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

This paper reviews a 3-year research project targetted towards processing and interpretation methodologies for borehole EM methods in the search for weak electrical conductors. Weak conductors are best energised by current-channelling methods, which include both MMR, and inductive loops off-set from the hole collar. The following results have been achieved:

• Recognition of the importance of using current-channelling energisation of weak conductors; a sphalerite-rich conductor is frequently too poor a conductor to support an inductive EM response of vortex currents, but any conductor showing a conductivity contrast relative to host rock, can produce an EM response from currents channelled through the conductor from the conducting host rock.
• Recognition of the equivalence of MMR and long-offset TEM data, in terms of information and interpretation of current-channelling energisation of conductors.
• Comparison of numerical modelling techniques for inductive and current-channelling responses in TEM surveys using state-of-art software (MARCO and LEROI); identification of instabilities in multiple-plate modelling in LEROI.
• Development of an algorithm for modelling the three-component MMR or current-channelling EM response of multiple 3D conductors.
• Recognition of the importance of correct modelling of the MMR response of a layered earth as a precursor to stripping background and modelling of residual MMR anomalies.
• Development of a software package for processing three-component MMR field data (calibration, stripping of wire field and layered-earth background field) and for modelling the data in 3 components, and with multiple 3D conductive bodies.
• Development of a 3D rotatable screen viewer for an arbitrary number of boreholes, transmitters and dipping tabular conductors (incorporated in the above software package).
• Numerical modelling of an MMR phase response.
• A comparison of noise characteristics of different receivers and borehole EM probes.

A method is presented for estimating residual statics that are computed prior to normal moveout correction. No velocity information is required, and the offset and structure terms are eliminated from the decomposition.

Model source (shot) records are formed using the EOM method of prestack migration to provide a one-to-one trace for cross-correlation. After cross-correlation, the estimated time shifts are decomposed directly into source and receiver statics.

The method is stable for ling time windows that are input to the cross-correlation, and provides solutions for structured data where other methods fail.

We have used numerical modelling to improve understanding of seismic reflection in basalt covered regions. Our models are based on hydrocarbon prospects in the Denison Trough (Queensland, Australia).

Reflectivity modelling has been used to assess the influence of a range of model and source parameters. Models that include a single near-surface basalt layer generally result in relatively noise-free reflection signals, provided the basalt is reasonably attenuative. Reflection quality is poorest for models with buried high-velocity basalts, or for multi-layered basalts interspersed with lower-velocity material. Such models result in strong reverberatory noise, apparently propagating between the surface and basalt, or within the multi-layered basalts. In these situations, reflection strength is significantly improved if the source can be positioned below the basalt.

Finite-difference modelling permits analysis of models incorporating lateral variations in basalt geometry. Shot records generated with this approach exhibit basalt-related features seen routinely in real field data. Simple stacking of the finite-difference records indicates that reasonable sections can be obtained in areas of near-surface, or thin, basalts. Poorer stack quality is associated with thicker, buried basalts although deeper reflectors may be imaged by undershooting the basalt.

The cost and risk associated with mineral exploration in Australia increases significantly as companies move into deeper regolith covered terrain. The ability to map the bedrock and the depth of weathering within an area has the potential to decrease this risk and increase the effectiveness of exploration programs. This paper is the second in a trilogy concerning the Grant's Patch area of the Eastern Goldfields. The recent development of the graVP potential field inversion program in conjunction with the acquisition of high-resolution gravity data over an area with extensive drilling provided an opportunity to evaluate three-dimensional gravity inversion as a bedrock and regolith mapping tool

An apparent density model of the study area was constructed, with the ground represented as adjoining 200 m by 200 m vertical rectangular prisms. During inversion graVP incrementally adjusted the density of each prism until the free-air gravity response of the model replicated the observed data. For the Grant's Patch study area, this model proved easier to interpret than the Bouguer gravity.

A regolith layer was introduced into the apparent density model and realistic fresh-rock densities assigned to each basement prism according to its interpreted lithology. With the basement and regolith densities fixed, the graVP inversion algorithm adjusted the depth to fresh basement until the misfit between the calculated and observed gravity response was minimised. The resulting geometry of the bedrock/regolith contact largely replicated the base of weathering indicated by drilling with predicted depth of weathering values from gravity inversion typically within 30% of those logged during RAB and RC drilling.

The Elang formation produces a weak event which is virtually impossible to pick on conventionally processed seismic sections. Its top is difficult to define on wireline logs. A rigorous approach to modelling its reflectivity is as a superposition of reflections from a sequence of layers at < 2m thickness. This formation responseverifies that AVO can be derived for interfaces near the top of the Elang if VD,VS and p are averaged over at least 10m above and 10m below this interface. Using a depth of 3264m for Top Elang this method predicts a polarity reversal at ray parameter, p=0.105 s/km (about mid-streamer).

However, at near-to-mid streamer offsets (p < 0.105 s/km) the reflectivity is too sensitive to depth to derive an AVO representative of the Elang. At Laminaria East, the full depth model predicts a peak/trough wavelet at the TWT near top Elang (3215ms) which is grossly influenced by the sidelobes of boundaries beneath the reservoir.

At far offsets (p > 0.105 s/km) a consistent trough develops in the x-p formation response which extends out to an equivalent 8km source/receiver offset. This trough is clear on the full model seismogram when multiples and guided waves are suppressed. It may provide the basis for defining a more interpretable signature for the Elang by weighting far offsets traces and perhaps acquiring data out to very long offsets.

Given a gravity profile, the location and size of a blocklike approximation to the source can be found with the use of simple formulas that do not involve any searching. This allows for a very fast algorithm and real-time results. The approach has proven robust when tested against different irregular sources and under noisy conditions. An applicati on to real data is also presented.

In April 1998 Normandy successfully flew a new helicopter borne TDEM system. This system has been in constant use and data comparing ground and airborne TDEM are presented.

In December 2000 a new non-metallic version of the system was flown which allows a combined transmitter and receiver to be flown at 30 metres terrain clearance. Data comparing the Mk I and Mk II versions are presented.

During 1999-2003, significant additional funding is being made available to the Northern Territory Geological Survey through the Northern Territory Exploration Initiative. Airborne geophysics is the cornerstone of the Initiative. The allocation of substantial expenditure to this activity recognises its importance to the early exploration protocols of project generation and area selection. The adoption of semi-regional airborne survey specifications is considered adequate for qualitative district-scale geological interpretation.

In its desire to attract and maintain global client contact, NTGS is focused on developing efficient mechanisms for access to and delivery of its spatial data and information over the Internet. Excellent early progress has been made using located geophysical imagery with the implementation of an image web server.

The production of territory-wide geological and magnetic maps was a key output achieved early in the life of the Initiative. The establishment of the Interpreted Geology map series, initially focused on poorly exposed mineralised terrains, provides explorers with basement geology derived largely from airborne geophysical data as part of the integrated multidisciplinary approach adopted by all NTGS regional geoscientific studies.

A new concept for acquiring calibrated towed streamer seismic data is introduced through a new acquisition and processing system. The specification of the new system has been defined by rigorous analysis of the factors that limit the sensitivity of seismic data in 4D studies and imaging. New sensor and streamer technology, new source technology and advances in positioning techniques and data processing have addressed these limitations.

Sensitivity analysis revealed that the most significant perturbations to the seismic signal are swell noise and sensor sensitivity variations. Conventional analog groups of hydrophones are designed to suppress swell noise however a new technique for data-adaptive coherent noise attenuation delivers even greater noise suppression for densely spatially sampled single-sensor data. Although modern source controllers provide accurate airgun firing control the signature of an airgun array may vary from shot to shot. This can be due to factors such as changes in the array geometry, air pressure variations, depth variations and wave action. A method for estimating the far-field signature of a source array is the Notional Source Method (proprietary to Schlumberger) which has been steadily refined since its first disclosure. A recent development compensates for variation in source array geometry by monitoring the position and azimuth of each subarray using GPS receivers mounted on the floats.

New calibrated positioning and streamer control systems are part of the new acquisition system. Active vertical and lateral streamer control is achieved using steerable birds and positioning uncertainty is reduced through an in-built fully braced acoustic ranging system. Calibrated marine seismic data are achieved through quantifying the source output, the sensor responses and positioning uncertainty. The consequential improvements in seismic fidelity result in better imaging and more reliable 4D analysis.

Surface seismic data are normally acquired using arrays of receivers; each array forms a group, which is recorded into a single seismic channel. Two of the main drawbacks of conventional analog arrays are that the outputs of each receiver element are simply summed without any preprocessing and that the spatial sampling is fixed at the survey design stage. A new acquisition system has been developed which can record up to 30,000 channels so each component of the array can be digitised and recorded individually, this is called single sensor recording. The system consists of the acquisition hardware and a suite of software tools for processing the data. Single sensor data will yield improved data quality as improved noise attenuation results from the process of digital group forming (DGF). In addition, eliminating intra-array static variations retains high frequencies lost when statics are averaged in conventional analog arrays. DGF also allows greater flexibility in the processing centre. Output sampling may be varied during processing so different processing schemes can be employed for different temporal and spatial parts of the survey. In a single survey, shot to image multiple targets, the workflow can be optimised for each target. The new system moves us a step closer to decoupling the geophysics of the imaging process from the acquisition hardware and reduces the levels of noise and distortion, which limit the bandwidth of seismic data

A recently acquired aeromagnetic survey over the predominantly Cainozoic covered Black Hill/Murray Bridge region of South Australia (SA) revealed three magnetic bodies that were poorly defined by previous, regional-scale BMR aeromagnetic data. Outcrop and mineral exploration company drilling of the Black Hill gravity and aeromagnetic anomalies indicate that they are layered mafic/ultramafic intrusive bodies that have potential for PGE, chromite and Ni/Cu massive sulphide mineralisation.

The current project of detailed ground geophysical surveys are part of the South Australian Governments phased, regional exploration strategy known as the Targeted Exploration Initiative South Australia (TEISA). The aim of this project and TEISA is to accelerate resource exploration in SA and send a strong message to the mineral industry that the SA Government is prepared to share some of the preliminary exploration costs.

The geophysical surveys of this project revealed anomalism within the core of the northern Black Hill Gabbroic Complex intrusive. The data were interpreted and the gravity anomalism modelled, resulting in a proposed diamond drill hole and down-hole geophysical survey to test a layered mafic/ultramafic magmatic mineralisation model.

The Hamersley Basin of Western Australia holds significant tonnages of detrital iron deposits. Formed by a cyclic weathering and eroding sequence of nominally barren banded iron formation, they may form rich proximal accumulations of iron cemented gravels distal from any obvious hard-rock iron mineralisation. While generally small, they nonetheless represent excellent economic targets as they are near surface and easy to mine. The relatively high density of iron detritals, as compared with their sedimentary hosts, makes the gravity method the primary method of exploring for these blind deposits. Identification of possible trap sites is used as a precursor to gravity surveying, with magnetics used as an ancillary method prior to drilling.

Application of the gravity method in this terrain is not straightforward. Detrital trap sites are best developed next to the spectacular cliffs that form the range fronts to the Hamersley Ranges. Precise terrain corrections and use of first vertical derivatives are routinely applied to gravity data in an effort to map every possible gravity high that may reflect the presence of a detrital accumulation. Extensions to Hamersley Iron Pty. Limited's BS2D deposit were discovered by the routine application of gravity. It has been used to create a model by which exploration for detrital deposits continue to the present day.

Following extensive negotiation and consultation with Anangu Pitjantjatjara (AP) stakeholders, Minerals and Energy Resources South Australia (MER) obtained permission to undertake a regional airborne geophysical program over the Mann, Woodroffe and Alberga 1:250, 000 map sheets within the Musgrave Block.

Commencing with the Woodroffe map sheet the surveys were flown at 400 metre line spacing and at a survey height of 80 metres.

The surveys provided an excellent opportunity for the AP stakeholders to view first hand the processes and mechanisms required to undertake a large regional airborne survey. As a result MER was further encouraged to continue the survey onto the adjoining map sheets of Mann and Alberga.

The result is a spectacular data set covering approximately 75% of the Giles Complex within the Musgrave Block. The new TEiSA data has provided an excellent insight into the geological structure of the region at a resolution never seen before. There is clear delineation of the mafic bodies and fault systems hidden beneath the shallow cover.

The radiometric "RGB" ternary image shows the contrasting geology clearly delineating such features as the Woodroffe Thrust Zone.

The digital terrain data provides good terrain detail successfully delineating the ranges and dune systems throughout the region.

Formation properties such as mineral composition, fluid contents, porosity, permeability and etc are accurately obtained from the petrophysical interpretation of well logs. After calibrating the petrophysical interpretation with well testing results, net pay zone is quantified.

Different seismic attributes (facies) are associated with different subsurface formation properties in different ways. These distinguished relationships are used to map the areal distribution of formation properties, particularly reservoir fluids.

The integrated interpretation of well logs and surface seismic adds value to the process of reservoir optimization. The in-situ measurements of the logs are the ultimate ground truth and are linked to surface seismic via the transformation from logs to seismic waves. This transformation is the vital link to the integrated interpretation of well logs and seismic attributes.

According to Dix' s formula, for the isotropic and short spread case, the moveout curves of a common shotpoint (CSP) gather or a common midpoint (CMP) gather can be approximated by a hyperbola. As a matter of fact, the spread of the field survey may not always meet the requirement of Dix's assumption and the real earth does not behave as a perfect isotropic medium. Releasing the limitation of spread length and considering the anisotropy the real earth, Byun et al (1989) derived the formula for nonhyperbolic analysis for horizontally layered, transversely isotropic, media. The formula is called skewed hyperbolic moveout formula and involves three measurement parameters: the average vertical velocity and horizontal and skew moveout velocities. Carry on Byun's skewed hyperbolic moveout formula; Sena (1991) derived analytical expressions giving traveltime-offset curves for multilayered weakly azimuthally isotropic and anisotropic media on terms of the elastic properties of each layer.

Inheriting to the idea of skewed hyperbola moveout that has been proposed, we assumed that the seismic ray follows an imaginary rectilinear path. Based on the fundamental concept of vector analysis, we decomposed the velocity of seismic wave into two components: vertical and horizontal components. Compare to Byun's moveout formula, the velocity parameters had been reduced from three to two. We named the new formula as a simplified nonhyperbolic moveout formula. To ascertain our ideology, the new formula is tested by laboratory data.

Laboratory walk away transmission experiments were done on both Plexiglas and phenolite that were homogenous and commonly adopted to stand for isotropic and anisotropic materials, respectively. A semblance analysis is adopted to process the observed data. Scanning the recorded gather using the new formula we proposed, laboratory works show the moveout velocity that was computed from reflection seismology could be decomposed into vertical and horizontal components with reasons. In the process of time-to-depth conversion, one can always estimate the depth or layer thickness by multiplying one-way traveltime to vertical velocity of the new moveout formula, and the horizontal velocity that can be used to diagnose the anisotropy of the explored area and to explore the lithology of the subsurface strata.

Various types of noise can obscure reflections and refractions on deep crustal seismic data. This study presents a simple but elegant approach to true wavefield processing where noise reduction and trace interpolations are applied implicitly during depth extrapolation. No velocity and source/receiver information is required and the proposed approach can be combined with conventional or available processing algorithms. Prestack processing is successfully applied to a complicated OBS data set, demonstrating an improvement in noise reduction and effective trace interpolation.

Significant information about the underground continuity of the surface geology of the Las Petas district has been obtained, and it is presented on a solid geology map.

The newly defined lithomagnetic units range from Precambrian basement, largely beneath non-magnetic cover, to Cretaceous sandstones.

The basement comprises three units, i.e. a high magnetic gradient, Lower Proterozoic unit of high-grade metamorphic rocks, a medium magnetic gradient, Middle Proterozoic unit of low- to medium-grade metamorphic rocks and a low magnetic gradient unit of Middle Proterozoic sediments and metasediments. In addition, a unit of Upper Proterozoic sediments is inferred to underlie the Quaternary sediments of the southeastern portion of the district.

Two circular shaped, previously unrecognized Cretaceous alkaline bodies have been identified.

An area of low magnetic gradient and high potassium content identified in the Middle Proterozoic gneisses would correspond to an aureole of hydrothermal alteration.

New data also include structural information, particularly foliation/schistosity of the metamorphic rocks covered by laterite, as well as the positions of faults and fractures of various scales.

In April 2000 BHP conducted six FALCON airborne gravity gradiometer (AGG) test surveys over the Cannington Ag-Pb-Zn ore body in NW Queensland, Australia

The purpose of the test surveys was to demonstrate the capabilities of the AGG instrument by comparison with known detailed ground gravity data, to investigate the effects of source distance on signal strength by flying surveys at various altitudes, and to estimate AGG instrument noise levels in survey conditions. The processed FALCON gD data compare very favorably with the upward-continued residual ground gravity data, capturing most geological features and clearly delineating the Cannington ore body

Analysis of the effects of source distance on signal strength indicates that with a nominal flying height of 120m, the FALCON AGG instrument can detect the gravity anomaly from a deposit with the size and geometry of Cannington through 130m overburden. By repeating a survey using the same acquisition parameters we can make an estimate of the RMS noise of the instrument under survey conditions. Analysis of repeat survey readings show that the FALCON AGG instrument attained a noise level of 10 Eotvos RMS in the bandwidth from 0.0 Hz to 0.125 Hz during the Cannington test flights

Petrophysical data are important for constraining the geophysical signatures of the Endeavour Cu-Au deposits within the Goonumbla Volcanic Complex (GVC). Susceptibilities vary systematically with lithology and, particularly, with alteration. Remanence tends to be subordinate to induced magnetisation in the GVC. Densities are predictably related to composition, Alteration effects on densities are generally minor, although rocks with particularly strong development of secondary magnetite, hematite or sulphides have higher densities.

Modelling of magnetic and gravity profiles over the Goonumbla volcanic belt and the GVC, constrained by geological information and petrophysical properties, suggests that the GVC is underlain by a large zoned intrusion, representing the parent magma chamber, which has a substantial low density, weakly to moderately magnetic, core of alkali feldspar granite to monzonite composition, enclosed by marginal mafic monzonite and monzodiorite phases. The mafic roof zone and marginal phases of the GVC have high susceptibilities (> 0.08 SI, > 2.5 vol % magnetite).

A prominent ridge of low density material occurs along the eastern margin of the mother intrusion. A zone of lower susceptibility occurs above the felsic ridge, probably representing magnetite-destructive alteration due to fluids emanating from inferred underlying felsic intrusion. This zone appears to be related to the Endeavour lineament, which is thought to control the emplacement of many of the mineralising intrusions in the GVC. Magnetic signatures of deposits tend to be obscured by the heterogeneous magnetic environment, but reflect variably developed halos of enhanced magnetite content, associated with early potassic alteration, surrounding a core of reduced magnetite content, which represents the combined effect of felsic mineralising intrusives, mineralising phase (K-feldspar dominated) alteration and phyllic overprinting. Different signatures can be expected for lava-dominated wall rock sequences (weak to moderate annular high with well-developed central low) and volcaniclastic-dominated sequences (unimodal weak to moderate high).