ASEG Extended Abstracts - ASEG2010 - 21st Geophysical Conference, 2010

Water that has been co-produced with coal bed methane in the Powder River Basin, Wyoming, is being applied to agricultural fields using subsurface drip irrigation (SDI). Ground-based frequency-domain electromagnetic (EM) data are acquired over several fields in order to monitor changes in subsurface electrical properties related to the SDI operations. These data indicate spatial variability in soil properties across the site, as well as a systematic increase in conductivity in one field observed on three repeat surveys carried out over one year.

A quantitative assessment of changes in subsurface properties requires inversion of the EM data to recover the true distribution of electrical resistivity with depth. Data calibration and filtering procedures are presented that correct for systematic and random errors in the data, which results in improved inversion estimates.

The Kondoolka Batholith is an early Mesoproterozoic Hiltaba Suite granite that intruded into the late Paleaoproterozoic to early Mesoproterozoic Yalbrinda shear zone, no reactivation of the Yarlbrinda shear zone in the south is apparent in the batholith.

The renewed investigation of the Kondoolka batholith is related to recent remapping of the area in the light of improved geophysical data. A review of the radiometric data, as part of preparing a state wide grid, discovered a geology boundary that incorporated two different granite units under the same unit in the historic mapping.

Several other features were also identified as part of the reinvestigation of the batholith including a potential fault incorporated into the current geophysical interpretation for the map sheet. A multidisciplinary approach was taken, geologist and geophysicist working together in the field and the office to better understand the Kondoolka batholith

We have implemented an estimation procedure whereby wireline data can be extrapolated away from existing well using geostatistical inversion of post-stack 3D seismic data. This procedure works directly in a fine-scale stratigraphic grid, and is conditioned by well and seismic data. It uses a Bayesian framework and a linearized, weak contrast approximation of the Zoeppritz equation to construct a joint log-Gaussian posterior distribution for Pwave impedances. Variograms are also estimated from well-log, seismic data and acoustic inversion results that define the expected degree of lateral smoothness away from the well. A sequential Gaussian Simulation algorithm is applied to sample the posterior PDF and generates multiple, high-resolution realizations of the acoustic impedance which can be utilized to generate stochastic realizations of petrophysical variables that not only honor the well-log data, but most importantly, that fully honor the 3D seismic.

Sensitivity analysis was also performed to find the optimum values for S/N ratio, Variogram ranges and prior standard deviation. We had just one well in the area and preferred to keep it to perform quality control of the inversion results. Without constraining with well, geostatistical inversion still can be used to estimate acceptable static reservoir model for the subsequent simulation and planning of in-fill drilling and/or enhanced-oil-recovery operations.

Integrated interpretation of high resolution FALCON airborne gravity gradient and magnetic data with 2D seismic data from the Chirete Block, onshore northern Argentina, provided enhanced understanding of the tectonic framework, basement configuration, and sedimentary structures.

Basement related faults/lineament maps were generated using several enhancements of gravity gradient and magnetic data. These were combined with magnetic depth estimates, seismic and gravity to create an integrated 3D basement model.

Seismic depth sections, velocities, and well logs from nearby discoveries, were used as constraints for 2.5D and 3D gravity and magnetic modeling. This enabled key sedimentary features to be identified and mapped both along and between existing seismic lines.

Several igneous provinces were identified, and are probably related to the rift and post-rift periods. A strong correlation exists between faults derived independently from seismic reflection data and enhanced Airborne Gravity Gradiometer data.

A number of structural features were identified with the mapped faulting, which may provide new targets for further interpretation or drilling

In this case study, we have employed post stack inversion, guided by rock physics, to estimate reservoir porosity in an Iranian oil field. The reservoir, based on the lithology and porosity information can be divided into 8 zones and 19 subzones with lithologies comprised of shale, sand and carbonates. Among them, three zones above WOC are carbonate and sandstone. We use different rock physics model to estimate porosity from wire line and seismic data. The procedure uses stack and estimated wavelet to determine the elastic parameter: Pwave impedance. Different rock physics model were applied to relate acoustic impedance (AI) to porosity. This allowed performing rock physics analysis, not only on well log data, but also on seismic data (post stack inversion results). The poststack inversion of AI within the target zone was projected onto the template to generate porosity volume. One of the most important issues in carbonate rock physics is fracture distribution. Petrophysical analysis in carbonate sections of the reservoir shows that fracture distribution is limited and production is mostly from matrix porosity. Fractures in the carbonate zone are mostly joints with 0.5 mm opening and 15 cm in length, so the isotropic medium assumption can be made. The results show that Nur-Dvorkin (1996) model and Raymer-Greenberg-Castagna (1997) are appropriate for predicting porosity in sandstone and carbonate zone of the reservoir respectively and predict porosity up to 30% which shows good agreement with the porosity values measured from well log data.

Under the NSW Government’s New Frontiers initiative, the Geological Survey of New South Wales has commenced an interpretation of regional geoscience datasets for the Ana Branch, Pooncarie, Booligal, Balranald, Hay and Deniliquin 1:250 000 scale map sheet areas. The aim is to encourage exploration in frontier areas of NSW by extrapolating the geology beneath covered areas using regional aeromagnetic, gravity, radiometric, Landsat7, seismic and borehole stratigraphy datasets.

The Pooncarie and Balranald 1:250 000 scale map sheet areas are the latest areas to be interpreted over the Murray Basin. They were predominantly interpreted using total magnetic intensity (TMI) data, first vertical derivative (1VD) TMI imagery, tilt-filtered TMI imagery, Bouguer gravity imagery, tilt-filtered Bouguer gravity imagery and Bouguer gravity multi-scale edge detection or “worms”.

Outcomes of this interpretation include: (1) pipe-like, magnetically susceptible sources that intrude the basement, interpreted as diatremes; (2) The Hay– Booligal Zone, which consists of Silurian–Devonian sediments overlying an interpreted basement of crystalline rocks; (3) A suture zone of S-type granites along the western border of the Hay-Booligal Zone. The eastern side is bounded by the Bootheragandra Fault; (4) Curvilinear magnetic trends, interpreted as the Stawell Zone in NSW, that wrap around the Hay–Booligal Zone, and represent turbidites deformed during the Delamerian Orogeny.

Results of this study provide minor adjustments to previously interpreted geological boundaries and that the sources, interpreted as diatremes, are not the origin of volcanic ash required for the formation of the Arumpo bentonite deposit.

We report on results from a coordinated geophysical and drilling program on highly conductive floodplains near Mildura, Victoria. High resolution ground TEM (the Zonge NanoTEM system, configured as a towed rig) and low frequency GPR (the Mala Pro Ex system, combined with a towed 25 MHz antenna) surveys were run over three lines to evaluate the effectiveness of near surface geophysical methods for resolving important floodplain characteristics, including the depth to water table, location of perched water lenses, and extent and location of flushed zones near waterways. We were specifically concerned with whether the results from the two techniques could be used together to better inform the hydrogeology of these environments.

The TEM data were processed using standard techniques, i.e. depth sections were prepared based on smooth-model inversion. The GPR data were initially processed using standard techniques to produce wiggle traces. Due to the conductive nature of this environment and the use of a relatively low frequency GPR system, the results from the standard processing were not satisfactory. These data were then reprocessed using conductivity information from the TEM section to improve velocity estimates at each GPR sounding. Results show improved resolution of water table elevation and delineation of river flush zones, while also providing information about lithological variations in the unsaturated zone.

In summary, improving velocity estimation by incorporating information about the conductivity structure of the survey area has improved the interpretation of GPR data collected in this study, allowing us to interpret the TEM and GPR data sets together.

Porphyry-style mineralisation often appears as near circular anomalies within magnetic data. This article presents an automatic grid analysis system to detect such responses. Our approach follows three steps: (1) Find circular features using the radial symmetry transform; (2) Validate the detected features by the presence of high magnetic contrast at the feature location; and (3) Highlight the alteration zone boundary using deformable splines. The outcome of this system is two-fold. First, provide estimates of the location of potential porphyry systems, and second, accentuate the appearance of potential exploration targets to aid manual data inspection.

Experiments were conducted on survey data from Reko Diq, Pakistan, a region known to contain numerous occurrences of porphyry-style mineralisation. The prediction results of our system closely matched the location of the known deposits in this region thus rendering confidence in the effectiveness of our approach. It is suggested that this system be used as an initial screening tool for large datasets, therefore reducing the time and cost imposed by manual data inspection in the exploration targeting process.

We investigate the ability of gravity and magnetic data to define the distribution of crustal types at continental margins, with specific focus on the conjugate Australia-Antarctica margins. Previous studies have used features in gravity maps as a proxy for the pre-rift location of plate boundaries. Instead, we discuss the use of potential field data to define the boundaries of stretched continental crust on a regional scale, and demonstrate this process for the conjugate margins of South Australia and Antarctica. In a companion paper (Whittaker et al, 2010) we show how these boundaries are used along with estimates of crustal thickness to determine the location of the plate boundaries prior to rifting, and ultimately to derive full-fit plate tectonic reconstructions.

The U.S. Geological Survey and its partners have collaborated to use airborne geophysical surveys for areas of the North Platte River valley in western Nebraska. The objective of the surveys was to map the aquifers and bedrock topography of the area to help improve the understanding of groundwater–surface-water relations to be used in water management decisions. Frequency-domain heliborne electromagnetic (HEM) surveys were completed, using a unique survey flight line design, to collect resistivity data that can be related to lithologic information for refinement of groundwater model inputs. To make the geophysical data useful to multidimensional groundwater models, numerical inversion is necessary to convert the measured data into a depth-dependent subsurface resistivity model. This inverted model, in conjunction with sensitivity analysis, geological ground truthing (boreholes), and geological interpretation, is used to characterize hydrogeologic features. The two-and three-dimensional interpretation provides the groundwater modeler with a high-resolution hydrogeologic framework and a quantitative estimate of framework uncertainty. This method of creating hydrogeologic frameworks improved the understanding of the actual flow path orientation by redefining the location of the paleochannels and associated bedrock highs. The improved models represent the actual hydrogeology at a level of accuracy not achievable using previous data sets.

The Tupinda porphyry prospect is located in the Tabar Islands Group of Papua New Guinea (PNG), approximately 80 km NW of the supergiant Lihir epithermal gold deposit (43 Moz Au). Geophysical data over the prospect include airborne magnetics and radiometrics, ground pole-dipole induced polarization and resistivity, gradient array induced polarization, petrophysics, and airborne time domain electromagnetics.

All geophysical datasets clearly identify the footprint of the porphyry body and associated alteration system. The magnetic data show a central high representing the magnetite altered potassic core surrounded by a circular zone of low response indicative of magnetite destructive alteration. The radiometric data show a discrete potassium anomaly overlapping the magnetic anomaly. The pole-dipole IP data display a resistive chargeable zone coincident with the magnetic anomaly. Mid to late time airborne EM data exhibit a broad sub-circular resistor coincident with the mapped limits of the alteration system.

Field mapping and drilling of the prospect are consistent with the geophysical data. Outcropping potassic alteration is observed at the location of the magnetic and radiometric anomalies. Drill core shows widespread disseminated sulphide mineralization consistent with the chargeable response observed in the IP data. Predominantly fresh felsic lithologies intersected in the drilling concur with the resistor measured in the airborne EM data. A more complex relationship exists between grade (Au and Cu) and magnetic susceptibility, with some areas displaying a good correlation and other demonstrating a more cryptic relationship which may be a function later overprinting geological events.

Given the sparsity of seismic data covering remote offshore frontier basins, Geoscience Australia’s studies of these areas rely heavily on gravity and magnetic data. These data complement 2D seismic reflection data by allowing interpretations to be extrapolated away from the generally widely-spaced seismic lines. However, interpreting potential-field data in frontier areas is challenging. Continuous gravity coverage is only available from satellite-altimetry-derived data, but the resolution of these data is often not sufficient for detailed geological interpretation and the data are subject to error in areas of shallow water or close to the coastline. Shipborne data provide higher-resolution data along track, but the sparsity of ship-tracks means that line levelling is difficult. Modelling gravity data to test seismic interpretations of sediment thickness is further complicated by poor constraints on the depth to the Moho. Efforts to understand frontier basins could be improved with better knowledge of the Moho (e.g. from seismic refraction data) and by better ship-track coverage. Extensive acquisition of airborne gravity and magnetic data over Australia’s margins would also provide better coverage and provide a seamless link between offshore and onshore areas.

This case study presents the CSEM results of 3D datasets acquired over three prospects in a frontier deepwater area in Southeast Asia. The survey was performed as part of the requirements for the drilling campaign in order to mitigate exploration risk. To obtain an understanding of the subsurface resistivity distribution, the acquired CSEM data were subjected to basic processing and inversion before further interpretation. Resistive features corresponding to the anticipated prospect outlines are observed in the attribute analysis and unconstrained 3D isotropic inversion results. Subsequently, post inversion modelling suggests that localized resistors are clearly required in the target area to explain the measured data for two out of the three prospects. Finally, the CSEM results obtained have provided useful input in conjunction with other G&G data for firming-up the drilling priority between the prospects.

The automatic velocity model building technology is a new velocity model building method researched all by ourselves, it had been supported by China Ministry of Geology and Mineral Resources and SinoPec petroleum corporation from 1992 to 2004. We have got Chinese Patent in 2004 after over ten years study. After that, we applied this technology successfully in several workareas of China with different geology conditions, such as foothill complex surface and subsurface structure in the southern part of Tianshan Mountain、carbonate platform edge in Tahe area and marine carbonate rock in southern China etc.

This technology includes two main procedures: High Density CDR Velocity Analysis and Constrained Interval Velocity Inversion.

High density CDR velocity analysis uses ray parameters at shot and receiver, and the corresponding traveltime to calculate the seismic velocity trace-by-trace at peak times. Its basic procedures include 1) Perform time-variant stacking on CMP gather, and automatically pick the traveltimes、amplitudes、frequencies、wavefield coherences, and ray parameters at shots and receivers for the regular waves. 2) Calculate the reflection velocities at peak times of trace for each offset using ray parameters at shots and receivers、traveltimes、as well as the coordinates of the shots and receivers. 3) Improve velocity accuracy via space-variant、time-variant velocity filtering、frequency filtering or dip filtering. Then we obtain a high dense rms-velocity model in each CMP Gathers.

Constrained interval velocity inversion is a stable inversion method to create a geologically constrained instantaneous velocities from rms-velocity model. The inversion includes five steps: 1) Building a global initial instantaneous velocity trend function; 2) Calculating the rms-velocity by Dix-equation; 3) Performing a constrained least-squares inversion; 4) Modifying the initial velocity and repeat 2 and 3 steps; 5) Interval velocity model establishing. Finally we obtain a smooth interval velocity model for time migration and initial macromodels for depth migration or tomography.

Conventional marine CSEM and tensor magnetic gradiometer measurements were compared over numerical models of idealised exploration targets in 1 km water columns. It was found that because of their low amplitudes, tensor gradiometer measurements present challenges. When background-normalised data were compared, it was found that tensor magnetic gradient data showed comparable amplitudes. It was also found that one tensor component showed little variation with the underlying model. This suggests that it might be used to estimate a background when seafloor topography or dipping strata are present.

Borehole seismic data acquired on wireline or during drilling have a variety of uses from industry standard corridor stack and sonic calibration to AVO, Imaging, real-time time-depth information and pore pressure prediction. This paper presents a novel workflow that is designed to identify seismic multiple generators using Vertical Seismic Profiling (VSP) data. The primary technique in identification of multiples in surface seismic data is assessment of periodicity and polarity. Similarly the major technique in identification of multiples in VSPs is assessment of the complete VSP wavefield for truncations and periodicities. In both cases these assessments are commonly made using a qualitative approach. Now, a more rigorous and structured approach that involves separation of the multiple energy from the primary energy is presented. This allows easier interpretation and a quantitative assessment of the relative strengths of primary reflections and their multiples, which in turn simplifies the interpretation of the surface seismic data. The results of this analysis is a list of tops and bottoms of major peg-leg multiple generators and an image of upgoing multiples. This VSP interpretation is transferable to the surface seismic data and can be used in surface seismic reprocessing to target specific formations for removal of seismic multiples generated in these formations.

Anisotropic pre-stack depth migration is becoming a common imaging practice in oil and gas industry because of its enhanced imaging capabilities. It is a slow, iterative process that greatly depends on a-priory knowledge of anisotropy in the imaged subsurface. Here a quick and robust workflow of estimation of anisotropy is presented. This workflow utilizes knowledge of lateral velocity variations, which can mask themselves as anisotropy, from the isotropic interval velocities of the surface seismic data and combines this with the VSP data. Acquisition of a simple zero-offset Vertical Seismic Profile (VSP) provides the starting place for validation of these seismic velocities, and hence input into the isotropic 2D velocity model, as well as a well tie that can be used in estimation of anisotropy from a CMP gather at the well location. The CMP gather is thus corrected for normal moveout using an isotropic 2D velocity model and then artificially moved out using an isotropic 1D model derived from VSP measurements. The resulting CMP gather can be used in interactive 1D anisotropy estimation. The results of this anisotropy estimation can then be used to provide a seed point for a 3D model for anisotropic pre-stack depth migration. This approach can also be extended to evaluate azimuthal variations in anisotropy.

Southeast Asia contains various continental fragments that sequentially rifted from East Gondwana. A number of these fragments were sourced from the northern margin of East Gondwana. East Gondwana's fragmentation was initialised with Late Jurassic rifting along the northwest Australian margin, forming the Argo Abyssal Plain, and followed by Early Cretaceous N-S oriented rifting along the entire western Australian margin, forming the Gascoyne, Cuvier and Perth Abyssal Plains. In a comprehensive study to address the issue of margin formation and terrane drift and docking, we revised the tectonic formation of the entire margin using an integrated analysis of marine magnetic and gravity anomaly data from all abyssal plains involved.

Our model highlights the necessity for a new continental fragment, the Gascoyne Terrane, due to the presence of tectonic features, such as margin age offsets, spreading rates, ridge jumps and isochron orientations, that are distinctly different north and south of the Wallaby Zenith Fracture Zone (WZFZ). Our model suggests that the northern extent of Greater India was limited by the WZFZ. The relative motion between Greater India and Australia formed the Perth Abyssal Plain, while motion of the Gascoyne block, located north of the WZFZ, formed the Cuvier and Gascoyne Abyssal Plains. Following rifting from the northwest margin north of the Exmouth Plateau, Argoland accreted to Burma at ~75 Ma. Our model suggests that in contrast the Gascoyne Terrane was accreted to West Burma at ~55 Ma, following about 20 million years of Tethys seafloor subduction after Argoland's accretion. Our model also suggests that the eastern tip of Greater India collided with West Burma at around 35 Ma. This is consistent with the onset of extrusion of the North Indochina Block around 34 Ma, expressed by leftlateral strike-slip along the Ailao Shan - Red River shear zone between 34 and 17 Ma.

Burma is rich in gems, lead, zinc, iron, tin, chromium, nickel and tungsten deposits, and has oil-bearing fields in its forearc basin. Our model has implications for the formation of the West Burmese margin, as it would also be comprised of a continental terrane once conjugate to the Bernier Platform and Exmouth Plateau, both proven hydrocarbon-rich resources. Advances in modelling the accretion history of Burma will help to distinguish possible continental fragments, which may reveal new resources.

In conventional (P) seismic-reflection surveys, a P-wave generated at the surface is reflected to provide an image of the subsurface. Upon reflection some of the energy is converted into a shear (S) wave. This reflection is known as a converted wave or PS-wave, and may improve the geological interpretation. Velseis has acquired a number of 2D PS-wave surveys at the shallow coal-scale (0-400m). Typically, the two images created from positive, and negative, offset rays can lead to differing geological interpretation. The effect has also been documented at the petroleum scale, and may be caused by diodic-illumination and azimuthal anisotropy. This effect is expected to be exacerbated in the 3D case.

To investigate these complexities we have recently acquired a small trial 3D PS-wave survey with the immediate aim of examining such azimuthal effects in detail. The ultimate aim is a commercially viable 3D-3C methodology for coal-scale targets. The trial dataset consists of an approximately 1000m x 300m swath, collected over a target that has previously been investigated with a conventional 2D survey. The geological environment consists of a target coal-seam with a depth ranging from 70-120m, and including significant faulting. The acquired 3D-3C survey has high shot density, compared to conventional single-component data. This will allow us to produce four full-fold datasets constructed using different azimuth rays. This will be used to examine the effects of anisotropy and diodic-illumination. We are simultaneously examining the P-wave sections to determine if these are also affected by anisotropic problems. The preliminary results from this investigation will be presented.