Lucky Mountain (LMMT), like the other stations in this area, is located at the boundary of the playground area of a primary ground. These sites made working conditions generally comfortable.
The Borehole at LMMT was quite good, there were no problems running the baler down to test the hole depth. There is a pore-pressure instrument installed into a hole about 7 m from our own hole, and at the time, this hole was producing water. However the GTSM borehole was not. The pore-pressure meter is buried at a depth less than 100m, whereas the GTSM instruments is at a depth of 166m.
Lucky Mountain has not been such a lucky site. It has received 2 lightning strikes to date. The first strike caused damage to our instrument, as well as taking out the entire phone system throughout the school. Analysis of the first strike revealed that the pore-pressure meter electronics deployed after the main deployment team had left had been connected to the GTSM instrument battery. This is most likely the cause of the electronics failure. When electronics are connected together to the same battery they share a common negative voltage reference. Actual grounding of the GTSM instrument occurs at the bottom of the borehole. In the event of a nearby lightning strike, the vertical voltage profile produced by the strike depends on depth. With instruments grounded at different depths, the voltage potential between the two instruments becomes very large. If they are connected uphole in the measurement systems, destructively large current flows are induced. This is summarized in here.
It is essential that all other systems have their own power supply, and own battery. If this procedure is followed, the GTSM system has protection which will minimize damage on all but the most direct lightning strikes.
During the site visit in 2006, it was observed that there was a “bubbling” noise coming from an opening in the ground approximately 4m from the borehole. The bubbling was a result of water rising to the surface.
GTSM at Hsinchu Region, Taiwan
Lucky Mountain (LMMT)