Exploration Geophysics - Volume 43, Issue 4, 2012

The dominant north–south strike of the Palaeozoic outcrop of central Victoria has been well documented, but to the north, these rocks are covered by the Cainozoic sedimentary deposits of the Murray Basin. Two magnetotelluric surveys were completed to assist in extrapolation of the known structure and to identify possible new targets for mineral discovery. Supporting the results from previous seismic interpretations for the region, the 2D MT inversion models substantiate an intrazone thrust fault system of listric geometries in the Bendigo Zone connected in the mid-crust. With the zone boundary clearly defined the electrical resistivity structure is distinct between the major subdivisions, indicating a different tectonic evolution for the Bendigo and Melbourne Zones. However, the conductive overburden in the region poses complications for the generation of the 2D resistivity models. Static shifts and electrical anisotropy were identified as distortions in the dataset, with further processing needed to attain a complete picture of the underlying geology. The difficulties caused by galvanic distortion were allayed by using the phase tensor response in place of the distorted amplitude response. Phase tensor analysis of MT data has been completed subsequently, the results of which we present here, along with the original 2D inversion models, confirming that electrical anisotropy persists into the mantle.

In regions where conductive overburden dominates the near-surface, static shifts and electrical anisotropy can present significant complications for the generation of 2D magnetotelluric resistivity models. Affecting results collected in southeastern Australia, such difficulties caused by these galvanic distortions have thus been allayed through phase tensor analysis of the data.

Common offset refraction (COR) traveltime attributes are derived from multi-fold data with novel adaptations of the generalised reciprocal method (GRM). COR GRM stacks are generated from a refraction equivalent of common midpoint (CMP) gathers, which are computed at each CMP with the COR GRM algorithms. The COR GRM stacks, which generate detailed spatially varying attributes for each layer detected in the near surface region, provide useful starting models for automatic refraction tomography.

The spatial resolution of the depth models of the wavepath eikonal traveltime (WET) refraction tomograms obtained with starting models derived with the COR GRM is similar to the WET tomogram obtained with the standard GRM, whereas the COR GRM seismic velocity model is a smoothed version of the standard GRM model. In all cases, the GRM-derived WET tomograms avoid the generation of undetectable artefacts with common implementations of automatic refraction tomography, which can occur with the use of default starting models consisting of smooth vertical velocity gradients and with the need to minimise misfit errors through over-processing.

The COR GRM attributes demonstrate that the traveltime data are consistent with minimal penetration within the sub-weathering, representative of uniform seismic velocities, and that the spatial variations in the time model and seismic velocities are more significant than any variations caused by vertical velocity gradients in the sub-weathered zone. However, the occurrence of vertical velocity gradients in the sub-weathering largely remains unresolved because minimal penetration of the first arrivals can occur even with large vertical velocity gradients, such as the hyperbolic velocity function.

The WET tomograms generated with the COR GRM time model and seismic velocity attributes are generally very similar visually to the starting models, even though the misfit errors may differ. It is concluded that COR GRM starting models can frequently be a useful alternative to refraction tomography.

Common offset implementations of the generalised reciprocal method generate detailed spatially varying models of the near surface from multi-fold seismic refraction data. These models facilitate the elimination of undetectable artefacts with automatic refraction tomography, the validation of vertical velocity gradients and the convenient evaluation of large sets of traveltime data.

This paper presents the outcome of reconnaissance surveys in metropolitan Manila (Metro Manilla), the Philippines, with the aim of mapping shallow shear-wave velocity structures. Metro Manila is a seismically active and densely populated region that is in need of detailed investigation of the subsurface structures, to assess local site effects in seismic hazard estimation. We conducted microtremor array observations and used the spatial autocorrelation method to estimate the shear-wave profiles at 32 sites in major geological settings in Metro Manila. We applied a hybrid genetic simulated annealing algorithm to invert phase velocity data from the spatial autocorrelation method to generate shear-wave velocity models near the global best-fit solution. The comparison between the inferred shear-wave velocity profiles and PS logging showed good agreement in terms of the fundamental mode of Rayleigh waves and site responses. Then, we utilised the inferred shear-wave velocity profiles to compute the site amplifications with reference to the motion in engineering bedrock. Subsequently, the site amplifications have been grouped, based on NEHRP site classes. The amplification factor has also been compared with the average shear-wave velocity of the upper 30 m at each site, to produce a power-law regression equation that can be used as a starting basis for further site-effects evaluation in the metropolis.

Microtremor array observations were conducted in Metro Manila, the Philippines, to estimate the shear-wave velocity profiles at 32 sites. The inferred shear-wave velocity profiles were utilised to compute the site amplifications with reference to the motion in engineering bedrock. Subsequently, the site amplifications were grouped based on NEHRP site classes.

New methods for inverting gradient tensor surveys, based on eigenanalysis and the normalised source strength derived from the eigenvalues, have been developed for point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models.

Acquisition of magnetic gradient tensor data is likely to become routine in the near future. New methods for inverting gradient tensor surveys to obtain source parameters have been developed for several elementary, but useful, models. These include point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The normalised source strength (NSS), calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over 3D compact sources, 2D compact sources, thin sheets and contacts, and is independent of magnetisation direction. In combination the NSS and its vector gradient determine source locations uniquely. NSS analysis can be extended to other useful models, such as vertical pipes, by calculating eigenvalues of the vertical derivative of the gradient tensor. Inversion based on the vector gradient of the NSS over the Tallawang magnetite deposit obtained good agreement between the inferred geometry of the tabular magnetite skarn body and drill hole intersections. Besides the geological applications, the algorithms for the dipole model are readily applicable to the detection, location and characterisation (DLC) of magnetic objects, such as naval mines, unexploded ordnance, shipwrecks, archaeological artefacts, and buried drums.

The palaeomagnetic analysis of the Late Cambrian Cupala Creek Formation suggests that the high temperature component is primary in origin. It is inferred, therefore, that this area of the Delamerian Orogen did not record any movement or rotation since the Late Cambrian and can be considered as part of Gondwana.

The Tasmanides of south-eastern Australia comprises the Delamerian Orogen, considered to be stable relative to the craton of Gondwana since the mid Cambrian, despite the presence of the Grasmere Knee Zone, a change of structural trends in the Broken Hill area. A palaeomagnetic study has been carried out on the Late Cambrian red sandstones of the Cupala Creek Formation, a post-Delamerian formation gently folded in the latest Ordovician–Early Silurian and/or Early Devonian. Anisotropy of magnetic susceptibility (AMS) measurements show that internal strain is negligible and low Königsberger ratios indicate that lightning effects can be discarded. The magnetisation observed is complex with four components, which are sometimes difficult to isolate clearly. The orientation of the first component corresponds to the present-day field and appears to be associated with weathering effects. The second component (~185°−350°C) is of reverse polarity relative to the third (350°−575°C). These two components are interpreted to represent a chemical remagnetisation, which lasted long enough to record a reversal as the reversal test is positive and classified C. The overprint must be Early Palaeozoic in age. The last component is carried by haematite and/or maghaemite and is believed to be primary, which is supported by the positive ‘pseudo-unconformity’ test with the underlying Early–Middle Cambrian Teltawongee beds, and by the good correspondence of the pole [PLong. 351.3°/PLat.+33.9° (dp = 3.6°; dm = 6.0°); African coordinates] with other poles of the same age for Gondwana. Despite the presence of the Grasmere Knee Zone, these results imply that this area of the Delamerian Orogen did not record any movement or rotation since the Late Cambrian and can be considered as part of the craton of Gondwana.