ASEG Extended Abstracts - 24th International Geophysical Conference and Exhibition – Geophysics and Geology Together for Discovery, 2015

Alternative technologies for the production of hydrophones using optical sensing are reviewed with respect to performance and manufacturability. Sensor designs utilising spectral shifts as a result of strain-optic interactions are uncommon, and we believe they merit further investigation as geophysical sensors due to good sensitivity and relative ease of manufacture Specifically, a Long Period Fibre Grating placed onto a mandrel appears to be as promising candidate as a future compact hydrophone sensor.

A mathematical model has been created for a compliant mandrel coupled with a Long Period Fibre Grating inscribed into plastic fibre. The modelling results indicate that such a sensor should provide a sensor of minimal size, with desirable sensitivity characteristics.Compared to the Rayleigh based optical fibre sensors being evaluated in geophysical applications currently the modelled sensor is predicted to have significantly greater sensitivity, with the mandrel acting as a mechanical amplifier. The main limitation of the spectral shift method is the number of sensors that can be multiplexed on a single fibre. However, a combination of time-domain and wavelength domain multiplexing could significantly increase the number of sensors per fibre to usable numbers for geophysical applications.

Hematite ore deposit loses magnetic susceptibility as it oxidizes. So, with the absence of significant magnetic anomaly, it doesn’t respond properly to magnetic method. As hematite ore deposit can be conductive due to its porosity. This characteristic leads electrical methods, especially Electrical Resistivity and Induced Polarization (IP&RS), to be applicable. Applying IP&RS methods is cost efficient and relatively fast.

In this study, we examined the applicability of IP&RS method to discriminate ore bodies and country rock in order to depict potential area of Iron ore. The interpretation of the geophysical results are made base on geological aspects and they confirm that this technique can be a good alternative for other geophysical methods.

The studies conducted at Adriyala longwall block of Singareni Collieries Company Limited (SCCL) in the state of Telangana, India conclude that geophysical logs comprising electrical, density, neutron, caliper, Full Waveform Sonic (FWS) and acoustic images can provide reliable geological and geotechnical models required for longwall mining. The P wave velocities obtained from sonic logs are correlated with the lab determined strength parameters such as uniaxial compressive strength (UCS), Tensile Strength (TS) and Young’s Modulus. The empirical equations provided a means to construct UCS map of interburden strata of coal seams from sonic data. The integrated study of various maps prepared from geological and geophysical inputs provided an effective means to analyse the competency of immediate overburden and roof of Seam-I, which is considered for longwall mining.

Back analysis of behaviour of strata will allow developing of predictive models and appropriate strata control strategies to be applied at Adriyala and other mines and also for multiseam extraction.

The principal objective of the research was to determine an exploration target estimate for the Xaudom Iron Ore project. Geophysical data inversion modelling was carried out and the results calibrated against local drill hole interpretation-based geological models. The results compared favourably and enabled a number of correction factors to be established. Subsequent drilling and geological modelling have yielded NI 43-101 compliant resources that are similar to the initial inversion based modelling estimates within optimised pit shells, showing the robustness of the Exploration Target technique. The approach discussed here may be useful for delineating exploration targets for other magnetite-rich iron mineralized areas faced with complex deformational histories.

The Wallaby Gold deposit is located 25km southwest of Laverton within the Eastern Goldfields Province of Western Australia. Gold mineralisation is hosted within a mafic conglomerate, intruded by a south-plunging magnetite-actinolite-epidote-calcite altered syenite pipe. Regions of low susceptibility within the pipe are associated with gold mineralisation.

Airborne magnetic data from the Wallaby Gold deposit was inverted using the University of British Colombia Geophysical Inversion Facility MAGINV3D code to produce a 3D model of the subsurface magnetic susceptibility.

Magnetic susceptibility measurements acquired at 1m intervals on diamond drill core were used to constrain the results of the inversion. This was facilitated using the Sparse Constraint Model Builder, which creates a physical property model based on existing geological, geophysical or geochemical measurements to then be applied within the UBC-GIF inversion code.

The constrained inversion defined regions of low magnetic susceptibility within the outer high magnetic susceptible zones of the alteration pipe and potential mine-scale structural features can be interpreted. This is useful information given the structural control on gold mineralisation at Wallaby and its association with regions of reduced magnetic susceptibility.

Embedded within Earth’s topography is a constantly evolving fluvial network sensitive to variations in horizontal and vertical motions, driving sediment transport from elevated sources to sedimentary basins. The notion that a river acts as a ‘tape recorder’ for positive vertical displacements suggests that changes in spatial and temporal characteristics of surface uplift can be deduced through the analysis of longitudinal river profiles. The relative tectonic quiescence of the Australian continent during the Cenozoic makes it an excellent natural laboratory to study recent large-scale variations in surface uplift, often linked with mantle convective processes. Here, we analyse X longitudinal river profiles from south Western Australia. Major knickzones in the longitudinal profiles of rivers in southwest Australia suggest recent surface uplift. Given the lack of recent large-scale tectonic activity in that region, this uplift requires an explanation. Applying an inverse algorithm to river profiles of south Western Australia reveals that this surface uplift started in the Eocene and culminated in the mid-late Neogene. The surface uplift rates deduced from this river profile analysis generally agree with independent geological observations including preserved shallow-marine sediment outcrops across the Eucla Basin and south Western Australia. The timing of this event is also to be compared with offshore stratigraphic sections to link onshore surface uplift to offshore sedimentation. We show that the interplay between global sea level and long-wavelength dynamic topography associated with south Western Australia’s plate motion path over the remnants of an ancient Pacific slab is a plausible mechanism driving this surface uplift.

Towed streamer EM data offer a possibility to generate a resistivity model of the earth within the sensitivity range of the EM survey, using procedures such as inverse modelling. There is, however, an inherent non-uniqueness in the problem due to noise, uncertainties, finite number of measurement positions and field components. To assess the model uncertainty, and analyse the data information content, we propose the Bayesian method of calculating probabilities for model parameters within a given set of models. Real towed streamer EM noise applied to synthetic EM data in a 3D model, similar to Barents Sea conditions, was used for the evaluation.

The result of this work is a formulation of the posterior probability distribution for a set of sub surface resistivity model parameters. By analysing these probability functions we find that we are able to evaluate how a change in the data, e.g. different frequencies, sensor positions, noise levels or complexity of background, affect the probability of finding a model close to the true model.

Higher than normal subsurface temperatures are found in the Perth and Carnarvon basins in Western Australia. Both basins are known to be underlain by granitoid rocks which may possess higher than normal levels of the elements uranium and thorium, which in turn contribute to heat generation in those rocks. Airborne and ground radiometric data confirm the presence of radiogenic-granitoid rocks surrounding the Perth Basin, which are believed to be the cause of the observed elevated subsurface temperatures in that basin. Heat generation in these granitoid rocks typically ranges between 2 and 20 μWm⁻³. New radiometric data for crystalline rocks, inferred to underlie the high subsurface-temperature regions of the Carnarvon basin, also indicate regions of anomalously high uranium and thorium. Brief reference to other high-radiogenic rocks in Western Australia and possible implications for heat flow is also made. One and two dimensional static heat-flow models, incorporating new upper-crustal radiogenic information, for the Perth and Carnarvon basins have been developed. The models are used to review the thermal history of these basins.

Making seismic acquisition quicker and/or cheaper requires removing one or more of four major components: the source, the sensors, cables connecting the sensors, or the recording system. In this paper we describe how fibre-optic cables can be used as sensors in a distributed vibration sensing system.

Most existing signal enhancement and random noise removal algorithms take no account of structure, and so tend to smear coherent events across faults and harm steeply dipping events.

A new noise suppression technique is presented here which uses 3D structure tensors, calculated from stacks or offset panels, to detect event edges and applies anisotropic diffusion filtering between those edges, to enhance signal at the expense of noise.

Using both synthetic and real data, we have demonstrated the capability of this technique to remove noise as effectively as existing methods whilst also preserving signal and honouring truncations (faults, etc).

We propose how this fast and flexible technique can be used not only on seismic reflection data as an aid to interpretation, speeding up automated horizon picking, but also on other attribute volumes, for example as a robust method of helping build initial velocity models that honour structure.

Natural fractures in the Carnarvon Basin’s Rankin Platform and Dampier Sub-Basin are identified using electrical resistivity image logs from 10 petroleum wells. In-situ stresses are diagnosed for the area using data from these and four additional wells, with these results indicating it likely that this study area hosts a relatively isotropic in-situ stress field.

Identified fractures occur at all orientations, and demonstrate no dominant trend. They do not reflect the in-situ stresses, nor the dominant north-northeast to northeast fault strikes. Rather, they most closely reflect the orientation of more local structures which the wells are adjacent to, demonstrating that natural fracture populations may be more dependent on local structure than dominant regional trends.

The moving coil geophone is still the most commonly used sensor for land seismic surveys despite the introduction of other sensors. Modern geophone development has reached the stage where the signal recorded is of very high quality, but it can still be affected by the geophone being placed on an angle relative to the vertical (‘tilt’).

This paper describes the acquisition and analysis of field measurements using tilted geophones. It is shown that the critical angle of the 10 Hz geophones used for this test is 55°. For data recorded using vertically placed geophones separated by only 10 cm the perturbation level (the difference between the data recorded by adjacent geophones) averaged 8% and increased to more than 50% at tilt angles of 40°. The level of perturbation is heavily dependent on the orientation of the tilt angle relative to the source-detector axis, for example, for a geophone tilted at an angle of 30°, the perturbation varied between 14% and 48%.

The obvious solution to these issues is to record data using sensors that have been planted extremely carefully or to use other sensors, such as digital accelerometers or 3C sensors that have the effect of tilt removed during processing.

Increasing levels of exploration along rifted continental margins, such as the southern Australian margin, has led to growing recognition of the detrimental impacts of magmatic activity on hydrocarbon prospectivity. Key exploration risks include the impact of intrusions on seal integrity, reservoir quality, source rock maturation and migration pathways. However, the extent and distribution of volcanic rocks along continental margins, such as the Australian southern margin, and the processes by which magma is transported through sedimentary basins are still poorly understood despite the wealth of available seismic datasets. Although classified as a ‘non-volcanic’ rifted margin, our analysis shows that an extensive and largely undescribed record of Cretaceous-Cenozoic magmatic activity is preserved within the sedimentary successions of the rift basins located along the southern Australian margin. The combination of seismic reflection data and geochronological and geochemical data shows that this magmatic activity cannot be solely attributed to continental break-up and related decompressional melting processes or the presence of a hotspot or plume.

Historically, arrays have been used to attenuate ambient noise under the assumption that the level of attenuation is directly proportional to the square root of the number of sensors in the array. Given the availability of high channel-count point-receiver systems and the cost associated with laying out large arrays this assumption of ‘spatial randomness’ requires further analysis. Using measurements of ambient noise made at various sites in Perth, Australia with closely spaced geophones we show that ambient noise is strongly correlated over distances of up to 10 m. This correlation reduces the signal-to-ambient-noise performance of an array considerably. The correlation coefficient can be modelled using an exponential function and the correlation-distance used to determine the efficient geophone spacing.

The optimum geophone spacing on days with a low wind speed (< 10 km/h, observed on 27% of days in the area) is 15 m. For days with a very high wind speed (> 80 km/h) the optimum spacing is 2.5 m, although this wind speed is very uncommon, occurring on average less than once each year. For more than 90% of days the wind speed is such that the optimum geophone spacing required for ambient noise suppression is 7.5 m.

During IODP Expedition 343: The Japan Trench Fast Drilling Project (JFAST), five boreholes were drilled from the D/V Chikyu in >6800 m water depth. Three of these crossed the main fault target. A logging-while-drilling (LWD) hole that penetrated to 850.5 meters below seafloor (mbsf) (total depth [TD] = 7740 meters below sea level [mbsl]) was documented using a suite of LWD tools. From an adjacent partially cored hole drilled to 844.5 mbsf (TD = 7734 mbsl) 21 cores were acquired that spanned the two main fault targets. During the follow-up expedition 343T a third borehole was drilled to 854.8 mbsf (TD = 7752.3 mbsl) and a simple temperature observatory was deployed in the wellhead. The drilling operation, which lasted 88 days, was very technically challenging. Notably, the drill string had to be withdrawn a number of times due to high seas, and technical issues.

In certain intervals, rather than core we recovered loose, subrounded fine gravel clasts of the two major lithologies penetrated to those depths (silt and mudstone). Particle shape and size of these clasts was analysed. Results demonstrate (1) particle shape variations apparent visually are not easily quantified, (2) there are distinct variations in particle size distributions. We discuss whether these relate to variations in drilling parameters.

Multiple is a tough issue in recent years, especially the internal multiples in deep earth. In this paper we proposed to construct virtual events to predict internal multiples. Then adopt the mean value multi-channel adaptive subtraction method for matching the multiple model and seismic data. However, the results depend on the parameters closely. We propose to apply the dynamic time wrapping method in order to accomplish the precise matching. And the regression of multiples can effectively improve the internal multiple elimination results when internal multiples are interfered with effective signal.

The present-day stress field is important for a range of earth science disciplines including petroleum and geothermal geomechanics, mine safety, neotectonics and seismic hazard assessment. So far, many studies have been carried out to understand the state of stress in different parts of the world and the results reveal that the contemporary tectonic stress field can range from being uniform over large areas (100s-1000s of kilometres) to being highly varied over short distances (10s-100s of meters) due to interaction of different parameters. One of the most well-known examples of a heterogeneous stress pattern is observed in the Australian continent, which displays a wide range of stress orientations from province to province that, unlike all other major plates, are not aligned with absolute plate motion.

The Australian Stress Map (ASM) project was started in 1996 to compile a public data set of maximum horizontal present-day tectonic stress information to determine and understand the state of stress in the Australian crust. The early phases of the ASM revealed that plate boundary forces provide the first-order control on the present-day stress pattern. However, all models of the stress field have failed to replicate the stress pattern in Eastern, and particularly north-eastern, Australia. The ASM project commenced again in 2012 with a primary aim of building up the database in Eastern Australia, such as new hydrocarbon provinces, and to help better establish the controls on the Australian stress field at scales ranging from tectonic plate down to individual fields and wells. To date, we have interpreted more than 400 borehole image logs in coal seam gas, mineral and conventional petroleum wells. The results show that local sources of stress (i.e. second and third orders) play a key role in the stress pattern of Australia which is an important issue for geothermal and unconventional exploration and production.

The seismic method is able to produce highly accurate images of the Earth’s subsurface. Having such detail is not only an important factor in mining, but also in civil engineering. Bauxite exploration attracts both government and industrialists to invest in it because of the high percentage of aluminum present. The economic importance of extracting aluminum from bauxite encouraged us to take this challenge; to image bauxite layers by using a high-resolution seismic reflection method at Al Qassim, Saudi Arabia. Since the subsurface structure of the area is complex, this high-resolution reflection method was carried out along a 2D line with geophone and source interval, with settings at 5m. The result for the seismic section shows that the depth and thickness of the bauxite layer varied between 20 to 34m, and 3 to 7m respectively. In addition, the bauxite layer was sandwiched between clay layers. In order to achieve an even more precise depth than presented by seismic section alone, we tied the drilled wells to the seismic data and we accomplished a well match with an approximation error of 1-2m, which may have been caused by the upper clay layer or by very shallow loose subsurface material. The seismic method thus applied shows the ability to detect significant details within the near surface of the earth, and is considered more cost-effective than only drilled wells.

During the drilling process core samples often are damaged and proper measurements on samples cannot be performed. The objective of this study is to investigate rock powders and evaluate how their seismic properties relate to the seismic properties of their corresponding rocks. Consolidated and poorly consolidated rocks and fine powders made of those rocks have been used in this study to assess such possibilities. A comparison between the seismic properties of dry powders to the properties of wet powders has been done. A correlation in mechanical properties (Young’s modulus and Poisson’s ratio) between compacted powder and samples from host rock has been found.