ASEG Extended Abstracts - 24th International Geophysical Conference and Exhibition – Geophysics and Geology Together for Discovery, 2015

The continental crust of southeast Australia is a complex and highly prospective area. Southeast Australia comprises the Delamerian and Lachlan Orogenies which, together with the Eastern Tasmania Terrane, are understood to have Phanerozoic basement. In contrast, the Western Tasmanian Terrane comprises areas of exposed Neoproterozoic basement which were assembled along the proto-Pacific margin of East Gondwana. In this study, the crustal structure across southeast Australia and Tasmania is considered using seismic and aeromagnetic methods. We use previous passive seismic results and present a new analysis of magnetic data. The Curie temperature, the temperature at which magnetic rocks lose their magnetisation, is investigated using spectral analysis of aeromagnetic data and the Curie point depth (CPD) is consequently determined. CPD is compared to the depth of the seismic Moho discontinuity throughout the study area.

The Moho depth and newly calculated CPD throughout the study area vary from ~20 to >38 km and ~25 to >45 km, respectively. The CPD is slightly shallower than the Moho across the study area. The Delamerian and Lachlan Orogenies are underlain by a 30-35 km and -40-50 km deep Moho respectively, while average CPD depths are ~30 and ~28 km for these regions. A relatively shallow CPD is observed in the northeast of the study area and corresponds to Cainozoic volcanism in eastern Australia. The shallow Moho beneath Tasmania supports the idea of crustal thinning during Gondwana breakup. In Tasmania, CPD increases in depth from ~21 km in the northwest to >31 km in north. This is consistent with variations in the depth of the Moho from 25 km in the northwest to 37 km in the north.

The Southern Thomson Orogen airborne electromagnetic (AEM) Survey was flown in 2014 using the Geotech VTEMplus® AEM system. The AEM survey was designed by Geoscience Australia, and its partners the geological surveys of New South Wales and Queensland, to help solve geological problems in the Southern Thomson Orogen as part of the UNCOVER Initiative of the National Mineral Exploration Strategy.

Survey results indicate variable depth of penetration governed by conductive cover, primarily the Cretaceous Rolling Downs Group, and saline groundwater in broad ephemeral drainage systems including salt lakes and channel country around the Paroo River. The unconformity between the Paleozoic rocks of the Eulo Ridge (a partially-exposed palaeotopographic high) and the overlying Mesozoic and Cenozoic cover is well mapped in the central part of the survey area. The survey data reduce risk to explorers in the area by decreasing uncertainty regarding depth of cover for drilling activities and advising where ground and airborne electromagnetic methods can be expected to produce reliable results.

We investigate a small (100 metre width) 150 nT amplitude magnetic anomaly delineated in a high-resolution aeromagnetic survey in the Teetulpa Goldfield of the southern Flinders Ranges. We believe that the anomaly is due to a kimberlite pipe, part of a field already known in the general region. As is quite common with kimberlites, the magnetization is clearly dominated by remanence. Modelling the anomaly reveals that the source is very shallow, and would have outcropped at some stage. Follow-up ground geophysical, geochemical and biogeochemical investigations are planned to establish a methodology for integrated studies as follow-up to high resolution aeromagnetic surveys.

A gravity survey has been undertaken on the Alcurra, Agnes Creek and Tieyon 1:100K mapsheets, in the eastern portion of the Musgrave Ranges, South Australia. A total of 821 readings were collected, incorporating 689 new stations, 88 repeats and 44 base measurements.

The data highlights new features giving insight into the underlying geological structure, including a gravity high near Doug’s Well.

The integration of geophysical data including deep seismic reflection, receiver function and gravity data provides the ability to image the deep crustal structure of Capricorn Orogen. In this study, we aim to reconcile seismic models of crustal structure with gravity measurements.

S-wave seismic velocities from receiver function models were converted to P-wave velocity and density. Geology interpreted from deep seismic reflection profiles was depth converted based on the velocity data.

An initial density model was constructed based on the depth converted seismc interpretation and densities estimated from velocities. To match observed variations in Bouguer anomaly the initial densities were modified based primarily on the comparability the wavelength of Bouguer anomaly and calculated gravity response of the model.

We found that the Bandee Seismic Province (BSP) has a high density compared to the density of the adjacent regions and the underlying lower crustal blocks. This province, a ‘deep crustal seismic terrane’ without surface expression, recognised from the seismic data. Sensitivity testing suggests the mantle, the lower crust and the upper crust are less likely sources of the coincident high gravity anomaly. This observation is supported by receiver function models that reveal a high S-wave seismic velocity in mid-crust.

Importantly, the region of higher density in the middle crust, unlike the BSP, does not extend south of the Talga Fault. This suggests the Talga Fault is a more significant structure than suggested by previous interpretations and the area around its surface outcrop may be more prospective than previously thought.

Wireline logs are a supplemental data source to conventional core logging. The recent explosion of machine learning algorithms has provided researchers with ample opportunity to develop automated statistical tools for classifying lithology from wireline logs, which geologists can use to produce first-pass interpretations or to validate existing interpretations. Such automated interpretations can be particularly valuable information in the case of missing or damaged core samples. There exists, however, a need to evaluate said machine learning algorithms in the case where available wireline logs contain a wide range of different logs which are highly-sampled.

This paper explores different machine learning algorithms and architectures for lithology classification using wireline data from project area Jundah East, 60 km north-west of Wandoan, Queensland, which is well known for coal mineralisation. We used seven well logs each containing 19 wireline logs sampled at 1 cm^-1, available through the Queensland Digital Exploration (QDEX) data system. Three popular supervised machine learners, namely the Naive Bayes classifier, Support Vector Machine, and Multilayer Perception (an artificial neural network), are tested under two architectures: committee (one classifier per well log) and singular (one classifier for all well logs). The results show the Naive Bayes classifier, although computationally simple, achieves good results in general when training using a committee architecture on a large data set. For coal classification in particular, it achieved the sensitivity score of 0.79 and the specificity score of 0.97. While the committee and singular architectures generated similar results, the committee architecture provides the benefits of faster computation time as well as a flexible platform for the training of additional well logs.

The integration of geophysical with litho-structural models represents a valuable tool for better understanding of subsurface geometries of lithological contacts. Improved subsurface models add value to mineral exploration projects. Geophysical data is used to enhance and validate litho-structural models. The regular distribution of geophysical data allows lithologies and faults to be extended from observed locations into the subsurface. Geological models are validated and improved by comparison of the geophysical signal calculated from the model geology with the observed signal. Discrepancies between modeled and observed signals highlight areas requiring refinements of the geological model.

The case studies present examples of how iterative modeling from geological and geophysical data will result in an improved final product. The calculated geophysical signal from two distinct geological interpretations shows how well a certain litho-structural model conforms with the geophysical data. Applications are in determine the position of rocks of distinct physical properties, checking the geometry of faults and extending mapped structures into inaccessible/covered areas.

A 3D high-resolution seismic dataset was acquired to investigate typically complex IOCG deposits in Hillside, South Australia. Petrophysical data measured from the core samples and the density data supplied by the mining company were utilised during the volumetric interpretation. However, petrophysical data show that the boundaries between gabbro and metasediments may not generate acoustic impedance contrast to be clearly detected by seismic reflection method. The base of the top cover is mappable throughout the cube and the tops of the major formations have agreement with magnetic data. The faults extracted from the seismic volume using ant-tracking attribute have good agreement with the company supplied geological interpretation based on the drilling information.

Original motivation of this study was to understand important structures with a proven geothermal signature associated with high temperatures in the shallower basin and high flow rates in the aquifers. Anomalous temperatures are recorded around the Beagle Ridge and significant flow rates observed near the Urella Fault, factors important to unconventional geothermal prospects. The focus of this study was therefore a detailed geophysical investigation of several Geothermal Exploration Permits (GEPs) in the North Perth Basin.

Two Magnetotelluric (MT) surveys are conducted over target areas in the North Perth Basin and adjoining tectonic domains to provide information about the electrical conductivity regime of the basin and western margin of the Yilgarn Craton. Existing geophysical data in this part of the basin are sparse and electrical data for the basin in general is limited to shallow Time Domain Electromagnetic (TDEM) data targeting superficial aquifers.

High-resolution MT data, acquired between 2011 and 2013, provide information on mid-deep crustal rocks. In addition, new gravity data and joint interpretation of these data sets are undertaken to improve the geological model of the North Perth Basin and test some existing hypotheses.

We present a best practice case study and workflow for data acquisition and filtering, robust dimensionality analysis and removal of distortion effects from impedance tensor estimates. 1D and 2D inversions are found to be largely sufficient for the majority of these data while 3D modelling provides an additional tool to verify results. Finally, modelling of gravity data and integrated interpretation ensures robust geological models for the area are consistent with all data available.

We conclude with several inferences about the geology in this area. 1) Electromagnetic (EM) and gravity data does not seem to support significant crustal thinning beneath the basin. 2) The Dandaragan Trough appears deeper than generally modelled and 3) extremely high conductivities persist to depth in the basin.

The Frome airborne electromagnetic (AEM) survey was designed to provide reliable pre-competitive AEM data to aid the search for energy and mineral resources around the Lake Frome region of South Australia. Flown in 2010, a total of 32,317 line kilometres of high-quality airborne geophysical data were collected over an area of 95,450 km² at a flight line spacing mostly of 2.5 km, opening to 5 km spaced lines in the Marree-Strzelecki Desert area to the north.

Interpretations of the data show the utility of regional AEM surveying for mapping crucial elements of sandstone-hosted uranium mineral systems as well as for mapping geological surfaces, structures and depth of cover over a wide area. Data from the Frome AEM survey allow mineral explorers to put their own high-resolution AEM surveys into a regional context. Survey data were used to map and interpret a range of geological features that are associated with, or control the location of, sandstone-hosted uranium mineral systems, and have been used to assess the uranium prospectivity of new areas to the north of the Flinders Ranges.

Modelling IP parameters, including dispersive resistivity, from AEM data showing clear IP effects is possible. Using the spatially constrained inversion approach, with forward response that account for the full Cole and Cole model, we recover realistic chargeability and “IP corrected “resistivities sections from two VTEM datasets, from Canada and Australia. The “IP corrected” resistivity sections often show better agreement with known geological features, while improving dramatically the data fit, with respect to those obtained without IP modelling. While the majority of the IP effect originate from shallow chargeable layers, there seems to be some positive correlation between an isolated deep chargeable anomaly and known base metal deposit location.

Wide-spread application of gamma ray logging system in mineral exploration has long been desired by the mining industry, but not achieved. New developments in borehole logging approaches the Deep Exploration and Technologies Commonwealth Research Centre (DET CRC) have recently created Logging-While-Drilling tools, named the Autosonde and Shuttle, to measure natural gamma ray activity during drilling process. The Shuttle in particular will allow the collection of high quality natural gamma spectra by placing a sensor on the core-barrel and logging whilst drilling; a much slower process than wireline logging. Thus, a new approach collecting gamma data provides opportunities to use natural gamma radiation data in ways not normally done with conventional wireline tools.

We have used a prototype spectral gamma sensor using BGO crystal on a wireline to simulate the data that will be collected by a shuttle system to demonstrate the data quality and to test whether more sophisticated data analysis of spectral attributes such as the ratio of Photoelectric to Compton gamma bands, or “heavy minerals indicator”, will further lithological information than the standard K, U, Th analysis. Our preliminary results indicate that spectral data collected by the Shuttle will allow the heavy mineral indicator to be used and opens the possibility of better lithology identification tools.

In this study, the fuzzy c-mean clustering method was used in an unsupervised manner to automatically classify the different lithologies present at the Hillside prospect (Yorke Penninsula, SA). The algorithm was applied to various combinations of petrophysical and geochemical data to identify the combination that returned the most accurate result and the smallest combination that provides a nearly identical success as the best. We show that by using a combination of geochemical and petrophysical data the likelihood of a correct classification increases by 5% compared to analysing only geochemical data, and by over 20% compared to analysing only petrophysical data. However, using a few common elements and a few petrophysical values we can achieve almost the same success rate as the best result. Improvements in pre-treatment and conditioning of the data should allow the fuzzy cluster algorithm yield even better results. In addition to showing that combining petrophysical and elemental analysis is more robust, we demonstrate that if we could add some targeted elemental analysis to logging while drilling (LWD) then robust automated lithological logging becomes feasible.

Myanmar’s onshore block EP-2 is a small exploration concession with high potential structures located on the eastern side of the Irrawaddy River. One of the major anticlinal structures can be mapped based on surface geological survey, but the recent 2D seismic data has extremely poor quality due to a severe static and strong side scattered noise. Although the size of this prospect is attractive, the subsurface uncertainties are considered too risky for further drilling operation. An alternative acquisition technique called “2D-wide line” was firstly implemented in Myanmar. Its results provide better stack sections, more reliable velocity function and higher geological probability of success.

The north-west margin of the Dampier sub-basin is characterised by a strongly segmented fault pattern. NE trending faults define the edge of the Rankin Platform, and separate it from the Kendrew Trough. However a secondary set of NNE trending faults define smaller scale graben on the edge of the Rankin Platform that preserve Lower and Middle Jurassic sediments. This strongly suggests oblique reactivation of an inherited NE trending basement fabric under WNW oriented extension during Middle Jurassic extension.

This paper presents our continuous effort on multiples attenuation in shallow water environment. The seismic data were acquired at various locations along the offshore of Vietnam, and the interpretation of the data is made difficult due to the presence of shallow water related multiples. Previously, we have demonstrated a two-step workflow of applying first shallow water de-multiple (SWD) and then surface related multiple elimination (SRME) to handle shallow water multiples. This workflow has been proven to be effective as it has been applied on many marine seismic surveys.

In this paper, we expand our current workflow by incorporating the data before and after SWD as the dual input for 3D SRME model prediction, namely 3D iterative SRME for shallow water for targeting higher order peg-leg surface related multiples that includes the seafloor as one of the multiple generators. Apart from being data driven, 3D iterative SRME for shallow water also takes into consideration the spatially-varying nature of subsurface structures.

We demonstrate, through the real-data examples, that our workflow provides an optimal multiple attenuation solution in the shallow water environment.

The interpretation of ‘vintage’ seismic data acquired in underexplored frontier basins is often challenged by their sparse coverage. This example from the Canning Basin illustrates how FALCON® Airborne Gravity Gradiometer (AGG) data greatly enhances the 2D seismic interpretation, facilitating exploration in such frontier basins.

The initial seismic interpretation was performed by Buru Energy, and given the ‘vintage’ data, was limited at best. The integration of the AGG, magnetic, well, and other available data allowed the improvement of seismic interpretation. A basement structure map, and two intra-sedimentary structure maps were produced, resulting in an overall geological model.

In particular, the initial seismic interpretation of seismic traverses perpendicular to strike across the AGG survey could be significantly improved by using images of the AGG data and AGG profile data (GDD and gD). The AGG data and the structure maps were used to constrain fault locations and depths as well as thickness distributions of geological units. The interpreted seismic traverses were validated by 2.5D gravity modelling, ultimately resulting in a conceptual geological model.

This is a key-method to constrain the interpreted geology, providing a more confident interpretation of ‘vintage’ reflection seismic data with sparse coverage.

It is very important to know the subsurface structure and depositional environment from the coastal to the shallow sea region, when studying groundwater flow. However, when we acquire geophysical survey data, since the data acquisition methods of both land and marine cannot successfully acquire the data at the domain from the coastal to the shallow sea regions, such regions are often left as blank of geophysical surveys. Furthermore, in our country, such domains are generally highly developed and even setting up geophysical survey lines are difficult. Therefore, we are investigating the geophysical survey methods appropriate for the surveys beneath the coastal to the shallow sea region. We are also developing the evaluation methods for such regions. Therefore, we conducted a seismic reflection survey to image the subsurface structure of coastal to the shallow sea region of the mouth of the Fuji River, Shizuoka, Japan where the Fujikawa-kako fault group exists, and checked applicability of the technique. As a result, we obtained subsurface structure down to the 5000 m.

The North-western Bonaparte Basin offers a very good opportunity to understand the nature of oblique extension system, where Neogene flexure-induced extension was superimposed obliquely to the Mesozoic rift-related structures. The Mesozoic trends strongly control the distribution and style of the younger Neogene structures, both at regional and local scale. The younger Neogene activity produced a new set of NE trending, right-stepping en echelon faults and reactivated the older faults. In addition, episodes of stratigraphic growth provide critical evidence regarding the timing of fault activity. Results demonstrate that, in the study area, main fault activity occurred in several pulses during the latest Miocene to Late Pleistocene. These episodes of fault activity correspond to recently constrained regional tectonic events i.e., the initial collision of the Australian Plate with the Banda Arc, the episodes of uplift of the Timor Island and the timing of lithospheric flexure.