Obtaining Surface-Wave Velocity Spectra from Seismic Refraction Recording Equipment

John N. Louie
Seismological Laboratory and Dept. of Geological Sciences
The University of Nevada, Reno, NV 89557-0141 USA
Phone: +1-775-784-4219 Email: louie@seismo.unr.edu Web: www.seismo.unr.edu

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. Channels 1-18 extend south from the Rock and Mill intersection along Rock Blvd., with 19-24 turning west. Channels 25-48 extend west from the intersection along Mill St. The dots in triangular arrays show the locations of 3-component accelerometers deployed by Iwata et al. (1998). The words labeling the airport show the location of the main runways in use during the experiment. The major intersection between Rock Blvd. and Mill St., the nearby Truckee River, and the irrigation ditch flowing along the site also appear.

2-Hz Recording on 8-Hz Geophones?

A Cluster Test

Results from Reno/Tahoe Airport Refraction Deployment

Log-log plot of the power spectrum summed from seven 48-second noise, blast, and hammer records taken by the 8 Hz refraction equipment during the Reno/Tahoe Airport deployment. The main peak spans the 3-8 Hz band.


Velocity spectral power-ratio plots of selected records and summed records taken by the 8 Hz refraction equipment during the Reno/Tahoe Airport deployment. In each plot apparent horizontal velocity along a segment or segments of the refraction array increases non-linearly upwards, and frequency increases linearly to the right. Warmer colors show a larger ratio of power at one slowness, in comparison with other slownesses at the same frequency. A typical dispersive surface wave would thus slope down from left to right in these plots, as indicated on the upper left plot, of the surface-wave record from a blast 16 km distant. Aliasing in the spatial frequency domain would occur below the dotted curve in each plot.

Upper: velocity spectrum of the Reno/Tahoe Airport noise record in the upper right of figure 3, plotting the period versus Rayleigh phase velocity of the picks made from spectral power peaks, and the response of a model (lower) fit to the picks.

Analysis of Reflection Survey Data from Central Nevada

Velocity spectral power-ratio plots of two 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.

Test of Limited Channels and Offsets

Velocity spectral power-ratio plots of a Reno/Tahoe Airport noise record, the same as in the upper right of figure 3, and of the Dixie Valley playa record, the same as the upper plot of figure 5. 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.

Test on a 1-Hz Array in Wellington

Test Against a Borehole Log

What is ``Engineering A'' Rock?

Conclusion

The tests here show that common seismic refraction equipment can yield accurate surface-wave dispersion information. Configurations of 12 to 48 single vertical, 8 Hz exploration geophones can give surface-wave phase velocities at frequencies as low as 3 Hz, and as high as 26 Hz. Recording of microtremor noise or point active sources are fruitful. This range is appropriate for constraining S-wave velocity profiles from the surface to 50 m depths.

Analysis by simple p-tau transform of single or multiple recordings followed by spectral-ratio computation shows clear dispersion trends in velocity spectrum plots. In microtremor noise recording tests at the Reno/Tahoe International Airport, Nevada, phase velocity estimates from this technique closely overlapped those of Iwata et al. (1998), who used 3-component broadband accelerometers. Analysis of an existing seismic reflection data set from a central Nevada basin was able to distinguish a clay playa environment from a piedmont sand and gravel setting.

The success of these tests shows that anyone possessing digital seismic refraction equipment having 12 or more channels, and single refraction geophones, can quickly and easily measure the S-wave velocities to 50 m depth at any site, however noisy.

Acknowledgments

This research was supported by the U.S. Geological Survey National Earthquake Hazards Reduction Program under contract #NI17292, with co-investigators J. G. Anderson, F. Su, and Y. Zeng. The W. M. Keck Foundation generously donated the seismic refraction equipment to the Mackay School of Mines, University of Nevada, Reno. T. Iwata, H. Kawase, T. Satoh, Y. Kakehi, and K. Irikura of the DPRI, Kyoto University, Kobe University, and Shimizu Corporation graciously made the accelerometer deployments, performed data analyses, and also helped support the author's sabbatical research. Refraction instruments were deployed with valuable assistance from K. Smith, R. Anooshehpoor, R. E. Abbott, G. Ichinose, M. Herrick, and C. Mann. Caithness Power Corp. kindly provided timing information on their 3000 kg construction blast. The Reno/Tahoe International Airport Authority generously provided access to the recording site.

References