Chapter 5. Physical installation

The goal of any seismic installation is to ensure that wave-trains arriving at the instrument accurately reflect the internal motion of sub-surface rock formations. To achieve this, the seismometer and its emplacement need to be considered as a mechanical system, which will have its own vibrational modes and resonances. These frequencies should be raised as high as possible so that they do not interfere with true ground motion: ideally, beyond the range of the instrument.

In particular, the sensor needs to be protected against environmental factors such as:

• fluctuations in temperature,

• turbulent air flow around walls or trees, or around sharp corners or edges in the immediate vicinity of the sensor;

• vibration caused by equipment in or near the installation, particularly computer equipment; and

• vibration caused by heavy machinery (even at a distance), or by overhead power lines.

In seismic vaults, instruments are often installed on piers. It is important to ensure that the interface between the pier and the floor does not introduce noise, and that the pier itself does not have resonant frequencies within the passband. Ideally, a seismic pier will be significantly wider than it is high (to minimize flexing) and will form a single piece with the floor, e.g. by moulding a poured concrete floor within a wooden frame.

Many situations do not allow for the construction of a seismic vault. For example, you may need to deploy quickly to monitor the activity of a volcano showing signs of rejuvenation, or to study the aftershocks of a major earthquake. In other cases the site itself may be too remote to ship in construction equipment.

Temporary installations can be protected against spurious vibrations by:

• selecting a suitable site,

• placing the instrument in a protective enclosure (an open-sided box of 5 cm expanded polystyrene slabs, placed over the instrument and taped down to exclude draughts, makes an excellent thermal shield),

• standing the sensor on bedrock where possible, or at least deep in well-compacted subsoil;

• clearing the floor of the hole of all loose material; and

• using as little extra mass as possible in preparing the chamber.

After installation, the instrument case and mounting surface will slowly return to the local temperature and settle in their positions. This will take around four hours from the time installation is completed. Note that the Certis will need to re-centre its masses during this period – possibly more than once. The process is automatic but may show up in the recorded data.

5.1 Vault deployments

You can install a Certis into an existing seismic vault using the following procedure:

• Unpack the Certis from its packaging, which should be saved for later use.

• Prepare the mounting surface, which should be smooth and free of cracks. Remove any loose particles or dust, and any pieces of loose surfacing. This ensures good contact between the instrument's feet and the surface.

• Inscribe an accurate North-South line on the mounting surface, if it is not already present.

• Place the sensor over the scribed line, so that the North pointer on the lid is aligned with the marked directions. This can be done by rotating the base of the sensor whilst observing it from above.

• Check that the Certis is level using the bubble indicator built into the top of the instrument. The bubble should be entirely within the black circle printed on the viewing window:

  

  If the instrument is not perfectly level, try moving it to a slightly different position, either along the scribed line or from side to side until a position is found where it is naturally level. Re-check the North/South alignment after moving the instrument.

• If the instrument cannot be levelled in this way, either

• • the pier surface can be re-shaped with an angle-grinder to create a level area; or

  • the resulting data can be rotated mathematically using an appropriate digitiser, such as the Güralp Minimus family

• Connect the sensor to the digitiser.

  Caution: The locking ring on the connector should by tightened by hand only. Do not use tools.

• Cover the instrument with thermal insulation – for example, a 5 cm expanded polystyrene box. This will shield it from thermal fluctuations and convection currents in the vault. It also helps to stratify the air within the seismometer package. Position the thermal insulation carefully so that it does not touch the instrument casing. The cable entry should be at the base in order to reduce transmission of vibrations to the instrument along the cable.

5.2 Post-hole deployments

In areas with non-trivial cultural noise, a post-hole deployment can provide significant benefits over a surface deployment. Post-holes can be used both for rapid, temporary deployments and for long-term stations.

5.2.1 Rapid deployment

The Certis is an ideal instrument for use in emergency rapid deployments for purposes such as aftershock monitoring or volcanology. It can be deployed with the minimum of preparation and tools.

A suitable hole can be drilled using a hand-portable, petrol-powered auger. A diameter of 150 mm is suggested although smaller diameter holes are usable. The depth must be less than ten meters. If possible, pour a little loose sand into the bottom of the hole and tamp it down to form a firm base.

Connect a suitable steel wire to the instrument’s lifting shackle. The wire should be long enough to reach the bottom of the hole plus an additional 5 m to allow for the use of a winch to remove the instrument at the end of the deployment, if necessary.

If possible, provide a long, wooden pole with a J-hook on the end for manipulating the instrument when it is at the bottom of the hole. An endoscopic camera with built-in LED illumination, of the type that connects to a mobile phone, can be taped to the pole near the bottom end.

Connect the sensor cable to the instrument, hook the pole to the instrument’s lifting shackle and lower it into the hole, maintaining tension on the suspension cable in case the hook slips. Using the pole and camera, rotate the instrument until the arrow on the top surface is facing North. A small laser fitted with a diffraction grating, as sold in D.I.Y. stores, can be used to project a line to the bottom of the hole, which can help with alignment.

Once aligned, manipulate the instrument until it is level, as indicated by the bubble on the top of the instrument. The bubble should be entirely within the printed black circle:

Without disturbing the instrument, unhook the pole. Check with the camera that the instrument is still level and correctly aligned before fully withdrawing the pole.

Gently pour a small quantity of sand into the hole: enough to just cover the instrument. This protects the Certis from convection currents and improves its coupling to the underlying bedrock.

Close the top of the hole with a rock or other suitable cover. Lead the sensor cable and the excess wire rope a short distance from the hole to where the power supply and digitiser can be located, using a shallow trench if desired. 12 V or 24 V vehicle batteries can be wrapped in plastic and buried to provide power for short- and medium-term deployments.

When the instrument is to be removed, do so by pulling on the wire rope only: never pull on the sensor cable. As the inside of the post-hole may have partially collapsed, considerable force may be required. A portable tripod winch is recommended for this purpose.

5.2.2 Permanent post-hole deployments

Post-holes make suitable vaults for permanent deployments. Many techniques are possible but the following description makes a good starting point.

Auger a 200 mm (8“) hole to the desired depth; up to 10 m maximum. Prepare an appropriate length of 150 mm (6”) PVC pipe, joining multiple sections with solvent-weld if necessary. Lower the pipe into the post-hole and insert a suitable grout into the space between the pipe and the surrounding earth. If the pipe floats, weigh it down until the grout has set.

Pour some bentonite or Fuller’s earth into the bottom of the pipe to provide a waterproof seal and tamp it down well. Add a layer of sand on top of the bentonite and tamp it down to provide a suitable surface for the instrument.

Remove the earth around the top of the pipe and provide a suitable well-head enclosure, either using brickwork, concrete form-work or a pre-fabricated steel box. In all cases, provision should be made to add a lockable lid. Fit a water-proof tee-piece near the top of the pipe, inside the well-head enclosure, to allow the sensor and suspension cables to exit through a 50 mm (2”) PVC pipe or metal conduit. This can be just below the level of the surrounding ground to allow burial of the cables to the power supply and digitiser enclosure.

Provide a wooden or aluminium pole with a J-hook on the end for manipulating the instrument when it is at the bottom of the hole. An endoscopic camera with built-in LED illumination, of the type that connects to a mobile phone, can be taped to the pole near the bottom end.

Provide a steel suspension wire with eyelets on both ends. Its length should be the depth of the borehole plus at least an additional 5 m.

Thread the sensor cable and the steel wire though the tee-piece, from the outside into the pipe, and attach both to the Certis, hand-tightening the cable connector and using a shackle to attach the suspension cable to the lifting hook. Hook the pole to the instrument’s lifting shackle and lower the Certis into the pipe, maintaining tension on the suspension cable in case the hook slips. Using the pole and camera, rotate the instrument until the arrow on the top surface is facing North. A small laser fitted with a diffraction grating, as sold in D.I.Y. stores, can be used to project a line to the bottom of the pipe, which can help with alignment.

Once aligned, manipulate the instrument until it is level, as indicated by the bubble on the top of the instrument. The bubble should be entirely within the printed black circle:

Without disturbing the instrument, unhook the pole. Check with the camera that the instrument is still level and correctly aligned before fully withdrawing the pole.

Gently pour a small quantity of sand into the pipe: enough to just cover the instrument. This protects the Certis from convection currents and improves its coupling to the underlying bedrock.

Seal the top of the PVC pipe with a suitable locking plug – this prevents moisture ingress – and lock the well-head cover to provide security.

If the power supply and digitiser enclosure are to be above-ground, take care that they do not function as an inverted pendulum. A simple steel box mounted on a single stake will have a strong natural resonance which will couple directly to the earth near the instrument. Depending on the location of the enclosure (and, in particular, the GNSS receiver if used) consider providing lightning protection for the installation.