ASEG Extended Abstracts - ASEG2012 - 22nd Geophysical Conference, 2012

Magnetotelluric (MT) techniques measure natural time variations of the Earth’s magnetic and electric fields to infer subsurface electrical conductivity structure. Data are collected over a range of frequencies, providing insights into how this structure varies with depth. Depending on the Earth conductivity and frequencies used, information can be obtained from the near surface to depths of hundreds of kilometres.

MT surveying has been used in a wide variety of geological scenarios, from investigations of continentalscale structures to mineral and geothermal exploration, and in the search for ground-water, and many such surveys have now been undertaken in South Australia. Recently, surveys have been conducted by Geoscience Australia (GA) under the Australian Government’s Onshore Energy Security Program (OESP) along deep crustal seismic reflection transects, in part in collaboration with the University of Adelaide (UA), the Geological Survey of South Australia, Primary Industry and Resources South Australia (GSSA, PIRSA) and the Australian National Seismic Imaging Resource (ANSIR) across the Gawler Craton and Curnamona Province. Given the wide range of applications for MT data, it is proposed to deliver these data online as industry-standard electrical data interchange (EDI) files, starting with the most modern datasets.

This paper presents an overview of the MT data and reports presently available for South Australia. All MT data are available for download online from the South Australian Resources Information Geoserver (SARIG), and both seismic and MT data acquired by GA and collaborators under the OESP are available for download from the GA web site.

3D reflection seismic survey was conducted over an area of about 9 km² at the Kevitsa Ni-Cu-PGE (platinum group elements) deposit, northern Finland. About 1000 active receivers and 3000 shots in nine overlapping swaths were used to acquire the data. The principal objective of the survey was to image major fault and fracture zones at depth. Understanding the geometry of these zones is important for designing a steep open-pit for mining. Geological structures are complex and a varying thickness of overburden combined with various weather conditions during the data acquisition pose the data processing very challenging. However, a careful processing design combined with our experience on this type of data helped to produce interesting results. Processing results suggest that the 3D seismic survey has been successful in imaging both gently dipping and steeply dipping reflections as shallow as about 150 m, many of which correlate with fault systems and lithological contacts observed at the surface. Several new target areas, bright spots, are identified in the seismic data that require further investigations for their mineralization potential.

Deep sedimentary basins across the globe hold massive untapped reserves of geothermal energy. The economics of developing these resources is yet to be fully understood however it is likely that project scale will be a key factor for success. I investigate the geophysical methods that may be of value in the design and monitoring of geothermal well fields. Such well fields may utilize networks of pumping and injection wells installed in aquifers at multiple depth for heat exchange and or power generation. The Perth basin in Western Australia has at least four distinct expansive aquifers levels below the city. I compute simplified multi-layer hydrothermal numerical models for parameter distributions broadly based on the Perth basin to depths of over 5000m. The possible radial extent of changes in pressure, solute concentration and temperature are considered. The modelling demonstrates that pressure changes spanning hundreds of square kilometres could occur for large projects with a 50 to 100 year life span. Given the above it is clear that surface seismic reflection methods should be routinely used for multilevel geothermal developments in sedimentary basins. The benefit of 2D and 3D seismic is that a robust base-line hydrothermal framework can be developed. This means the entire well field and its impacts can be mapped out over hundreds of years. Australia is highly urbanized and there are good arguments for locating large geo-thermal projects in or proximal to cities where suitable deep basins exist. However cities like Perth tend to be dead zones for deep wells and seismic coverage. So it will be necessary to design and execute seismic surveys within cities. If the geothermal industry is to be a serious provider of energy it will need to build on lessons already learned in the petroleum industry.

The relationship between electromagnetic velocities derived from in-hole radar surveying and soil saturation can be exploited to map changes in recharge from rainfall infiltration in the vadose zone against time. We have completed time-lapse cross-well radar and vertical radar Profiling (VRP) experiments with the objective of monitoring rainfall infiltration during the winter season at two sites on the Gnangara Mound in the Perth Basin, Western Australia. Depth-velocity profiles have been derived from the direct transmission measurements. Results obtained from Vertical Radar Profiling and Zero vertical offset cross well profiling are evaluated and the influence of different geometries and test-site conditions are discussed. We find that zero vertical offset cross well radar experiments were highly repeatable. Further changes in ground conditions such as an increase in moisture content can be observed with great confidence. The interpretation of vertical radar profiles was more challenging. However both techniques successfully reveal the time-lapse response of water migrating through the unsaturated soil profile for the two trial sites.

Carbon Dioxide (CO2) sequestration is one proposed solution to the possible detrimental effects of increased CO2 emissions into the Earth’s atmosphere. A proposed method for CO2 sequestration is capture and storage below the earth’s surface in deep saline reservoirs. Australia’s CO2CRC research group is currently trialling this method of CO2 sequestration by injection into the Paaratte formation in the Otway Basin Australia. As CO2 is injected into brackish or saline water saturated sediments it is expected to create a zone of increased electrical resistivity around the injector well. The cross-well controlled-source electromagnetic method may be capable of mapping the movement of injected CO2 as it expands out from the injection interval.

We simulate time-lapse in-hole controlled source electromagnetic surveys using expected change in electrical resistivity that might be associated with CO2 injection. We demonstrate that controlled source electromagnetic methods will successfully monitor CO2 injection given; (i) suitable transmitter type and frequency range; (ii) a monitoring well design that can facilitate the electrical methods and (iii) correct monitoring well location relative to the injection well. In particular we find that, because of the large volume of CO2 that would likely be injected during a large sequestration project even relatively small changes of less than 10% in electrical resistivity associated should be readily detectable. We provide images of the time lapse cross well electromagnetic response for an expanding disk representing 0.1 to 10 kilo tonne of CO2.

Reflection seismic method has a number of constraints to obtain suitable images in hard rock environment. For many seismic migration algorithms it is necessary to know the values of propagation velocity of seismic waves. In sedimentary basins, where layers are usually continuous reflectors, that information could be obtained directly from acquired data. However in hard rock environments there are a lack of such horizontal boundaries. This presents a difficulty for an estimation of the velocity model, so obtaining a satisfactory image in hard rock environments is not a trivial problem. Imaging algorithms that do not require prior construction of a velocity model might be promising for such media. Here we describe an application of a velocity-less time migration to hard rock conditions and demonstrate its effectiveness on synthetic data. This approach is based on an estimation of horizontal slownesses in commonshot and common-receiver gathers and a following calculation of the migration attributes. The algorithm was tested on simple convolution synthetic models and applied to 2D synthetic dataset, which simulate a common geological section in hard rock media.

A ZTEM (Z-Tipper Axis Electromagnetic) helicopter AFMAG test survey was conducted in April, 2011, over the Gold Springs Project, in south eastern Nevada, for low sulphide epithermal gold exploration. Previous ground CSAMT surveys at Gold Springs revealed a positive correlation between the known epithermal gold systems and buried, subvertically-dipping resistivity high features that extend to depth. Synthetic forward modeling showed that ZTEM could also be successful, provided topographic effects were accounted for.

The ZTEM test results performed over the known epithermal gold prospect in southeastern Nevada appear to correlate very well with the CSAMT results and the known geology, in particular the presence of all the known epithermal centers over resistivity highs. Similar anomalies have been identified in previously unexplored areas of thicker overburden cover. 2D inversions of the airborne ZTEM, using a newly developed Avert2d code that accounts for bird clearance and 2d topography, appear to agree very well with the inversions obtained from ground CSAMT. These results provide encouragement for the application of ZTEM for epithermal gold deposits elsewhere in the district.

Early times are of interest for near-surface surveys and are object of developments during the last decade both in the electronics and the processing domains. The new SkyTEM system, called SkyTEM101, has a very short turn-off of 3 μs which authorizes the use of gates between 3 and 10 μs, and to increase the vertical resolution of the near-surface. However, a reliable monitoring of the Coil Response (CR) is necessary to get these very early time gates usable. For this, a new Coil Response Correction (CRC) has been developed which allows removing the CR in the first early gates before the inversion process. Application of this CRC correction to high altitude and production measurements has shown its efficiency to reduce the CR to the noise level. A new adaptive data filtering has also been developed to maintain the best lateral resolution above different areas like open fields or forests above which the signal to noise ratio varies due to the change of the helicopter altitude. This improvement of the lateral resolution has a great value for environmental surveys where definition of local and shallow heterogeneities is a critical issue.

In the computation of the forward response for Time Domain Induced Polarization the incomplete description of the transmitter waveform causes dramatic errors in the estimation of the magnitude and time characteristic of the IP phenomenon. Both the duration of the current pulse and the sequence of pulses used for the stacking procedure have a strong effect in the magnitude and shape of the IP decays. Furthermore, it is important to model low-pass filters of the receiver system, in order to extract all the information contained in the acquired data. For these reasons, a new 1D forward and inversion algorithms have been developed using the full time decay of the IP response and the receiver transfer function to reconstruct the distribution of the four Cole-Cole parameters of the earth. The waveform implementation in the forward response for TDIP is a significant improvement that allows moving from a qualitative interpretation of TDIP data for recognition of anomaly patterns towards a quantitative analysis, able to discriminate soil lithotypes and, if present, some contamination plumes.

Two airborne EM surveys were conducted over the Drybones kimberlite, NWT, Canada. A VTEM helicopter time domain EM survey was flown by Geotech in 2005, followed by a ZTEM helicopter tipper EM survey flown in 2009. The data sets collected over the kimberlite were inverted and interpreted first by Geotech, Ltd using a 1D TDEM conductivity-depth transform for VTEM data and a 2D inversion for ZTEM data. The data sets were transferred to UBC-GIF for 3D inversions and comparative analysis. Both the VTEM and ZTEM data have now been inverted in 3D and the results compared with the drill data over the kimberlite and with older results of previous geophysical interpretation. The 2D and 3D ZTEM inversions show the same general structure but the 3D inversion has greater electrical conductivity contrast and may have fewer artefacts in zones of low sensitivity. On a large scale the conductivity structure, recovered from the 3D inversion of time domain data, is comparable to that from the CDI transform, but the 3D result shows a more complicated distribution of the conductive properties. The 3D magnetic inversion had not been done previously and it shows susceptible material to depths of approximately 400 m, which is consistent with the known geometry of the kimberlite.

The ZTEM airborne EM system was introduced into commercial service by Geotech Ltd. in late 2007. ZTEM is unlike any other commercial EM system in that it relies on the measurement of natural occurring EM fields in the Afmag frequency range of 25-720 Hz.

As a result of using natural EM fields that pass through the earth as plane waves, the ZTEM system response shares similarities and important differences to traditional inductive source EM systems such as VTEM or MegaTEM, used extensively by the minerals industry to explore for targets of high conductance. While ZTEM can detect discrete conductors like inductive systems, it also responds to bulk changes in resistivity and conductivity gradients that often characterize geological contacts or structures.

One important deposit style that typically lacks a discrete conductivity response are porphyry copper-gold deposits, such as commonly found in many locations around the circum Pacific region. The present study will examine the ZTEM responses for several porphyry deposits in light of other exploration data including drilling, mapped geology and other forms of airborne and ground geophysics.

The Gilgai Granite and the Tingha Monzogranite are located south of Inverell in northeastern NSW (see Figure 1). Potential field data has been modelled to determine the morphological relationship of these two granites. Total Magnetic Intensity (TMI) data, acquired by the NSW government Discovery 2000 program, was used to generate a 2.5D model. Presented within this paper are the results of modelling the southwestern corner of the Gilgai Granite, which accounts for approximately one third of the Gilgai Granite that is visible in TMI imagery.

The Gilgai Granite is highly mineralised with disseminated and vein-type cassiterite and polymetallic sulfide occurrences. Tin has been historically mined, mainly from shallow workings. Better understanding the mineralisation and formation controls may increase exploration in this area.

Eleven TMI cross-sections were modelled. The results indicate that the Gilgai Granite is steeply dipping and tapers with increasing depth. It has a vertical extent of approximately 1000–1400 m and intruded around and over the Tingha Monzogranite, but not beneath the Tingha Monzogranite. The Gilgai Granite has sill-like bodies, isolated masses and/or roof pendants that intrude the Tingha Monzogranite. The source rock has magnetic zonation, with modelled magnetic susceptibilities ranging from 4.8–14.0×10^-3 SI with a mode of 5.5×10^-3 SI.

Modern underground coal mining requires certainty about geological faults and other structural features. Even a fault with a throw of a few metres can create safety issues and lead to costly delays in mine production. In this paper, we investigate the detectability of small faults by the seismic reflection method through numerical modelling in an ideal noise-free environment with homogeneous layering. We find that 1) the smallest faults that can be identified in a 2D survey have throws of 1/8 of the wavelength; 2) faults are more difficult to detect when they occur within other structures.

In typical seismic exploration for coal mining, the dominant seismic frequency is about 100 Hz and the seismic velocity of the overburden ranges from 3000m/s to 4000m/s. The corresponding wavelength is 30m to 40m. This suggests that the detectability limit for faults is about 4 – 5m. However, in the case of 3D seismic surveying we suggest that this can be redefined to 1/16 of wavelength (2 - 2.5m) because of the benefits offered by computer-aided horizon identification and the improved spatial coherence in 3D seismic surveys. In all cases, the actual fault detectability will depend on the quality of the seismic data and the geology of the area under investigation.

Magnetic field interpretation is often conducted on an incorrect assumption that remanent magnetization is insignificant and that the resultant magnetization direction is in the local geomagnetic field direction. For compact anomalies various methods exist to test this hypothesis and return estimates of magnetization direction utilising trial reduction to pole (RTP) transforms. We have developed an analysis to return the magnetization direction which generates the most symmetric RTE anomaly and have shown that this approximately also matches input magnetizations of synthetic compact anomalies. Estimation of magnetization direction from elongate anomalies is more problematic and intrinsically less reliable, but nevertheless we found that we were able to recover approximate magnetization direction from these anomalies using cross-correlation of an analytic signal function computed from vertically integrated gradients (which we term the ‘total vertically integrated gradient’ or TVIG) with RTP and RTE grids computed for trial magnetization directions. The various methods are readily and automatically obtained from scanning TMI grids. The resulting magnetization direction estimates are empirical rather than analytic and are approximate. They are best used as initial estimates prior to application of more rigorous, manual methods.

The direction of magnetization of a compact source causing a magnetic field anomaly can be found without concern for the details of its shape using magnetic moment analysis (MMA). This provides assistance in the successful inversion of magnetic anomalies due in part to remanent magnetization of unknown direction. However, the success of MMA is dependant on the analysis being positioned appropriately over the horizontal centre of the source body. MMA itself can derive an estimate of the horizontal centre of magnetization and in this presentation we investigate the conditions for stable convergence of an iterative solution that progressively steps the analysis to a revised horizontal centre of magnetization. We find that reliable magnetization directions can be recovered for grid widths down to twice the source depth and less. One, or at most two iterations of the analysis should locate the horizontal centre of magnetization over a compact source to within 5% of its depth and with a resulting uncertainty in magnetization direction of the order of 5°.

If Correlation of Gravitational Data with Topography is minimized correctly and coherently, Interpretation of Land-Based Gravitational Data will be a valuable clue leading us to detecting Lateral Variations of underground Mass-Density Anomalies particularly for Oil Exploration Purposes. For this reason, the author recommends a New Idea based on new mathematical definition of “Nearest Reference Equi-Gravitational Surface (NREGS)” for making uncorrelated gravitational data with topography. NREGS is Reference Equi-Gravitational Surface in exterior of earth where the gravitational data will be upward-continued to and interpreted, too. The Mathematical Theory of the Idea explains that gravitational data on NREGS should be the most uncorrelated with topography. Hence, gravitational data on it can easily illustrate lateral density variation of geological substructure with a lot of clear contrasts being so useful for horizontally detecting underground density anomalies e.g. salt structures. In a case study based on 6350 gravitational observations in Coastal Fars of Iran, the author applied NREGS Idea for exploration of salt structures. There, NREGS idea detects all outcropped salt structures and also, it shows some other Low Anomalies which can be underground hidden salt structures. In addition, the case study shows that the gravitational data on NREGS is statistically more uncorrelated with topography than other data e.g. Bouguer Anomaly, Free-Air Anomaly, etc. It means that the case study practically proves the theory of the New Idea, too.

The remote determination of magnetic remanence in rocks is a method that has largely been ignored because of the ambiguity associated with the estimation of both the Koenigsberger ratio and remanent magnetization direction. Our research shows that the resultant magnetization direction can be derived directly through inversion of magnetic data for an isolated magnetic anomaly. The resultant magnetization direction is a property of the target magnetic rocks and a robust inversion parameter. The departure angle of the resultant magnetization vector from that of the inducing magnetic field is an important indicator of the existence of remanent magnetization and the inversion process can detect departures that are not easily detected by visual inspection. This departure angle is called the Apparent Resultant Rotation Angle or ARRA.

The induced field vector, remanent magnetization vector and resultant magnetization vector lie on a great circle. We find the intersection of the polar wander vector trace with the great circle to obtain one or more possible solutions for the remanent magnetization direction. Geological deduction will normally allow us to reduce the ambiguity for multiple solutions to obtain the most likely remanent magnetization direction. Once the remanent magnetization direction is established, it is then possible to determine the Koenigsberger ratio and magnetic susceptibility for the target.

We illustrate the methodology with some synthetic models and targets from Australian magnetic surveys. Magnetic remanence is a physical property of the rock that is distinct from susceptibility and this methodology provides a new tool to help with the categorization and prioritization of exploration targets.

Understanding the relationship between mineralisation and magnetic intrusive complexes is pivotal in an exploration program. With the recent release of high resolution airborne geophysical data over the southeast of New South Wales, it is timely to complete a detailed study of the magnetic data that will assist the geological mapping and further understanding of the structure of the area.

Preliminary studies in determining the depth to magnetic source have been completed, including analysis of both manual and semi-automatic methods. Results are consistent and viable, and show general agreement between various modelling applications. A more detailed and exhaustive study is underway that will include comparisons using both line and grid data, and with various commercial semi-automatic magnetic modelling packages.

We have designed and tested compact magnetic B and dB/dt sensors suitable for geophysical operation through monitoring the current and voltage induced in a test conductor within the sensor. Laboratory and field tests confirm that a 50 cm long sensor of mass less than 1 kg can be constructed with noise levels between 1 pT and 10 fT per √Hertz over the 10 Hz to 100 kHz bandwidth respectively. The sensors are robust, and a rigid 3 component mounting box with accelerometer orientation permits rapid field deployment without the need for levelling