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

The key computational kernels of most advanced 3D exploration seismic imaging and inversion algorithms involve calculating solutions of the 3D acoustic wave equation, most commonly with a finite-difference time-domain (FDTD) methodology. While well suited for regularly sampled rectilinear computational domains, FDTD methods seemingly have limited applicability in scenarios involving irregular 3D domain boundaries and mesh interiors best described by non-Cartesian geometry (e.g., surface topography). Using coordinate mappings and differential geometry, I specify a FDTD approach for generating numerical solutions to the acoustic wave equation that is applicable to generalized 3D coordinate systems and (hexahedral) structured meshes. I validate the method on different computational meshes and demonstrate the viability of the modelling approach for 3D non-Cartesian imaging and inversion scenarios.

Broadband acquisition aims to improve the bandwidth of seismic data, which in practice means extending the low-frequency end of the spectrum without limiting the high-frequencies beyond the natural earth response (Q-factor). These “unconventional” techniques focus on the receiver-side ghost, and commonly used are co-located velocity and pressure sensors and dual-depth hydrophone or variable depth hydrophones, which either capture phase or timing differences respectively of the receiver ghost. All these methods rely on processing to achieve the final receiver side de-ghosted data as the “dumb sum” of the measurements will lead to poor results, or post-stack broadband data in the case of slant streamer. With sufficient signal-to-noise in the data it is possible to de-ghost the receivers towed at a moderate single depth by tuning the acquisition design, with consideration of the source emission response in combination with the streamer reception response.

A test line was acquired that shows the equivalency of slant streamer and flat depth streamers in terms of post-stack amplitude spectra, showing that the acquisition design and pre-stack deghosting processing methodology is effective in providing broadband data.

This research project uses 3D geological modeling software to build a 3D structural surface model of the Permo-Carboniferous rocks in the northern Hastings Block (NHB). The model is being built using comprehensive strike and dip structural data and a digital elevation model. It is designed to unravel a comprehensively mapped, complexly folded, extensively faulted geological sequence where there are no well-log data. The new 3D model will enable testing of the validity of existing tectonic models, which will assist in constraining the relative timing of fault development, testing fault emplacement of the block, and verification of the number and orientation of folding events in the NHB.

Fault-block analysis has highlighted shortcomings with the existing geological map of the NHB. Fault movement history shows early movement south of the NHB and later initial movement around the northern and northeastern margins of the NHB. Fault movement termination was probably during the Hunter-Bowen Orogeny after folding of the NHB. Preliminary 2D restoration indicated the NHB was compressed (folded) and then extensively faulted.

ExxonMobil’s development of the Hides area to the northwest, and Inter Oil’s giant gas discoveries at Elk and Antelope to the east, have revitalized exploration in the intervening area of PPL 319-PRL 13, southern Papuan Basin. With only limited seismic and well data available, the most time- and cost-efficient exploration option for the permit holder was to fly and interpret an airborne gravity and magnetic survey covering the permits and the adjacent surround.

After completion of acquisition and processing, the gravity/magnetic data were analysed both qualitatively and quantitatively. Existing seismic data were reprocessed and reinterpreted. We then integrated the results by means of 2D structural models incorporating surface geology, seismic, and subsurface data in order to reach solutions compatible with all data sets.

The final interpretation revealed what appeared to be a large, deep Jurassic basin which we have named Kikori Basin. If confirmed, it could be a hydrocarbon kitchen feeding both internal and surrounding prospective fold and fault structures. Several target leads in and around the deep basin were selected for detailing by a new seismic program which is not yet completed.

Understanding the geological risks is an essential process while exploring for hydrocarbons and seal risk is considered the primary reason for unsuccessful wells around the world. This paper focuses on a simple seismic interpretation workflow to address fault-seal sand juxtaposition risk in a structurally complex area. The geological nature of the fluvial reservoirs in the study area, combined with tilted fault blocks provides an effective sealing mechanism to trap hydrocarbons but also enhances leak points. The workflow uses 3D seismic data to analyse juxtaposition of sand bodies across faults, with some geophysical limitations. The presented method has been successfully applied to the study area and has significant implications in exploration pre-drill risks and post-drill evaluations. The results of this study reinforce the necessity to integrate a multidisciplinary evaluation with latest technology to obtain reliable subsurface assessments that effectively translates to better business decisions and improved exploratory success rates.

To better understand the dispersion of seismic velocities arising from stress-induced fluid flow, broadband laboratory measurements have been conducted on a range of synthetic samples. Forced oscillation methods providing access to low frequencies (mHz - Hz) were combined with measurements at MHz frequencies with ultrasonic methods. Either fully dense soda-lime-silica glass or aggregates of sintered glass beads were subject to broadband tests before and after thermal cracking under dry, argon- and water-saturated conditions in sequence. Crack closure effects under pressure are observed on all samples. A systematic increase in shear modulus, attributed to the suppression of ‘squirt’ flow, has been monitored on the low-porosity (approximately 2%) cracked glass-bead specimen with both argon and water saturation at ultrasonic frequency. The use of samples with different porosities varying from 0 to 6% promises to distinguish the roles of pores and cracks in fluid-flow-induced dispersion.

The UltraTEM Deep metal detection system combines a rugged, fast-switching transmitter with a powerful custom designed generator and one to ten three-component receiver coils that collect time-domain electromagnetic induction data across a wide time-range. The roving receiver is operated inside a fixed loop of copper cable, typically 30-150 m long and 5-50 m wide. The large size of the loop results in a slow falloff in primary field with depth and effective excitation of deep buried unexploded ordnance as well as other metal objects such as Ground Engaging Tools (GET). We describe the results of an extensive trial of the UltraTEM system at a magnetically challenging site in Laos. The UltraTEM was able to detect all seeded items buried at depths down to 5 m across a wide range of site conditions. Subsequent work with the system at a different site demonstrated an ability to detect GET to depths of at least 3.0 m in a magnetite stockpile.

Burial sites have extreme cultural significance to societies around the world. Until recently, insufficient recognition of Aboriginal heritage in Australia has led to a very poor understanding and documentation of many culturally significant locations, including burial sites. In some cases, sites have been preserved through the efforts of local people; however, others were subsequently redeveloped or even completely destroyed. Local Aboriginal people are usually the best source of information regarding these locations and can identify broad regions with historical significance, but seldom do they provide precise details about individual grave locations. There are still many Aboriginal gravesites throughout Australia where the exact burial locations are unknown. Locating gravesites - and doing so in a way that minimises site disturbance - is paramount to any investigation and preservation program. For efficient investigation of large areas, geophysical remote sensing provides practical and non-invasive tools for investigation of large poorly documented burial areas.

The UWA Society of Exploration Geophysicists Student Chapter, in conjunction with the South West Aboriginal Land and Sea Council, acquired several near-surface geophysical surveys over a known aboriginal burial site near Quairading, Western Australia. Multiple techniques were used to delineate possible grave locations, including ground penetrating radar (GPR), magnetics and conductivity. While work is ongoing with the data processing and integration, and future surveys are planned, early indications show anomalies that may be related to burial locations.

As a topic “Australian archaeology” is immensely vast and covers close to 60,000 years of history with initial colonisation by the ancestors of Aboriginal Australians through to the maritime and industrial archaeology of more recent immigrants. During this keynote I will delve into the history and prehistory of Western Australia as evidenced through the results of exiting new research projects ranging from the enigmatic rock art of the Kimberley through the Pleistocene use of dusty caves in the Pilbara to the shipwrecks in the depths of the Roaring Forties.

Velseis Pty Ltd acquired and processed an ultra-shallow seismic reflection survey designed to image targets with a depth of less than 50m, including the structure of the weathering layer. Several experimental sources were implemented, each with unique frequency and amplitude characteristics.

Reflection processing was not routine since the target of interest was the weathering zone itself. Due to this, a combination of reflection and refraction processing was used in order to develop an integrated image and interpretation of the near-surface.

The results from the different processing techniques, including refraction (reciprocal method and tomography), reflection, and a depth converted stack, provide an internally consistent interpretation of the base of weathering and layering within the weathering.

A seismic survey across a river with refraction and multichannel analysis of surface waves (MASW) methods was carried out to investigate the ground condition for design of a bridge across Iron Creek near Hobart, Tasmania.

The survey had onshore and offshore components. Therefore it was necessary to use a hydrophone cable as well as land geophones. A sledge hammer was used as an onshore seismic source and a small airgun across the creek.

The result is presented as P-wave velocity section from the refraction analysis and S-wave velocity section from MASW. Two boreholes onshore indicated the depth of basalt with very high strength at 8 metres on west bank and 3 metres on east bank. These depths correspond to P-wave velocity about 1400 m/s and S-wave velocity about 600 m/s. The sections showed the depth of this strong basalt increases in the creek up to about 10 metres, and it is the deepest in the eastern side of the creek. With this information, necessity of expensive offshore drilling was eliminated.

It is becoming increasingly evident that our understanding of the geological conditions in the shallow subsurface is limited. This is especially apparent in large cities and areas covered by lakes where underground infrastructure such as tunnels, subways and train stations have to be constantly developed or expanded to facilitate the daily life and transportation. The degree to which we can understand geological conditions such as these also has great economical and environmental effects for mine planning. What makes these environments similar and challenging targets for geophysical investigations are the various sources of noise and restriction (both in time and space), which require the equipment to be versatile and to produce minimal disruption as well as fast to set up and pack. Direct observations of the subsurface are cumbersome, expensive and sometimes impossible. However, if properly designed and implemented, geophysical methods are capable of imaging detailed subsurface structures and can successfully be used to provide crucial information for site characterizations, infrastructure planning, brown- and near-field exploration and mine planning. To illustrate the potential of geophysical methods in these environments, I will show prototype seismic and EM instrumentation and their applications that are especially geared for noisy environments and areas where high-resolution images of the subsurface are needed. The presentation will be supported by several examples from these two areas.

Historically, near surface refraction seismology has focused almost exclusively on inverting first arrival traveltimes to generate spatially varying models of the seismic velocities in the weathered and sub-weathered regions. This study describes two approaches to full waveform near surface refraction seismology, using common offset gathers (COG) and the refraction convolution section (RCS). Full waveform refraction methods can improve the resolution and characterization of the routine mapping of the base of the weathering, through stacking, flattening and spectral analysis.

Full waveform refraction methods can usually reveal first and later events wherever reflection events are recorded within the refraction Fresnel zone. In most cases, full waveform refraction methods can provide more detailed images of the sub-surface structure than can be obtained with low resolution 1D refraction traveltime tomography.

The amplitudes of first and later events are related to the head coefficient, which in turn, is a simple ratio of the specific acoustic impedances. Both the density and the P-wave modulus models of the near surface, which are derived from the head coefficient and the seismic velocities, can be employed for more comprehensive characterization of the regolith for geotechnical and groundwater investigations, as well as for starting models for full waveform inversion.

Full waveform methods represent a new frontier for the modernization of near surface refraction seismology. They offer the opportunity for more effective implementation of exploration refraction seismology through extracting greater value from the data.

Over the years interest in sports science has boomed with current research in using technology to monitor athlete performance and the motion of balls or other equipment during a game. The contributions of Geophysics to sport are, as far as we have found, only indirect until now. We used the seismic method, specifically a 48-channel seismic acquisition system, coupled with basic processing, to locate the position at which a cricket ball impacted the pitch with an accuracy of ±10 cm. Previously this could only be done using expensive television-based systems.

A recent ultra-shallow 3C survey provides an attractive dataset for evaluation of surface-wave dispersion analysis, for improving knowledge of the near-surface. The primary motivation is for S-wave reflection processing, but with potential for P-wave static control. Finite-difference modelling and real data analysis suggests maximum-offset should be set at several times the investigation depth. This study suggests the geophone interval should be less than 10m, and single phones are preferred. An inverted near-surface S-wave section provides structural information complementary to that available from P-wave refraction.

In the last decade the analysis of surface wave dispersion has become a standardly applied technique in near surface seismic exploration. The method has evolved from the local estimation of 1D VS profiles, based on the inversion of surface wave fundamental mode, to more sophisticated approaches that can provide reliable velocity models in complex geological settings presenting 2D/3D velocity distributions, with the inversion including higher modes and other guided waves. To retrieve comprehensive velocity models, surface wave and body wave data can be extracted from the same seismic records and inverted jointly, imposing structural and petrophysical constraints and overcoming the inherent limitations of both body and surface wave techniques. More recent developments of surface wave methods are aimed at adapting tomographic techniques used in earthquake seismology to small scale exploration data.

The scope of this paper is to highlight improvements in Ground Penetrating Radar (GPR) as an infrastructure condition assessment tool, in particular through the use of multichannel 3D GPR.

Multichannel 3D GPR is a relatively new and alternative infrastructure scanning tool which can assist geophysicists and engineers in providing 100% sub-surface coverage of an investigation area, where site access is possible. Advantages of an increased level of subsurface coverage using multichannel 3D GPR includes providing the user with improved accuracy in highlighting and quantifying regions that may require further invasive testing for future maintenance programs and also possible long-term monitoring.

This paper briefly discusses current applications of standard GPR for infrastructure condition assessments and how multichannel 3D GPR can improve knowledge of the sub-surface in these application areas.

Visualisations of multichannel 3D GPR data outputs with interpretations have been presented to illustrate the improved subsurface information made available from this method. The example presented is an approximately 4 m long section of multichannel 3D GPR data acquired along the surface of a reinforced concrete-lined tunnel.

SRK Consulting (SRK) conducted a geological and geophysical modelling study of the Darlot-Centenary Gold Mine, Western Australia. This study defined geological boundaries in areas where drilling was limited, allowing for targeting of potential extensions to the gold mineralisation. 3D geological modelling of the structural setting and geometry of gold mineralisation within the deposit has shown a strong relationship between gold grade and a magnetic sub-domain within the Mount Pickering Dolerite. The magnetic dolerite domain is considered as a more prospective unit and a target for exploration drilling.

To define the boundaries of the dolerite multiple unconstrained magnetic inversion, models were conducted. This was followed by geologically constrained inversions. The resulting models were consistent and both methods are capable of resolving the prospective magnetic dolerite domain. However, the constrained inversion model ultimately provided a better representation of the geometry of the folded dolerite units. Using the inversion modelling, SRK was able to provide greater certainty in the subsurface geometry of the prospective Mount Pickering Dolerite, providing greater accuracy for the potential near mine exploration targets.

The lack of modern mining in Charters Towers is linked to the difficulties associated with accurately pinpointing high-grade gold-bearing lodes on host fractures. A geophysical survey of the Charters Towers area has been carried out by ADROK using a non-destructive, non-invasive surface-based technique termed Atomic Dielectric Resonance (ADR). A vertical log is generated for selected sites and the resonance energy (E-ADR) used to pinpoint sulphide-bearing lodes within granitic host rocks below the site. Preliminary results show that the technique can successfully pinpoint sulfide and associated gold mineralisation to a depth of up to 1000m. A total of nine scans or “Virtual Boreholes” over three main ore-bearing fractures in Charters Towers have correctly identified the depth and presence of known sulfide ore zones with a maximum depth error of 13m. In some scans, the presence of anomalies at unexpected depths is interpreted to represent potential sulfide targets. The ADR technique is particularly useful in Charters Towers, for example, where other techniques such as TEM magnetics, gravity or seismic reflection surveying cannot be used due to access or other anthropogenic factors. Results so far indicate that the technique represents a significant advance in the pre-drilling identification of target sulfides.