Shear-wave Velocity Profile for Holocene Sediments Measured from Microtremor Array Studies, SCPT, and Seismic Refraction

Abstract

The microtremor method was trialed on soft Holocene sediments overlying glacial gravels near the Hutt River, Wellington New Zealand. A triangular four-station array of radius [Formula: see text], processed using a modified spatial autocorrelation (SPAC) method was sufficient to establish shear velocity for sediments and gravels, and a thickness of [Formula: see text] for the soft sediments. The interpretation was performed blind to geological and geophysical data, and subsequent comparison with seismic cone penetrometer and seismic refraction data shows the accuracy from the SPAC interpretation to be better than 5%. Interpretation of horizontal: vertical spectral ratio data assists with estimates of thickness of the glacial gravels (hundreds of meters) but accuracy in thickness is poor, perhaps due to uncertainty in the 2D nature of the base of the gravel-filled glacial valley.

On bias and noise in passive seismic data from finite circular array data processed using SPAC methods

“…Figure 6 quantifies the effect of the biased SPAC spectrum of Figure 5a. A best-fit model for array ACE requires a reduction in both shear velocity ͑−10%͒ and thickness ͑−25%͒ of layer 3 ͑see Table 1͒, which differ significantly from the verified results given in Asten et al ͑2005͒. In contrast, array BDF does not show the bias in the SPAC spectrum and does not produce any changes in the layered-earth model.…”

“…In both cases, c r ͑ f͒ is readily compared with a model SPAC spectrum using the MMS-PAC method noted above. The earth model used is shown in Table 1 ͑documented by Asten et al, 2005, from the interpretation of data acquired with a hexagonal array͒. The curve c i ͑ f͒ shows strongly developed oscillations ͑maxima-minima exceeding 0.5 in amplitude͒ which, by analogy with Figure 2, clearly indicate that microtremor sources are distributed in azimuth over less than 60°.…”

Determination of the Site Characterization Properties in Eastern Segment of the North Anatolian Fault Zone in Turkey based on the MMSPAC Method

“…At Site 2, there is a close fit between observed coherency curve and model coherency curve where the standard deviation is 0.72 for r1 and 0.69 for r2.The useful low-frequency limit of the SPAC data at Site 2 is approximately 0.1 Hz whereas the useful high-frequency limit is 10 Hz (Figure 7). Where the seismometers are buried, useful SPAC data is typically obtained down to frequencies of order 0.1 Hz (see for example Asten, 2005;Stephenson et al 2009, andSchramm et al, 2012). In Figure 8, there is a clear peak at 1 Hz, which is shown on both observed and model HVSR curves.…”

Blind Test of Methods for Obtaining 2-D Near-Surface Seismic Velocity Models from First-Arrival Traveltimes

“…There are many different methods for obtaining a velocity model from seismic refraction first-arrival times, ranging from forward modeling to analytic and geometrical methods to inverse and tomographic methods (e.g., Palmer, 1980;Zelt and Smith, 1992;Sheehan et al, 2005;Ellefsen 2009). Near-surface P-and S-wave velocity models provide essential constraints in applications ranging from hydrologic characterization to site hazard evaluation (e.g., Deen and Gohl, 2002;Asten et al, 2005;Zelt et al, 2006;Powers et al, 2007;Martí et al, 2008;Yordkayhun et al, 2009), but often these models include little or no quantitative estimation of uncertainty, resolution or non-uniqueness. Furthermore, it is seldom possible to know the true velocity model that is being sought, thereby leaving open the question of how robust these methods are and how confident we can be in the results.…”