> From:    sean@...........
> Date:    Wed, 15 Nov 2000 01:28:05 -0600 (CST)

> The normal background noise for a seismometer like a Lehman are the
> 6-second microseisms, usually caused by storms off the east coast.
> Away from the immediate shore (100km) these run 2 to 4 microns peak-
> to-peak, but can be ten times that during a hurricane or a Nor'easter.
> For a seismometer sensitivity of 100 V/m/s, an amplifier with a gain
> of 100 is needed to raise the signal from the microseisms to about 20
> millivolts. This would provide about 100:1 signal to noise if the least
> count of a 10 volt, 16-bit digitizer is about 0.30 millivolts.

Here is some noise and signal data from my first Lehman which I
am still debugging.  It looks like the noise is peaked near 6 seconds as
you suggest.  Does this look OK to you?

 http://www.infiltec.com/seismo/nois-amp.gif         noise data
 http://www.infiltec.com/seismo/nois-fft.gif             noise spectra

 http://www.infiltec.com/seismo/qake-amp.gif         7.5 quake data
 http://www.infiltec.com/seismo/qake-fft.gif            7.5 quake spectra

I have increased the gain about 5x since I made the measurements
estimating a sensitivity of 120 nm/s/bit and my absolute noise
levels are up quite a bit, but it looks like I still have lots of adc
dynamic
range (unless the big one hits VA).

My system sits on the basement slab of a VA office building about
100 feet from an continuously operating HVAC unit, with a major
road about 100 ft on one side, and a McDonalds drive through about
50 ft away on the other.  During the day the record shows many spikes
similar to what happens when someone walks near the pendulum and
deforms the slab. I get some pretty quiet data in the middle of the night
when the Big Macs go home.

My pendulum pivots on a ball bearing resting on a glass plate, and
it seemed pretty easy adjust it for an 18 second period. My home brew
electronics board only has 3 chips: one low noise opamp, one switched
capacitor 8 pole bessel filter chip, and one PIC microprocessor
with built-in 16 bit adc and serial output ports.  I run a serial cable to
my PC directly from the mpu, and system power (~3ma) is drawn from
the serial port.  I log the serial data at 16 sps with the Amaseis freeware
which can produce files for input to WinQuake. My data logging
computer is an old P90 that simultaneously runs SETI@HOME and
is used for other odd jobs, so Amaseis does not seem to be using
much of its resources.

Caio,

Dave Saum

__________________________________________________________

Public Seismic Network Mailing List (PSN-L)