Borehole installations

A common way of reducing the effects of cultural and surface noise on a seismic instrument is to install it in a borehole.

In sites away from buildings, drilling a borehole is often cheaper and easier than constructing an underground vault. Boreholes also provide greater contact between the instrument and bedrock than is possible in most vault installations.

For general seismic experiments, a shallow borehole (2 – 5 m) is normally sufficient to provide a low noise environment whilst still responding to surface waves. Some experiments require the use of very deep boreholes—in some cases over 500 m.

Güralp Systems' instruments are suitable for deployment in cased or uncased boreholes, either mid-hole using a hole lock mechanism or at the bottom of the borehole and installed with sand backfill.

Our hole lock mechanisms are suitable for cased boreholes between 89 and 229 mm in diameter. Single-jaw and three-jaw hole lock units are available. Normally, a single-jaw hole lock is used for triaxial borehole instruments, whilst uniaxial instruments are mounted on top of a three-jaw hole lock unit. Slimline CMG-3T instruments can be converted for installation in boreholes by combining them with a three-jaw hole lock unit.

The motorized hole lock mechanism is controlled from the surface by a dedicated control unit. The motors are powered separately from the sensor to minimise noise during operation.

Instruments

CMG-3TB weak motion velocity sensor

The CMG-3TB uses the same sensor mechanics as the CMG-3T. It is ideal for scientific studies in low noise sites.

The CMG-3ESP may also be supplied in borehole form.

CMG-40TB medium motion velocity sensor

For medium noise sites, the CMG-40TB is a cost-effective velocity sensor based on the CMG-40T.

CMG-5TB strong motion accelerometer

The CMG-5TB uses the mechanics of the CMG-5T in a compact borehole sonde.

CMG-3T/5TB complete borehole station

The CMG-3T/5TB combines weak- and strong-motion instruments in a single borehole casing, with a supplied downhole digitizer and networking options.

Digitizers and data modules

The CMG-DM24 digitizer, CMG-DCM communications module and CMG-AM authentication module can all be supplied in integrated borehole configurations, either as separate sonde sections with integrated strain relief (as left), or directly fixed to the sensor housing.

Using these components, you can build complete downhole solutions, using digital communications to ensure high-fidelity data recording from our range of broadband borehole seismometers.

For deep boreholes in particular, we recommend the use of a downhole digitizer, or digitizer/DCM with modem output, to minimize the length of the analogue signal cable. Borehole digitizers can be supplied with either RS232 or RS422 output interfaces.

Accessories

We can also provide winches and other equipment for your borehole installation, or a complete installation service.

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