ASEG Extended Abstracts - ASEG2003 - 16th Geophysical Conference, 2003

Conductivity-depth images are finding application in salinity, ground water and environmental mapping. Hydrological modeling demands are for a much higher vertical resolution than the 10+m accuracy that was adequate in CDIs used for mineral exploration. Contractors are increasingly confident of system waveform, geometry, and some provide corrections for factors such as pitch, roll and yaw. This increased system accuracy is the trigger for efforts in increasing the accuracy of processing.

The CDI process makes a number of approximations in order to increase the speed of processing. One of the most critical in program EMFlow is an assumption that the transmitter and receiver are entirely within the current system induced in the ground at all delay times. This assumption equates to all components of the secondary field decaying monotonically with time. For typical fixed-wing AEM geometries, this assumption is poor for the z component of the response, and in fact on a CDI, z component data may predict to top of a surficial conductor to be several metres in the air. Allowing for part of the induced currents to lie between the transmitter and receiver leads to an accurate prediction of surficial conductors to lie at or below surface.

Conductivity-depth images are finding application in salinity, ground water and environmental mapping. Hydrological modeling demands are for a much higher vertical resolution than the 10+m accuracy that was adequate in CDIs used for mineral exploration. Contractors are increasingly confident of system waveform, geometry, and some provide corrections for factors such as pitch, roll and yaw. This increased system accuracy is the trigger for efforts in increasing the accuracy of processing.

The CDI process makes a number of approximations in order to increase the speed of processing. One of the most critical in program EMFlow is an assumption that the transmitter and receiver are entirely within the current system induced in the ground at all delay times. This assumption equates to all components of the secondary field decaying monotonically with time. For typical fixed-wing AEM geometries, this assumption is poor for the z component of the response, and in fact on a CDI, z component data may predict to top of a surficial conductor to be several metres in the air. Allowing for part of the induced currents to lie between the transmitter and receiver leads to an accurate prediction of surficial conductors to lie at or below surface.

Airborne gravity gradiometer technology has been successfully used to explore for a range of ore types (IOCG, Iron Ore, Kimberlite) and for geological mapping. BHPBilliton has successfully demonstrated that airborne gravity gradiometer can be used to map coal seams.

Geological sequestration of CO2 into brine-saturated reservoirs is an immediate option to reduce anthropogenic CO2 emissions into the atmosphere. It is anticipated that time-lapse 3-D seismic technology will form the foundation for monitoring CO2 migration within the subsurface. The success of seismic monitoring will be determined by the magnitude of the change in the elastic properties of the reservoir during the lifecycle of CO2 storage. In the short-term, there will be a strong contrast in density and compressibility between ‘free’ CO2 and brine. The contrast between these fluids is greater at shallower depth and higher temperature. The change in the elastic moduli of the reservoir will enable time-lapse seismic methods to readily monitor structural trapping of CO2 below an impermeable seal. However, because the acoustic contrast between brine saturated with CO2 and brine containing no dissolved CO2 is very slight, dissolved CO2 is unlikely to be detected by any seismic technology, including high-resolution borehole seismic. The detection of porosity increases associated with dissolution of susceptible minerals within the reservoir may provide a means for qualitative monitoring of CO2 dissolution. Conversion of aqueous CO2 into carbonate minerals should cause a detectable rise in the elastic moduli of the rock frame, especially the shear moduli. The magnitude of this rise increases with depth. Forward modelling suggests that the optimal reservoir depth for seismic monitoring is between 1000 and 2500 meters. Higher reservoir temperature is also preferred so that ‘free’ CO2 will resemble a vapour.

Edge detection and automatic trend analysis using potential field gradients are methods for producing unbiased estimates of sharp lateral changes in physical properties of rock packages. Where the points lying on the maximum horizontal gradients of potential field data show a lateral continuity, they can be mapped as "strings". These strings may be generated for many different levels of upward continuation.

When analysed in three dimensions, the strings provide information about the strength of the gradients, the locality of source-body edges, and possibly their dip directions and depth. For automatic analysis, sets of string-points may be converted into poly-lines, or curves, for each level of continuation. The best-fitting straight lines for curves with high linearity can be plotted as circular histograms (rose diagrams), or balloon diagrams, to provide a statistical representation of their orientations. Discrete areas can be windowed separately to show how dominant trend directions change in different geological settings. Balloon diagrams calculated for many levels of upward continuation may show how interpreted fracture sets evolve vertically.

Examples of analyses are shown for two areas of Australia, using gravity data. The first is from the Olympic Cu-Au province of the eastern Gawler Craton, mapping NW- and NE-trending structures, and the second maps the broad domains of an area of northern Australia.

Three hypotheses are tested for the pre-conditioning of airborne gamma-ray spectra to improve the accuracy of principal component-type (PC) spectral noise-reduction methods. First, I show that the distribution of the input variables (channel count rates) has little effect on the accuracy of the noise-reduction methods. Second, if there are insufficient spectra of a particular shape to form a statistically significant sample, then this shape will not be resolved by the noise-reduction methods, and will be removed as noise. However, by padding the data space with spectra exhibiting the full range of possible spectral shapes, an improvement in accuracy can be achieved. Third, the low signal-to-noise ratio in raw gamma-ray spectra limits the effectiveness of PC methods for removing the noise. If the signal-to-noise ratio in the input spectra is improved, the PC methods better remove the noise. Along-line summing of spectra improves the signal-to-noise ratio by exploiting the high correlation in signal between successive airborne gamma-ray spectra along each flight line. Summing spectra to optimum channels also improves the signal-to-noise ratio, but at the expense of spectral resolution. In both cases, spectral summing prior to the application of PC-type noise reduction results in a significant improvement in the accuracy of the noise-reduced spectra.

The NSW Department of Mineral Resources has been evaluating the use of geophysical techniques as a tool to better understand controls on opal formation in the Lightning Ridge district of New South Wales. A detailed study over the Gurley Ridge, approximately 25km southwest of the Lightning Ridge township, commenced in June 2001. This Cretaceous sedimentary ridge hosts the Coocoran Opal Field and includes the productive Allahs and Natelies Dream prospects. Current geological understanding predicts opal occurrence at the interface of clay and sandstone horizons within the sedimentary package. The study included a compilation of historic drill records, detailed surface geological mapping, lithofacies mapping, and acquisition of detailed airborne electromagnetic data. An electrical model of the subsurface generated from previous ground work was used to guide selection of the transmit-receive frequencies employed in the airborne survey. The acquired multifrequency airborne data were inverted and conductivity depth images for each flight and tieline produced. The CDI sections revealed the existence of an ubiquitous conductor located between 10m and 20m below the surface, covering the majority of the ridge area. A ground follow-up program was instigated to investigate the identified conductive zone. Eight high-resolution, detailed transient EM ground lines were acquired coincident with various conductive signatures identified in the airborne data. A small drill program was conducted to ground truth both the airborne and TEM subsurface conductive features. Drill results, downhole geophysical logs, and the detailed ground TEM data have been used to validate and refine inversion of the airborne data.

Moving platform 3D Full Tensor Gradiometry (FTG) is now available in both airborne and marine system versions. Current marine systems recently modified to go airborne are proving the adaptability/utility of FTG in transitional and land environments for both petroleum and minerals objectives. Two such surveys in North America, one targeting mineral deposits and the second, a hydrocarbon accumulation at a salt dome illustrate some current applications of this innovative technology.

The minerals deposit data was flown using/comparing both constant altitude and drape survey acquisition methods over an area of rugged topography (7.5x15.5km). Two drilled/sampled ore body targets (250-350m in diameter) were successfully detected with the Tzz tensor and compared/integrated to a dense grid of existing land gravity, Tz. Additionally, the horizontal tensors Txz and Tyz were utilized to determine lineament geometries from grid peak analysis, linking local tectonic elements to prospective subsurface geology and providing insight to ore body formation and hence detection. Frequency-filtering techniques are also employed to further enhance and separate geologic features.

The oil field data was gathered at constant altitude (300m AMSL) and covers an area of a shallow piercement salt diapir (9.7x10.3 km). The primary survey objectives are to image complex salt geology and detect areas of complex overhang geometries where 3D seismic data are inherently ambiguous. In this manner 3D FTG analysis is utilized to reduce risk and geologic uncertainty with respect to ongoing development/directional drilling programs. Further, it is anticipated that the FTG derived salt isopach will be used to reduce valuable iteration time during pre-stack depth migration (PSDM) of the 3D seismic.

These two case studies clearly demonstrate the diverse utility of this exciting new technology.

Helicopter electromagnetic (HEM) and magnetic survey data were acquired over the Cawse district in the Eastern Goldfields of Western Australia, to help define the structural controls that influence supergene mineralisation and delineate areas favorable for further exploration. The rationale for conducting the EM survey was that previous work in the area had demonstrated that local discontinuities, represented by stratigraphic and structural variations in the regolith, or related textural and/or mineralogical changes have influenced hydrogeological process and consequently the distribution mobile elements such as Ni, Co and Mn within the profile.

The observed conductivity structure in the HEM data is related to regolith heterogeneity and to the presence of hydromorphic barriers (structural, material) behind which salts and water accumulate. In places these barriers are spatially associated with Ni enrichment.

The Riverland area, located on the southern bank of the River Murray in South Australia, is a priority area for intervention under the National Action Plan. As part of the South Australian Salinity Mapping and Management Support Project, airborne geophysics was recognised as having potential to provide valuable biophysical data relevent to the management of irrigation development and groundwater recharge reduction in the area. We examine this potential, giving particular regard to the process involved in understanding the target, defining an appropriate geophysical tool and testing whether the desired output could be delivered economically and at an appropriate resolution.

The target in the Riverland area is the near surface Blanchetown Clay unit. Forward modelling suggested that a frequency domain helicopter electromagnetic (HEM) system could map spatial variability associated with this unit. A test survey was conducted using the RESOLVE HEM system further demonstrated that potential. CDI's of these data were compared with shallow drilling, EM31, EM34 and broadband ground TEM data. Results confirmed that near surface conductivity variations mapped by the airborne EM system were associated with the clay and that a product which could be incorporated with hyrogeological models to help predict rechage and influence management decisions in the area could be generated for the whole area.

The South Australian Salinity Mapping and Management Support Project (SA-SMMSP) developed a strategy for the application of geophysics from an understanding of regional variations in landscape, hydrogeology, the scale and nature of land use and the requirements of particular communities across the state. There was a clear appreciation that airborne geophysics by itself was not likely to define salinity management strategies, rather its value lay in better defining the biophysical attributes of the landscape thereby giving greater confidence in planning and designing remedial activities.

Data from a combination of airborne electromagnetic, magnetic and radiometric (magspec) systems were acquired to improve our understanding of five catchments/areas. Recognition was given to benefits of acquisition by particular systems at particular scales, given cognisance of the targets and resource needs of the decision makers. Preliminary data suggest that airborne geophysics will add value to existing regional-scaled datasets, allowing catchment planning to be based on biophysical data at more appropriate scales. Nevertheless, the economics involved remains to be determined.

Anomalous conductive responses that could not be related to obvious geological sources were observed in field data from a CSAMT survey in the Cobar area, New South Wales. Repeat measurements over the anomalous stations were made, yielding a substantially different resistivity response. Approximate positioning of the E-field dipole stations due to the presence of thick scrub was believed to be one possible source of error contributing to the differing responses.

Subsequent test measurements in which the E-field dipole angle was varied by 10 degrees east and west of the correct direction demonstrated that such an orientation change resulted in resistivity variations of up to 50% from the true value. This is significantly larger than expected from simple theory, which suggests that the results from location errors up to 10 degrees from correct should only be a few percent. This large variation observed in the Cobar area is believed to be due to a strong geo-electrical anisotropy within the steeply dipping and highly cleaved meta-sedimentary rocks of the Cobar Basin.

It was concluded that relatively small errors in receiver orientation could have a large effect on the magnitude of the received signal, which can potentially generate 'false' anomalies in the derived resistivity pseudosections and inversions. It is possible that such location errors are commonly made when a standard GPS is used to locate stations on a grid. Based on the experience from the Cobar area, accurate station positioning is clearly a pre-requisite for reliable electrical surveys in any geological terrain.

This paper presents field examples demonstrating the potential errors, and the results of applying more-accurate positioning techniques. Recommended methods to gauge the magnitude of the potential errors in any survey area are proposed.

We analyze true amplitude weights in time migration and demigration for a v(z) medium based on Bleistein's Kirchhoff inversion and modeling formulas and we discuss proper anti-aliasing formulas honoring sampling theory to preserve fidelity for 3-D migration and demigration.

To measure the land deformation, we applied SAR interferometry to a geothermal field, where a geothermal plant has operated since 1995. Interferometric fringes were generated by the difference in radar phase acquired at different times by the JERS-1 SAR. We constructed interference patterns from the difference of two pairs of SAR images of '94/09/19-'95/04/27 and ‘95/04/27-’95/09/06.

The coherence of the ‘94/09/19-’95/04/27 pair is degraded probably by the spatial baseline noise. The interference patterns partly suffer from a spatial change of moisture. To evaluate the results, we compare the interferometric fringes of ‘95/04/27-’95/09/06 pair with the level changes during 1994 - 1995 observed by leveling.

The leveling shows a local subsidence of several centimeters. There is a good correlation in the comparison, however, the amplitude of the leveling during September 1994 - October 1995 is about two times greater than the slant range of interferometry.

I interpret that the ground has been subsiding constantly during September 1994 - October 1995 and that the interferometric fringes correspond to the land deformation in the order of cm during about a half period of the leveling observation.

Existing surface wave modelling methods fail to correctly interpret some critical engineering scenarios, for example Earth models comprising 'soft' under 'hard' layers. Numerical and experimental repeatability analyses and the use of synthetic seismograms to reproduce the field data exactly, accounting for all superposed modes, leads to a better understanding of resolution and accuracy of the inversion results.

An inversion scheme based on the observation of an 'effective mode' phase velocity, whilst employing realistic error envelopes, has proved more successful than conventional methods, and demonstrated the need for array length at least 2.5 times the depth of investigation.

Recently the UWA PulseEKKO GPR system has been used for a variety of research applications. Earthquake seismology, physical geography and historical archaeology have all benefited from high-quality images of the shallow subsurface. These were for delineating a shallow Devonian fault scarp near Wave Rock, mapping fluvial channels and erosion processes in the Irwin River catchment and locating unmarked 19th century graves at the East Perth cemetery. While the radar does not provide literal underground structure, this was appreciated by all the clients, who's specialised knowledge of their field considerably aided the GPR interpretation.

As UWA science restructures to a more cohesive unit, similar multidisciplinary applications are anticipated, such as in agriculture and soil science. In addition, some of the acquisition problems and processing observations are discussed. Often seen with the unshielded PulseEKKO antennas is an unrecoverable clipping before DEWOW, due to signal saturation. Whether this problem is a conductive, (super-)paramagnetic or IP effect is still debatable. However, especially in a high dielectric earth, PulseEKKO seems to perform well.

Broadband VHF borehole radars (BHR) can be used as a tactical tools to map orebodies, faults and other marker horizons; to identify hazards in advance of mining and to stop unnecessary mine development. Ground penetrating radar (GPR) operations in boreholes have a number of advantages over those conducted in mine galleries. Boreholes give freedom from gallery reverberation and offer access to the third dimension. Mine boreholes are often drilled in fans, and can be used to reconstruct targets in 3D.

Significant progress has been made in recent years in using synthetic aperture radar interferometry (InSAR) to reconstruct 3D images from sparse arrays. SAR systems work with backscatter echoes and specular/nadir reflections from smooth surfaces are deliberately avoided. Hard-rock boreholes are thin requiring slimline BHR systems. The fact that slimline BHR systems are not directional makes it difficult to avoid recording nadir data.

Automatic methods of projecting data into 3D image space such as migration and InSAR make stringent demands upon rock homogeneity, translucence and the accuracy of borehole trajectories. These demands can be relaxed by kinematic mapping using geologically plausible 3D primitives such as cylinders, planes and hollows. Field data quality and object illumination constraints set the point of balance between object identification in image space and interactive interpretation in observation space. The balance sought must be able to take advantage of specular reflections, as BHR time sections are frequently dominated by nadir glare.