ASEG Extended Abstracts - ASEG2007 - 19th Geophysical Conference, 2007

Conventional interpretation of three dimensional potential field inverse models usually involves thresholding the model volume to create isosurfaces that hopefully outline the subsurface distribution of geological units. Unfortunately models are inherently smooth and the choice of the most appropriate value for isosurface generation is seldom clear-cut. However if both density and susceptibility models are available for an area then the combined dataset can be interpreted by classification using techniques developed for multi-band image data. Classification can be conducted using unsupervised or supervised techniques using either hard or soft classification algorithms. Supervised classification can be based on measured petrophysical data or on model values in areas where the surface or subsurface geology is well established. Soft classifiers are generally more appropriate than hard classifiers for this purpose since they better reflect the inherent geological ambiguity associated with often overlapping physical property distributions. Geological classification of potential field inverse models is illustrated with examples from the Northern Territory

The NSW Department of Primary Industries recently acquired over 200,000 line kilometres of high-resolution airborne geophysical data. As part of this process, significant resources were allocated in monitoring the data quality, which resulted in contractors delivering data to a very high standard.

Upon release of the new data, it was timely to update the state-wide grids for magnetic, radioelement, gravity and digital elevation data. After a mammoth effort in overcoming problems in merging aerially extensive data sets, merged state-wide grids of geophysical data is now available as a set of four DVDs.

Novel approaches in imaging aeromagnetic data in the Broken Hill region are assisting in the interpretation of the geology and structure in poorly outcropping areas.

Anglo Platinum, the world’s largest primary PGE producer conducts world-wide Greenfields and Brownfields exploration. In support of conventional methods, Anglo Platinum supplements the mapping and drilling with geophysical and remote sensing datasets. These datasets are subsequently integrated with all other available datasets and are captured in 3D software. This results in a most comprehensive structural ore body model which is subsequently available for an improved mine planning.

Datasets acquired comprise Landsat TM and ETM, Aster, Quickbird and Ikonos for remote sensing, geophysical and geotechnical wireline logging, airborne magnetics, radiometrics, gravity gradiometry, Spectrem’s EM and most importantly 3D Seismics, including VSPs.

Anglo Platinum is the world’s largest PGE producer and is currently exploring in several countries on various continents. Current exploration ranges from grassroots to near-mine expansions. Anglo Platinum uses an integrated exploration approach. Conventional geological exploration methods such as mapping and drilling are routinely supplemented with sophisticated geophysical and remote sensing data.

Geophysical wireline logging is a fundamental tool for determining the physical properties of the target and country rock. Knowledge of the physical properties is used to determine the optimum geophysical technique for subsequent exploration and ore body definition.

The wireline logging is complemented by geotechnical borehole logging to provide data aiding rock stability assessments for future underground development or slope stability assessments for open pits.

A variety of airborne surveys have been flown over Anglo Platinum’s concessions, amongst them high-resolution magnetics, EM surveys and gravity gradiometry. The magnetic surveys are the most significant for identifying disturbances. Consequently Anglo Platinum is a major participant in the Anglo Group Low-Temperature SQUID development project.

Prior to shaft sinking, risk reduction is undertaken using 3D seismic surveys. Survey design varies depending on the local geology and depth of the economic horizons. Surveys to date have vastly increased geological confidence.

Detailed satellite imagery is used for field-mapping. The availability of high resolution Quickbird and Ikonos imagery has assisted exploration in defining geological features. After comprehensive data acquisition and verification all geological and geophysical datasets are integrated to produce a detailed 3D ore body model for mine planning.

Technical Area: Case Histories Rest of the World

ROMPEL Andreas, has been with Anglo American since 1988 where he worked on mines and exploration for several commodities world-wide. Andy currently holds the position of Manager Geology & Geophysics in the Anglo Technical Division in Johannesburg, South Africa. He has been extensively involved in the integration and interpretation of geophysical and remote sensing data for Anglo Platinum. He has a Ph.D. in structural geology on the “Tectonic History of the Welkom Goldfield” in South Africa.

The sensitivity of RESOLVE data to highly conductive targets is examined. Modelling results from synthetic data across horizontal plates suggest that RESOLVE data detect highly conductive structures, but don’t resolve their conductances and time-constants if those values exceed 500 S and 8 ms, respectively.

A RESOLVE data set flown initially for kimberlite exploration has been re-analysed for nickel sulphides. Since the latter are highly conductive, the standard processing products such as apparent resistivity grids, deemed useful for mapping kimberlites, did not provide the optimum resolution for data analysis. The survey area is characterized by strong magnetic and EM responses due to the presence of banded-iron formations and pyrrhotite-rich massive sulphides. In order to determine the strongest conductors, time-constants and apparent conductivities were derived from the RESOLVE survey data. Due to the strong magnetic response, correction of the RESOLVE data for magnetic permeability resulted in better-resolved time-constants. This was achieved by inverting the data for the conductivity  and magnetic permeability μr of a layeredearth, followed by the forward modelling of the layer conductivities with μr=1. A comparison of independently derived time-constants and apparent conductances suggests that reliable values do not exceed 0.5 ms and 200 S, respectively. Conductors with values above 200 S / 0.5 ms are detectable, but not resolvable.

The Virtual Source Method (VSM) has been proposed for removing the effects caused by heterogeneities in the near surface. The method involves acquisition geometries similar to that of VSP, with receivers below the most troublesome part of the overburden. The time reversal principal is utilised to focus down-going energy through the overburden into useful primary energy at the virtual source location. The time reversal process is performed during data processing, and requires no knowledge of the velocity model between sources and sub-surface receivers. The result is a downward continued-dataset with virtual sources at the sub-surface receiver locations. The concept is verified using numerical and physical modelling, which demonstrates that the method can be used to accurately detect reflectors at depth, where conventional seismic fails.

A SQUID (Superconducting Quantum Intereference Device) rotating tensor gradiometer has been test flown in a helicopter-towed bird over a magnetic dipolar source. The bird was instrumented with fluxgates, tiltmeters, a gyroscope and GPS receivers to assist in levelling and positioning. After correcting for pitch, roll and yaw, the gradient tensor measurements along flight lines compare well with those calculated for the flight lines relative to the magnetic source.

The uses and advantages in mineral exploration of magnetic tensor gradiometry have been discussed at previous ASEG meetings (2001 and 2004) and include the benefits of vector surveys without the disadvantage of extreme sensitivity to orientation, desirable mathematical properties of true potential fields (important in areas with strong anomalies), allowing rigorous continuation, RTP, magnetization mapping and redundancy of tensor components giving inherent error correction and noise estimates.

A novel inversion/deconvolution procedure has been developed for locating and characterising dipole sources. Inversion using Euler deconvolution has been developed for locating a wider set of sources generally encountered in mineral exploration, such as spheres, sheets and pipes. A wide range of new types of processed data are available, including invariants, directional filters and depth slicing.

In Perth, Western Australia, there has been both an increasing demand for water and decreasing rainfall over recent years. Managed Aquifer Recharge is a water recycling method identified as having the potential to reduce pressure on Western Australia’s surface and ground resources. For this reason a treated waste water injection trial is planned for the Beenyup Waste Water Treatment plant. The treatment plant is located in the northern Perth suburb of Craigie. The trial will include detailed hydraulic flow and reactive transport modelling of the injected water. Accurate modelling requires precise knowledge of the hydrostratigraphy below the injection site. Consequently a high resolution 3-D seismic reflection survey will be used to assist in building a detailed groundwater flow model. Optimal 3D survey geometry has been designed based on a preliminary 2D survey, VSP data and the resolution required for the target injection zone within the Leederville formation. 3D survey design was faced with various difficulties as it needed to be designed with a number of exclusions zones related to topographic mounds, vegetation and existing infrastructure. Future construction plans at the site were also factored in to allow for future time lapse seismic surveys after long term injection has pressurised the target aquifer. Forward models of the expected seismic response of the injection process are computed using borehole information and velocity-pressure tests from core sample tests.

Technical area: Environmental-groundwater

An experimental device for measuring the transverse isotropic (TI) elastic properties of sediments under uniaxial strain conditions has been developed. The phased array compaction cell utilizes matched sets of Pand S-wave ultrasonic transducers located along the sides of the sample and an ultrasonic P-wave phased array source with pinducer receiver on the ends of the sample. The phased array provides plane P-waves that are used to measure phase velocities over a range of angles. From these measurements, the five elastic constants for TI media can be recovered as the sediment is compacted, without the need for sample unloading or reorienting. Descriptions of the apparatus and data processing and an application to an unconsolidated sand sample are provided in this paper. P-wave anisotropy of 20% has been observed in dry sand under an axial stress of 5 MPa.

Time-lapse surface and borehole seismic surveys are planned for monitoring the injection of CO2 in the CO2CRC Otway Basin Pilot Project in Victoria. Critical to the success of this is to ensure optimum repeatability of acquisition parameters and ground conditions. In order to assess the relative influences of source types and environmental conditions, a series of repeated test surveys have been undertaken.

The study utilised repeated high-resolution seismic surveys along the same 2D line. The first test line was acquired with mini-vibroseis (6000 lb) in wet conditions when the top soil and the weathered layers were fully saturated. The line was subsequently re-recorded in dry conditions where we utilised the same mini-vibrator, but in addition repeated the line using a free-fall weight drop. Both sources have similar total energy output but a vibrator is a controlled frequency source while a weight drop is not. Despite differences in the frequency content and phase of the signal generated by these two sources, and positioning differences for source and receiver locations of up to 1m, almost identical stacked sections were obtained after phase matching and scaling of the two datasets. Far greater differences in total energy, frequency content and phase of the signal were observed between the two vibroseis lines recorded at different times of the year (wet and dry periods).

Our results clearly demonstrate that near surface conditions has a first order effect on repeatability of land seismic surveys. A common belief that deployment of the same seismic source and positioning errors are crucial for successful time-lapse seismic needs to be reexamined in light of our results, which show that these factors are of secondary importance when it comes to land seismic surveys.

Technical Area: Time-lapse/CO2 sequestration, Seismic acquisition

Application of Integral Equation Method to calculate the electromagnetic induction in multiply folded sheet conductors (many folds) is simplified by replacing the conductor with trial source currents (two dimensional polynomials) of unknown amplitude. Using the Galerkin method to solve the integral equation reduces the problem to inverting for the amplitudes of the current basis (trial) functions .This results in calculation of two matrices, one the resistance matrix and only a function of the sheets dimension and conductivity, the inductance matrix related to the self and mutual inductance of the trial currents, and only function of sheet’s geometry,and a vector describing the interaction of the primary magnetic field with each trial function . In comparison to the solution for a flat (not folded) sheet conductor, the folded conductor solution involves changes to the inductance matrix . Using these solutions, computing the EM induction (forward model) requires less than one second of CPU time using current computing units. Including this forward model solution in an inversion scheme to produce parameters of multiply folded and plunging sheet conductors is easy to apply and results is inversion solutions requiring (typically) less that one minute of CPU time using a 2 GHz processor . This is expected to be an orders of magnitude improvement on any inversion scheme using for example smooth model voxel (cells) or finite element inversion algorithms .By using approximate solutions to show that at appropriate sampled times or frequencies, EM response of multiply folded sheet conductors in a layered medium, could be largely controlled by the changes of the primary magnetic field at the conductor, similar quick forward models and inversion algorithms can be applied to sheet conductors in conductive layered earth . A number of forward models and practical inversions of field data are used to demonstrate the effectiveness of the developed forward modelling and inversion algorithms.

This paper will present results from the combined use of hyperspectral data with airborne magnetics and radiometrics for mineral exploration in Western Australia. Due to the remoteness of Western Australia, it is often difficult to gain ground access to exploration leases for the intial phase of exploration. Hyperspectral data allows us to map the lithology of rock outcrops, while airborne magnetics enables us to interpret the structures of rock units to depth.

This paper will provide case studies demonstrating how in-mine geophysics is now being routinely used in a number of mines in Western Australia to map mineralisation and structures ahead of mining. Refined equipment and procedures enable in-mine geophysics to be run in any drillhole or tunnel in an underground mine with minimal impact on the mining process. These geophysical techniques provide a much higher resolution than traditional exploration techniques, mapping interfaces to an accuracy of less than 1 m.

Two of these in-mine geophysical techniques are downhole electromagnetics and Borehole radar. Downhole electromagnetics is a proven geophysical technique that can be used to map planar conductive mineralised zones, for example massive sulphide. Borehole radar is a proven geophysical technique to map the topography or ore zones and structures that control mineralisation. Borehole radar can also be used in transmission mode to map breaks and offsets in ore ahead of mining.

This paper will show how in-mine geophysics can greatly increase the confidence in the delineation of an orebody directly ahead of mining.

Large part of the northwest and central India is covered with Deccan volcanic of Cretaceous origin. Presence of thick sequence of Mesozoic sediments underneath the volcanic cover has opened a new frontier for hydrocarbon exploration in India. Recently, detailed gravity measurements were carried out over the Deccan Syneclise of central India as a part of integrated geophysical studies for the delineation of subtrappen Mesozoic sediments.

Bouguer anomaly map of the region depicts number of significant short wavelength anomalies due to shallow sources superposed on long wavelength regional anomalies due to deep-seated sources. Important among them are (i) an E-W trending broad relative gravity high associated with the Satpura mountains belt indicating presence of high density magmatic material at the deeper level (ii) a large wavelength regional gravity low in the southern part bears an inverse correlation with topography of Ajanta hills indicating mass deficiency beneath the excess topography load due to isostatic compensation (iii) short wavelength nearly E-W trending gravity low suggest the presence sediments below the volcanic cover.

The residual gravity field obtained using frequency domain filtering reveals number of linear positive anomalies due to basement upwarp apart from short wavelength negative anomalies aligned in E-W direction probably due to sediments. Quantitative modeling of residual gravity field constrained by other geophysical information has brought out prominent subtrappean Mesozoic sub-basins along the Tapti rift. It is observed that thickness of sediments increases towards the east reaching to a maximum of about 2.0 Km. It is therefore inferred that the Deccan Syneclise region of central India has large potential for hydrocarbon exploration.

The aim of this study was to investigate potential for the fixed wing Spectrem AEM system to detect perfect conductors. The methodology extends previous results in successful detection of perfect conductors in the twin helicopter Gemini AEM experiment. Primary fields in the square-wave Spectrem response are conventionally estimated by using the latest time samples in each halfcycle. Measured Spectrem aircraft attitude, coupled with physical constraints using tow-cable geometry and drag, were collectively used to devise a new primary field prediction for high altitude data. The new prediction method was then applied to survey data to produce a secondary field. Time constants predicted from the new secondary field estimation were sensitive to values 10 times greater than time constants using conventional late-time reference approach.

The Enfield oil field, located in the North West Shelf, Australia, began production in July 2006. In 2007 a 4D monitor survey was acquired to obtain a better understanding of injector pathways, stratigraphic and fault controlled reservoir connectivity, reservoir pressures and water front movement.

A dedicated baseline survey was acquired in 2004 with good image quality. The monitor survey, acquired 7 months after production, is the first dedicated 4D monitor survey in Australia. Both base and monitor optimise 4D repeatability and they were processed in parallel to enhance production-related effects.

A series of 4D modelling studies were conducted to decide the criteria for successful 4D interpretation. Pressure and saturation changes were modelled to determine the impact of production-related effects on the 4D. The competing effects of pressure and saturation on seismic amplitudes are complex, therefore an additional 3D swath was acquired over a water injector whilst injecting. This approach helped to calibrate pressure and saturation effects and identify preferential pathways within the main producing interval. The rock properties vary across the field with the eastern most area requiring AVO interpretation to determine fluid movement.

In this paper we will present the initial 4D interpretation results using both conventional seismic and AVO volumes and describe how the results were integrated with production and geological data.

There are varieties of velocity models and velocity estimation methods in seismic data processing, but each has some drawbacks to use specially in complex structures and curved reflectors. NIP tomography inversion method is a new method which works with kinematic wave field attributes. Common-reflectionsurface (CRS) stack provides us with three attributes, among them normal-incidence-point wave curvature RNIP will be used for the NIP tomography inversion. The method consists of three steps; automatic picking, modelling and inversion. One of the drawbacks of traditional methods is the huge points need to be picked and the difficulty of hand picking. With NIP tomography, we performed automatic picking on coherency section obtained from CRS stack. Picked data were plotted against other parameter and outlier data were omitted from dataset. These outliers are almost related to multiples which could make instability on inversion process. Then the corrected picked points were used for inversion process. In this step, the information velocity was interpolated between picks and trend of velocity changes was cleared. The traveltime for each point was calculated and the final 2D smooth velocity section was modelled. As it was expected, the velocity model performed good improvement in departing velocity variation in small scale and also in complex structures where it does not let conventional methods to differ two velocity values from each other.

While near-surface and classical seismic explorations obey the same laws of physics, the relative importance of those laws is different for the two types of surveys. These differences have led to some eccentric experiments with unexpected and occasional serendipitous outcomes. Progress attained by our research group has occurred through a mixture of stupid experiments that turned out to be clever and clever experiments that turned out to be stupid. Shallow seismic methods have matured noticeably since the time 25 years ago when the world’s scientific literature contained few refereed papers on shallow reflection. Much of the maturation is related to the revolution in microelectronics and the associated several orders of magnitude decrease in computational costs, while developments in sources, seismographs, and field methods have all played a role to differing degrees. However, other driving factors in this improvement have included demonstrable attainment of objectives such as providing structural contour maps of bedrock beneath alluvium, delineating shallow faults, evaluating near-surface stratigraphy to detect preferential groundwater flow paths, and detecting underground cavities. By 1999, we had demonstrated seismic reflection images from depths of less than a meter, easily within reach of a marginally competent grave digger. Detecting such shallow reflectors is expensive, however, because of the requirement to plant geophones at intervals of 10 cm or less. The effective resolution potential of classical seismic exploration data recorded on land is often determined by geologic conditions in the upper few tens of meters; in addition, the majority of statics problems commonly occur in the upper 30 meters. We are currently experimenting with methods of making near-surface threedimensional seismic imaging more cost-effective.

The Elk Trend Gas field in the Elk Reservoir was discovered in October of 2006 in a frontier area within the Eastern Papuan Basin in the Gulf Province, Papua New Guinea.

To overcome a lack of well and seismic data in this lightly explored area, InterOil has engaged in a comprehensive appraisal 2D and well seismic program. The stated objectives of this seismic program were to determine the overall structure of the Elk Reservoir with seismic data of dramatically increased quality and in addition to acquire seismic data in the follow-up appraisal Elk2 well to tie seismic character to well data including formation tops.

The increase in data quality is necessary to accurately define the dimensions of the Elk structure and, with well seismic from VSP programs, to enable strong correlation between the seismic data and reservoir properties from well bore measurements. To enable better quality data a series of parameter tests were performed which included analysing previous 2D data for noise content and strategies for noise removal, evaluating various shot and receiver arrays and also testing various shot hole depths, and charge sizes.

These parameter tests were highly successful with the data exceeding expectations. The demonstrated significant increase in data quality has allowed not only the gross reservoir structure to be mapped but has even allowed internal detail in the reservoir to be imaged. This data has allowed sequence stratigraphy to be attempted on land data from PNG for the first time.

VSP data collected from both zero offset and simultaneous walkaways has allowed robust correlations between formation tops and seismic to be performed. Future work with the VSP datasets and the new 2D seismic is intended to include AVO and inversion enabling petrophysical modelling of the Elk Reservoir.