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We present a method for measuring the dispersion of higher mode surface wave phase velocities from a single seismogram using a hierarchical transdimensional Bayesian approach. The 1-D shear velocity profiles down to 800 km depth between sources and seismic stations are regarded as the controlling parameters to tune the phase velocities of fundamental and higher modes. The misfits between synthetics and real waveforms indicate whether the phase velocities are recovered well from the data. We use Monte Carlo Markov chains to approximate the posterior distribution of each model parameters and assess the uncertainties from these probability density functions. These techniques can test models of varying dimensions while being parsimonious, thereby letting the data themselves control the complexity of the solution. Another advantage is that the algorithm can decide how much data noise is needed in order to explain the data without overfitting them. The data noise can be treated as an unknown and different noise levels can be applied to the different time windows considered. The posterior noise distributions can then be used as an indicator of the quality of the waveform fit within each frequency–time window. We considered phase velocities between 50 and 200 s for each mode and performed a reliability analysis to determine which modes and periods are reliably constrained. In this paper, we first present the method and demonstrate its feasibility with synthetic tests, which show that the technique is robust. We then illustrate it with applications to real data. We applied the method to two paths sampling Australia using earthquakes at regional distances and obtained results that agree well with previous studies. The new method can be used in regional and global tomographic studies to obtain phase velocity maps and 3-D models of seismic velocities and anisotropy at depths that are not well resolved by fundamental mode surface waves or body waves. [ABSTRACT FROM AUTHOR]

In the absence of quantitative relationships, a cross-gradient function can be used to correlate unknown physical properties in a joint inversion of geophysical data sets. It introduces a structural correlation between properties. A commonly used, inexact approach adds a weighted cross-gradient term as a penalty to the cost function being minimized during inversion. This weighting factor needs to be tuned to balance the regularization and cross-gradient terms. In this paper we propose nonlinear weighting for the cross-gradient function which addresses the very different magnitudes of the cross-gradient and regularization terms. This approach also couples the weighting factors for the regularization and correlation terms reducing the number of tuning parameters. The approach is investigated for a synthetic case. Results are also shown for the 3-D joint inversion of high-resolution magnetic and gravity anomaly data from Southern Queensland in Australia with over 30 million cells. [ABSTRACT FROM AUTHOR]

The article presents information about various papers related to geophysics. Some of the papers are "The Seismic Structure of the Saurashtra Crust in Northwest India and Its Relationship With the Reunion Plume," by G.S.P. Rao and H.C. Tewari, "Maximum Horizontal Stress Orientations in the Cooper Basin, Australia: Implications for Plate-Scale Tectonics and Local Stress Sources," by S. D. Reynolds, S. D. Mildren, R. R. Hulls, J. J. Meyer and T. Flottmann, "A Magnetic Field Model With Daily Variations of the Magnetospheric Field and Its Induced Counterpart in 2001," by V. Lesur, S. Macmillan and A. Thomson.

Crustal scale seismic images provide information on the geometry of subsurface structure. In this paper we examine shear zones as they provide geometrical constraints on the evolution of the crust and as they provide pathways for the migration of mineral-rich fluids from the lower crust. However, they typically appear in seismic images of the deep crust as laterally continuous bands of discontinuous reflections with individual reflections often having high amplitudes. Geological mapping of exposed shear zones show them to have a complex 3-D structure yet crustal-scale seismic reflection surveys use single or at the most only a few profiles, and therefore only create 2-D images of these structures. The processing and imaging of the multifold common midpoint (CMP) data assumes that the seismic energy comes entirely from within the plane of the section. In this paper, we use full-waveform 3-D synthetic data to consider the effects that 3-D topography on a reflector has on reflection character on a 2-D profile. We base our synthetics on an observed shear zone and test models with both a single layer and anastomizing layers. We show that topography on the reflector out of the plane of the section may cause spurious events both above and below the expected target depth. We derive the basic understanding using a simple isotropic homogeneous model, however, we then demonstrate that this is a robust phenomenon and is endemic on all 2-D sections even if the overburden is not homogeneous. We demonstrate that we obtain similar results with a velocity gradient or, for a more extreme case, with a 2-km-deep basin filled with low-velocity sediment. For crustal scale seismic profiles, in particular, the effect is pervasive as neither stacking nor migration can discriminate against out-of-plane energy and the 2-D stack represents the 3-D seismic response of a broad swath centred on the profile. However, we conclude that using the modelled data it is possible to identify qualitatively where there is significant contamination from out-of-plane topography and show examples from a shear zone in the Archaean Yilgarn Block in Western Australia. [ABSTRACT FROM AUTHOR]

In this paper, we report the 3-D electrical conductivity distribution beneath the Australian continent in the depth range 410–1600 km, which we have imaged by inverting C-response estimates from a regional network of geomagnetic observatories. The inversion scheme is based on a quasi-Newton optimization method while the forward algorithm relies on an integral-equation approach. To properly account for the ocean effect in responses at coastal observatories we included a high-resolution (1° × 1°) fixed thin laterally varying surface conductance layer. As starting model in the inversion we considered a laboratory-based 3-D conductivity model of the region obtained from seismic surface wave data and thermodynamic modelling. This model provides a good fit to observed C-response estimates supporting its choice as initial model. The most striking feature of the obtained 3-D model is a high-conductivity anomaly in the lower part of the mantle transition zone (MTZ; 520–660 km depth) beneath southeastern Australia implying considerable lateral as radial heterogeneity in the conductivity structure. The high-conductivity region appears to be 0.5–1 log units more conductive than previous global and other regionalized 1-D models. Further analysis using laboratory-based conductivity models combined with thermochemical phase equilibrium computations shows that the strong conductivity anomaly implies water contents of around 0.1 wt per cent in the upper part and >0.4 wt per cent in the lower part of the MTZ. This implies a large MTZ water reservoir that likely totals one to three times that which currently resides in the oceans. The amount of water in the lower MTZ appears to exceed the experimentally determined water storage capacity of the main lower MTZ mineral ringwoodite, which, as a result, undergoes dehydration-induced partial melting. Including contributions to conductivity from a thin melt layer (20 km thick) located in the mid-MTZ increases conductivity locally in the melt layer to ∼1 S m−1, that is, about 0.5 log units more conductive than the average surrounding mantle. This provides an adequate explanation for the strong conductivity anomalies observed beneath part of the continent and points to lateral variations in melt in the MTZ. [ABSTRACT FROM AUTHOR]