Jim, What a lucky man; its too bad that all your Geotech LP seises are not verticals. The vertical data is much more interesting, since the horizontal generally shows very little of the P-wave. Also, the pier/site tilt problem is much greater for a horizontal, which will affect where you put them. The LP seises are good candidates for converting to a VBB instrument if the coil resistance is low enough. I don't know the parameters of the Geotech units, but a coil resistance of more than a few hundred ohms, when used as the feedback coil, will be unstable. You may have to wind new coils, or find a way to tap the existing winding. One of the first LP sensors that I converted to a VBB is the old Sprengnether 5100 of the WWNSS design: 15 seconds To, 500 ohm coils, and generator constant Gn of 83 N/A. The high coil Rf works in the transfer function because Gn is high as well as the mass is relatively large at 11.2 kg. I use an integrator at 400 seconds with the other parameters of RI = 402k, Cf = 12.29uf , Rp = 2.1 megohm, for a Tn of 280 seconds; Rf is actually 469 ohms, and r is 612900 volts/meter from the LVDT. This gives a velocity output of 11000 Volts/meter/second. I couple the output through a 3000 second high-pass filter to the display recorder in the lobby of our building. It has been operating this way since 1996. The very long period response readily shows 30 to 60 second barometric noise, especially when tornado weather is in the area. If you are going to convert a number of units to a VBB system, we need to make some headway on getting the circuit boards available. I have laid out new boards (voltage regulator, oscillator, VRDT amp/demodulator, and the quad amp board) for the new instrument, but I have not had any actually produced yet. When that happens, I can make a set available at cost, and then maybe Larry can assemble them into a working system. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>