How To: properly build up a seismic station
During our last fieldtrip we finally installed all of seismographs in the field. In this blog I want to focus on the installation of such a station itself. I already (un)installed seismic stations before. The number of installed stations by my supervisor, who joined the trip, is countless. But both of us learned a lot the last three weeks… from our local guide. In an earlier post I already described the necessity of local guides, but this time we appreciated his accompany a lot again.
The most important thing is planning. To set up a seismic network (we have 13 single stations) you have to search for a nice place for them. Seismometers are very sensitive instruments to detect very small shaking of the ground. Hence, you should go far away from sources who produce shaking. Mostly anthropogenic noise influences our measurements the most. For example traffic: big trucks on bad paved highways or freight trains makes a lot of noise and you should move away from them some kilometers. But often (we have the problem on one station) you will enter cropping fields when you go away from roads and villages and then you have even bigger and louder machines than on the highway. Pumps, generators or other engine like machines: all these things are producing a lot of noise in a similar frequency bandwidth in that we record earthquakes (actually we only record the seismic waves generated by earthquakes). It’s the same when you are sitting in a car and starting the engine, it will make your car chassis vibrating. Humans and animals at all: try to avoid cows!!! They will kill you… actually no. But they can destroy a seismic stations by just stepping on them (in Argentina they are walking around everywhere and weight a lot) or they start to chew on the cables because of the plasticizers in them. But all animals including humans are always bad seismic sources, because you can record the steps over about a hundred meters. Avoid civilization at all also can be risky for security reasons. It’s nice to have someone around to have an eye on the instruments. In very very rural locations it’s helpful to contact locals to be sure nobody steals your solar panel. When you dig several thousands of Euros into the ground and mark them with a metallic plate, you cannot avoid the illegal hunters who maybe damage your stuff or event rob it. A lot of things to keep in mind and also in the end, all positions you found should build up a nice network. That means you have to take care that the interstation distances are the same for all surrounding stations. This will give you the opportunity to better locate the epicenter (good azimuthal coverage of the network) and the depth (one station as close as possible to the epicenter).
2.) Dig a hole
Sounds easy. And often it is quite easy to make a 40cm deep cylindrical hole. But we seismologists prefer to place our seismometers directly on rock. This gives you a better coupling of the instrument to the ground, that means the energy from the seismic wave are better transmitted into the instrument and bedrock often provides a better frequency content in the signal. Digging a hole in bedrock is much more exhausting, but when you look at the GIF of our local guide it looks very easy to “explode” the bedrock.
3.) Place the instrument in the hole
We use 3-component seismometers. These instruments consists out of three orthognal pendulums to record the movement in vertical and both horizontal directions. Therefore you need to level the instrument properly in the hole (Z-component) and orient it towards north (N- and E- component). They are completely water resistant and we use the plastic bags only to keep them a little bit clean and to avoid earthing of the electrified instrument.
4.) Connect everything
A seismometer is an electrical instrument and needs power supply. This is preserved by a lead battery which is recharged by a solar panel. To transform a seismometer station into a seismograph you need a recording device, too. We use a Cube³ digitizer/datalogger for that purpose. The seismometer output is a voltage (depending on the strength of shaking the output-voltage increase) and has to be transferred into a digital value (digitizing) to be stored somehow (datalogger). Thanks to the internal storing of the data on that low power-consuming device, the system runs fully automatically for up to 6 months.
5.) Protect the station
The number one reason for protection are animals like cows, dogs or some wild animals. This can be done in a “nice” and time consuming way with a fence (we just did it once because the owner asked us to do it like that), with a simple and effective solution of branches with spikes (that avoids also humans) or with planting “agaves” around the station. Especially the last solution was new for us. It was a little bit funny when our guide started to dig out the plants and place them around the station. In the end he put some soil on the roots and promised to us the will grow on their new places. Let’s see in November.