Markus Eisel, Gary Egbert College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331.
In the Fall of 1995, UC Berkeley installed magnetotelluric (MT) observatories at two locations along the San Andreas Fault in California to monitor possible changes in the electromagnetic fields before, during and after earthquakes (e.g. Fraser-Smith, 1994). Since then MT data have been recorded continuously and archived at the UC Berkeley Digital Seismic Network (BDSN). This data set provides the rare opportunity to investigate long term variations of MT parameters at short periods.
The MT observatories are located at Parkfield (PKD1), 300 km south of the San Francisco Bay area (Figure 5.1), and Hollister (SAO), halfway between San Francisco and Parkfield. Both sites are equipped with 3 induction coils and 2 x 100 m electric dipoles connected to a Quanterra digitizer. Time synchronization is provided by GPS. The data are sent to BDSN via telephone lines.
Five components of the electromagnetic fields are recorded at 40 Hz and 1 Hz sampling rates. The multiple station processing code (Egbert, 1997) is used for the time series analysis. The continuous data stream is subdivided into daily sets from which MT transfer functions (TF) and interstation magnetic TF are estimated to study daily and seasonal variations. The 1 Hz data covers the years 1996 and 1997. Due to system outages 588 of the 731 days of data are available. (Figure 5.2)
Sections of signal power (Figure 5.3), noise power (Figure 5.4) and signal to noise ratio (Figure 5.5) of Hy at site PKD1 have been derived from the daily averages of the 1 Hz data. The sections cover the time 1/96 to 12/97 and clearly show a strong seasonal variation. The other field components and those at site SAO show the same seasonal variations.
During the summer, short period (1 - 10 s) signal and SNR are increased while at long periods these values tend to decrease in correlation with the magnetic activity indices. For comparison the daily sum of three-hour Kp indices is plotted (Figure 5.6). The black line gives the median filtered values which emphasize long term variations. A general decrease of magnetic activity from 1996 to 1997 visible in the magnetic indices plot is reflected in the MT data as well.
Apparent resistivities and phases and interstation magnetic transfer functions were calculated for 7 days during the night-hours (0am - 4am PST) and 7 days during rush hour (6am - 10am PST) during the Bay Area Rapid Transit (BART) strike from day 251 - 257 of 1997. To emphasize the bizarre nature of the bias effects, we computed transfer function estimates in very narrow bands.
The magnetic transfer functions (SAO predicted by PKD) calculated from the daytime windows (Figure 5.7) shows the enormous bias due to coherent noise between the two sites. TF derived from time windows after midnight (same days) (Figure 5.8) are much smoother, but there is still some bias present.
The plane wave source assumption corresponds to a response space of dimensionality two. Likewise the spectral density matrix (SDM) has 2 dominant eigenvalues corresponding to the two plane wave source polarizations. Other eigenvalues above background noise levels show that the plane wave response space is not entirely appropriate for the Parkfield-Hollister array. These additional eigenvalues of the SDM represent noise, some of which is coherent across the array. The significantly larger third and fourth eigenvalues may be a result of BART (Figure 5.11).
Egbert, G. D., Robust Multiple-Station Magnetotelluric Data Processing. Geoph. J. Int.,it 130,, 475-496, 1997.