Comparison of Rayleigh wave dispersion relations from three surface ...

Comparison of Rayleigh wave dispersion relations from three surface wave measurements in a complex-layered system X. Jin1, B. Luke1, J. Louie2 1

Civil and Environmental Engineering, University of Nevada Las Vegas, NV 89154-4015; [email protected];

[email protected] 2

Seismological Laboratory, University of Nevada Reno, NV 89557; [email protected] Abstract: Three widely-available surface wave measurement techniques, including two active-source methods, the Spectral Analysis of Surface Waves (SASW) method and the Multi-channel Analysis of Surface Waves (MASW) method, and one passive-source method, the Refraction Microtremor (ReMi) method, were applied at a complex-layered site. Dispersion relations were compared to one another. Outcomes from the MASW measurement reveal a fundamental and first-higher mode. All three methods agree closely in the frequency range over which only one mode is present. The dispersion curve from the SASW method, which represents a superposition of modes, shifts gradually from the fundamental mode to the first higher mode as frequency increases. The ReMi outcome tracks only the fundamental mode over its range of analysis.

Introduction With respect to geotechnical boring or drilling, seismic surface wave methods (SWM) provide a way to obtain subsurface detail non-destructively and non-intrusively, hence saving time and money. Due to those advantages, SWM have become popular and continue to develop rapidly. For example, a special edition of the Journal of Environmental and Engineering Geophysics (O'Neill, 2005) contains work by sixteen research teams from around the world, each using SWMs in different configurations and venues. Surface wave methods capitalize upon the dispersive nature of Rayleigh waves in layered media to derive shear wave velocity (VS) profiles of the subsurface. The SWM can be divided into two categories: active-source and passive-source methods. Different as they are, SWM are all based on the following three steps: (1) field testing, in which seismic waves are detected by mechanical sensors and recorded; (2) signal processing and construction of dispersion curves or panels; and (3) inversion of the dispersion data to obtain a single VS profile or a vertical slice, compiled from a series of VS profiles, of the subsurface. Construction of the dispersion relation is a critical step. The accuracy of the inverted VS profile depends largely on the purity, specificity and accuracy of the dispersion relation. The Spectral Analysis of Surface Wave (SASW) method (Stokoe et al. 1994) has proven to be a valuable tool for determining detailed VS profiles. This method uses receiver pairs and the phase spectral method to generate an “effective” dispersion curve (Gucunski and Woods 1991), which comprises a superposition of surface wave modes and other types of waves, when present. The Multi-Channel Analysis of Surface Waves (MASW) method (Park et al. 1999) uses a multi-channel array, which makes it possible to distinguish the fundamental-mode Rayleigh wave from higher modes and body waves.

The MASW and SASW methods usually involve active sources, which are optimized for the purpose of the test. Passive-source methods capitalize on ambient noise instead. With respect to active methods, passive methods cover broader and deeper volumes. However, used alone, the passive method can lack resolution in the near-surface and is poorly suited for seismically quiet locations (Tokimatsu 1995). The Refraction Microtremor (ReMi) method (Louie 2001) is a passive-source, multi-channel method that uses a linear array configuration. The linear configuration has the advantage of simplicity – the test configuration is the same as might be used for refraction testing – but it has the drawback that the predominant noise-arrival azimuth is unknown. Since energy oblique to the array travels faster than energy along the array, the peaks in the frequency-slowness (f-p) image would yield incorrect, higher velocities than the true Rayleigh-wave phase velocity, if enough noise is not traveling in all directions. Therefore, the dispersion curve is picked along a “lowest-velocity envelope.” Upperand lower-bound values are also picked, to define a range. The upper boundary is along the energy peaks, and the lower boundary is where the spectral ratios approach those of uncorrelated noise. Each of the three SWM has been compared favorably against independent VS measurements, e.g., Stokoe et al. (2003), Xia et al. (2002), and Stephenson et al. (in press) for SASW, MASW, and ReMi methods respectively, though some discrepancies have been found, e.g., Brown et al. (2002). The three SWM have also been compared against one another. In a test of a normally-dispersive hydraulic-fill site in Los Angeles, California (GeoVision 2005), dispersion curves from these SWM matched one another very closely. Here, dispersion relationships from the three SWM are developed for a complex-layered site. Complex layering should give rise to the prominence of higher Rayleigh modes (Gucunski and Woods 1991; Roësset et al. 1991; Tokimatsu et al. 1992). Therefore, differences between multi-mode and superposed “effective” dispersion curves should appear.

Site description The Engineering Geophysics Test Site (EGTS), dimensions 80 m × 70 m, is located on the campus of the University of Nevada Las Vegas (UNLV; see map at http://www.ce.unlv.edu/egl/test-site/). The site has been characterized using different surface-based and intrusive geophysical methods. The shallow stratigraphy comprises stiff, lightly- to heavily-carbonate-cemented fines, sands and/or gravels interspersed among much softer uncemented clays and sands (Tecle et al. 2003). Such lithology is not uncommon in arid settings. Here, a significant heavily-cemented layer, approaching two meters thick, occurs at approximately 2.5 meters depth. The test line is located along the south edge of the EGTS. The three measurements have different array lengths but share the same center point.

Data acquisition The SASW data were collected by UNLV researchers in partnership with colleagues from Utah State University (USU). One-Hz vertical geophones were placed in a linear array to test nine spacings ranging from 1 to 80 m (Liu et al. 2005). Data were collected using a four-channel signal analyzer. One of the channels is used to record the source function so that the receiver functions can be convolved with it, thus reducing effects of

uncorrelated noise. An instrumented sledgehammer was used to excite the wave energy for short receiver spacings. For longer receiver spacings, USU’s 2040-kg trailer-mounted drop-weight source was used. The MASW data were collected by UNLV researchers using sixty, 4.5 Hz, vertical geophones at 0.5 m spacing in a linear array. A sledgehammer was used to generate the source energy and a seismograph was used to record the dataset. The sampling rate and the recording length were 0.5 ms and 1 s respectively. Separations between source and nearest sensor varying from 7 to 15 m were tested. Records were stacked to improve signalto-noise ratio. The ReMi data were acquired by UNLV researchers using twelve, 4.5-Hz vertical geophones at 10 m spacing, in a linear array, and a seismograph. Ten, 30-second records were collected with 2 ms sampling rate using a refraction seismograph. This is a minimal number of channels for a ReMi survey, providing a stringent test of the technique.

Data processing For the SASW data, individual dispersion curves generated for each receiver spacing are superimposed (Fig. 1). The composite is then condensed to approximately one hundred points. The software package SurfSeis (Kansas Geological Survey) was used to process the MASW dataset. The outcome with the 15-m source offset yielded the best outcome. Fundamental and first-higher modes can be clearly distinguished in the amplitude image (Fig. 2). The ReMi dataset was processed using the software SeisOpt ReMi (Optim LLC). Figure 3 shows the f-p image with best picks and ranges. The dispersion curve is computed by inverting each slowness pick to obtain a velocity value.

Figure 1. SASW method: Dispersion curves for each receiver spacing and condensed composite

Figure 2. MASW method: Dispersion panel with picks, 15-m source offset

Figure 3. ReMi measurement: Dispersion panel with best-estimate and bounding picks While all three techniques described here are used commercially, each requires a measure of subjective input; in other words, the data interpreter should be knowledgeable, skillful, and experienced. Comparisons The dispersion relations from SASW, MASW and ReMi measurements are superimposed in Fig. 4. Frequency spans for the three methods differ, due to the sources used: the SASW method covers the broadest range, the ReMi method resolves the low-frequency end of the spectrum, and the MASW method covers a midrange. The three methods agree closely in the frequency range over which only the fundamental mode is present, up to 30 Hz. The ReMi method’s error bars illustrate a commonality among surface-based methods: uncertainty

Figure 4. Comparisons of dispersion relation from SASW, MASW and ReMi measurements increases with decreasing frequency, which correlates to increasing depth. The SASW dispersion curve matches well with that from ReMi except at the lowest frequency, where uncertainties are highest. The fundamental-mode MASW data match the others for frequencies up to that where the first-higher mode appears. Here, the SASW dispersion curve starts a gradual transition from the fundamental to the first-higher mode. At frequencies above 56 Hz, it falls back below the first-higher mode response. The ReMi data from this experiment were not resolved at the higher frequencies where multiple modes were observed. This comparison confirms expectations that a profile with a high stiffness contrast will elicit considerable higher-mode response. In this case a dispersion relation generated by superposition of energy spectra will contain significant higher-mode energy. In theory, as long as the wave propagation phenomena are modeled correctly in the inversion, both the multi-modal approach (e.g., MASW) and the energy-superposition approach (SASW) should yield correct VS profiles.

Acknowledgments We thank UNLV students and associates J. Jernsletten, Y. Liu, J. O’Donnell, S. Saldaña, V. Skidmore, and J. Wixon, for the their assistance with data collection. We thank USU’s J. Bay and J. Gilbert for assistance and use of equipment. We thank UNLV’s C. Snelson and the IRIS consortium for use of equipment. Parts of this work were supported by Lawrence Livermore National Laboratory (LLNL) under contract no. W-7405-ENG-48.

References Brown, L. T., Boore, D. M. and Stokoe, K. H., II (2002). “Comparison of shear-wave slowness profiles at 10 strong-motion sites from noninvasive SASW measurements and measurements made in boreholes.” Bull. Seismological Soc. America, 92, 3116-3133. GeoVision (2005). “Active and passive surface wave techniques.” Application note, (viewed Aug. 11, 2005). Gucunski, N., and Woods, R. D. (1991). "Use of Rayleigh modes in interpretation of SASW test." Proc., 2d Int’l. Conf. on Recent Advances in Geotechnical Earthquake Eng. and Soil Dynamics, St. Louis, Missouri, 13991408. Liu, Y., Luke, B., Pullammanappallil, S., Louie, J., and Bay, J. (2005). “Combining active- and passive source measurements to profile shear wave velocities for seismic microzonation.” Geo-Frontiers2005, Am. Soc. Civil Eng. Geotechnical Spec. Pub. 130-142, 977-990. Louie, J. (2001). "Faster, better: shear-wave velocity to 100 meters depth from refraction microtremor arrays." Bull. Seismological Soc. America, 91 (2), 347-364. O'Neill, A. (2005). “Seismic surface waves special issue guest editorial.” Journal of Environmental and Engineering Geophysics, 10 (2), 67-86. Park, C. B., Miller, R. D., and Xia, J. (1999). "Multichannel analysis of surface waves (MASW)." Geophysics, 64, 800-808. Roësset, J. M., Chang, D. W., and Stokoe, K. H., II (1991). "Comparison of 2-D and 3-D models for analysis of surface wave tests." Proc., 1st Conf. on Soil Dynamics and Earthquake Eng., Computational Mechanics Publ., Southampton, England, 112-126. Stephenson, W. J., Louie, J. N., Pullammanappallil, S., Williams, R. A., and Odum, J. K. (in press). “Blind shear-wave velocity comparison of ReMi and MASW results with boreholes to 200 m in Santa Clara Valley: Implications for earthquake ground motion assessment.” Bull. Seismological Soc. America. Stokoe, K. H., II, Rosenblad, B. L., Bay, J. A., Redpath, B., Diehl, J. G., and Steller, R. A. (2003). “Comparison of VS profiles from three seismic methods at Yucca Mountain.” Proc., Soil and Rock America 2003, ed. P. J. Culligan, H. H. Einstein, and A. J. Whittle. Verlag Glückauf GMBH, Essen, (1), 299-306. Stokoe, K. H., II, Wright, S. G., Bay, J. A., and Roësset, J. M. (1994). "Characterization of geotechnical sites by SASW method." Geophysical Characterization of Sites, ed. R. D.Woods, Oxford & IBH Pub. Co., New Delhi, India, 15-25. Tecle, M. T., Giorgis, A. Y. and Luke, B. (2003). “Comparison of seismic downhole to crosshole measurements in a complex-layered system.” Proc., 38th Ann. Symp. Eng. Geology and Geotechnical Eng., Idaho State Univ., Pocatello, 217-232. Tokimatsu, K. (1995). "Geotechnical site characterization using surface waves." Proc., 1st Int’l. Conf. Earthquake Geotechnical Eng., IS-Tokyo '95, Balkema, Rotterdam, 1333-1368. Tokimatsu, K., Tamura, S., and Kojima, H. (1992). "Effects of multiple modes on Rayleigh wave dispersion characteristics." J. Geotechnical Eng., 118(10), 1529-1543. Xia, J., Miller, R. D., Park, C. B., Hunter, J. A., Harris, J. B., and Ivanov, J. (2002). “Comparing shear-wave velocity profiles from multichannel analysis of surface wave with borehole measurements.” Soil Dyn.and Earthquake Eng., 22 (3), 181-190.