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

If the major rock forming minerals are divided into three categories, and, if the density, magnetic susceptibility, and the proportions of each category are known, then the density and susceptibility of a mixture of the three categories can be determined. Conversely, if the physical properties of a mixture are known along with the physical properties of each of the three components, then the proportions of each component can be calculated. This reasoning can be used to superimpose category percentage contours onto a density-susceptibility scatter diagram. Such a diagram can be used to relate petrophysical measurements on a rock specimen to its mineral category content, the veracity of which can be appraised by a geologist with a hand lens. Several hundred qualitative tests suggest that the approach is valid. The analysis is independent of scale, and can be applied to the bodies of a density-susceptibility model developed to simulate exploration data. Gabbro with magnetite plots along a specific locus on the combined phase/scatter diagram and can often be distinguished from denser accumulations of hematite and/or sulfides. Case histories for a number of economic and non-economic density-susceptibility anomalies give support for the usefulness of the method.

In November 2001, Minotaur Resources announced the results of discovery drillhole URNOOl. The drillhole intersected 107m averaging 1.94% copper and 0.66 g/t Au from 200m depth and a further 152m averaging 1.20% Copper and 0.61 g/t Au from 429m, hosted in a hematite breccia. URNOOl was targeted at a gravity anomaly, located to the south of an intense magnetic anomaly. Targeting of the gravity anomaly utilised 3D smooth model inversion.

Following the announcement of the assay results from URNOOl, potential field and electrical surveys were completed over the prospect area. Downhole induced polarisation logging of URNOOl, showed that the high grade copper mineralisation was associated with chargeability and resistivity anomalies.

In an attempt to highlight zones of high grade copper mineralisation, a dipole-dipole induced polarisation (IP) survey was performed. A strong phase anomaly, coincident with a gravity anomaly was highlighted by this survey.

Drill testing of this anomalous source resulted in the intersection of massive hematite, containing little to no copper mineralisation. Downhole IP logging, and petrophysical work has confirmed that the hematite is chargeable and has a low resistivity. The electrical properties of the "barren hematite" are comparable to the high grade copper mineralisation. The barren hematite is also a significant contributor to the gravity anomaly seen at Prominent Hill. As a consequence it is difficult to distinguish between mineralised and unmineralised breccias by geophysical methods.

The Yallalie impact structure is located within the Mesozoic sedimentary rocks of the Perth Basin, Western Australia. A high-resolution aeromagnetic survey across the structure at a 200 metre line spacing and 60 metre flying height has defined a series of concentric anomalies centred on the structure.

The causes of magnetic anomalies associated with meteorite impacts in sedimentary basins are poorly understood in comparison with those into crystalline basement rocks. Modelling suggests the most likely source of the magnetic anomalies at Yallalie is either an injected melt sheet or new magnetic species created by hydrothermal alteration of pyrite or glauconite within the Cretaceous marine sediments of the Leederville Formation or Parmelia Formations.

Within the Mesa J pisolite iron ore mine, clay is distributed unevenly and often forms pods and channels less than 10 m in width. The mine resource model, based on 50 m x 50 m spaced drilling, has been unable to accurately delineate the clay for scheduling and mining purposes. Therefore, alternative methods for mapping the clay-contaminated ore were sought.

Predictive clay mapping will significantly reduce production costs at Mesa J through improved selective mining practices, more accurate scheduling, reduced wear on mining equipment and minimising the need for maintenance of haul roads and ramps.

High lateral resolution electromagnetic (EM) surveying using the Geonics EM31 tool can quickly delineate the extent and orientation of the clays. A block model based on the interpretation of the EM data accurately predicts the volume and tonnage of ore and waste material. While rip-line mapping, infill drilling, ground penetrating radar and gravity can delineate clay to varying degrees of success, none were as efficient as EM31.

Predictive deconvolution is widely treated as a universally applicable tool for multiple removal and wavelet compression. The fundamental assumption of random reflectivity is seriously compromised in geological situations where the reflection sequence comprises a small number of dominant horizons. This situation is not uncommon in coal environments.

Where the primary seismic objective is high quality imaging of particular target horizons, an improved result can be achieved if the deconvolution is designed according to assumptions more relevant to the geological situation. We outline a simple example of this approach, aimed at imaging a production coal seam, of thickness 5-10 m, at a mine in the Bowen Basin, Australia.

Using horizon time picks from a preliminary volume, the full reflection package associated with the seam is extracted and deterministically filtered to obtain an estimate of the intrinsic wavelet. A Wiener spiking filter, designed on the extracted wavelet, is then used to deconvolve the seam package.

In comparison to the predictive deconvolution approach, this model-based procedure provides improved resolution of the top and base coal interfaces. In addition, derived amplitude and frequency attributes are more robust in terms of known geology. Variants of this simple model-based procedure should have relevance in a range of dominant-horizon situations where predictive deconvolution is invalid.

Algorithms have been constructed in MATLAB for the inversion of potential field tensor (vector gradient) data using Monte Carlo and downhill simplex processes. Such algorithms have been used to invert artificially created data sets generated from simple geological structures, such as linear dykes and faults. Both magnetic and gravity tensor data can be examined using the algorithms. The algorithms have been tested over a range of depths, from metres to kilometres. They stop inversion once ideal parameters are obtained (RMS error equal to one), or can be altered to continue, in order to overfit the model.

Within fractured rock, the irregular and often unpredictable distribution and geometry of hydraulically conductive fractures produces large spatial variations in bore yield and groundwater quality. As fractures act as conduits for flow of both groundwater and electrical charge, methods that can efficiently detect the distribution of electrical pathways can be used to infer characteristics of significant hydrological parameters. This study compares the capabilities and limitations of electrical data obtained from direct current (DC) and electromagnetic (EM) surface azimuthal measurements, and from DC borehole-to-surface and cross-borehole measurements, for the interpretation of major hydrological structures.

Data obtained from surface methods, while limited by poor depth sensitivity and overburden provides a useful tool for determining variations in directional conductivity at sites free of bedrock exposure and boreholes. Application of borehole-to-surface methods yielded a better resolved interpretation of sub-vertical fracture strike and was useful in identifying lateral variations in bedrock heterogeneity. Improved flexibility and sensitivity to measurements at depth permitted cross-borehole electrical tomography data to be used in accurately reconstructing the spatial distribution of sub-horizontal, laterally extensive, electrically conductive zones. While the technique was restricted to sites with multiple boreholes, the data obtained was used to constrain specific flow paths over a regional scale.

Conventional multi-component seismic analysis simply relies on appropriate component selection to provide P-and S-wave images. However, this ignores the potential cross-contamination of P-wave energy on the horizontal components, and S-wave energy on the vertical component that may occur in certain geological situations.

Where wavefield cross-contamination occurs, there is potential to achieve cleaner P- and S-wave images by more fully exploiting the true vector nature of multi-component seismic data. Vector processing for exploration-scale data typically combines frequency and slowness information, together with particle motion, to distinguish different wave types. Three such multi-trace, multi-component wavefield separation schemes, termed MUSIC, rvVSA and PBVI, are considered here. These vector techniques all utilise a parametric approach whereby wavefield slowness and polarisation are modelled simultaneously in the frequency domain. The PBVI algorithm is considered to be the most generally useful of the three algorithms.

Synthetic and ocean-bottom data examples are used to demonstrate practical issues relating to the use of these vector separation schemes. In cases where there is significant cross-contamination, vector wavefield separation produces P- and S-wave records that differ significantly from the vertical and horizontal components, respectively. Where cross-contamination is less problematic, production vector processing is not warranted. In these cases, however, vector processing still provides valuable quantitative validation of the natural-separation assumption.

The recent availability of 14 band multi-spectral and 15 metre DEM data from the satellite borne ASTER (Advanced Spaceborne Thermal Emission Reflection Radiometer) instrument has introduced new possibilities for geological mapping and integration with traditional geophysical data sets. Calibrated radiance at the sensor data and atmospheric corrected ASTER products, such as surface reflectance and emissivity imagery, have been made available by NASA/USGS and the Japanese agency, ERSDAC. ASTER radiance data measures five bands at 30 metre resolution, within the short-wave infrared 2.1 to 2.5 urn wavelength region, and also five bands at 90 metre resolution within the 8 to 12 urn thermal infrared region. This compares with only one band as measured by Landsat TM for each of these regions. An investigation into the capabilities of ASTER and strategies for its integration with geophysical data (using ER Mapper) was undertaken at Broken Hill, given its high spatial geological control and extensive airborne geophysical data sets. Mineral (group) maps derived from multi-scene ASTER data, collected over the Broken Hill Block, proved useful for discriminating stratigraphic units, regolith and areas of alteration. In particular ASTER imagery highlighted several sericite-rich as well as quartz-rich colluvial regolith. Variations in ASTER's derived AIOH mineral spectral signatures correlated with higher potassium radiometric responses and indicated a change in the muscovite chemistry, possibly due to retrograde metamorphic alteration. However structural features associated with retrograde shear zones were identified most clearly using aeromagnetics. Overall ASTER data products provided complementary mineralogical information to the structural interpretation afforded by geophysical data sets.

Australian Water Environments (AWE) has been engaged by the Bookpurnong - Lock 4 Environmental Association (BL4EA) to investigate the interception of saline groundwater along the River Murray downstream of Berri, South Australia (Figure 1). Groundwater discharge contributes approximately 70 tonnes per day of salt to this reach of the river. Salt loads are predicted to rise to more than 200 tonnes per day over the next 30 years. The proposed Environmental Enhancement Scheme (EES) comprises a series of shallow pumping wells constructed primarily on the floodplain to intercept saline groundwater before it reaches the river. Intercepted groundwater will be pumped to the Noora Disposal Basin located some 25 km inland. A detailed understanding of the hydrogeological environment is essential to the design and effective operation of the EES and similar groundwater interception schemes.

In March 2001, Zonge Engineering conducted a fast-sampling TEM (time-domain EM) traverse to investigate changes in sub-surface resistivity across the highland area adjacent the River Murray and the floodplain within the riverine trench. The highland is planted with citrus and has been irrigated since the 1960's. The shallow depth to saline groundwater within the Monoman Formation aquifer has seriously degraded the health of native vegetation health across the floodplain.

In May 2002, twelve shallow wells were constructed along a narrow stretch of floodplain located approximately one kilometre downstream of the first traverse. Accurate formation samples were collected and logged. Zonge Engineering conducted a second fast-sampling TEM survey in June 2002 along a line parallel to the trial borefield to investigate the correlation between TEM data and variations in hydrogeology identified during drilling.

This paper reports the results of the two land-based geophysical surveys and comments on the utility of fast-sampling TEM as a tool for mapping subsurface hydrogeology across the highland and floodplain at Bookpurnong.

The Pajingo Epithermal System is an area of low sulfidation epithermal veining and alteration about 15 km in diameter at the northern margin of the Drummond Basin. Tertiary and younger conductive sediments cover about 80% of the area.

The gold mineralisation is within thin quartz veins and most of the ore bodies discovered to date are along the NW trending Vera-Nancy structure.

The host intermediate volcanics are magnetic and the epithermal alteration that extends up to 50m from the veins along the Vera-Nancy structure is magnetite destructive. Results of a high resolution magnetic survey clearly delineate the major structures including the Vera-Nancy structure.

The quartz veins are within broader zones of silicification and gradient array resistivity surveying has been used to map these zones. Generally the high resistivity zones due to silicification are coincident with the structures identified in the magnetics.

High resolution magnetics and resistivity continue to be the most useful geophysical tools in the ongoing exploration of the Pajingo Epithermal System for additional mineralised structures.

The airborne electromagnetics (AEM) technique has been used very successfully for over 50 years to locate conductive ore bodies. For most users of the technique, the primary purpose of AEM has been to provide a rapid and inexpensive means to locate targets, whereupon the best features were then followed up with ground geophysical techniques prior to possible drill testing.

In this role as a first-pass detection tool, AEM equipment development has focused primarily on spatial resolution, depth of detection and rejection of conductive cover. Traditionally little emphasis was placed upon the need to provide quantitative information about a target's conductance.

This situation has been changing in the last 10 years, driven in large part from concerns expressed from those groups exploring for magmatic nickel deposits. Drawing upon both petrophysical studies and field observations, a case is developing that magmatic nickel deposits have conductances that place them far outside the conductance bandwidth of effectively all AEM systems. This being the case, then the use of AEM as a primary target identification tool for new nickel deposits is a seriously flawed strategy.

So as to better understand this issue, AEM data sets acquired over the Voisey's Bay deposits, Labrador, Canada have been analysed using standard commercial processing techniques such as CDIs and time constant analysis. Additionally, a series of numerical models have been generated to try to better explain the observed field results, as well as simulate the response that other AEM systems would produce over the deposit.

While the Voisey's Bay deposit undoubtedly contains mineralisation of very high conductivity, the processing and modelling shows the AEM technique to be an effective means to locate and discriminate targets of high conductance. Although such conductance discrimination may only be achievable on a relative scale, it is seen as generally adequate for most exploration situations.

The industrialist Peter Scotese once said, "Integrity is not a 90 percent thing, not a 95 percent thing; either you have it or you don't." In the case of seismic data, structural integrity derives from the combined accuracy of the migration algorithm and velocities used. Indeed, these two components are complimentary. For example, even the most accurate migration method cannot provide reliable structural images if the velocities used are obtained by simply scaling stacking velocities by 90 percent or 95 percent!

The nature of the velocity field is, of course, a major factor in determining the detail, sophistication and intensity required for velocity analysis. However, it also influences the accuracy of the structural image in other, subtler ways. In particular, although the range of velocity variations dictates the complexity of the imaging method that should be used, it can also influence what parts of the earth can be imaged, and, by implication, even place constraints on the data acquisition. Finally, because the focusing aspect of migration is relatively insensitive to certain (anisotropic) aspects of the velocity field, this limits the accuracy with which migration velocities alone can be used to convert directly to depth, and can even introduce lateral mispositioning in the final image. Indeed, this is one of the major reasons why migrated data often do not tie wells correctly.

In this paper, we consider the impact and interaction of velocity variations and migration algorithms on structural imaging. Specifically, we review different migration algorithms, and show how more detailed and accurate consideration of the velocity behaviour on the underlying physics can lead to more reliable images and improved velocity determination. We also show that fundamental limitations in data-derived velocities can be overcome using well information, and that, when this is done, anisotropic prestack depth migration can provide exceptionally accurate images in depth. As General Norman Schwartzkopf once said about integrity, "The truth of the matter is that you always know the right thing to do. The hard part is doing it."

Nuclear Magnetic Resonance (NMR), also known as Proton Magnetic Resonance, is observed when the magnetic dipole moments of single protons resonate in a magnetic field with a frequency proportional to the strength of that field. Typically, an artificial magnetic field is used to align the proton moments in a direction different from the ambient field and terminated abruptly, after which the resonant frequency can be observed with a decaying amplitude. This effect is exploited in proton precession magnetometers, as well as in medical imaging.

The magnetic field of a large electric current in a wire loop on the surface of the earth will excite protons in the subsurface (usually in the form of water) in the same way. The relaxation of the protons can be observed and used to determine water content as a function of depth.

The aquifer at WMC's Phosphate Hill Mine in northwest Queensland is associated with a siliceous facies of both the Monastery Creek Phosphate and Lower Siltstone Members of the Beetle Creek Formation with a maximum thickness of 60-80m, and is confined by the overlying Inca Shale. The groundwater is contained in fine joints, bedding partings and interstices of sandy beds, and the water table depth is about 30m below the surface throughout the area. In this article we examine the results of NMR measurements using 100m loops to delineate the aquifer and determine its hydrogeological parameters.

The test indicated an excellent correlation between the results of one dimensional layer inversions of NMR results data and the known aquifer geometry. The inversion utilised the amplitude E0 of the proton relaxation field after turn-off of the excitation (directly indicative of the water content), the decay time constant T2* of the relaxation field (related to pore size), and phase shift (p0 of the relaxation field with respect to the excitation current in the loop (related to resistivity). The top of the aquifer and the water content were both defined within known parameters from drilling, while the base of the aquifer was inferred to be consistent with current estimates of its position in almost 50% of the locations.

In hard rock regions, a large range of stacking velocities is required to correctly stack reflectors of different dips. Typically, horizontal reflectors stack at 6000 m/s, whereas reflectors with dips of 60° stack at 12,000 m/s. For high fold (vibrator) data, correct stack of conflicting dips can be achieved by dip moveout (DMO) correction. However, for lower fold (dynamite) data, the sparse offset distribution complicates application of DMO. An alternative technique involves producing stacks with different stacking velocities and stacking these stacks. This technique was applied to two seismic reflection data sets, low fold dynamite data from Broken Hill and high fold vibrator data from the Lachlan Fold Belt. The Lachlan data set was used as both full 60/120 fold and reduced 10/20 fold. Velocity analysis, both analytical and empirical, was carried out to determine the range of stacking velocities. Stacking velocity increases with dip angle (cos" 8), but the velocity range across which an event stacks coherently increases more rapidly (approximately cos" 8 for velocities typical of hard rock). The most critical area for analysis is the first two seconds of data, due to greater sensitivity of NMO to stacking velocity. The optimum number of stacks is an important consideration, based on the number of stacks in which an event contributes coherently to the sum The Broken Hill stack data showed simultaneous imaging of horizontal and dipping events. For the Lachlan reduced fold data set, horizontal and moderate to steeply dipping events were stacked successfully, although not as well as the post-DMO stack of the full fold data. The technique has some problems at the shallowest levels, where the stack can be degraded due to time shifts of events in the individual stacks, but is a useful tool for low fold seismic data typical of older regional profiles.

Success of airborne gravity survey mainly depends on determining the three-dimensional (3-D) position of the moving platform. Recent advances in technology, especially in Global Positioning System (GPS) have made it possible to determine the velocity and position of the moving platform with greater accuracy. Taking the advantage of these advancements in GPS technology and using a newly developed system, helicopter-borne gravity measurements were successfully carried out over the Kanto and Tokai districts of Japan. This new gravimeter system is composed of servo accelerometer sensors, stabilized platform, optical-fibre gyroscope to control the stabilized platform, GPS receivers and a data processor.

The 3-D position of the helicopter at every second was accurately determined by interferometric GPS method. These GPS data were also used to compute various correction factors which are applied on the measured gravity acceleration. Real-time differential GPS positioning were also conducted using a separate receiver mounted on the helicopter. These real-time positioning data were used for controlling the optical-fibre gyroscope. The gravity acceleration data were processed and necessary corrections were applied. Numerical filtering was carried out to remove high frequency noises present in the data. The observed free-air gravity anomalies were then compared with upward continuation of the ground truth. We also made an attempt to compile airborne gravity anomaly map.