ASEG Extended Abstracts - ASEG2001 - 15th Geophysical Conference, 2001

Airborne measurement of gravity tensor components provides a new means of high-resolution mapping of the gravity field. The tensor components attenuate more rapidly with increase in source depth than does gravity, so these methods are best suited to the investigation of shallow sources. A primary use of gravity studies in both petroleum and mineral exploration is the mapping of basement relief.

In this study gravity tensor variations have been computed over a complex model representing a horizontally and vertically faulted basement surface. Simulated measurement noise was added to the model output to create synthetic data sets. This synthetic data was then inverted to investigate the limitations of recovering basement structure from the data.

The model results presented in this study show that gravity gradients over wide and extensive vertical steps in a 0.4 gm/cc interface with amplitude +/- 10% of depth can be reasonably inverted up to noise levels of approximately 10 E.u rms. Multi-profile inversions substantially outperformed single profile inversions, indicating their value in suppressing random profile to profile noise.

A comparison between acousto-elastic modeling, surface seismic, and wireline responses to overpressure suggests that acoustic responses to overpressure are not uniform among the data of different sources. Acousto-elastic modeling underestimates the effect of the in-situ formation pressures on acoustic properties of waves. That seismic energy decreases with increasing pore pressure is evident for either very low overpressure or extremely high overpressure, but becomes less definite for moderate overpressure. Sonic logs in conjunction with wireline pressure measurements in a number of wells are used to establish a common understanding of acoustic-pressure relationship in the Muderong Shale of the northern Carnarvon Basin. The velocity-pressure relationship in the Muderong Shale seems to be controlled by stratigraphic factors. In the same stratigraphic category, acoustic velocity changes across wells are found to be consistent with pore-pressure variations in our examples. Investigation of velocity-pressure relationships for the Muderong Shale indicates that pore-pressure prediction should be based on the detailed lithologic classification.

Conductivity-depth imaging is a convenient form of presentation for preliminary interpretation of ground and airborne EM data. This paper describes the airborne EM adaptation of the Emax conductivity-depth transformation, originally developed for ground TEM. The transformation proceeds in two stages: first the apparent conductivity is determined at a given delay time; then the depth of the current maximum in a half-space with conductivity equal to the apparent conductivity is adopted as the apparent depth at that time.

The advantage of the Emax transformation is that it is readily adaptable to a wide variety of TEM data. The disadvantage is that apparent conductivity is not unique, nor always defined. In practice this does not usually pose difficulties for transformation of airborne EM.

The utility of the Emax transformation to airborne data is illustrated via application to GEOTEM_DEEP total field data. The total field provides a degree of immunity to receiver mis-orientation.

The Australian Department of Defence and AGSO aimed to provide a calibration facility for three-component (vector) magnetometers and thereby a cost-effective service for defence and geoscientific purposes. Defence was the main provider of funding and would be spared the expensive and long-winded process of sending its instruments overseas for calibration. Australia would gain from having a world-class facility available to all magnetometer users.

Three sets each of four coils are mounted orthogonally and connected to programmable current sources enabling the creation of magnetic fields between 0 |iT and 100 |iT in any direction. A sequence of field values is generated and compared with the corresponding outputs of the magnetometer under test. The system computer then automatically prints the main parameters describing the instrument’s sensor angles, sensitivities, linearities, and test conditions. Background field variations over the measurement period are subtracted.

Our system is capable of generating fields to a magnitude accuracy of 20 ppm and a directional precision of about 10 seconds of arc assured by checking the generated fields with a standard scalar (proton) magnetometer.

The Defence Department’s needs and its policy of encouraging general use have enabled AGSO, in close collaboration with its suppliers, to create a unique facility for terrestrial and space exploration, for general scientific research, and a foundation for future client needs. Already AGSO is developing, with funds from the CRC for Satellite Systems, the capacity to magnetically characterise FedSat which will carry a three-component fluxgate magnetometer into polar orbit early next year. FedSat is an Australian Government Centenary of Federation project.

High definition microgravity surveys have been conducted at Fosterville in an attempt to delineate major fault structures associated with gold mineralisation in Central Victoria. More specific local gravity anomalies were also anticipated for highly prospective zones of sulphide mineralisation along the margins of the fault.

Long wavelength features in the resulting data appear to be associated with the Fosterville fault but there are no obvious anomalies associated with the high level oxide zones. This result is consistent with the disseminated nature of the mineralisation within the host material and the subtle nature of any density contrasts close to the surface. However two distinct gravity anomalies have been detected to the east of the fault-line. These are associated with sulphide zones and estimates of excess mass are consistent with the geological constraints.

In addition to the gravity surveys, high density magnetic images have been obtained to provide complementary data over the same area. These indicate several linear contacts identified as possible splay faults. However there is no apparent correlation with the previously identified gravity anomalies.

Simple planar filaments can be used to provide a reasonable approximation to compound TEM eddy current decay systems. An optimum solution can be found using inversion theory to constrain the location and physical dimensions of these filaments in order to duplicate any spatial anomalies observed with fixed loop surveys.

Multiple filament solutions are possible for any traverse using different windows in the total decay curve. Any systematic migration or progression in the results as a function of time can provide an indication of target or host configuration.

Filament tracking techniques are developed here to provide a basis for data stripping designed to isolate a target response from the effects of highly conductive host materials. The results can provide for more complex geological constraints in place of a simple half-space model.

Spectral component analysis is commonly applied to airborne gamma ray spectral data to reduce statistical noise in the measurements of potassium, uranium and thorium. This technique has been applied to continuous 5-second ground measurements with a portable gamma ray spectrometer using a 7.6 cm x 7.6 cm (3 inch x 3 inch) sodium iodide detector. The results have shown that the reduction in statistical noise is much greater than for large airborne detectors. Even with a short sampling time of 5-seconds, continuous ground measurements have practical significance for both geological mapping and mineral exploration.

High-resolution aeromagnetic surveys were acquired for the Albuquerque basin in the central Rio Grande rift, a basin filled with poorly consolidated sediments. The surveys proved successful in efficiently and economically mapping previously unknown hydrogeologic features of the shallow subsurface. This success suggests that aeromagnetic methods may be useful in hydrogeologic studies of other sediment-filled basins.

The aeromagnetic surveys were used primarily to delineate buried igneous rocks and to locate faults within the basin fill, both important for understanding the subsurface hydrogeology. Buried igneous rocks were recognized from their high-frequency, high-amplitude anomalies and anomaly patterns. The horizontal-gradient and local wavenumber methods were applied to these anomalies to obtain estimates of their source depths.

The aeromagnetic surveys were also successfully used to locate faults within the basin fill. Anomalies associated with faults are produced by the juxtaposition of sediments having differing magnetic properties, contradicting the traditional belief that magnetization of poorly consolidated sediments is negligible. Expression of faults is abundant throughout the basin, revealing patterns that cannot be mapped at the surface due to widespread cover.

A fault signature recognized in the high-resolution data that has multiple inflection points is best explained by a fault with a thin magnetic layer on the upthrown block and thick magnetic layer on the downthrown block, called the thin-thick layers model. Geologically, this signature indicates erosion of the upthrown block or a growth-faulting scenario: fault-controlled sedimentation for faults that offset sediments, and successive accumulation of basalt on the downthrown block for faults that offset volcanic rocks.

Explicit expressions for the frequency dependence of the velocity and attenuation of shear waves in a periodic system of flat solid and viscous fluid layers have been derived by solving exact Rytov’s dispersion equation in the long-wavelength approximation. The dispersion and attenuation are related to the well known mechanisms of dissipation in porous media: viscoelastic mechanism (viscous shear relaxation) and visco-inertial Biot’s mechanism. The expressions describing the effects of these two mechanisms in a broad frequency range have been derived from the same standpoint. The asymptotic expressions for various limiting cases coincide with the results of previous studies.

I study the attenuation of an elastic wave propagating in a macroscopic heterogeneous poroelastic medium due to the scattering (conversion) of the passing wave’s energy into the highly attenuative Biot’s slow wave. This is done by studying two particular geometrical configurations: (1) a thinly-layered porous medium and (2) porous saturated medium with ellipsoidal inclusions. The frequency dependence of the so-called mode-conversion attenuation has the form of a relaxation peak, with the maximum of the dimensionless attenuation (inverse quality factor) at a frequency at which the wavelength of the Biot’s slow wave is approximately equal to the characteristic length of the medium (layer thickness or size of the inclusion). The width and the precise shape of this relaxation peak depend on the particular geometrical configuration. Physically, the mode-conversion attenuation is associated with wave-induced flow of the pore fluid across the interfaces between the host medium and the inclusions. The results of our study demonstrate how the local flow (or squirt) attenuation can be effectively modeled within the context of Biot’s theory of poroelasticity.

A new system for precise monitoring of surface and underground deformations in rock mass surrounding a mining operation has been developed and successfully deployed by CSIRO Exploration and Mining since 1993. The new system is based on a borehole strainmeter, which has the sensitivity to enable measurements remote (for example at distances of 1-5 km) from the mining operation, and long term stability to provide accurate measurements over periods of years.

Strainmeters traditionally used for deformation measurement have neither the long-term stability nor the high sensitivity to perform measurements in these applications. This paper describes the new precision borehole strain monitoring system (GTS) currently deployed by CSIRO Exploration and Mining, which has the potential to solve these new issues in rock mass deformation monitoring. This technology was originally developed for use in hard rock mines (Gladwin 1977) and refined considerably for earthquake research (Gladwin et al. 1994, Gwyther et al., 1992). A case study of the use of the GTS in monitoring longwall coal mining over the period 1993-1999 is presented.

Accurate measurement of rock mass response during the mining of massive underground ore-bodies is essential as mines become larger and deeper. The CSIRO GTS system is immediately applicable in circumstances such as: provision of quantitative data for optimal pit slope design and engineering; measurement of the stability of deep pit slopes, monitoring the long term integrity of shafts, large scale underground infrastructure and surface infrastructure, and long term measurements of deformation or rock mass creep following the mining process to provide data for environmental management.

It is well known that conventional Radon demultiple may fail when there is limited differential moveout between primaries and multiples or when the input data contain aliased events. These limitations can be overcome by an extension of the conventional Radon transform which uses data-derived constraints to enhance the focusing of energy in the transform domain. This leads to better separation of primaries and multiples and an improved resistance to errors due to noise and aliasing.

Data examples show the benefits that result in practice from this high-resolution version of the transform:

• a)   Better multiple removal and signal preservation when primaries and multiples have small differential moveout.
• b)   Removal of aliased multiples without the need for pre-interpolation.
• c)   Removal of aliased noise in the course of demultiple processing.

An interpretation of the bedrock topography of virtually the whole of Sydney Harbour has been undertaken to estimate the bedrock configuration, identify paleochannels, and determine the nature and thickness of overlying sediment layers. The interpretations are based on an extensive series of marine seismic reflection data recorded in 1975 covering the harbour entrance and tributaries, and data recorded in August 2000 covering the harbour entrance and adjacent areas.

Sydney Harbour has a bedrock configuration defined as a meandering channel that follows a generally deepening path as it reaches the harbour entrance with the bedrock configuration deepening to 85 m below sea level (b.s.l.) at the harbour entrance. Within the harbour, the paleochannel maintains a depth of about 30-40 m b.s.l. in the southern region near Point Piper and in Middle Harbour, and a depth of about 40-70 m b.s.l. in the middle region around the Sow and Pigs reef. In the northern survey area, paleochannel depths of 40-50 m b.s.l. were found in the Manly region. In Parramatta River, bedrock depths occur between 20-30 m b.s.l. in upstream sections of the channel and 40-45 m b.s.l. in downstream sections of the channel. In Lane Cove River, the depth of the paleochannel extends to 20-35 m b.s.l.

Meandering paleochannels occur throughout Sydney Harbour with tributary paleochannels clearly defined. Sydney Harbour sediments interpreted as Holocene marine sand were found to be thickest (65 m) near the harbour entrance. Average estimated sediment thicknesses were 17.4, 10.8, 14.3 and 13.6 m in Port Jackson, Parramatta River, Middle Harbour and Lane Cove River respectively.

Magnetite-pyrrhotite mineralisation at the Bull Creek Prospect in North-West Queensland is both magnetic and conductive. The mineralisation of Proterozoic age is buried beneath 30-50m of conductive younger cover. Overburden conductance values of 10 to 30 S make this a challenging area for airborne electromagnetic systems. The presence of this conductive cover and availability of ground EM and drilling information make this a valuable case study for evaluating the performance of airborne electromagnetic systems.

This paper presents data from both ground and airborne time domain electromagnetic systems and compares conductivity sections derived from the data with drilling information.

The moving loop ground EM system produced a very clear response from the basement conductor. This conductor was also detected by the 25 Hz GEOTEM and TEMPEST systems flown over the prospect in 1997 and 1998 respectively. Conductivity sections derived from measured response data were useful for visualising the response variations. Due to the width of the discrete conductor and the continuity of the conductive cover, conductivity sections based on a 1-dimensional approximation provided reasonable quantitative estimates for the thickness of conductive cover and the depth to the conductor.

While sphalerite is an important ore-forming mineral, the difficulties of exploring for this mineral target due to its low petrophysical contrast with gangue material have been well documented. Fortunately, many large zinc deposits also contain dense and conductive secondary sulphide minerals, which are good targets for standard geophysical tools.

The low-grade metamorphic, sediment hosted Century deposit of northwest Queensland represents a style of mineralisation rich in zinc, but with relatively few accessory sulphide minerals. Mineralisation is generally stratiform, consisting of fine-grained laminae of sphalerite, pyrite and galena cross cut by veins of sphalerite and galena in the upper layers of the deposit. Pyrite replaces sphalerite as the main sulphide mineral at the base of the deposit. Of the wide range of techniques applied to detect the deposit, including the gravity, magnetic and electromagnetic methods, only the induced polarisation technique shows a clear anomaly.

Conventional wisdom would suggest this induced polarisation anomaly was sourced from galena in the upper part of the deposit or pyrite enrichment in the footwall shales. The authors suggest that sphalerite may be a significant contributor to the observed chargeability response. Further study of the electrical properties of this mineral is required to assess the potential for using induced polarisation as direct targeting tool for zinc-rich orebodies with a low content of secondary sulphides.

The optimum bandwidth for shallow, high-resolution seismic reflection differs from that required for conventional petroleum reflection. An understanding of this issue is essential for correct choice of acquisition instrumentation. Numerical modelling of simple Bowen Basin coal structures illustrates that, for high-resolution imaging, it is important to accurately record all frequencies up to the limit imposed by earth scattering. On the contrary, the seismic image is much less dependent on frequencies at the lower end of the spectrum. These quantitative observations support the use of specialised high-frequency geophones for high-resolution seismic imaging. Synthetic seismic inversion trials demonstrate that, irrespective of the bandwidth of the seismic data, additional low-frequency impedance control is essential for accurate inversion. Inversion provides no compelling argument for the use of conventional petroleum geophones in the high-resolution arena.

An integrated geophysical study of salinisation at Cape Portland, NE Tasmania, has mapped the distribution of saline areas, and has identified constrictions in the hydrogeologic basement and a possible source and transport mechanism for the salt.

EM-31 data collected in the area not only clearly delineate the extent of salt scalds, but also highlight areas of elevated conductivity not visibly affected by salt. EM-31 data has enabled the distribution of the salt to be mapped at a much higher resolution than was previously possible using shallow drilling.

Results from time-domain electromagnetic surveys confirm the responses seen in the EM-31 data and provide additional information about the subsurface distribution of saline material. Conductivity depth pseudosections and layered earth inversions indicate depressions of up to 140 m in the resistive basement are infilled with more conductive material.

Ground magnetic data show the distribution of shallow Jurassic dolerite basement features and define a major negative amplitude anomaly which transects the study area. This feature is coincident with depressions identified from the time-domain electromagnetics and with a negative Bouguer gravity anomaly.

The electromagnetic and potential field interpretations are consistent and indicate the presence of a major palaeochannel infilled with more conductive material. This feature is inferred to be the major control on salinisation in the study area.

It has been shown by others working in the field of time-domain electromagnetic induction, that the late-time step response of the ground can be very useful for the detection of large, slowly decaying secondary fields. (Lamontagne, 1975; West et. al., 1984). A simple method is outlined here to calculate the step response from pulse-type time-domain EM data. Pulse-type systems are often described as impulse systems, but a measured impulse response cannot be used to derive the step response. These systems would better be described as hybrid systems - lying somewhere between a pure impulse and pure step - because their current termination is of short duration, but not instantaneous. By taking at least one measurement during the current turn-off time, and by ensuring that the turn-off is a very linear, controlled ramp, a simple step response calculation can be made.

It can also be important to determine the impulse response of the ground since many interpretation schemes are based on this measurement (for example, Nabighian, 1979). It is shown that with a few additional calculations while the step response is being calculated, the impulse response can also be easily determined.

In the Northern Territory, natural resource assessments are performed before land is released for horticulture or for intensive agriculture.

With the advent of detailed airborne geophysical surveys, funded by the NT Geological Survey, advanced assessment methodologies have been introduced. The new methods combine traditional site assessment skills with advanced image processing and GIS analysis.

The assessment process targets both water and land resources, with the intention of planning and managing development in a sustainable manner.

Initial tests of Enhanced Resource Assessment tools have shown the value of using airborne data to reduce the amount of ground truthing, and have been invaluable in modelling hydrogeological environments.