The seismometers for refraction exploration weigh less than one kilogram each. Many universities and engineering consultants possess seismic refraction systems that can digitally record only 12 such geophones, or channels, simultaneously. A complete 12-channel system has a capital cost as low as $10,000 (as from GISCO. Here I case histories of how such inexpensive equipment may be easily deployed to record background noise, or ``microtremor,'' and how such data can yield surface-wave velocity dispersion measurements that constrain shallow S-wave velocity structure.
With sponsorship by the U.S. Geological Survey and the U.S. National Science Foundation, and through collaboration with colleagues from Shimizu Corporation, Kobe University, and the EPRI of Kyoto University, this project carried out a noise survey with 716 m linear arrays at the northeast corner of the Reno/Tahoe International Airport (figure 1) in western Nevada, USA. These surveys were made with 48 compact vertical seismometers designed to record at 4 Hz and above. Analysis including all 48 channels is described by Louie in a paper submitted to BSSA. Refraction shots were recorded with the same equipment and NSF sponsorship in Dixie Valley, central Nevada (with all channels analyzed in a explosion refraction case history.
Figure 1:
Map of microtremor experiment deployment near the Reno/Tahoe International
Airport, western Nevada, USA.
The lines of 48 dots total show the placement of the 8 Hz single refraction
geophones.
(Click on image for plot in Adobe
Acrobat PDF format.)
Figure 3:
Velocity spectral power-ratio plots of two 48-channel
records from the March 1998
Dixie Valley fault reflection survey (Abbott et al., 1998).
Both records had a 2 kg explosion source just off the east end of a 48 channel,
716 m linear array of the 8 Hz single refraction geophones.
The ``playa'' record generating the upper plot was taken in the middle of the
Dixie Valley Basin, where Quaternary alluvial and lacustrine sediments
are >1 km thick.
The lower ``piedmont'' record was taken with the west end of the array against the
basin-bounding Dixie Valley fault, with granitic basement no more than
350 m below the east end of the array.
Aliasing in the spatial frequency domain would occur below the dotted
curve in each plot.
(Click here for a larger image;
on the image for plot in Adobe
Acrobat PDF format; or here
for a monochrome PDF plot.)
Clearly the spectral-ratio peaks below 6 Hz are at high apparent velocities. The two peaks between 6 and 12 Hz still represent accurate velocities. From 12 to 26 Hz, the peaks are at velocities about 10% lower than those on the best multiple-record airport analysis (figure 2 lower right). For microtremor noise recording, recording and stacking the spectra of a large number of records may well overcome the inaccuracies of a 12-channel refraction instrument. Recording additional records will even take less time than setting more geophones in the ground.
The lower plot of figure 4 is the analysis of the Dixie Valley playa record (figure 3, upper), using the 12 channels closest to the blast of the original 48. As might be expected, the surface-wave dispersion now shows more strongly, with higher spectral ratios against other velocities at the same frequencies. The body waves are not as strong. There is, however, some minor velocity smearing, reducing velocity picking accuracy by 5-10% at these values.
Figure 4:
Velocity spectral power-ratio plots of a Reno/Tahoe Airport noise record,
the same as in the upper right of figure 2, and of the Dixie Valley
playa record, the same as the upper plot of figure 3.
To generate these plots, I used only twelve channels of the original 24
(for the Airport noise record) or 48 (for the Dixie Valley playa record)
that I analyzed above.
(Click here for a larger image;
on the image for plot in Adobe
Acrobat PDF format; or here
for a monochrome PDF plot.)
