ASEG Extended Abstracts - 25th International Conference and Exhibition – Interpreting the Past, Discovering the Future, 2016

We use resistor network models to explore the relationship between electrical resistivity and permeability in fractures filled with an electrically conductive fluid. The fracture aperture distribution is determined by generating fracture surface pairs that are constructed based on characteristics measured on rock samples. We use these to generate and solve resistor networks with variable hydraulic and electrical resistance. The aperture is incrementally increased, to analyse the changes in both properties as a fault is opened. At small apertures, electrical conductivity and permeability increase moderately with aperture until the fault reaches its percolation threshold. Above this point, the permeability increases by four orders of magnitude over a change in mean aperture of less than 0.1 mm, while the resistivity decreases by up to a factor of 10. The permeability increases at a greater rate than the conductivity, and therefore the percolation threshold can be defined in terms of the matrix to fracture resistivity ratio, M. The value of M at the percolation threshold, MPT, varies with the ratio of rock to fluid resistivity, the fault spacing, and the fault offset but is always less than 10. GreaterM values are associated with fractures above their percolation threshold and therefore open for fluid flow.

The Artemis Cu-Au-Zn-Ag deposit is located approximately 43 km southeast of Cloncurry and 19 km west of Eloise in northwest Queensland. The deposit, hosted within the Paleoproterozoic Mount Norna Quartzite unit which is part of the Lower Soldiers Cap Group, is a steeply-dipping massive sulphide body. The Artemis deposit was discovered by Minotaur in July 2014, only 9 months after taking possession of the project. The deposit is a new type of mineralisation that has previously not been identified in the Cloncurry district. The discovery was made by meticulously piecing together the historical geophysical and geological data, complimented by new geophysical data sets including airborne EM and ground EM to improve the understanding of prospective targets throughout the area. The dominant sulphide within the deposit is pyrrhotite, hence the deposit is highly conductive. Airborne and ground EM techniques were therefore the main tools in resolving the mineralisation. The pyrrhotite is non-magnetic and therefore the deposit has no discernible magnetic signature and magnetics played no part in the discovery process. The deposit had no associated surface geochemical anomalism and very little alteration, making this deposit geologically difficult to find and essentially reliant on EM techniques in making the discovery.

The Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) has the goal to map the electrical resistivity of the Australian lithosphere to constrain the geodynamic framework of the continent. Data acquisition in South Australia has covered two-thirds of the state to date. Three-dimensional resistivity models of subsets of the AusLAMP grid across the Gawler Craton show a generally electrically resistive crust and lithosphere, but an area of low resistivity between depths of 100 km and 200 km beneath the Gawler Range Volcanics exists. A possible explanation is a fertilised mantle signature as a result of metasomatic events in the Proterozoic. The continuous low resistivity connection along the margins of the Gawler Craton core to the surface coincides with the prospective IOCG belt along the eastern margin of the Gawler Craton. The results support the importance of the AusLAMP project to define the lithospheric architecture of the continent and the value of primary lithospheric architecture for mineral exploration.

Exploration is becoming harder, at depth or under cover and decisions need to be made in model rather than data space; supported by multiple data sets. Geophysics plays an ever increasing role and integration of information from various geophysical data sets in tight collaboration with geological control is required to maximise the return from the individual data sets.

In terms of integrating geological and geophysical data, the essential goal is to interpret the available geophysical data in terms of geological domains. The process requires a common sense approach to interpretation that is flexible, adaptive and objective driven. It is not an exact formula or procedure; particularly when multiple geophysical surveys are involved. Understanding the relationships between geology, geophysical responses and rock properties is the key to develop a geological basis for your integrated interpretation. Following this, rapid 3D geological modelling and geologically based forward modelling and inversion are essential for model validation and quantitative integration of data. An integrated interpretation is not necessarily the simplest approach, but does provide answers to geoscientific questions that are stronger than individual elements interpreted on their own.

This paper presents a review of the mechanics involved in integrated interpretation and demonstrates the results with selected case study examples.

The West Foxhound 3D survey is located approximately 200km from the north-west coast of Australia within the Exmouth Plateau, which represents the outermost structural element of the Northern Carnarvon Basin. The survey covers one of two known, isolated, Rhaetian pinnacle reef complexes that exist within the Exmouth Plateau making it of particular interest to enhance understanding of a new and under-explored exploration play.

The results presented herein are derived from the focused interpretation and analysis of the West Foxhound 3D, which makes up part of a larger, comprehensive regional Triassic study, covering the entire North West Shelf including the Northern Carnarvon, Browse and Bonaparte basins. Results of the interpretation and analysis confirm that the Late Triassic interval within Foxhound hosts a significant number of isolated pinnacle reefs with potential access to charge from underlying Triassic source rocks. The pinnacle reefs represent a new and emerging play with only one reef to date drilled as a primary target.

We use 74 stations from the long period eventual-Australia-wide AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) dataset to image the electrical resistivity beneath the Neoproterozoic Ikara-Flinders Ranges and adjacent Paleo-Mesoproterozoic Curnamona Province. Results from 3D inversions using ModEM software show a relatively resistive Ikara-Flinders Ranges, with two parallel arcuate conductors at 20 to 80 km depth in the Nackara Arc. There is a good correlation of diamondiferous kimberlites occurring over conductors, which we interpret as evidence for these conductors to be residing on large lithospheric structures that have been conduits for partial melt and volatile movement in the Jurassic period. The Curnamona Province is remarkably conductive for a region that is thought to have a cratonic core, with Delamerian reworking only at its edges. We see an enriched crust that covers most of the province at depths of 10-40 km. The presence of the conductor at lower crustal depths suggests that conductive sediments cannot entirely explain the conductor. We suggest that fluids associated with subduction have pervasively modified the crust in the past, resulting in an enrichment of carbon and sulphides, enhancing conductivity. Additionally, we conclude that the notion of a single continuous arcuate Flinders Conductivity Anomaly is unlikely and that the anomalous response observed is instead a result of the combined response of three separate anomalies; the Curnamona Province Conductor and the two Nackara Arc Conductors.

In 2016, Geotech completed a test of a helicopter-borne GT-2A gravimeter combined with ZTEM^™ (Z-Axis Tipper Electromagnetics) and aeromagnetic towed bird system over the Vredefort Dome impact structure in South Africa. The survey consisted of nine (9) approx. 70 km long NW-SE oriented flight lines, totalling 640 km, acquired at a nominal spacing of 500 m over an area of approximately 650 km². The successful test demonstrates the feasibility of integrating density measurements using airborne gravimetry on a helicopter platform while combining the superior depth of investigation of ZTEM^™ and aeromagnetics for regional geophysical survey applications.

The stress anisotropy in Polymethyl Methacrylate (PMMA) subjected to triaxial stress has been investigated using True Triaxial Cell .True Triaxial Cell with 4 sealed S wave transducers inside each actuators facilitates the wave velocities measurement in different set of stresses. 108 positions of measurements have been selected and comprehensive test sequences has been allocated. The experiment analysis results confirm that the wave velocities increasing by increasing the stress in all sequences on different rates. Moreover, this experiment shows the designed True Triaxial Cell operates symmetrically and the results in both sets of transducers show a good correlation to one another which confirms the uniformity of applied stresses.

ATP 636 is a lightly explored permit on the eastern margin of the Cooper-Eromanga Basin, more than 60km northeast of the Jackson Oil Field in South-West Queensland. Prior to award of the permit, heritage seismic data was very limited and no wells had been drilled within the permit area.

Low-fold 3D seismic was acquired (the “Gumbo 3D”) in place of a more conventional program of 2D seismic followed by 3D seismic. The location of the low-fold 3D seismic survey within the permit was determined using play-based exploration principles.

The seismic data quality compares favourably with full-fold 3D seismic from a structural perspective and provides good imaging of faults and key stratigraphic units. The result is a robust interpretation and a well-defined inventory of prospects and leads.

Integrated data sourced from drill core geochemistry, spectral logs, petrophysical logs from available holes and geophysical data in and around the Punt Hill Copper-Gold prospect was used to characterise and map an IOCG system and predict prospective areas using a multi-disciplinary approach. Mineral assemblages associated with copper mineralisation were identified from geochemical analysis and spectral logs; density ranges associated with favourable mineral assemblages were noted and density inversions were fitted to these ranges in an attempt to predict similar densities in untested parts of the study area.

Mineral assemblages associated with both prograde- and retrograde-dominated assemblages were found to be associated with copper mineralisation at Punt Hill. Densities associated with the prograde mineralising event were generally higher than those associated with the retrograde mineralising event; both are higher than background density. This provided a basis for fitting the results of density inversions to the known data to map inferred prograde and retrograde alteration within the study area. Inversions were performed at both the regional and deposit scale and were used to identify as yet untested regions with density values that are consistent with known zones of mineralisation and therefore showing high potential for further copper mineralisation.

Elastic moduli of rocks derived from its powder is a new concept and can be applied in practical geophysics studies. To develop this concept, we make ultrasonic velocity measurement on granular packs of quartz sand. We calculate dynamic elastic moduli from that measurement and invert afterwards to find the shear modulus of quartz. The inversion technique follows Extended Walton Model that relies on the grain’s contact surface condition between infinitely rough and perfectly smooth. We use different coordination numbers from previous studies (for different samples) in the process of forward modelling and inversion. Forward models have good match with the laboratory measurements both in bulk and shear moduli of the granular pack. Our overall inverted results for the shear modulus are stable and close to actual shear modulus of quartz. However, the coordination numbers that has better match in forward modelling a little bit overestimates shear modulus. On the contrary, the coordination numbers that predicts the higher effective moduli of the pack is giving closest result. As the experiment set up and procedure are simple and robust, this technique can be extended and run in very rigorous situation such as at hard rock drilling rig site to get the elastic properties of the penetrated rocks in real time, where the effective elastic moduli of a grain can be represented as a statistical averaging of elastic moduli of hard rock minerals. This information can be helpful for planning and monitoring the ongoing drilling procedure. It can also be a replacement of solid cores that are missing or damaged for elasticity study.

A large (c. 2000nT amplitude, 50 km diameter) negative amplitude TMI anomaly under the Eucla Basin in southwestern South Australia, long known and referred to as the “Coompana Anomaly” has been well resolved by a new high resolution (200 metre line spaced, 80 metre ground clearance) survey flown for the Geological Survey of South Australia. We use parametric inversion to simultaneously derive magnetization direction and spatial distribution of magnetizations giving rise to satellite anomalies both above and around the main anomaly. For the two most prominent satellite anomalies these magnetization estimates agree well with estimates derived from the historic data, but the new survey data provides greater confidence, and reliable mapping of many smaller bodies. The improvement in resolution has allowed us to attempt isolation of the anomaly due to the more voluminous, deeper magnetization, but to date we have not yet been able to recover repeatable results for that deeper magnetization. Magnetization intensity estimates for all investigated anomalies are very high, ranging from about 5 A/m to over 20 A/m (equivalent in intensity to magnetizations from susceptibilities of 0.1 to 0.5 SI). The deeper magnetization is well correlated with a negative gravity anomaly, suggesting that the material generating the main magnetic anomaly has a relatively low density.

Increased hydrocarbon exploration along rifted continental margins indicates the need of a better understanding of the impact and influence of igneous rocks on hydrocarbon systems as they are often present in these tectonic settings. The southern Australian margin contains several petroliferous sedimentary basins which contain Cretaceous-Cenozoic igneous rocks and is therefore an ideal study area to investigate both the positive and negative effects on hydrocarbon systems. In particular, the Kipper Field in the offshore Gippsland Basin forms an excellent example of a volcanic play, as it holds a 328 m gas column and 14 m oil leg sealed by a >100 m thick basaltic lava flow juxtaposed against a sealing fault. The basin also contains a number of cross-cutting and layer-parallel type intrusions, although their impact on the petroleum system is as-yet unclear. Seismic interpretation techniques such as spectral decomposition and opacity rendering combined with electrical log signatures allowed us to identify the lava flow and intrusions down dip from the fault. Whether this fault has acted as a conduit for the magma responsible for the lava flow is still unclear. Future work will aim to further delineate and constrain flow paths of the intrusive and extrusive rocks near the Kipper Field and their influence on the Kipper play. This study highlights the importance of volcanic rocks in hydrocarbon basins and the possible effect they can have on hydrocarbon systems.

The Juha Anticline in the jungle-covered highlands of Papua New Guinea was drilled by three crestal wells in the 1980’s and discovered gas-condensate in a Lower Cretaceous clean quartz sandstone reservoir. The Juha-4 and Juha-5 wells drilled in 2007 further delineated the structure and defined a separate North Juha compartment. The Juha structure is 25 km long and up to 8 km wide and is traversed by a number of seismic lines, some of which are of moderate to good quality allowing the structure to be interpreted. Unlike most structures in PNG the seismic lines reveal the nature of the overlying Pliocene-Pleistocene sediment which help to define the depth of burial and timing of deformation. The wells and seismic data suggest that the Lower Cretaceous sandstone reservoir was buried by 1.5 km of Cretaceous shale, the regional seal, and 1.5 km of Miocene limestone as well as more than 1.6 km of Pliocene-Pleistocene sediment prior to uplift and erosion. To constrain the timing and style of extensional and compressional deformation, 25 2D seismic lines were interpreted aided by forward modelling of the structure. The seismic interpretation revealed basement-involved structures that were predominantly influenced by two major events, rifting in the Triassic-Jurassic and compression in the late Pliocene-Pleistocene. The deep structure remains uncertain, but gravity data indicate a very deep underlying graben a concept that has recently been investigated and validated by 3D analogue modelling. A key seismic section indicates inverted basement faults beneath Juha flattening upwards into a detachment horizon creating triangle zones in the Cretaceous mudstones such that the overlying Miocene Limestone in part deforms independently. The Juha Anticline is part of the PNGLNG project operated by ExxonMobil which commenced production in 2014, 32 years after drilling of the Juha-1 discovery.

This paper presents a review of integrated interpretation of geophysical surveys with geological data for interpretation and exploration targeting for detrital iron deposits at the Koodaideri Project, Western Australia. Significant previous exploration has been conducted in the area and this has identified a number of detrital iron deposits. The aim of this project has been to integrate all available geoscientific data in order to assess remaining prospectivity of the area and provide a framework for future exploration and evaluation projects.

Previous exploration at Koodaideri has used a variety of techniques including drilling, downhole geophysical logs, sparse refraction seismic, airborne and ground gravity, airborne magnetics and time-domain airborne electromagnetics. Spatial coverage of the individual exploration datasets is irregular, and the first stage of this project has focussed on an area of approximately 42 km x 12 km within which there is reasonably good coverage of all data types. The relatively high data density has allowed the relationships between the various data types to be assessed and effective exploration parameters to be defined.

The larger detrital iron deposits at Koodaideri occur within palaeochannels or depressions within the basement, which is mainly comprised of units of the Wittenoom Formation. The detrital iron deposits are considered to have been sourced from erosion of bedded iron deposits of the Brockman Iron Formation which outcrops on the high ground both upstream and immediately to the southwest of the area of interest. The known deposits generally occur beneath cover of variable thickness of up to 50 m. The detrital deposits themselves may have thicknesses in excess of 100 m in major palaeochannels and sinkholes. The detrital deposits have a higher density than other cover units due to their high iron content (>50% Fe). However, gravity alone is not an effective exploration technique because the gravity signature is complicated by significant variability in the depth to higher density Wittenoom Formation bedrock. A more recent development is use of a modified seismic refraction method (Sparse Refraction Seismic) to constrain the basement topography, and then to model and remove the basement response from the observed gravity data to identify areas of anomalous excess mass. This approach has allowed cost-effective semi-regional exploration and has been successful in identifying all known major detrital iron deposits.

This study extended the excess mass approach by constructing revised basement models from the sparse refraction seismic and drilling and from interpretation of the SkyTEM airborne electromagnetic and drilling. The results show that the basement interface can be interpreted from either the seismic or airborne electromagnetic datasets, although the airborne electromagnetic interpretation is complicated by highly saline groundwater in the northeastern quadrant of the area and by conductive shale units within the Wittenoom Formation bedrock. Almost all of the known detrital and channel iron deposits are spatially associated with an overlying pisolite unit, which can be identified from the magnetic data via its characteristic magnetic texture.

These studies have shown that the derived excess mass is spatially associated with the known detrital and channel iron mineralisation. Significantly, almost all of the known deposits were also successfully identified from the simple geological model, in the absence of drill hole constraints. A number of untested areas of possible mineralisation have been identified, as well as potential extensions or alternate trends to known mineralisation.

The modelling scenarios tested confirmed that the minimum elements for exploration targeting at Koodaideri are a geological model incorporating basement topography, interpreted magnetic domains and geologically constrained inversion of the gravity data.

Passive seismic techniques utilise the properties of ambient seismic waves to infer information about the structure of the subsurface, namely the depth(s) at which significant impedance contrasts occur. Geoscience Australia has recently conducted three passive seismic surveys to assess the suitability of two passive seismic methods, the horizontal over vertical spectral ratio (HVSR) and spatial autocorrelation (SPAC) techniques, for estimating cover depth over crystalline basement. Both techniques rely on ambient seismic noise in the form of surface waves. The HVSR technique involves measurement of the horizontal and vertical components of ambient seismic noise at an individual site. Where an impedance contrast exists, a maximum is observed in the HVSR value at a frequency directly dependent on the interface depth. The SPAC technique utilises dispersion observed in surface waves, which is also dependent on interface depth. Shear wave subsurface velocity profiles are constructed through inversion of the HVSR and/or SPAC curves.

The logistically simpler HVSR method, requiring only one seismometer, was found to produce estimates with significantly lower error than estimates from SPAC for depths up to 300 m in the Murray Basin, where unconsolidated to semi-consolidated Cenozoic sediments overly Paleozoic crystalline basement. Both HVSR and SPAC methods failed to resolve the target interface with the exception of one site in the Gawler Craton, located at a depth of 900m. Further work is planned for these data with different processing techniques. The HVSR technique produced estimates consistent with other geophysical techniques (airborne electromagnetic, refraction seismic, and magnetotelluric methods) for the majority of sites in the Thomson Orogen, where the Mesozoic Eromanga Basin, Cenozoic cover, and regolith associated with both these stratigraphic groups overly crystalline basement to a depth of up to 550 m. The accuracy of these profiles will be verified with stratigraphic drilling.

Results from these three surveys provide strong support for the passive seismic technique, in particular the single station HVSR method, as a highly effective and logistically low-cost and simple tool suitable for mapping cover depth in many regions of interest across Australia.

The present-day stress field of Australia has been the subject of great interest in the three past decades because it shows a variable pattern for the orientation of maximum horizontal stress (SHmax) that is not parallel to absolute plate motion. The last prior release of the Australian Stress Map (ASM) project, published in 2003, contained 549 data records in 16 stress provinces and highlighted the role of plate boundary forces on the regional stress pattern of continental Australia. However, smaller scale rotations of the SHmax orientation in Australian sedimentary basins were not investigated in great detail in previous studies. Herein, we present the latest release of the ASM with 2140 data records in 30 stress provinces, with a particular emphasis on newly compiled data in eastern Australian basins. The new release of the ASM has stress data from 20 Australian sedimentary basins, which further confirms the regional variability of SHmax orientations in the Australian continent, and reveals four major trends for the orientation of SHmax including NE-SW in northern, northwestern and northeastern Australia, E-W in southern half of Western Australia and South Australia, ENE-WSW in most parts of eastern Australia and NW-SE in southeastern Australia. In addition, the 2016 ASM reveals significant rotation of stress within various sedimentary basins due to different geological structures, including basement structures, faults, fractures and lithological contrasts. Understanding and predicting local stress perturbations has major implications for determining the most productive fractures in petroleum and geothermal systems, and for modelling the propagation direction and vertical height growth of induced hydraulic fractures in unconventional reservoirs.

Linear strings of circular negative magnetic anomalies in the greater McArthur Basin of the Northern Territory are interpreted as due to holes in an underlying sheet of Kalkarindji flood basalts. Individual anomaly inversion results provide an estimate of the diameter, depth to top, and depth extent of holes in the volcanic sheet. The effective magnetization of the hole is its contrast against the more strongly magnetised sheet. Estimated magnetization contrast values are mostly rotated from a direction antiparallel to the local geomagnetic field, which we interpret as due to the contribution of remanent magnetization within the sheet. We support interpretation of the anomalies as due to holes in the sheet by comparing them with the magnetic expression of distant sheet edges. The linear arrangement of the anomalies is believed to arise from fractures in the sheet, suggesting that the holes developed after the sheet was emplaced, most probably by local escape of fluids, which altered the sheet and destroyed its magnetization.

Passive seismic methodology is becoming a popular tool for estimation of cover depth, cover velocity structure and basement topography for mineral exploration applications. Analysis of passive seismic array data gives information on wavefield characteristics and constrains shear-wave velocity structure. Single station techniques are useful as a tool to rapidly estimate cover thickness but require further information from drill holes or array techniques for robust interpretation. The combined use of array and single station techniques provides an effective cover depth mapping methodology and allows for highly flexible survey design, adaptable for a range of applications. This paper provides a summary of relevant passive seismic techniques and overview of their effective use with several example mineral exploration scenarios.