Michael W. Asten

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

The finite nature of typical small seismic arrays used in conjunction with spatial autocorrelation SPAC processing for observing the microtremor wavefield causes predictable perturbationsoftheSPACspectrumwhensourcesofseismic noiseareconfinedtoarestrictedrangeofazimuths.Suchperturbations are especially evident at higher frequencies where wavelengths are on the order of the array radius. The effects arereadilymodeledandshowthatthetriangulararraygeometries commonly used for microtremor studies require azimuthal distributions of wave energy on the order of 60° or greatertohaveahighprobabilityofbeingfreeofsuchperturbations. The imaginary component of the SPAC spectrum, which is ideally zero for a sufficiently dense circular array and/or a sufficiently isotropic wavefield, is in practice often nonzero and provides three quality-control indicators: 1 an indication of insufficient spatial averaging, 2 an empirical measure of the level of statistical uncertainty in SPAC spectralestimates,and3anindicationofdeparturesfromplanewavestationarityoftheseismicnoisewavefield.

Comparisons of Shear-Wave Slowness in the Santa Clara Valley, California, Using Blind Interpretations of Data from Invasive and Noninvasive Methods

Many groups contributed to a blind interpretation exercise for the deter- mination of shear-wave slowness beneath the Santa Clara Valley. The methods in- cluded invasive methods in deep boreholes as well as noninvasive methods using active and passive sources, at six sites within the valley (with most investigations being conducted at a pair of closely spaced sites near the center of the valley). Although significant variability exists between the models, the slownesses from the various methods are similar enough that linear site amplifications estimated in several ways are generally within 20% of one another. The methods were able to de- rive slownesses that increase systematically with distance from the valley edge, cor- responding to a tendency for the sites to be underlain by finer-grained materials away from the valley edge. This variation is in agreement with measurements made in the boreholes at the sites.

Full transmitter waveform transient electromagnetic modeling and inversion for soundings over coal measures

Modeling of transient electromagnetic soundings requires consideration of the full bipolar transmitter waveform in order to obtain full measurement accuracy from field instruments. The full‐waveform transient may be computed using existing impulse‐response forward algorithms followed by a convolution procedure with truncation and residual estimation after an even number of terms, which results in average increases in run time of about 80 percent. Simple calculations based on approximations for the behavior of transient decay over (a) a conductive layer over a resistive basement, (b) a homogeneous half‐space, and (c) a resistive layer over a conductive basement show that the effect of the full waveform is greatest at sample times near the end of the transmitter off‐time, and for the case of a conductive basement. Failure to consider the full waveform is expected to yield errors in the amplitude of the computed transient in the range of 4 to 100 percent or more, depending upon the sample time and type of ea...

Metalliferous mining geophysics—State of the art in the last decade of the 20th century and the beginning of the new millennium

The downturn in mining activity experienced during the 1990s did not preclude significant new developments in various areas of mining geophysics. The methodology for acquiring and compiling data has kept pace with the latest technological developments, from Global Positioning System navigation to raster displays and parallel computing. Wavelet transforms, principal component analysis, and fractals have begun to find successful applications in both processing and interpretation of geophysical data. Methods of quantitatively interpreting/inverting anomalies in terms of 2‐D and 3‐D models of causative bodies are becoming common.For the first time it is possible to make airborne gravity gradient measurements suitable for use in mineral exploration. Lower transmitter frequencies for airborne time‐domain electromagnetic (EM) systems have enabled surveys in areas where conductive cover previously screened basement conductors. The use of approximation algorithms has allowed the transformation of either time‐domai...

SHEAR-WAVE VELOCITY PROFILE FOR HOLOCENE SEDIMENTS MEASURED FROM MICROTREMOR ARRAY STUDIES, SCPT, AND SEISMIC REFRACTION

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 40 m, processed using a modified spatial autocorrelation (SPAC) method was sufficient to establish shear velocity for sediments and gravels, and a thickness of 21.8 m 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.

Comparison of Shear-velocity Profiles of Unconsolidated Sediments Near the Coyote Borehole (CCOC) Measured with Fourteen Invasive and Non-invasive Methods Editorial

A set of fourteen invasive and non-invasive geophysical methods are compared for the measurement of the shear-velocity (Vs) profile at or near a borehole at the Coyote Creek outdoor Classroom, Santa Clara Valley, California. The borehole with geophysical logs provided opportunity for a series of blind trials of methods for measurement of the Vs profile in thick, soft sediments. The various methods are important in the task of establishing shear-velocity profiles for purposes of earthquake hazard site classification in California. Results of the trials were presented at a one-day workshop at the US Geological Survey in May 2004; this paper summarizes results, relative depths of penetration and …

FAST APPROXIMATE SOLUTIONS OF TRANSIENT EM RESPONSE TO A TARGET BURIED BENEATH A CONDUCTIVE OVERBURDEN

We derive fast transient electromagnetic (TEM) solutions for a thin‐plate conductive target buried in a resistive host rock overlain by thin conductive overburden. For this model, the total TEM field is composed of the overburden response, which is known analytically, and the target response that is approximated by a wire loop. Thus the transient response of the target in free space is approximated with a single exponential function. The interaction between the target and the overburden is approximated by convolving the overburden‐delayed driving field with the exponential function. The target response is then computed from the wire loop by the Biot‐Savart law in free space, thus neglecting the second overburden‐delay effect. Including the second overburden‐delay effect does not yield significantly improved results. The approximate solutions agree reasonably well with scale model and numerical results, but their computation is more than 100 times faster than conventional methods and can be easily done on ...

Site characterisation in north-western Turkey based on SPAC and HVSR analysis of microtremor noise

The geology of the north-western Anatolia (Turkey) ranges from hard Mesozoic bedrock in mountainous areas to large sediment-filled, pull-apart basins formed by the North Anatolian Fault zone system. Düzce and Bolu city centres are located in major alluvial basins in the region, and both suffered from severe building damage during the 12 November 1999 Düzce earthquake (Mw = 7.2). In this study, a team consisting of geophysicists and civil engineers collected and interpreted passive array-based microtremor data in the cities of Bolu and Düzce, both of which are localities of urban development located on topographically flat, geologically young alluvial basins of Miocene age. Interpretation of the microtremor data under an assumption of dominant fundamental-mode Rayleigh-wave noise allowed derivation of the shear-wave velocity (Vs) profile. The depth of investigation was ~100 m from spatially-averaged coherency (SPAC) data alone. High-frequency microtremor array data to 25 Hz allows resolution of a surface layer with Vs < 200 m/s and thickness 5 m (Bolu) and 6 m (Düzce). Subsequent inclusion of spectral ratios between horizontal and vertical components of microtremor data (HVSR) in the curve fitting process extends useful frequencies up to a decade lower than those for SPAC alone. This allows resolution of two interfaces of moderate Vs contrasts in soft Miocene and Eocene sediments, first, at a depth in the range 136–209 m, and second, at a depth in the range 2000 to 2200 m.

Further investigation over Quaternary silts using the Spatial Autocorrelation (SPAC) and Horizontal to Vertical Spectral Ratio (HVSR) microtremor methods

This study extends a previous investigation using the passive seismic Spatial Autocorrelation (SPAC) and Horizontal to Vertical Spectral Ratio (HVSR) methods on microtremor observations in the Port Melbourne area. The area has up to 80 m thickness of Quaternary gravels, sands, and silts, with the near-surface Coode Island Silt being so soft that the base of the unit could not be resolved in a previous study. In order to extend the previous work, further surveys using instrumentation more suited to low-frequency measurements (SPAC and HVSR methods) and a larger array radius (SPAC method) were undertaken. Despite the use of significantly larger array radii and long-period seismometers, coherencies in the sub 1–2 Hz range were poor and allowed only marginally deeper shear-velocity information to be obtained. We have attributed this deficiency in the SPAC spectrum to a lack of energy in the vertical component of Rayleigh wave motion, as indicated by a high-amplitude peak in the HVSR spectrum around 1–2 Hz. Despite the poor low-frequency SPAC results, a combined modelling approach using both HVSR and SPAC results, along with the results from the large array survey, allowed deeper shear-velocity values (to depths of 70–100 m) to be estimated with greater precision than the initial surveys. We note that the HVSR spectrum is more sensitive to variations in shear velocities at these depths than the SPAC spectrum, providing an additional constraint on interpreted shear-velocity profiles from SPAC data. Lack of a secondary HVSR peak below 1 Hz that could be attributed to the sediment-bedrock interface leads us to believe that this expected peak is masked by the high amplitude peak resulting from the large shear-velocity contrast closer to the surface in the sediment profile (a silt–gravel interface).

Using the SPAC Microtremor Method to Identify 2D Effects and Evaluate 1D Shear-Wave Velocity Profile in Valleys

The requirement of a layered-earth geology is a restrictive assumption when using the spatially averaged coherency spectra (SPAC) method. Numerical simulations of microtremors and SPAC observations recorded in the Tamar paleoval- ley, Launceston (Tasmania, Australia), are used to assess the potential of the SPAC method to identify two-dimensional (2D) effects and evaluate one-dimensional (1D) shear-wave velocity (SWV) profile in a valley environment. The Tamar Valley is approximately 250 m deep by 700-1000 m wide. It is filled with soft sediments from the Tertiary and Quaternary periods above hard dolerite bedrock of Jurassic age. Observed coherency spectra of the vertical component are analyzed at two sites in the Tamar Valley; using two 50-m-radius centered triangular arrays above the deepest point of the valley at site DBL, and above the east flank of the valley at site RGB. Simulated and observed coherency spectra suggest the propagation of Rayleigh waves of first higher mode at the SV frequency of resonance of the Tamar Valley affects the coherency spectra recorded with pairs of sensors perpendicular to the valley (transverse-COH). Simulated and observed coherency spectra recorded above the deepest point of the valley (site DBL) with pairs of sensors parallel to the valley axis (axial-COH) are not affected by these edge-generated Rayleigh waves and agree well with the theoretical coherency spectrum computed from the preferred 1D SWV profile. The simulated and observed results from this paper suggest that differences between the observed axial-COH and transverse-COH give an indication of the exis- tence of the 2D buried valley. Results also suggest that the observed coherency spectra recorded on pairs of sensors oriented parallel to the valley axis can provide a reliable evaluation of a 1D SWV profile above the deepest point of a deep and narrow valley, such as the Tamar Valley.