ASEG Extended Abstracts - ASEG2013 - 23rd Geophysical Conference, 2013

Toolibin Lake is located southeast of Perth in the WA Wheatbelt. Land clearing since the early 1990’s changed the hydrologic balance, which has produced rises in groundwater levels, water logging and salinisation. To manage this and to protect the environmental values of the lake system, pumping bores were installed in aquifers to allow water (and if possible salt) to be drawn down from the root zone. Whether these have been optimally located and whether others could be installed to greater effect has been the subject of more recent investigation. To assist with that SkyTEM data acquired over Toolibin Lake in October 2006 were reprocessed and re-inverted using borehole elevation and conductivity data as a-priori information, with the aim of improving the accuracy of the interpretation. These data were combined with information from a borehole NMR survey across the lake, allowing for the mapping of a more complex sequence of channels and valley fill deposits. A new understanding of the palaeohydrology of the Toolibin Catchment shows a complex palaeovalley sedimentary package with higher yielding aquifers located beneath Toolibin Lake. The present distribution of pumping bores suggests that they do not appear to tap these aquifers as effectively as possible. The updated hydrogeological conceptual model, resulting from a re-interpretation of the geophysics, will be employed to build a new numerical model to test management scenarios that include the installation of new groundwater pumping bores on the floor of Toolibin Lake

Coded-impact seismic sources, including Mini-SOSIE and SIST, deliver a sequence of impacts in a controlled pattern. These sources generate seismic data with acceptable bandwidth and signal-to-noise, but also introduce correlation noise proportional to the rate of production.

We examine a method for correlation-noise attenuation utilising a Weiner filter designed on the known impact sequence. Numerical models demonstrate that the algorithm can achieve very significant attenuation of correlation noise in both Mini-SOSIE and SIST records. The practical viability of the approach is examined with reference to real-data examples.

This procedure has the potential to provide records of acceptable quality from rapid impact sequences, yielding significant productivity benefits over current Mini- SOSIE and SIST approaches.

The Kevitsa nickel-copper deposit in northern Finland is a large, low-grade, mafic-hosted accumulation of disseminated sulphides with rare, spatially restricted occurrences of net-textured to semi-massive sulphides. The mineable limits of the resource grow or shrink with commodity prices, but it has also been recognised that subtle mafic layering in the intrusion controls sub- horizontal layering of sulphides. The net-textured and semi-massive mineralisation styles occur near the base of the intrusion. Data from a 3D seismic survey demonstrate the unpredicted ability to image the sub-horizontal mafic layering, as well as the expected reflections at the base of the intrusion in contact with interlayered volcanic and sedimentary country rocks. The ability to trace the lateral extents of the mafic layering, backed up by analysis of borehole sonic and density logging, offers the possibility to predict the ultimate envelope of the resource. The interpretation of the base of the intrusion provides a horizon along which to target the net-textured to semi- massive contact mineralisation.

Variable-depth streamer acquisition, as one of the key marine broadband solutions, has shown great advantages in providing high resolution seismic imaging and better low frequency penetration than conventional data from examples around the world. By utilizing the notch diversity, the receiver ghost was fully removed through proprietary joint deconvolution method in the imaging domain which produces broadband imaging with bandwidth up to six octaves. However, this post- migration deghosting method requires the receiver ghost to be well preserved prior to final migration thus introduces complexity in key steps like multiple attenuation, velocity analysis and migration than the conventional processing flow.

The objective of this work is to simplify the processing flow by applying bootstrap deghosting method right after shot domain de-noise. After this step, processing flow will be very similar to the conventional flow. This new flow has been tested on the variable-depth streamer data from NWS Australia. Compared to the post-migration deghosting flow, the new results not only provide similar benefit as to broader bandwidth and rich images, but also show improvement on multiple attenuation and primary preservation as well as seismic inversion.

Conventional geological mapping using aerial photo interpretation coupled with field visits is often handicapped in areas with tropical weathering or sand cover in arid regions. High resolution airborne geophysical data acquisition over large areas covered by overburden can augment geological mapping of these areas provided information extraction is done in an adequate, automated and objective way. We suggest that the integration of airborne geophysical data with space- borne remote sensing data suites using fuzzy partitioning clustering meets these criteria.

The outcome of unsupervised clustering is a classified zonal map which, in combination with some field inspections, produces a high resolution lithology map, in this case 200 m by 200 m pixel size, which is by power of spatial resolution far superior to any conventional mapping. It can therefore provide new incentive for geological understanding of the area, modelling or mineral exploration.

Our new approach to map hard rock geology hidden beneath overburden has been applied to a portion of largely sand and sediment covered southern Namibia, the so-called Sperrgebiet, by integrating airborne magnetic, radiometric and Landsat 7 data suites. The classified zonal maps obtained from these data were converted to a (pseudo) lithology map by attributing what is known from previous geological investigations of this area.

The ZTEM airborne EM system was introduced into commercial service by Geotech Ltd. in 2006. ZTEM is unlike other EM systems in that it relies on the measurement of natural occurring EM fields in the AFMAG frequency range of 25-720 Hz. As a primary survey outcome, a ZTEM survey produces in-line and cross-line tipper data; Tzx and Tzy, at a series of frequencies. The contractor provides a suite of filtered X and Y spatial grids and 2D conductivity depth inversions have typically been provided using the Tzx component.

More recent, full 3D inversion of data has begun to be applied which provides concurrent information about cross-line (derived from the Tzx) and along line (derived from the Tzy) data.

The exploration value of inversions whether 2D or 3D is not yet well established due to 1) the limited surveys available to date and 2) limited availability of inversion codes and their application. In the present assessment, 2D and 3D inversion results over three deposits will be examined and considered in terms of the geological knowledge available at each site.

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 of the key means assess how a new technology performs is to compare the new outcomes with results from well-understood techniques. In this review, ZTEM data will be compared with data from three different airborne TEM systems over three deposits.

Lalor Lake is a VMS deposit in central Manitoba, Canada. The deep ore body is buried under the cover rocks up to 1000 m. Multiple EM data sets were collected to delineate the compact and conductive alteration zones and two data sets are available to us. The first is HELITEM, an airborne time-domain EM survey that covers the entire exploration area. The second is a ground loop EM data measured by SQUID magnetometers that have high precision at late times. The two data sets map the conductivity structures at Lalor Lake in different ways: the airborne survey covers a broad area but has limited resolving power at depth; the ground survey provides information about the deep targets through very late times but the measurements were made in a smaller area. Individual 3D inversions were carried out for both data sets assuming little a prior information. Both are able to recover the trace of the expected ore body, but the airborne model is smooth and the ground model contains highly conductive anomalies. Then we invert the ground data again with the airborne model as the reference model. The new inversion again confirms the existence of the VMS ore body but also rearranges the conductive material according to the constraints from the reference model. The new model differs significantly from the blind inversion model at the deposit scale. Based on the information from the inversion so far, we conclude both surveys have picked up signals from the ore body in different levels of detail. More analysis and further data are still required to better delineate the target’s geometry.

True 3D IP is defined to mean acquisition where the current flow is sampled in multiple directions up to and including orthogonal from a single injection point.

To achieve this, orthogonal receiver electrodes must also be used to prevent losing coverage where electrodes might otherwise be null coupled.

Using this configuration, Quantec Geoscience’s Orion system has demonstrated far greater geological definition than any conventional configuration including offset IP or multiple 2D lines inverted as a 3D dataset.

The reasons for the greater definition lie in several key aspects of the Orion layout including not only the large number of data points recorded but also that these data points are recorded in a true omnidirectional fashion. All receivers on the grid record the response for each current injection. This results in multiple intersecting current paths over the entire survey area.

3D inversions can use the multiple intersecting current paths to create a more geologically realistic inversion model with no acquisition directional bias

Results from actual field surveys show this increased definition through a high correlation with drill results.

An ORION 3D DC/IP survey was conducted over Blackthorn Resources’ Kitumba IOCG deposit in Zambia. The survey results provided 3D models that successfully delineated the known deposit and provided an enhanced understanding of the three-dimensional geometry of the mineralization. With this improved understanding of their deposit, Blackthorn was able to refocus their ongoing drilling program to best target possible extensions of the existing mineralization.

Presented is a case history of a seven year-plus project to acquire a 3D seismic data over the Long Beach Field. The Long Beach field is a faulted anticline along the Newport-Inglewood Fault Zone and presents challenging survey design considerations just for proper imaging; however, data acquisition is complicated by its location in the dense urban environment of the greater Los Angeles area. Presented is a brief history of discovery and development of the field, lessons-learn from a failed attempt in 2005 to acquire a 3D survey, application of lessons-learn and new nodal technology in the design and operation of the successful 2011 survey. Also, some unexpected benefits of autonomous recording are discussed.

The Kauring Test Site in Western Australia was established in 2009 to provide a public benchmarking and comparison venue for new and existing airborne gravity and airborne gravity gradiometry (AGG) technology.

Fugro Airborne Surveys flew the fixed-wing FALCON® AGG system over the Kauring AGG Test Site over three periods in July 2011, November 2011 and February 2012.

Comparison between the FALCON AGG survey data and the high resolution ground gravity data over the Kauring AGG Test site indicates that the FALCON vertical gravity gradient, GDD, has an error of +/- 5.6 Eo, and that the FALCON vertical gravity, gD, has an error of +/- 0.18 mGal,.

Comparison between the digital elevation model (DEM) derived from the fixed wing FALCON survey laser scanner data, and a high-resolution third party DEM, indicates that the error of the vertical position of the FALCON differential GPS is less than 0.5 m. At 60 m terrain clearance this corresponds to a subsequent error in AGG terrain correction of less than 2 eotvos. This terrain correction error is well within the Kauring AGG Test Site FALCON survey noise envelope of 5.6 Eo.

Electrical resistivity methods were applied to delineate leakage pathways and to investigate the condition of core material in earth fill dams. In other to evaluate the engineering geological properties of the soil deposits, two boreholes a dam were drilled to the bedrock that exceeds the height of the dam. A large set of field tests including standard penetration test(SPT) and in-situ permeability tests were carried out along the boreholes.

A series of laboratory tests were also conducted on the undisturbed soil samples obtained using the split-spoon sampler and thin wall tube sampler to determine their engineering characters. The resistivity values which were estimated from the previous inversion result for each depth were compared with the N values from SPT for each borehole. It could be classified in two groups where were not showed in general trend and most of the sites had some seepage problem and decrepit facilities need to be improved.

We have also measured resistivity values of undisturbed soil samples obtained from boreholes at 311 different dam sites. We confirmed low resistivity values of soil in core material were distributed at the Gyeongsang basin and some regions in Korean peninsula. As a result of these studies, it was possible to get more quantitative interpretation of seepage problem.

Faults are the most important geological structures which need to be detected in any modern underground coal mining project. Even a fault with a throw of a few metres can create safety issues and lead to costly delays in mine production. While locating faults with throws greater than 5-10 m is quite successful by seismic survey, techniques to resolve the more subtle faults, shears and features, which exploration programs should also locate are needed.

Faults cause breaks in continuity of seismic horizons. These discontinuities generate diffraction patterns. Before the days of seismic migration and generation of very high fold data, diffraction patterns were sought by seismic interpreters as an indication of faulting, especially for small faults where the discontinuities of the seismic reflections are less evident. Most processing is now aimed at suppressing these diffractions. However, in recent years, techniques for diffraction imaging developed for petroleum seismic data processing, makes small fault detection possible by separating the diffraction events from the reflection seismic events.

In this paper, we apply the diffraction imaging techniques to enhance the detectability of the small faults for coal seismic environments and demonstrate the feasibility of a new fault imaging method with numerical examples. The effects of noise and migration velocity to fault imaging will also be discussed.

A simple and fast method to perform the final time-to- depth conversion is to use 3D grid tomography, in which the inversion aims only at minimizing the misties between wells and seismic data. In the tomography only the normal ray needs to be shot, making the process very fast so that it can easily be run on a single workstation. This methodology avoids lengthy processes of correction using externally derived functions and/or transformation of velocities from interval to average and back.

The proposed workflow is applied to the PSDM data from the Pluto field, in the Australian North-West shelf. The input data had large residual misties with important lateral variability. The final results were robust, with tomography producing a smooth model which tied all the markers for all wells within a velocity depth sample.

The program package escript is a module in python for solving mathematical modelling problems. It is based on the finite element method (FEM) and scales on compute clusters for thousands of cores. In this paper we will discuss an extension to escript for solving large-scale inversion problems, in particular the joint inversion of magnetic and gravity data. In contrast to conventional inversion programs escript avoids the assemblage of the -in general- dense sensitivity matrix which is problematic when it comes to large-scale problems. Moreover, we will show how the FEM approach can easily be used to solve the adjoined forward problems required for the gradient calculation of the cost function.

Wavepath eikonal traveltime (WET) refraction tomograms are generated with the generalised reciprocal method (GRM), a novel medium resolution common offset gather (COG) implementation of the GRM, both using uniform velocities and vertical velocity gradients, and the low resolution default starting model consisting of smooth vertical velocity gradients. All tomograms have comparable misfit errors, which illustrates the ubiquity of non-uniqueness and the necessity for validating all starting models.

Nevertheless, the use of even the maximum vertical velocity gradients in the weathered region does not produce any improvement in the spatial resolution of the seismic velocities in the sub-weathered region with either the default or the COG GRM starting models. It is concluded that if a low resolution starting model is used, then the most likely outcome will be a low resolution WET tomogram, irrespective of whether or not vertical velocity gradients are employed.

Vertical velocity gradients can be represented as part of a continuum of seismic velocities in the weathered layer, which range from uniform to hyperbolic velocities, and which are consistent with the traveltime data. Acceptable models employ seismic velocities in the weathered and sub-weathered regions computed with the same XY value. The optimum XY value is representative of uniform seismic velocities, whereas the maximum XY value, which is the average cross-over distance, is representative of default and hyperbolic velocities. Intermediate XY values indicate more moderate vertical velocity gradients and/or undetected layers.

This paper describes how a large, 12,030km2, exploration 3D seismic survey in the Great Australian Bight was designed in order to maximize the efficiency of the survey. The entire survey area was completed within a single acquisition weather window of 7 months with low infill rates.

Challenges for this particular survey included deep targets, very large swells, stormy weather patterns, variable ocean currents and remoteness of the survey area itself.

The acquisition geometry was carefully designed to optimize the efficiency of the survey given the challenging operational constraints. The design of the acquisition parameters helped the acquisition to continue in severe swell conditions without introducing detrimental noise in the data.

Unnecessary infill lines were reduced through combining active infill management with Fanned, steerable streamer coverage. The required coverage was analysed using real data in the survey design stage and the achieved coverage was actively monitored during the survey.

The Magnetic Induced Polarization (MIP) method uses the measurement of magnetic fields to directly detect internal and external current flow from IP-generating targets, rather than the resultant surface currents as with conventional Electric Induced Polarization (EIP). Magnetometric Resistivity (MMR) measures the magnetic field produced by galvanic current flow to detect horizontal variations in resistivity. We focus primarily on the MIP method but since MIP and MMR data are collected simultaneously, we will treat them together where appropriate. MIP/MMR is insensitive to horizontal layering, and is especially suitable for regions with highly conductive cover where EIP and resistivity responses are sharply attenuated. Magnetic fields easily propagate through such conditions; therefore MIP/MMR is minimally impacted by conductive cover. The other major advantage of MIP/MMR, over traditional electrical IP and resistivity, is that it completely eliminates the need for measurement electrodes. Hence, it is effective in difficult ground contact conditions such as dry sandy soils, frozen ground, and rocky scree slopes. For inversion purposes, MIP has an additional benefit that magnetic fields can be measured in all three axes simultaneously, which provides significantly more information about target position and attitude. By using SQUID technology and remote referencing, we are able to improve the data quality and extract useful three component MIP and MMR data. We present a number of field trials using both frequency and time domain methods to analyze the MIP and MMR responses from porphyry copper, and unconformity uranium ore bodies.