Exploration Geophysics - Volume 38, Issue 3, 2007

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).

In this work we present a methodology for extracting valuable information from several seismic attributes by computing seismic similarity values through a pattern recognition approach, which is based on self-organising maps. This methodology allows for identifying regions with seismic properties similar to a pre-defined reference location of interest, for instance, a good well producer or a dry well; and it can be used during the exploratory phase when only limited and scarce well information is available. The methodology we propose improves the classical seismic similarity analysis and can be used on two-dimensional seismic maps or three-dimensional seismic volumes for frontier exploration, i.e. where there are scarce or limited well data but much seismic information. Using two case studies, we show how the proposed method constitutes a valuable tool for exploration geophysics and reservoir characterisation.

Synthetic experiments are used to test the applicability of coda wave interferometry (CWI) as a means for estimating distance between sources of nearby earthquakes. Acoustic waves for 45 sources are propagated through a Gaussian random medium. A pair-wise analysis of resulting waveforms illustrates the applicability of CWI as a tool for estimating source separation. Results suggest that, when the waveforms are filtered between 1 and 5 Hz, CWI provides accurate estimates of the separation for source-pairs separated by δ < 250 m. The technique provides a lower bound on the actual separation when δ > 250 m. The CWI breakdown distance of 250 m is likely to vary with frequency content in the waveforms.

The interpretation of CWI source separation estimates is aided by the construction of a conditional probability density function (PDF) P(δt ∣ δCWI), which describes the probability of actual separation δt for given CWI estimates δCWI. The conditional PDF provides a constraint on event separation that is asymmetric. It can be used independently of, or combined with, standard travel-time techniques to improve earthquake location.

The Centenary gold deposit is a concealed ore body located 110 km north of Leonora, Western Australia. The orebody is associated with sulphides and is hosted in the magnetic portion of the Mount Pickering Dolerite. Due to its sulphidic nature, both gravity and induced polarisation (IP) were trialled soon after discovery.

The gravity survey showed major structures and delineated the host magnetic dolerite, and a trial dipole–dipole IP and resistivity survey detected a significant chargeability anomaly over Centenary. Interestingly, both forward and inverse models showed an IP anomaly that was broader than, and displaced from, mineralisation. Down hole IP and resistivity surveys also showed an elevated chargeability response shallower and broader than the intersected mineralised zone. Pyrite is the main sulphide associated with Centenary and is spatially related to gold mineralisation. These data therefore suggested that pyrite was not the sole contributor to the chargeability response of Centenary.

Petrophysical results, integrated with examination of thin sections, found that the five samples giving the highest chargeability response contained at least 5% pyrite and 5% magnetite, and at least 15% magnetite and pyrite combined. Samples with comparable amounts of pyrite, but less magnetite, gave a lower chargeability response. This supports a hypothesis that rocks containing both magnetite and pyrite at Centenary can generate a larger IP response than rocks containing pyrite or magnetite alone.

Numerical inversion of geophysical data does not normally require user interaction apart from the selection of initial inversion parameters. However, such an inversion often returns a single solution based upon default parameters. While this solution will be geophysically correct, assuming convergence of the algorithm, it may not be the most geologically reasonable answer. It is necessary to incorporate human interaction in selecting inversion solutions, this being the most efficient method for adding qualitative geological constraints. An automatic system provides a user-directed search of the space of geophysical solutions. Rankings assigned to numerical inversion results guide a genetic algorithm in advancing towards a conceptual target. Our example uses resistivity and chargeability data from a pole-dipole induced polarisation survey collected during a mineral exploration program. We invert for specific geological features: a defined, conductive top layer, sharp geological boundaries in the resistivity, and greatest depth of resolution of the inversion algorithm. The interactive system is an organised way to investigate the solution space for valid inversion results that emphasise these geological possibilities.