Passive Seismic (HVSR)

Basic Concept

Though considered superfluous and a “nuisance” in other seismic methods, seismic noise is an acoustic-energy source that can be used to estimate valuable subsurface information. Seismic noise is comprised of naturally occurring vibrations caused by atmospheric and anthropogenic disturbances and, because it exists everywhere, is an extremely convenient signal source. The horizontal-to-vertical spectral ratio (HVSR) passive-seismic method exploits seismic-noise signals to estimate properties of unconsolidated, overburden sediments.

By measuring three components of seismic noise over time, the HVSR method can determine the fundamental resonance frequency (f₀) of the overburden at a point location. This frequency is related to the layer thickness (h) (i.e., depth to bedrock) and average shear-wave seismic-velocity (Vs) of the corresponding unconsolidated materials. Thus, the HVSR method can be used to estimate the thickness or shear-wave velocity of the sediments that sharply overlie bedrock.

Theory

The HVSR method is a type of passive-seismic method, and, as such, the need for an active (i.e., controlled and artificially induced) seismic-source signal is eliminated. Instead, the HVSR method considers how seismic noise, which is ambient-acoustic energy caused by human activities, atmospheric events, and ocean waves, interacts with subsurface materials. These low-frequency waves, which are categorized as microseisms and microtremors, penetrate the subsurface and can travel through the entire unconsolidated layer to bedrock.

A material naturally vibrates at a fundamental resonance frequency and amplifies incident waves with the same frequency upon contact. This phenomenon (i.e., resonance) can be excited by seismic noise encountering the overburden-bedrock interface, at which acoustic impedance generally changes. Acoustic impedance is a measure of resistance to acoustic energy and depends upon seismic velocity and density. A sufficient (i.e., 2:1) contrast must exist at the bedrock-sediment interface for it to be identified by the HVSR method (Bard and others, 2004).

The HVSR method uses a single, broadband, three-component seismometer that measures seismic noise in three-orthogonal directions (i.e., two horizontal and one vertical). The raw seismic-trace data show the amplitudes of acoustic noise in the time domain (i.e., over time) for each direction. Seismic-noise amplitude is time-varying, and recorded traces are often filtered to remove high-frequency impulse- and anthropogenic-noise prior to being transformed to the frequency domain (Koller and others, 2004).

The two-horizontal components are averaged merged into a single frequency spectrum, which is used to compute the ratio of horizontal-to-vertical (H/V) spectra. This H/V spectral ratio remains consistent despite time-varying factors and, thus, can be reliably be used to estimate the fundamental resonance frequency (f₀). Typically, f₀ is the lowest value and is evidenced by the highest amplitude peak on the H/V frequency spectrum (Arwananda and others, 2017).

The fundamental resonance frequency (f₀) is related to the layer thickness (h) and average shear-wave velocity (Vs) such that f₀ = ^Vs/_4h. Thus, if either parameter is measured directly (e.g., borehole logs, seismic-velocity survey) or estimated using local information, f₀ can be used to directly estimate the unknown variable. Furthermore, f₀ can be used to estimate layer thickness using a local power-law regression equation developed from f₀values measured at sites with a wide range of known thicknesses (Johnson and Lane, 2016).

Applications

HVSR seismometers are designed as handheld units with small footprints, and, as such, the potential for and convenience of subsurface investigations have increased. The HVSR method provides the ability to collect data that would have previously required expensive and/or time-consuming field efforts (e.g., drilling, large-scale, invasive surveys). Furthermore, this method expands the application of geophysics into urbanized zones, which typically prove challenging or are impossible with more traditional methods (Benjumea, 2011).

However, anthropogenic noise and modified landscapes (e.g., pavement) within urban settings can still be challenging, as ground coupling is one of the three-vital data-collection parameters.

The other two factors involved in HVSR data collection are the leveling and orientation of the unit. Though important, improper leveling or orientation will not render data unusable as would insufficient coupling. In large-scale studies, however, common instrument orientation becomes more critical for consistency, especially if the site has anisotropic subsurface properties.

Note that the abovementioned relation between fundamental resonance frequency, average shear-wave seismic velocity, and layer thickness is only valid in a simple two-layer system. The horizontal-to-vertical frequency spectrum may exhibit multiple peaks, and such occurrences can be indicative of a multilayered system and require more advanced analysis (Bard and others, 2004). Furthermore, complications may also arise from the assumptions made about the seismic-velocity structure of the subsurface.

The HVSR method assumes statistically similar seismic-velocity properties across the study area. Depending on the equation used to relate the fundamental resonance frequency to the layer thickness, the average shear-wave velocity (Vs) is assumed to be constant or vary uniformly with depth. Because earth materials lack complete homogeneity, errors are produced in circumstances where Vs varies irregularly with depth/location as well as when stratigraphic contacts are gradational or irregular (Delgado, 2000).

Locally and regionally expansive studies have relied on HVSR data to map unconsolidated deposits over bedrock (e.g., Ibs Von Seht and others, 1999; Parolai and others, 2002; and Lane and others, 2008). Additionally, the HVSR method can be easily integrated into various types of geophysical surveys to spot check overburden thickness and constrain models and/or interpretations. As either a primary or supplementary technique, the HVSR method has been useful in applications that include the following:

Bedrock-surface/sediment-thickness/reconnaissance mapping
Glacial/ice-sheet mapping
Shear-wave velocity structure determination
Microzonation/seismic-response analysis
Geotechnical/seismic engineering projects
Embankment integrity determination
Dynamic infrastructure characterization

Examples/Case studies

Benjumea, B., Macau, A., Gabàs, A., Bellmunt, F., Figueras, S., and Cirés J., 2011, Integrated geophysical profiles and H/V microtremor measurements for subsoil characterization: Near Surface Geophysics, v. 9, no. 5, p. 413-425, doi: 10.3997/1873-0604.2011021.

Abstract: Mapping bedrock structure beneath overburden is crucial for understanding geological and hydrogeological processes. Acquiring this information is generally done using well drilling or geophysical surveys; but these studies are expensive and require large periods of acquisition and processing time. In addition, geophysical data acquisition can be logistically challenging in urban zones with limited available areas for instrumentation deployment. Under favourable conditions (1D structure and high acoustic impedance contrast) the H/V microtremor technique can provide estimates of bedrock depth. This technique is used to obtain the soil resonance frequency in seismic microzonation studies. It is based on the computation of the horizontal to vertical spectral ratio of microtremor recordings acquired at a single station. The soil resonance frequency is related to soil shear-wave velocity and thickness. Here we investigate the capability of combining microtremor and traditional exploration geophysical techniques (electrical resistivity and seismic tomography) to obtain an empirical relationship relating soil resonance frequency and overburden thickness. Subsequently we propose to extend microtremor measurements to adjacent areas that have not been covered by geophysical surveys. This methodology has been applied at a test site located in a granitic environment where alluvial/colluvial sediments cover the granite weathering profile. This area is characterized by urban development and sectors having rugged topography. A priori, this area has suitable conditions to apply the H/V microtremor technique. Overburden thickness has been estimated to range between 20–50 m. The proposed methodology has been validated at the test site, encouraging us to apply the H/V method as an exploration tool in similar geological environments.

Mahajan, A.K., Mundepi, A.K., Chauhan, N., Jasrotia, A.S., Rai, N., and Gachhayat, T.K., 2012, Active seismic and passive microtremor HVSR for assessing site effects in Jammu city, NW Himalaya, India—A case study: Journal of Applied Geophysics, v. 77, p. 51-62, doi: 10.1016/j.jappgeo.2011.11.005.

Abstract: 1-D shear wave velocity structure is important for site effect studies and geotechnical engineering, but it is quite difficult and expensive to derive from the conventional geophysical techniques. Active (MASW) and passive (microtremors, HVSR) methods were conducted at 30 sites in the frontal part of the Himalaya which is characterized by soft sediments and strong seismological effects. Shear wave velocity (Vs) in the range of ~ 238 m/s to ~ 450 m/s has been obtained from 30 m thick layer of quaternary sediments overlying Lower Miocene bed rock (Upper Siwalik Conglomerate) in Jammu city, NW Himalaya. The shear wave velocity (Vs) along with seismic input motion of Chamoli earthquake (mb 6.8) has been used to obtain site response spectrum. The response spectrum suggests five to seven times increase in peak ground acceleration for single or two storey buildings and by eight to twelve times increase in amplification ratio with respect to input ground motion. The amplification spectrum shows peak amplification of ~ 2 Hz–~ 3 Hz in the central part and ~ 1.75 Hz–2 Hz in the northern, southwestern and southeastern parts of the city. The advantage of microtremor HVSR is that it yields direct estimate of the fundamental frequency which is found to vary from ~ 1 Hz to ~ 3 Hz for same sites. Further, the 1-D velocity models obtained from ModelHVSR Matlab routine have been compared with the soil models prepared by derived using MASW. The comparison shows correlation between soil models for sites having high shallow impedance contrast between the overlying sediments and very stiff material (bedrock) underneath as than sites having less impedance contrast.

Panou, A.A., Theodulidis, N., Hatzidimitriou, P., Stylianidis, K., and Papazachos, C.B., 2005, Ambient noise horizontal-to-vertical spectral ratio in site effects estimation and correlation with seismic damage distribution in urban environment: the case of the city of Thessaloniki (Northern Greece): Soil Dynamics and Earthquake Engineering, v. 25, no. 4, p. 261-274, doi:10.1016/j.soildyn.2005.02.004.

Abstract: The validity of the estimation of seismic site response characteristics from ambient noise measurements was investigated in the downtown district of the city of Thessaloniki (Northern Greece), which was strongly affected by the 20/6/1978 (M=6.5) damaging earthquake. For this purpose 250 ‘single site’ ambient noise measurements were performed in a dense grid of points covering the center of the city. The ambient noise H/V spectral ratio for each site was calculated and the fundamental frequency (fo) and corresponding H/V amplitude level (Ao) were estimated. Contour maps of both, fo and Ao, were compared with results from geological and geotechnical studies as well as with macroseismic data of the 1978 earthquake and were found to be well correlated. These comparisons provide strong evidence that ambient noise measurements properly processed with the (H/V) spectral ratio technique can be used as an inexpensive and fast tool for microzonation studies in urban environments.

Picotti, S., Francese, R., Giorgi, M., and Pettenati, F., 2017, Estimation of glacier thicknesses and basal properties using the horizontal-to-vertical component spectral ratio (HVSR) technique from passive seismic data: Journal of Glaciology, v. 63, no. 238, p. 229-248, doi:10.1017/jog.2016.135.

Abstract: Microtremor measurements and the horizontal-to-vertical spectral ratio (HVSR) technique, generally used for site effect studies as well as to determine the thickness of soft sedimentary layers, can effectively be applied to map the thickness of glaciers. In this work the radio-echo sounding, geoelectric and active seismic methods, widely employed to image the earth interior, are applied to verify the reliability of the HVSR technique in Alpine and Antarctic glacial environments. The technique has been used to analyze passive seismic data from glaciers of the Adamello and Ortles-Cevedale massifs (Italy), the Bernese Oberland Alps (Switzerland) and from the Whillans Ice Stream (West Antarctica). Comparing with the results obtained from the different geophysical imaging methods, we show that the resonance frequency in the HVSR spectra correlates well with the ice thickness at the site, in a wide range from a few tens of meters to more than 800 m. The reliability of the method mainly depends on the coupling of sensors at the glacier surface and on the basal impedance contrast. This passive seismic technique offers a logistically efficient and cost effective method to map glacier and ice-sheet thicknesses. Moreover, under certain conditions, it allows reliable estimations of the basal seismic properties.

Picozzi, M., Parolai, S., and Richwalksi, S.M., 2005, Joint inversion of H/V ratios and dispersion curves from seismic noise: Estimating the S‐wave velocity of bedrock: Geophysical Research Letters, v. 32, no. 11, 4 p., doi:10.1029/2005GL022878.

Abstract: A joint inversion of phase velocity and H/V ratio curves, both obtained from seismic‐noise recordings, permits the retrieval of the shear‐wave velocity structure of local sedimentary cover. Our inversion scheme uses a genetic algorithm and considers the influence of higher modes on the data sets. Encouraged by the results published previously on joint inversion (Parolai et al., 2005) we went one step further. We found, using a synthetic data set, that the impedance contrast at the sediment‐bedrock interface has a strong influence on the shape of the H/V ratio curve, which therefore allows the bedrock S‐wave velocity to be well constrained in the joint‐inversion procedure. Our observations were further confirmed using a real data set.

References

Arwananda, A.P., Aryaseta, B., Dezulfakar, H., Fatahillah, Y., and Rochman, J., 2017, Horizontal vertical Spectral Ratio Method in Microtremor to Estimate Engineering Bedrock Thickness at Sedati Mud Volcano, in Proceedings, IOP Conference Series: Earth and Environmental Science 62, September 2016: Bali, Indonesia, Institute of Physics, 7 p., doi:10.1088/1755-1315/62/1/012010.

Bard P.-Y., Acerra, C., Alguacil, G., Anastasiadis, A., Atakan, K., Azzara, R., Basili, R., Bertrand, E., Bettig, B., Blarel, F., Bonnefoy-Claudet, S., Bordoni, P., Borges, A., Bøttger-Sørensen, M., Bourjot, L., Cara, F., Caserta, A., Chatelain, J.-L., Cornou, C., Cotton, F., Cultrera, G., Daminelli, R., Dimitriu, P., Dunand, F., Duval, A.-M., Fäh, D., Fojtikova, L., de Franco, R., di Giulio, G., Grandison, M., Guéguen, P., Guillier, B., Haghshenas, E., Havskov, J., Jongmans, D., Kind, F., Kirsch, J., Koehler, A., Koller, M., Kristek, J., Kristekova, M., Lacave, C., La Rocca, M., Marcellini, A., Maresca, R., Margaris, B., Moczo, P., Moreno, B., Morrone, A., Ohrnberger, M., Ojeda, J.A., Oprsal, I., Pagani, M., Panou, A., Paz, C., Querendez, E., Rao, S., Rey, J., Richter, G., Rippberger, J., Roquette, P., Roten, D., Rovelli, A., Saccoroti, G., Savvaidis, A., Scherbaum, F., Schisselé, E., Spühler-Lanz, E., Tento, A., Teves-Costa, P., Theodulidis, N., Tvedt, E., Utheim, T., Vassiliadès, J.-F., Vidal, S., Viegas, G., Vollmer, D., Wathelet, M., Woessner, J., Wolff, K., Zacharapoulos, S., 2004, Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations—Measurements, processing and interpretation: SESAME European research project WP12—Deliverable D23.12, 62 p., Available at: ftp://ftp.geo.uib.no/pub/seismo/SOFTWARE/SESAME/USER-GUIDELINES/SESAME-HV-User-Guidelines.pdf.

Benjumea, B., Macau, A., Gabàs, A., Bellmunt, F., Figueras, S., and Cirés, J., 2011, Integrated geophysical profiles and H/V microtremor measurements for subsoil characterization: Near Surface Geophysics, v. 9, no. 5, p. 413-425, doi:10.3997/1873-0604.2011021.

Delgado, J., López-Casado, C., Giner, J., Estévez, A., Cuenca, A., and Molina, S., 2000, Microtremors as a Geophysical Exploration Tool: Applications and Limitations: Pure and Applied Geophysics, v. 157, no. 9, p. 1445-1462, doi:10.1007/PL00001128.

Ibs-von Seht, M., and Wohlenberg, J., 1999, Microtremors measurements used to map thickness of soft soil sediments: Bulletin of the Seismological Society of America, v. 89, p. 250-259.

Johnson, C.D., and Lane, J.W., 2016, Statistical comparison of methods for estimating sediment thickness from Horizontal-to-Vertical Spectral Ratio (HVSR) seismic methods: An example from Tylerville, Connecticut, USA, in Proceedings, Symposium on the Application of Geophysics to Engineering and Environmental Problems: Society of Exploration Geophysics, p. 317-323, doi:10.4133/SAGEEP.29-057.

Lane, J.W., White, E.A., Steele, G.V., and Cannia, J.C., 2008, Estimation of bedrock depth using the horizontal‐to‐vertical (H/V) ambient‐noise seismic method, in Proceedings, Symposium on the Application of Geophysics to Engineering and Environmental Problems: Society of Exploration Geophysics, p. 490-502, doi:10.4133/1.2963289.

Koller, M.G., Chatelain, J.-L., Guillier, B., Duval, A.-M., Atakan, K., Lacave, C., Bard, P.-Y., and the SESAME participants, 2004, Practical user guidelines and software for the implementation of the H/V ratio technique: measuring conditions, processing method and results interpretation, in Proceedings, 13^th World Conference on Earthquake Engineering: Vancouver, B.C., Canada, WCEE, 10 p.

Nakamura, Y., 1989, A method for dynamic characteristics estimations of subsurface using microtremors on the ground surface: Quarterly Report Railway Technical Research Institute, v. 30, no. 1, p. 25-33.

Parolai, S., Bormann, P., and Milkert, C., 2002, New relationships between Vs, thickness of sediments, and resonance frequency calculated by the H/V ratio of seismic noise for Cologne Area (Germany): Bulletin of the Seismological Society of America, v. 92, no. 6, p. 2521-2527, doi:10.1785/0120010248.