Example: seismicF program

This is an example program which produces power spectra of seismometer ground motion. It is intended for use with a Guralp CMG-40T Broadband Seismometer http://www.guralp.demon.co.uk/ http://www.guralp.demon.co.uk/, with the velocity sensing output set for the $0.033\rightarrow100$Hz band and the gain set to 400 volts-sec/meter. The normalization constants are defined with a line that reads something like this.

const float norm=((0.25/65536.0)*(1.0/400.0)*(1.0/(2.0*M_PI)));

Here the constants assume that the ADC that the seismometer is connected to records 65536 ADC counts per $0.25$ volts input and that the gain (single-ended) is set to 400 volts-sec/meter. The $2\pi$ and a factor of frequency $f$ arise in the code in converting velocity to position. The program outputs postscript and/or jpeg files labeled by GPS time.

Figure: A seismometer (one-sided) power spectrum produced by the seismicF example program. The x,y, and z (vertical) motion are shown in black, red, and green. The blue curve shows the LIGO standard noise power spectrum, for comparison. This spectrum was taken at the LIGO Hanford site, during the installation. The portable clean rooms may be responsible for much of the excess noise. The file name is Spec.610452023.ps