Jim, Carl, Chris, and co. Regarding seismic low-pass filters vs urban noise: (I have touched on this previously:) For years we have found that an acceptable compromise for the low-pass filter for a seismic station in an urban setting is about 3 hz. We actually use 2.7 hz because it is the mechanical period of the Wood-Andersion torsional optical seismometer that has been used even recently in major stations because it is the actual basis for the Richter scale, with a static magnification of 2500 at 1 hz. By urban setting, I mean the St. Louis Univ. campus vault under the old gym on a pier that Macelwane built in 1923. It is within a km or so of freeways, railroads, and a foundry. And of course the gym and new recreation center aren't exactly quiet settings. Nonetheless, we are able to record M 3.0+ events at New Madrid, 200 km away. As Chris pointed out, 60 hz AC noise is well attenuated by a 3 hz filter. As for quake data, nearfield events (< 50 km), will arrive with higher frequencies, but still have a spectrum rich in frequencies below 3 hz, so are well recorded. With the smooth roll-off of the phase-linear Bessel response, 10 hz is not that far down. For some time I have posted the schematic of the seismic preamp and 4-pole filter that has been used in over 100 telemetry stations from Alaska to Greece. The filter design is from the NASA filter handbook (details were posted). The schematic shows options for frequency scaling, as for a 2.7 second "Wood Anderson Seismometer" response that works well in an urban setting with a 15-second long period seis like a Lehman horizontal as input. at: http://www.eas.slu.edu/People/STMorrissey/index.html stmmisc.html" PSN INFO ... SLU Seismic Network For the "station" here in my basement, about 10 meters from a street, I use a corner at about 2 hz for the broadband vertical just to keep the car/truck noise to less than 2 mm on the drum record. I am interested in teleseismic events (1 second to hundreds of seconds) anyhow. Even so, the latest M3.9 in NE Arkansas recorded clear P and S phases, with Lg about 10 mm. Unless you are running a tight (10 km spacing) network of stations to record micro- earthquakes (M<2), you don't need high frequencies. You also run into seismometer resonances (the famous L4-C gets very ringy at 16 and 22 hz), so even networks have peak responses at 10 to 20 hz, which still requires sample rates of 20 to 40 per second. The "standard" instrument for the new global network is the STS-1, which has a peak frequency response of 10 hz, so the VBB sample rate is 20 per second. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>