Creating low noise environments for surface seismometers

General notes

For the best possible results, a seismometer should be installed on a seismic pier in a specially-built vault, where conditions are near perfect. Here, wave-trains arriving at the instrument reflect very well the internal motion of subsurface rock formations. However, this is not always feasible. For example,

  • instruments may need to be deployed rapidly, perhaps to monitor the activity of a volcano showing signs of rejuvenation, or to study the aftershocks of a major earthquake;
  • installations may be required in remote locations, or otherwise in circumstances where it is unfeasible to build a vault.

In these situations, 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. This is done by

  • 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.

In temporary installations, environmental factors are also important. The sensor needs to be well protected against

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

This can be done by selecting a suitable site, and 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.

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. If you require long-period recording, you should re-zero the instrument after this time.

Installing in vaults

You can install a sensor in an existing seismic vault with the following procedure:

  1. Unpack the sensors from their container, saving the shipping boxes for later transportation.
  2. 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.
  3. If it is not already present, inscribe an accurate North-South line on the mounting surface.
  4. Place the sensor over the scribed line, so that the brass and steel pointers are aligned with the marked directions, with the brass pointer facing North. This can be done by rotating the base of the sensor whilst observing it from above. The brass pointer can be found next to one of the feet.3T-stud-and-serial-number

    If you cannot easily see the pointers, you should align the sensor using the north arrow on the handle. However, the alignment of the handle with the sensors inside is less accurate than the metal pointers, so they should be used wherever possible.

  5. The top panel of the sensor includes a spirit level.
    3T-bubble-level

    Level the sensor by adjusting each of the three feet of the instrument in turn, until the bubble in the spirit level lies entirely within the inner circle. (The instrument can operate with up to 2 ° of tilt, but with reduced performance.)

    To adjust the height of a foot, loosen the brass locking nut and screw the foot either in or out. When you are happy with the height, tighten the brass locking nut to secure the foot. Gently check the foot to ensure that it is locked.

  6. Connect the sensor to the breakout box, or a Guralp digitiser if you are using one.The remainder of these instructions assume you are using a breakout box. The Guralp DM24 digitiser allows you to access all the functions of the seismometer through the digitiser instead; you should refer to its documentation for more details.
  7. Connect a 12 V power supply to its POWER socket through the breakout box.
  8. Check the mass position outputs using a digital multimeter, digitiser or the handheld control units. If required, re-centre the masses by pressing the ENABLE and CENTRE buttons on the breakout box together.
  9. 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 in the seismometer package. Position the thermal insulation carefully so that it does not touch the sensor package.
    polystyrene-shield
  10. Ensure that the sensor cable is loose and that it exits the seismometer enclosure at the base of the instrument. This will prevent vibrations from being inadvertently transmitted along the cable.

Installing in pits

This is the simplest method of providing an instrument with a suitable environment for high-quality seismic signals.

pit-cutaway-bedrock

Depending on the time and resources available, this type of installation can suit all kinds of deployment, from rapid temporary installations to medium-term telemetered stations.

Ideally, the sensor should rest directly on the bedrock for maximum coupling to surface movements. However, if bedrock cannot be reached, good results can be obtained by placing the sensor on a granite pier on a bed of dry sand.

  1. Prepare a hole of 60 – 90 cm depth to compacted subsoil, or down to the bedrock if possible.
  2. On granite or other hard bedrock, use an angle grinder to plane off the bedrock at the pit bottom so that it is flat and level. Stand the instrument directly on the bedrock, and go to step 6.
  3. On soft bedrock or subsoil, you should install a pier as depicted below.
    pit-cutaway
  4. Pour a layer of loose, fine sand into the pit to cover the base. The type of sand used for children’s sand-pits is ideal, since the grains are clean, dry and within a small size range. On top of the sand, place a smooth, flat granite plinth around 20 cm across, and shift it to compact the sand and provide a near-level surface.
    granite-plinth

    Placing a granite plinth on a sand layer increases the contact between the ground and the plinth, and improves the performance of the instrument. There is also no need to mix concrete or to wait for it to set, as in step 4.

  5. Alternatively, if time allows and granite is not available, prepare a concrete mix with sand and fine grit, and pour it into the hole. Agitate (“puddle”) it whilst still liquid, to allow it to flow out and form a level surface, then leave to set. Follow on from step 6.Puddled concrete produces a fine-textured, level floor for emplacing the seismometer. However, once set hard, the concrete does not have the best possible coupling to the subsoil or bedrock, which has some leeway to shift or settle beneath it.
  6. Alternatively, for the most rapid installation, place loose soil over the bottom of the pit, and compact it with a flat stone. Place the seismometer on top of this stone. This method emulates that in step 3, but can be performed on-site with no additional equipment.
  7. Set up the instrument as described above, in steps 4 to 9.
  8. The instrument must now be shielded from air currents and temperature fluctuations. This is best done by covering it with a thermal shield.An open-sided box of 5 cm expanded polystyrene slabs is recommended. If using a seismic plinth on sand (from steps 3 or 5), ensure that the box is firmly placed in the sand, without touching the plinth at any point. In other installations, tape the box down to the surface to exclude draughts.
  9. Alternatively, if a box is not available, cover the instrument with fine sand up to the top.The sand insulates the instrument and protects it from thermal fluctuations, as well as minimizing unwanted vibration.
  10. Ensure that the sensor cable is loose and that it exits the seismometer enclosure at the base of the instrument. This will prevent vibrations from being inadvertently transmitted along the cable.
  11. Cover the pit with a wooden lid, and back-fill with fresh turf.

Other installation methods

The recommended installation methods have been extensively tested in a wide range of situations. However, past practice in seismometer installation has varied widely.

Some installations introduce a layer of ceramic tiles between a rock or concrete plinth and the seismometer:

pit-installation-with-tiles

However, noise tests show that this method of installation is significantly inferior to the same concrete plinth with the tiles removed (right). Horizontal sensors show shifting due to moisture trapped between the concrete and tiling, whilst the vertical sensors show pings as the tile settles.

Other installations have been attempted with the instrument encased in plaster of Paris, or some other hard-setting compound:

installation-in-plaster-of-paris

Again, this method produces inferior bonding to the instrument, and moisture becomes trapped between the hard surfaces. We recommend the use of fine dry sand (right) contained in a box if necessary, which can also insulate the instrument against convection currents and temperature changes. Sand has the further advantage of being very easy to install, requiring no preparation.

Finally, many pit installations have a large space around the seismometer, covered with a wooden roof. Large air-filled cavities are susceptible to currents which produce lower-frequency vibrations, and sharp edges and corners can give rise to turbulence. We recommend that a wooden box is placed around the sensor to protect it from these currents. Once in the box, the emplacement may be backfilled with fresh turf to insulate it from vibrations at the surface, or simply roofed as before.

By following these guidelines, you will ensure that your seismic installation is ready to produce the highest quality data.

installation-complete

 

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