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

Two 2D deep seismic reflection survey lines totalling 229 km in length were completed in western NSW by the Geological Survey of New South Wales (GSNSW) under the New Frontiers initiative. Contract management for seismic acquisition and data processing were conducted by Geoscience Australia (GA), with funding for data processing contributed by Coal Innovation, NSW Industry. The survey aimed to detail the stratigraphy of thick Devonian sequences within the Yathong Trough in the eastern Darling Basin.

Survey line YT2 was extended eastwards across the southern Cobar Basin over rift-phase volcanic sequences of the Cobar Supergroup in the Mount Hope and Rast Troughs. The objective of acquiring 20 s two-way time data was to identify major structural elements, crustal architecture, and to improve understanding of this mineralised region. Preliminary interpretation of the deep 2D seismic data by GSNSW and GA has recognised reflector domains and discontinuities below the surface outcrops (in the east) and also within basement rocks beneath the basin sequences. These were considered in the context of regional aeromagnetic and gravity data, and previous deep seismic profile interpretations for the Cobar region. The key results are definition of crustal thickness and interpretation of prominent fault structures in the upper and middle crust. In particular, good correlation was found between near-surface fault zones (from geological mapping or interpreted from potential field data) and faulting related to a major west-dipping high-angle seismic discontinuity that penetrates to the middle levels of the lower crust.

This presentation describes the interpretation of major reflective horizons and structures, identifying upper, middle and lower crustal features. The profiles and interpretations will provide valuable input for regional geodynamic studies. Increased structural understanding may assist the search for additional gold and copper deposits in the southern Cobar Basin.

New physical rock property data from the Plutonic Well Greenstone Belt are presented. P-wave velocity (Vp), magnetic susceptibility and density measurements have been taken from rare stratigraphically complete drill core. Preliminary results show that variations of physical rock property data are successful in resolving most lithologies. However, questions remain regarding the interpretation of physical property data involving ultramafic and mafic rock, and, in particular, the effect of alteration.

Variable alteration is suggested to be the cause of the wide ranges within physical property data populations. This problem is not exclusive to the Plutonic Well Greenstone Belt, and is a common feature within many greenstone terranes. Geochemical and mineralogical data are also available from the study area and may allow a better understanding of the effects of common types of alteration (serpentinisation, talc-carbonate alteration) on the physical properties of mafic and ultramafic rocks from granitoid-greenstone terrains.

The Carnarvon Basin has experienced three distinct phases of extension - in the Carboniferous-Permian, in the Upper Triassic to Middle Jurassic and in the Lower Cretaceous. Detailed mapping of fault patterns associated with each event is possible at a regional scale using widespread, publically available 3D seismic data sets.

The complex interaction between NNE and NE-SW oriented Carboniferous and Permian age faults provides the structural framework for subsequent rift events. Both sets of faults show evidence of oblique reactivation under a WNW oriented extensional regime in the Jurassic, somewhat at odds with the general perception of NW oriented extension associated with separation of Argoland from the NW shelf at this time.

Lower Cretaceous extension is much shorter lived, is primarily confined to the SW part of the Northern Carnarvon Basin and is associated with significant uplift and erosion. The relationship of this event to the separation of Greater India from Australia is less clear, but a proposed mantle plume goes some way to addressing some of the observed structural and stratigraphic relationships.

The Australian Lithospheric Architecture Magnetotelluric Program (AusLAMP) is a multi-year collaborative project aimed at resolving the first order electrical structure of the Australian continental lithosphere through the acquisition of long-period magnetotelluric data at -55x55 km spacing. Here we present the results of the first deployment of AusLAMP which was in Victoria in 2014. Previous MT coverage over Victoria comprised limited 2D profiles in the western portion of the State. Three-dimensional inversion of AusLAMP data provides a context for these isolated profiles while revealing interesting and unexpected results with evident correlations with the mapped geology and well established seismic tomography trends. Along the eastern and southern edge of Victoria, the resistivity structure is resolvable into the asthenosphere, in contrast beneath the central and western part the State the base of the thicker lithosphere is not resolved. Resistivity of the asthenosphere beneath the Victorian eastern highlands conforms with global values (-1,000 Dm) and becomes more conductive (-200 Dm) beneath the Newer Volcanic province. The seismologically defined lithospheric mantle beneath the central and western part of the State is relatively resistive (-200 Dm) compare to the east (-20 Dm). This anomalously conductive lithospheric mantle we tentatively attribute to metasomatism during Palaeozoic accretion of oceanic terranes. Vertically, this conductive lithospheric mantle merges upwards into a series of northeast trending conductive anomalies within the mid to lower crust. These trends correspond with the surficial distribution of Devonian granite intrusions suggesting they represent fossil metasomatised ascent pathways of the granitic melts, which cross cut the older dominant north-south structural trend. The western limit of these linear conductive trends maps out the boundary of the Delamerian and Lachlan Orogens.

Given its importance, shallow water demultiple has been under constant investigation for many years. Significant progress has been made, effective processing flows have been established, and excellent results have been achieved in different basins across the world. There remain however significant challenges with demultiple in shallow water environments, especially when it comes to broadband acquisition with a wide tow configuration. In this paper, we discuss a shallow water demultiple processing flow used on a recently acquired wide tow broadband dataset in the Northern Carnarvon basin, North West shelf Australia. We demonstrate that the removal of shallow water multiples can be optimized in data acquired in this manner by using a combination of demultiple techniques.

At a trial field site in the Bowen Basin previous seismic surveys had difficulty imaging the coal seams near the base of weathering. It was suspected that these may be highly structured which could complicate future open-cut mining.

To improve the understanding of the geology a multicomponent 2D trial was conducted. This used the Mini-SOSIE technique to simultaneously generate P-wave and transverse S-wave energy. This allowed for the processing and interpretation of three separate datasets (P, S, and PS).

In this case, the PS image provided the best structural interpretation. This was achieved using information (e.g. statics and velocities) gained from processing the pure P and S datasets.

A regional reconnaissance AEM survey in the west Kimberley region (lower Fitzroy River valley and May-Meda Catchments) has successfully mapped the extent of regional Canning Basin aquifers (e.g. Pool Sandstone, Grant Group and Erskine Sandstone) confirming these as significant potential groundwater resources. The survey has successfully mapped a multi-layered hydrostratigraphy to depths of ~400m (in resistive areas), also revealing significant tectonics manifested as large scale basin-scale tilting of stratigraphy and more localised folding and faulting.

The survey also mapped other key objectives including: the seawater intrusion (SWI) interface for the lower Fitzroy River valley and the May and Meda River Catchments; river flush zones along the Fitzroy, May and Meda Rivers; groundwater salinity variations and potential salt stores in the shallow alluvium of these rivers and proposed irrigation areas. River-parallel surveys also identified reaches of the May and Meda Rivers with direct contact between the rivers and underlying regional aquifers of the Pool Sandstone and Grant Group. These reaches are areas of potential recharge, and/or groundwater discharge, with the AEM used to target hydrochemical investigations, drilling and aquifer testing. The AEM data reveal more complex relationships between perennial in-stream pools along the Fitzroy River, underlying aquifers, tectonics, and river alluvium.

AEM survey design incorporated time series analysis of surface water availability through time (Water Observations from Space (WOfS)) derived using the Australian Geoscience Data Cube (AGDC). This has facilitated investigations of surface-groundwater interaction through ensuring the AEM transects coincide with permanent water holes and river reaches considered to be potentially sustained by groundwater discharge. The AEM data provide a framework for hydrogeological process understanding, while the knowledge generated in this project will inform water resource allocation planning, help assess risks to culturally and environmentally sensitive riverine ecosystems, and de-risk investment in agriculture and water infrastructure more broadly.

The Ord Bonaparte Plains area is a priority area for irrigated agriculture development as part of the Ord Stage 3 development in the East Kimberley region of Western Australia. Irrigated agriculture in this area will depend on access to groundwater resources in underlying bedrock aquifers. A program of airborne electromagnetics (AEM), drilling, ground and borehole geophysics and hydrogeological investigations is being undertaken to confirm the presence of suitable groundwater resources, map the connectivity between surface and groundwater systems, and identify potential risks to agriculture and water infrastructure including salt stores, groundwater salinity and seawater intrusion.

Preliminary analysis shows that the AEM survey has successfully mapped key elements of the groundwater system, including aquifer and aquitard extent, groundwater quality (salinity) distribution, hydraulic properties, compartmentalisation and inter-connectivity, the seawater intrusion (SWI) interface in coastal zones, and key tectonic elements of regional hydrogeological significance. The survey has mapped significant faulting within the Cockatoo Sandstone and Point Springs Sandstone aquifers, while conductivity distributions suggest that faults within and bounding major stratigraphic units display both fault barrier and conduit behaviour. The survey has also found that fresh groundwater in the aquifer system continues offshore as discontinuous lenses.

Initial inversions have been used to target drilling, hydrochemical investigations, and a program of ground geophysics (including Surface Nuclear Magnetic Resonance (SNMR)). Further analysis and groundwater modelling is required to determine appropriate development and management of any groundwater resource and the potential risks to agricultural development.

Multichannel analysis of seismic surface waves (MASW) is well developed for shallow geotechnical applications. The technique has received much less attention in 3D reflection. The primary goal of this investigation is to develop an improved methodology for extracting S-wave receiver statics in 3D converted-wave (PS) reflection surveys.

The main challenge of using MASW in this context is the increased receiver spacing. To overcome this we need to include multiple sources in each dispersion analysis. Unfortunately, adding more sources increases the chance that the dispersion image will be degraded due to lateral near-surface variations. We present a methodology which incorporates interferometry into the MASW method (IMASW). This effectively repositions sources at receiver locations, reducing the region over which lateral variations can contribute to the data.

Application of this approach to a 3D-3C trial dataset provides an alternate approach for determining the S-wave receiver statics solution for converted-wave reflection surveys.

Early exploration of the frontier Amadeus Basin in the Northern Territory has been limited due to its size and remoteness, along with complexity of structuring and limited early success. The primary exploration targets of the southern Amadeus are the sub-salt and intra-salt plays of the Neoproterozoic lower Gillen-Heavitree petroleum system. Two wells have tested the sub-salt play, both flowing gas with high helium content, confirming the sealing capacity of the Gillen evaporates despite two significant orogenic events.

In 2013, the first regional framework of 2D seismic was acquired over the southern Amadeus. In areas where seismic coverage is sparse, or of poor quality due to halotectonics, higher spatial density magnetic and gravity surveys have been used to interpolate trends between seismic and well control and create a high resolution depth-to-basement model. Alternate methods of seismic processing have also been trialled to improve sub-salt imaging.

The newly acquired seismic and depth-to-basement model are revealing the architecture of the basin and provide a regional perspective of the sub-salt and intra-salt plays. With improved seismic imaging large sub-salt structures are emerging beneath the complex folds resulting from thin-skinned deformation.

Learnings from the challenges associated with acquisition and processing, along with a better understanding of the complex halotectonics, will have significant implications for future exploration in the region.

We illustrate that rapid acquisition of audio magnetotellurics (RAMT) is an alternative methodology to construct the impedance tensor. A solenoid is used as the magnetic sensor and a capacitively coupled, long wire antenna (LWA) as the electrical sensor. Recent improvements in data quality, automated processing flows and apparent resistivity to depth transforms resulted in a cost effective and practical alternative for quickly acquiring audio magnetotelluric 1D soundings. Case studies for groundwater, engineering and mining illustrate how the method may be useful for decision making, and hopefully enable more widespread adoption of low cost resistivity measurements.

We introduce and test a workflow that integrates petrophysical constraints and geological data in joint geophysical inversion in order to decrease the uncertainty of the results. This workflow uses statistical petrophysical properties to constrain the values retrieved by the geophysical inversion and geological prior information to decrease the effect of non-uniqueness. We integrate the different sources of information in a Bayesian framework, which takes into account the state of information. This permits us to quantify the posterior state of knowledge, the reduction of the uncertainty and to calculate the influence of prior information using quality indicators based on fixed-point statistics. This workflow was first tested using simple synthetic datasets to validate the method and assess the robustness of the workflow. As a result, the use of petrophysical constraints permits us to retrieve sharper boundaries, while prior structural information from geology permits to retrieve the geometry more accurately. Overall, the integration of the different constraints provides a model, with reduced uncertainties and better resolved parameters.

The resistivity structure of the crust is broadly expected to be homogeneous, with highly resistive lower crustal rock overlain by more conductive rock in the upper crust. However, observed data shows that although the upper crust is typically resistive, the lower crust can be much more conductive. The presence of such high electrical conductivity in the lower crust is remarkable and suggests a substantial highly connected material, melt or fluid. Has the low resistivity structure been present since inception, or is it the result of a later overprinting event. The secondary objective to establish how such a low resistivity region can be preserved over such an extended time scale.

Data has been collated from magnetotelluric (MT), and geomagnetic depth sounding (GDS) surveys collected over the last thirty years. Three different methods have been used to model thousands of data points. A thin-sheet inversion of thousands of GDS data has been used to place constraints on the regional scale electrical conductance. Inversions of the MT data in both 2D and 3D have provided more detailed models of how the Moho is connected to the upper crust.

Strong correlations were observed between major tectonic domains (such as the Gawler Craton) and regions of high resistivity within the crust. The 2D profiles show broad regions of low resistivity at the boundary between the upper and lower crust (10-15 km depth), with low resistivity zones extending for tens of km. Above the boundary, the low resistivity regions transform in to narrow pathways penetrating through the resistive upper crust and the areas with the lowest resistivity were found to have a strong correlation with known major mineral provinces. This leads to the suggestion that crustal low resistivity anomalies are likely a product of fluxes of fluid and possibly melt from the upper mantle and lower crust.

A heavy rainfall of May 2014, caused an extensive disaster to the catchment of River Sava in Serbia and Bosnia and Herzegovina (BiH). The number of landslides exceeded 2000 in Serbia and BiH. This prompted an urgent investigation of the areas which have potential of further landslide.

Association of Geoscientists and Environmentalists of Serbia (AGES) initiated a project of geophysical investigation of these areas supported by Geoscientists without Borders (GwB) of Society of Exploration Geophysicists (SEG). Over twenty students participated from three countries and more than ten technical professionals from six countries joined in the project.

Reflection seismic, MASW and resistivity surveys were carried out in the Vrazici area in BiH in June 2015. Two survey lines were surveyed totaling about 120 meters along a slope of grassland and orchard.

A clear increase of S-wave velocity from 250 to 350m/s at about 3 to 5m below the ground surface is observed by the MASW analysis, which is consistent with the reflection profile at the same area. This depth roughly corresponds to the resistivity boundary at 80 ohm-m. It is interpreted the interface between soft soils and relatively competent rock. The depth profiles of these boundaries show that the soft soils are thicker toward the lower part of the slope. This landslide can be classified as Varnes’s classification “earth slide or earth flow”.

An increase in demand for commodities coupled with a decrease in world-class ore deposit discoveries in the last three decades is the driving force for exploring for a new generation of world-class ore deposits at depth. Exploration through cover is becoming one of the critical challenges for the mineral exploration industry.

This paper explores methods of integrating geophysical, geological and landscape data so as to reduce uncertainty in landscape evolution models interpreted from inversion of electromagnetic (EM) data. Inversion of EM data is, in general, non-unique: many different models will be able to fit the EM data equally well, resulting in large uncertainty. However, EM model uncertainty can be significantly constrained when geological and landscape context are taken into account. This study aims to characterise the regolith (weathered and transported cover), and understand how its conductivity varies with the landscape and with the regolith architecture. This is assisted by logging information from 104 boreholes penetrating through the regolith and into the basement rocks. The study area is associated with the DeGrussa Cu-Au deposit located in the Capricorn Orogen of Western Australia, a regolith-dominated terrain where the regolith varies in thickness between <5 and ~150m.

We first select EM decay curves, extracted from an airborne EM survey, whose locations are close to those of the boreholes. We then use a layered-earth (1D) forward model, and invert those data for the electrical resistivities of each of the lithologies identified on the geological borehole logs. Layer boundary depths are fixed to the borehole depths. We show how the non-uniqueness associated with EM inversion can be reduced by the inclusion of decay curves from geology with different layer thickness ratios.

Lithological models derived from the integration of electromagnetic, geological and landscape data show less uncertainty and are therefore more reliable for mineral exploration targeting.

AEM Geophysical surveys are excellent tools for mapping conductivity variation over large areas. Common workflows involve inverting data using 1D models on a per-station basis, then gridding those results over lines to produce maps. In the absence of other filtering, gridding operations must combine finely-sampled along-line data with sparsely-sampled between-line data. Independent of the choice of gridding technique and map cell dimension, such maps will obscure the two scales. Obtaining a spatially coherent map will often involve reducing the resolution along the survey lines. Indeed, it could be argued that if the goal of an AEM survey is mapping, then inverting the data on a station by station basis is not necessary.

We show that large-scale structures are preserved when data are summarised using the arithmetic mean over a number of stations. This allows practitioners to objectively determine map cell dimensions since it provides an indication of the distance over which 1D models which are typically used to process large data sets are valid. Practically, it makes little difference whether this summary takes place before data are inverted or after. Summarising data before inversion may provide a practical estimate of spatial and temporal variation of the data at a particular scale. In contrast, summarising inverted models may provide an estimate of model variability at a particular scale.

Groundwater presence and salinity both correlate with electrical conductivity, a property readily mapped over a swath of depths by various geophysical instruments. Thus, it would seem then that exploration for groundwater could be a simple matter, however, for most groundwater investigations the level of detail of geological complexity demands detailed acquisition and multi-dataset interpretation.

Towed or airborne electromagnetic survey permits multi-layer map coverage of a site as is necessary for revealing geological detail that would make little sense when viewed as individual transects. Further, because electrical conductivity responds to so many natural variables related to groundwater, multi-disciplinary information must be included in the interpretation. In many cases, the footprint of an airborne system will be too large to sufficiently resolve complexity of geology and cultural interference to successfully site bores. In other cases the mobilization cost of an airborne system will, on its own, exceed the exploration budget. In these cases, towed electromagnetic devices including AgTEM4^™ cart have a niche.

Exploration depth and resolution of practical towed devices is very limited by their practical size constraints so great care must be taken in other design aspects including effective, robust primary field nulling and maximizing of transmitter loop - receiver loop separation.

Case studies of groundwater exploration demonstrate these geological interpretation and physical design challenges and limitations.

Magnetic exploration plays a significant role in regional geological investigation and detection of underground geological bodies with magnetic anomaly. At the moment, aeromagnetic survey is widely applied for its high efficiency, low cost and less subject to terrain restrict. Magnetic compensation is a key step in pre-processing survey data and several methods have been used. In this paper, we would use partial least square method to complete aeromagnetic compensation. Partial least square regression is frequently used to find the fundamental relations between two matrices. It combines linear regression analysis, canonical correlation analysis and principal components analysis. It can be applied into data with multicollinearity among independent variables and the number of variables is larger than that of observations. Before compensation, we should have several pre-processing steps such as parallax correction, diurnal variations correction, geomagnetic field correction and high-frequency noise removal. This will provide us magnetic data with higher quality and make compensation process more accurate. We set synthetic aeromagnetic data with interference of aircraft’s maneuvers and used partial least square method to do compensation. From the results of simulation, we can see that the interference signal is reduced to a low degree and satisfied compensation effect is obtained. Partial least square regression is a stable and effective method in the application of aeromagnetic compensation.

The Jemmys Point Formation - uppermost member of the cool-water carbonate Seaspray Group - records a terrestrial drainage system that traversed the now submerged continental shelf of the Gippsland Basin during the latest ice age. A partial network of large sinuous channels that includes a few diffuse areas of arced broadening (probably recording local slack-water lacustrine conditions), has been imaged by aeromagnetic survey data and now large-scale 3D exploration seismic data. Channels are well resolved in narrow-band quadrature phase exploration seismic data by a surface probe that runs along the first zero crossing of the seismic wavetrain. This corresponds closely with the modern-day bathymetric surface when assuming a seawater acoustic velocity of 1,500 m/s.

The stratigraphy of these channel features has not been ground-truthed despite the basin being a highly mature and productive petroleum province. In fact, it seems that their distribution is a ‘negative’ of the distribution of petroleum field areas. This fact leads to the consideration of two hypotheses: ongoing structural inversion of existing trapping structures at depth pushes channels away; or intervening zones of relatively high differential compaction subsidence pull channels towards them. No evidence was found to confirm the former hypothesis but this may be because the rate of local uplift does not outpace shelf-wide sedimentation (so cannot be resolved in seismic or seabed bathymetry data). By contrast, the loci of drainage channels appear to correspond well with regions of the thickest post-Eocene stratigraphy. This suggests that differential compaction subsidence has continued to hold the lowstand drainage system in place since a pre-cursor canyon head network was developed by the Mid-Miocene inversion maximum.