Maxime Claprood

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.

Initial results from spatially averaged coherency, frequency-wavenumber, and horizontal to vertical spectrum ratio microtremor survey methods for site hazard study at Launceston, Tasmania

The Tamar rift valley runs through the City of Launceston, Tasmania. Damage has occurred to city buildings due to earthquake activity in Bass Strait. The presence of the ancient valley, the Tamar valley, in-filled with soft sediments that vary rapidly in thickness from 0 to 250 m over a few hundreds metres, is thought to induce a 2D resonance pattern, amplifying the surface motions over the valley and in Launceston. Spatially averaged coherency (SPAC), frequency-wavenumber (FK) and horizontal to vertical spectrum ratio (HVSR) microtremor survey methods are combined to identify and characterise site effects over the Tamar valley. Passive seismic array measurements acquired at seven selected sites were analysed with SPAC to estimate shear wave velocity (slowness) depth profiles. SPAC was then combined with HVSR to improve the resolution of these profiles in the sediments to an approximate depth of 125 m. Results show that sediments thicknesses vary significantly throughout Launceston. The top layer is composed of as much as 20 m of very soft Quaternary alluvial sediments with a velocity from 50 m/s to 125 m/s. Shear-wave velocities in the deeper Tertiary sediment fill of the Tamar valley, with thicknesses from 0 to 250 m vary from 400 m/s to 750 m/s. Results obtained using SPAC are presented at two selected sites (GUN and KPK) that agree well with dispersion curves interpreted with FK analysis. FK interpretation is, however, limited to a narrower range of frequencies than SPAC and seems to overestimate the shear wave velocity at lower frequencies. Observed HVSR are also compared with the results obtained by SPAC, assuming a layered earth model, and provide additional constraints on the shear wave slowness profiles at these sites. The combined SPAC and HVSR analysis confirms the hypothesis of a layered geology at the GUN site and indicates the presence of a 2D resonance pattern across the Tamar valley at the KPK site.

Statistical Validity Control on SPAC Microtremor Observations Recorded with a Restricted Number of Sensors

Abstract The interpretation of an observed spatially averaged coherency (SPAC) spectrum assumes spatial and temporal stationarity of the microtremor wave field. This hypothesis gains in importance when recording coherency spectra (COHs) with a limited number of sensors. SPAC observations were recorded at three separate sites in Launceston (Australia) using a pair of sensors, triangular arrays, and hexagonal arrays to study the effect of the number of sensors, length of time series, and frequency interval for evaluating the shear-wave velocity (SWV) profile. The imaginary component of the observed complex COH is separated into roughened and smoothed parts. The root mean square of the roughened imaginary COH (rms(Im)) is an expression of the statistical noise in the observed coherency, while the behavior of the smoothed imaginary COH gives some indication of the distribution of the microtremor wave field. The mean square of residuals (MSR) between observed and theoretical COHs is an indication of the confidence level on the SWV profile interpreted. MSR values evaluated at pairs of sensors of different orientations give some indication of the azimuth distribution of the microtremor wave field and provide some guidelines on the field procedure when using a limited number of sensors to record SPAC observations. Observed COHs at site KPK demonstrate the importance of recording longer time series to increase the stability of the observed COHs. The increasing level of statistical noise with increasing frequency at site DBL suggests the SPAC method can be used with a single pair of sensors by restricting its upper frequency limit to the first minimum of the COH. Low values of MSR on observed COHs from most pairs of sensors further strengthen that hypothesis. SPAC observations at site RGB give further insights into the capabilities of the SPAC method to evaluate the SWV profile in limited azimuth microtremor wave-field distribution.

Rapid detection and classification of airborne time-domain electromagnetic anomalies using weighted multi-linear regression

Abstract We propose a rapid and efficient methodology for the detection and interpretation of airborne time-domain electromagnetic anomalies generated by thin sheet-like volcanogenic massive sulphides (VMS) deposits in a resistive environment, which are representative of VMS deposits in the Canadian Shield. In the first step of the approach, we use high-order statistics for the detection and the recognition of a MEGATEM anomaly as indicating a thin sheet-like VMS deposit with respect to three criteria of detection: the minimum level of detection, the length of detection, and the coherence of detection over time. We adapt these criteria in order to optimise the detection of thin sheet-like VMS deposits against geological noise models. Once the anomaly is detected and recognised as the response to a thin sheet conductor, we interpret the model geometry and physical property using attributes calculated from the MEGATEM anomaly. We develop a system of weighted multi-linear regression to find the most significant attributes to estimate the dip, depth, conductance, and dimensions of a thin sheet-like VMS deposit. Stepwise regression suggests that shape attributes are most significant to estimate dip while depth is most strongly estimated by size attributes. The most significant attribute to estimate the conductance is the time constant. The size is best estimated by attributes related to the size of the anomaly. We test the regression system on thin sheet models with excellent performance. Most of the parameters of the thin sheet models were estimated within an interval of confidence about the initial property. We further test the system by estimating properties of three VMS deposits in the Abitibi Greenstone Belt, Québec, Canada, for which the geometries and geological properties are known. Most parameters are estimated within the interval of confidence for ISO, a thin sheet body, while the estimates for New-Insco and Gallen show more variability caused by departure from the reference thin sheet model.