The solar panel pole is supported by two star pickets driven into the ground with wire ties passed through holes in the pole to prevent twisting. The solar panel maounting sits on the top of the pole along with the GPS antenna. The cables are brought down the face of the pole, secured with tapes and then protected with a third star picket secured with two hose clamps. The connections are bound to prevent ingress of water. The solar panel should be 8 metres or more from the seismometer pit.
The power cable from the solar panel and GPS cable are brought down the pole and then run in a shallow trench extending from the solar panel to the instrument package and then on to the seismometer pit. This trench is usually started with a pick but may need to be dug deeper.
The trench may need to have the cables snaked back on each other (depending on length). The sections of the cables nearest to the instrument package are protected with plastic tubing. The seismometer cable is similarly protected to the lip of the seismometer pit.
The trenches are back filled to ensure coverage and protection of the cables. it is important to make sure that the power and GPS cables are well protected as they enter the ground, either with extra dirt or with stones etc. Slabs of flat stone, when available, provide a useful protection of the cables.
The Tasman Line experiment uses 4 types of seismometers whose response
is flat to ground velocity of a broad band in frequency:
Streckeisen STS2 :- triaxial seismometer, very sensitive to tilt
Guralp CMG-3ESP :- with corner at 0.03 Hz, ESP+ corner at 0.015 Hz
- tilting causes problems on horizontal components
Guralp CMG-40T :- less sensitive and smaller
Nanometrics Trillium :- triaxial design
STS-2
The emplacement of the seismometer is a critical part of the site development. As far as possible the seismometer pit should not be close to trees, and best results are achieved when the ground is not soft, even though this makes digging the pit more difficult. The hole needs to be excavated to a depth about 20 cm greater than the height of the seismometer to allow room for the base plate and plastic cover.
The seismometer needs to be aligned so that the horizontal components are oriented NS and EW, the details vary with the instrument but a siting bar set up with compass bearing usually enables a field precision of 2 degrees or better. The correction for magnetic north is important and needs to be applied in the correct sense.
The cable configurations differ between the different models, but in each case care need to be taken to have a clean entry to the seismometer so that the plastic cover can be put on with good contact with the base plate. Centering of the mass requires a seismometer specific approach.
Once the seismometer is in position and the channels have been tested through the recorder (with gentle ground movement) the plastic cover is put over the top to provide mechanical protection and thermal insulation. For the STS2 an extra wrap of insulation goes over the seismometer before the cover is applied. Backfilling of earth around the seismometer cover needs to be carried out gently and the material should be built up above the original ground surface.
In some circumstances it may be necessary to have the seismometer cover protuding above the ground surface (e.g. shallow hard rock). The mound should then be carefully shaped to minimise wind interaction.
Earth Data
Nanometrics Orion
