Guralp Systems Limited
MAN-BHO-0001 Güralp 3TB Operator's Guide

Chapter 4. Installing the 3TB in a borehole

Before installing any instrument in a borehole, you should prepare the installation site.

Most installations are equipped with a strain-relief unit, which consists of a metal arm that swings out from the load-bearing cable to wedge against the side of the borehole. This removes any strain in the load-bearing cable and prevents vibrations from the surface from being transmitted to the instrument.

Note: In installations with a down-hole digitizer, the strain-relief arm is fitted to the base of the digitizer sonde; the phrase “strain relief unit” in the following instructions should be taken to refer to the digitizer's strain relief arm.

Note: In installations with a surface digitiser, the strain-relief arm is built into the surge suppression sonde; the phrase “strain relief unit” in the following instructions should be taken to refer to the surge suppressor's strain relief arm.

4.1 Installing a sensor with hole lock unit

4.2 Installing a sensor using sand backfill

Dry sand backfill is a convenient and effective way of installing a borehole or post-hole 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 300 mm 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 30 mm 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:

3TB-sand-backfill-1     3TB-sand-backfill-2     3TB-sand-backfill-3     3TB-sand-backfill-4

4.2.1 Procedure

To install a sensor at the bottom of a borehole of known depth using sand backfilling:

4.3 Assembling the winch

If required, Güralp Systems can provide a winch suitable for installing a borehole sensor. The winch and tripod are supplied as a set of parts which you can assemble on site. The precise model supplied may vary so please use this section as a guide only and, if in doubt, contact Güralp System technical support (via an email to <a "target="_blank" href="support@guralp.com">support@guralp.com) for advice.

winch-unpacked

There are two sections for each leg of the tripod. The upper sections are pre-attached to the head of the tripod; the lower sections are supplied detached.

4.4 Earthing a borehole sensor

To achieve the best performance from any borehole instrument, you must make sure that the sensor electronics, its casing and the power supply share a common, local ground, and that all power and data lines are adequately protected against lightning and other transients.

This section describes techniques for grounding sensor equipment which have proved effective in many installations. However, local conditions are always paramount, and you should design your installation with these in mind. Any regulations in force at your chosen location must also be followed.

4.4.1 Installations with AC power supplies

If you are using mains (outlet) power, or some other AC power distribution system, we recommend installing a fully isolating transformer between it and the power supply for the instrument. This will allow full control of the local ground.

A spark-gap surge protector should also be installed on the mains side of the transformer, so that transient over-voltages are not transmitted across it. Suitable protectors are available off the shelf from several suppliers. On the sensor side, surge protection is installed as standard within all new Güralp borehole sensors and control equipment. If your surface installation includes third party electronics, digitizers, etc., you may need to install additional protection where power and data lines enter the surface enclosure. Contact Güralp Systems if you are unsure.

Within the installation, a single ground point should be established, which is connected to a local ground plate. All earth lines for equipment in the installation, such as the casings of the transformers and of the sensor electronics, as well as the signal ground line from the sensor, should be connected to this plate.

The best local earth point in many installations is the borehole itself. For this to work, the borehole must have a conductive casing and be situated close (<30 m) to the surface installation. In such an installation you need only connect a cable (green wire in the photograph below) from the local ground plate to the borehole casing.

borehole-casing-grounded

An earth strap can be used to ensure a good connection.

If the lower borehole is filled with salt water, the instrument will be adequately grounded without any further action. Fresh water is an inferior conductor.

In a dry or sand-filled borehole, or one with a non-conducting casing, you will need to ensure the sonde is grounded by some other means. The best option is often to attach the sensor housing to an earth line brought out to the surface and attached to a metal stake driven into the ground nearby.

The sensor's load bearing cable is suitable for this purpose, provided it is secured to the sensor's lifting loop with a metallic clamp as shown below. This provides an additional firm electrical contact between the sonde and the load-bearing cable.

If your system has a separate surge-suppressor sonde, this technique should be used to ground the load-bearing cable to the surge-suppressor. A separate grounding cable must then be provided between the surge-suppressor and the sensor. The short, intermediate load-bearing cable between the two units can be used for this purpose.

Installations with down-hole digitizers will need similar arrangements at the top and bottom of the digitizer module, or a separate cable for this purpose.

For boreholes with a metallic casing at the bottom and plastic above, we recommend connecting a cable between the sensor housing and the ground plate so that the lower borehole casing acts as the earthing point.

If there is a significant distance (>30 metres) between the borehole and the surface installation, the resistance of the earth cable may make it impractical to use the borehole as an earthing point. In these cases, you will have to connect the local ground plate to an earth stake near to the enclosure; any coupling between this sensor-local earth line and ground lines for other parts of the system must be minimized.

4.4.2 Installations with DC power supplies

Güralp sensors require a 12 to 36 V DC power supply. In most cases, this is provided by an isolating DC/DC converter installed at the surface. This converter can be earthed to the local ground plate as above.

However, DC/DC converters contain sensitive electronics, which must be protected thoroughly. We recommend installing a full surge protection unit in addition to the spark gap protector. This protection is installed on the supply side of the isolator, so it must be earthed separately from the borehole installation. Otherwise, transients in the power supply will couple to the sensor.

As with AC installations, if the borehole is more than around thirty metres from the surface enclosure, you will need to provide a second earthing point for the local ground plate.

DC power is most commonly available at self-contained installations with power supplied from batteries, solar panels, or a wind generator. In these cases, the power supply may already have protection from transients installed, in which case you may not need such comprehensive protection (although some form of protection is always necessary.)

4.4.3 External lightning protection

The surface installation building and, if possible, the borehole should both be protected by lightning conductors.

These should lead to grounding point well away from the borehole. As a rule of thumb, a lightning mast provides a “zone of protection” within a 45° cone descending from the top of the mast.

If you are using two earthing points, for example in the DC installation shown above, it may be convenient to connect the lightning conductor to the supply-side earthing point. In any case, the lightning earth must be well separated from the borehole (and its earth, if it needs one.)

4.5 Levelling and Centring

Once it is installed, you should level and centre the instrument ready for use. This can be done using the various surface control units:

4.6 Down-hole orientation

Once the sensor is installed inside the borehole, you will need to measure its orientation with respect to True North or Magnetic North. There is no need to rotate the sensor itself, since the data can be rotated algorithmically after it is digitised.

Note: The most common problem affecting sensor orientation is the difficulty of determining an accurate North at the installation site. Local variations in the Earth's magnetic field affect magnetic compasses, as do many local structures, including including the metal bore-hole casing itself.

A simple method for determining the orientation of a sensor package using the sensor's own horizontal component sensors, has been used effectively by the Blacknest Seismological Centre, UK, with down-hole and surface equipment from Güralp Systems (AWE Report O 10/93, 1993.)

In this experiment, signals received by the N/S component of the reference sensor are correlated with those received at the N/S and E/W components of the sensor being studied, after different amounts of mathematical rotation. The highest correlation will occur when the N/S component of the reference sensor matches the rotated N/S component of the borehole sensor.

Once you know the deviation of the borehole components, you can instruct the digitizer to rotate the signals algorithmically.

4.6.1 Installing the Scream! extension

The Relative Orientation extension is supplied in the standard Windows distribution of Scream.

The extension uses Matlab libraries, which are currently only available for Windows. However, you do not need the full Matlab package to use the extension. The Matlab runtime libraries are installed as part of the Scream! distribution.

4.6.2 Installing the reference instrument

To measure the orientation of a sensor, you will need a second instrument which is known to point precisely North. We recommend the use of a Güralp H3, a single-component horizontal instrument with an elongated "North" pointer. This is supplied as part of our Borehole Orientation Kit. The instrument should be located on a solid surface as close to the other instrument as possible. Most boreholes are constructed with a concrete base around the top of the borehole; If this is present, we recommend installing the reference sensor there. A good coupling to bedrock is, however, more important than proximity so, if there is a nearby seismic vault or basement, you may get better results installing the reference sensor there.

Ideally, the two sensors will be directly connected to the same 6-channel digitizer.

Note: If you are using separate digitizers, you will need to ensure they are exactly synchronized. This can be done by connecting GPS receivers to both digitizers and waiting for the control system of each one to settle. This process takes at least 12 hours.

Record at least an hour and, ideally, 24 hours of data from both the reference sensor and the borehole sensor. Try to pick a period when local, cultural noise is at a minimum: the procedure works best with teleseismic data.

4.6.3 Measuring the orientation

The Blacknest orientation method generally provides a reliable indication of the sensor's orientation. In most cases, the greatest source of error is in the installation of the reference sensor.

If you have particular difficulty in deriving a stable value, additional information is contained in a third output window, which shows a waterfall plot of Coherence vs Frequency vs Angle. The frequency range used in the calculations is indicated with a black outline.

This plot can be used to select a more advantageous frequency range where the coherence curve is smoother and easier to interpret. In the example above, the frequency range 0.23 to 0.6 Hertz looks particularly promising: the surface within this range forms a smooth, symmetrical arch. Ideally, the peak should be very close to unity and the lowest points, at the margins, should be close to zero.

The chosen frequency range can then be entered into the Between X and Y Hz boxes at the top of the Coherence vs Angle window before clicking again; the data will be filtered accordingly before the coherence is recalculated, increasing the accuracy of the result.

4.6.4 Applying automatic rotation

You can configure a DM24 mk3 digitizer to apply an automatic rotation to the digitized data and output streams representing ground motion on true North/South and East/West axes.

This is done within the DSP to minimize the reduction in data quality.

To set up the rotation: