisers, as well as minimising the length of lossy cabling.

Full installation instructions are given in the documentation for the borehole sensors.