ASEG Extended Abstracts - ASEG2004 - 17th Geophysical Conference, 2004

The conversion of data to conductivity for fixed wing transmitter loop - towed bird receiver coil time-domain airborne electromagnetic (AEM) systems, such as TEMPEST, would ideally utilise complete knowledge of the system geometry and measurements for all three mutually perpendicular components of the received signal. In practice, not all of this information is available.

We use a layered inversion that integrates TEMPEST survey data with a priori conductivity information from the survey area. Total (primary plus secondary) field data from both the X (horizontal in-line) and Z (vertical) components are used. Receiver coil pitch angle and transmitter loop to receiver coil horizontal and vertical separation parameters are included as unknowns in the inversion. Borehole conductivity data are used to build a reference conductivity model that acts as a constraint to stabilise the partitioning of the measured signal into primary field and ground response contributions. Smoothness constraints are applied to the conductivity values in the ID model.

The quality of the inversion output was assessed through comparison of the conductivity predictions with borehole conductivity values and shallow single-frequency ground EM measurements. This showed that the new formulation more accurately predicted conductivity than two previous sets of conductivity predictions.

Mining of the Broken Hill Ag-Pb-Zn deposit has substantially modified what was originally a positive anomalous mass. When an airborne gravity gradiometer (AGG) survey was flown over the Broken Hill region in early 2003, the measured response reflected the modified mass distribution. To answer the questions "What was the original response of the orebody?" and "Would this response have been detected had the survey been flown prior to mining?", an estimate of the changes in response brought about by mining activities was made and added to the survey data to produce an image of the pre-mining gravity response.

To estimate the change in response, a 3D model of the mined portion of the deposit was built. An estimate of the change in mass due to mining activities was made and this mass was distributed with uniform density throughout the model. The gravity and vertical gravity gradient response of the orebody model was then calculated, filtered to match the characteristics of the AGG data and added to the observed survey data.

The ‘corrected’ data show distinct gravity and gravity gradient highs over the northern and southern parts of the orebody which hosted the bulk of the reserves. Although the anomalous response is close to the noise levels of the survey data, we can conclude that the AGG survey would have detected an anomalous response from the Broken Hill orebody had the survey been flown prior to mining. However, there are other geological features in the survey area that produce similar anomalies, notably a number of amphibolite units.

This paper presents an example of geological exploration significantly increased by the integration of a large spectrum of seismic attributes.

A workflow combining a classic seismic stratigraphic approach integrated with analysis of seismic attribute maps enables a significant increase in the readability of the seismic data and the recognition of subtle structural and stratigraphic features. Therefore, despite the lack of stratigraphic control from well data, a coherent model can be proposed for the upper Palaeozoic depositional system of the proximal part of the Dampier Sub-basin (Australian Northwest Shelf).

This enables the definition of 6 repetitive "stacked units" interpreted as 6 phases of glacial advance/retreat. A thorough analysis of the last "stacked units" allows to refining the interpretation. It presents: a basal unit interpreted as basal moraine eroding and draping the underlying deposits and characterised by a highly reflective seismic facies; and a series of paraglacial sequences which are interpreted as the product of the deglaciation with proximal ice-marginal fans, submarine (glacio-influenced?) channels, intermediate to distal fans and a terminal marine sequence.

A recent increase in the environmental usage of Airborne EM has shown the need to accurately provide values of depth and conductivity. Calibration problems in helicopter EM data produce imprecise conductivity depth images (CDIs), maps and sections, which are essential requirements in order to target smaller, near–surface objectives, such as salinity outbreaks.

To ensure agreement between ground–truth, such as conductivity logs and CDIs, data recalibration is often applied before processing. Ground-based methods, if available, have spatial limitations. An alternative statistical average method has been developed to provide theoretical consistency. In conductivity-independent αβ domain, the median response of a variety of expected synthetic models based on expected geology is calculated and compared with the median of the larger amplitude field data. The data are then rescaled in the data domain so that the recalibrated median response lies exactly on the theoretical curve.

The amplitude rescaling was applied to an HEM dataset, collected in the Riverland area in South Australia. The results were compared using maps and CDI images of the raw and recalibrated data. The original delivered data produced CDI images that were generally inconsistent with borehole conductivity data. However, rescaling to ensure ‘thin-sheet’ consistency has produced remarkable agreement between ground truth and the CDI sections.

A person lucky enough to have worked in different aspects of geophysics in Australia over the last fifty years is sure to have a treasure trove of rich memories. Australia has the great good fortune to be a whole continent within one national boundary, and to be a marvellous laboratory for geophysical methods. The developments in geophysics which have taken place in Australia have been based, and have indeed been possible, because of strong rigorous traditions existing in mathematics, physics and geology.

A perspective to the present state of geophysics is obtained be re-visiting various geophysical experiences over fifty years. The path followed commences with trigonometrical surveying (which allows the fifty years time span!) and progresses through various aspects of magnetic and electromagnetic measurements. Part of the journey takes place on land, some is airborne, and some is by sea. Developments in electronics, and computers, have made geophysics a rapidly-developing and exciting subject.

The last fifty years of exploration work in Australia have taken place against the proving and acceptance of continental drift, plate tectonics and mantle convection. It has therefore been a time of remarkable intellectual stimulation and activity, of the widest possible importance to humankind.

The effective removal of surface multiples is critical for imaging subsalt structures in the deepwater Gulf of Mexico. The widely used 2D surface-related multiple elimination (SRME) is inadequate for rugose reflectors. We extend the SRME methodology to three dimensions through the construction of high density and wide azimuth data. We demonstrate the success of our method with results from a case study.

Magnetic and gravimetric measurements have been carried out in Lusatia in Eastern Germany. Here a old geological massiv comprising granodiorits, graywacks and lower paleocoic sediments exist. The boundaries are characterized by big tectonic faults in western, northern and southern parts. In Tertiaery basaltic vulcanism appears in connection with young movements.

The objective of our research was to explore known and hidden basaltic intrusions by magnetic methods. Gravity measurements were carried out to find the thickness of overlying loose sediments. An estimation of the magnetic susceptibility from basaltic rocks reaching the surface was made as well.

We found the figure and the depth of the basaltic intrusions by 2-3/4D forward modelling. Most of the rocks are showing remanent magnetic behaviour. A maar depression was detected and interpreted by joint inversion of gravity and magnetic anomalies.

Multi-technique geophysical investigations have been conducted on the convict-period cemetery on the Isle of the Dead, Port Arthur, Tasmania. With the exception of historic photographs and some limited historical research, very little is known about the layout of burials or physical characteristics of the subsurface. Approximately 1100 burials took place on the island and less than 10% of these were formally marked. Apparent conductivity and magnetic surveys were conducted across the accessible portion of the island to locate subsurface artefact that may be associated with individual burials. The results show gradual variations in soil conditions over the site, some surface cultural features such as pathways, and numerous near-surface unidentified ferrous objects. Although some of these features showed apparent linear patterning, this did not correlate with individual areas of disturbance visible in the ground penetrating radar (GPR) profiles. 500 MHz and 250 MHz GPR surveys provided detailed subsurface information and was used to define the lateral extent of individual anomalous responses and areas containing complex multiple reflectors. The former were classified into various types, according to amplitude, shape and continuity of the reflectors. They were interpreted as possible or probable graves and located on a cultural sensitivity plan as zones of high archaeological potential. Of the techniques used so far, GPR appears to be the most effective method for detecting disturbances associated with graves on the Isle of the Dead.

The Boris oil field was discovered in 2001 in Green Canyon block 282 in the deep water mini-basin area of the Gulf of Mexico. The Boris discovery was followed by the drilling of the Boris North appraisal well in 2002. First oil from the Boris field was produced in early 2003 with recoverable reserves estimated at 10 to 35 million barrels of oil equivalent. The Boris reservoir shows up as a bright seismic amplitude anomaly in Pliocene-age sand. Seismic reprocessing for robust amplitude fidelity and the use of Kirchhoff prestack time migration were the main geophysical tools that led to the discovery of the field.

The Boris reservoir is steeply dipping at a depth of approximately 4500m. The original seismic data across the Boris field consisted of post-stack time migrated data that showed a broad, poorly defined amplitude with poor conformance to structure. After careful amplitude processing was input to prestack time migration, a bright well-defined amplitude ‘appeared’ on the seismic data with an excellent down-dip fit to structure. Concurrent with the reprocessing, a lithology and fluid prediction project was undertaken. Nearby well control was used to define rock property trends such as Vp versus depth, Vp versus Vs, and Vp versus density. The rock property trends were used to stochastically model the AVO response and the results were compared to the measured AVO response on the reprocessed seismic data. The results of the modelling showed that the fluid type at Boris was consistent with hydrocarbons. The Boris discovery well was drilled within three months of completing this reprocessing.

Electrical imaging surveys that are now widely used in many environmental and engineering studies have also been carried out in water-covered areas. These surveys include conventional surveys with multi-electrode resistivity meter systems where part of the survey line crosses a river. In some surveys that are located entirely within a water-covered environment, the electrodes are mounted on a streamer that is towed behind a boat. The streamer is dragged along the river/sea bottom, or floats on the water surface. The smoothness-constrained least-squares inversion method that is used to interpret the data from land surveys is adapted for underwater surveys. To accommodate the underwater topography, a distorted finite-element grid is used to calculate the apparent resistivity values for the inversion model. The first few rows of elements are used to model the water layer, while the lower part of the grid is used for the sub-bottom resistivity distribution. For an accurate inversion, the water resistivity as well as the depth to the bottom surface must be accurately known since a large proportion of the current flows through the water layer. The section of the Earth below the bottom surface is subdivided into a large number of rectangular cells. The water resistivity in the model is fixed, and the inversion program attempts to determine the resistivity of the cells that would most accurately reproduced the observed measurements. Examples from surveys in rivers, marine environments and across rivers are shown.

The Trefoil prospect is a low relief structural closure in the Bass Basin defined by a vintage 2D seismic grid. For such features, geological uncertainty can be reduced when a number of geophysical tools applied to a seismic data set all independently support a single model of the subsurface. This is particularly relevant for 2D data where the benefit of continuous structural and amplitude coverage of 3D data is not available. Geophysical techniques used to mature this prospect for drilling included pre-stack depth migration (PSDM), horizon-based stacking velocity analysis (HSVA), amplitude versus offset (AVO) analysis/modelling, and frequency attenuation mapping.

The application of these methods improved the understanding of the structure compared with previous work based on the same vintage 2D seismic dataset. These analyses consistently support the geological model that Trefoil is a low relief, four-way dip closed anticline, containing several gas columns that may be filled to spill. It is shown that AVO anomalies at prospective levels closely match the final depth closure, as does a frequency attenuation anomaly. The convergence of this information from different methods has reduced the perceived risk to the point where the prospect is viewed as economically viable and is planned for drilling in 3Q 2004.

Interpretation challenges in noisy data regions are well known. In such locations, slow improvements in drilling success have historically been based upon incremental improvements in seismic processing technology, and gradual improvements in interpreter experience and competence.

Using several 3D data examples, I demonstrate a variety of means by which interpretation confidence and success in difficult areas has been significantly improved by viewing the entire acquisition/processing/interpretation process as one entity. The use of immersive visualisation technology throughout the exploration process has proved to be invaluable, providing powerful QC of all acquisition and processing stages, and enabling the interpreter to overcome historical difficulties establishing what data components are noise, and which are valid primary events. This approach therefore allows an objective review of the key acquisition and processing issues affecting data quality, and provides a platform for 3D survey planning, 4D reservoir monitoring, processing QC, interpretation, and reservoir exploitation.

Recent marine case studies have demonstrated that a significant component of the seismic "noise" contaminating 3D images actually arises during processing, as an unfortunate, and inescapable artefact from poor 3D spatial sampling. When the cross-line acquisition dimension is sampled at an equally small interval as the inline dimension, a much larger frequency bandwidth than typical of standard 3D acquisition can preserved throughout all stages of processing, free of aliasing, and free of artefacts. Hence, it is observed that once the random noise component is suppressed below a certain threshold, other factors than mere fold are clearly contributing to the quality of a seismic image. It is quite poorly established how more complicated acquisition parameters, such as multi-streamer spread dimensions and shooting templates, influence the "S/N ratio" of seismic data - particularly after the application of multichannel pre-stack processing algorithms, notably pre-stack migration.

Historically, efforts at towing the source and streamer at shallower depths rather fruitlessly delivered higher dominant signal frequencies, at the cost of degraded lower frequency amplitudes, increased survey noise, and with minimal perceivable improvements in target resolution. Even if means can be found to reduce the inherent noise incurred, resolution remains frustratingly restricted, and the emphasis upon higher frequencies during acquisition was largely wasted. The solution is to sample densely in both the shot and receiver domains, particularly in the cross-line direction.

Several case study examples demonstrate significant improvements in resolution and signal-to-noise content are routinely achieved by high-density seismic acquisition. Depending upon local geological conditions, high frequency amplitudes can be increased by up to 15 dB, frequency bandwidth can be doubled, 3D steep dip imaging can be significantly improved, and overall signal-to-noise ratio is improved, further contributing to better resolution. Hence, a powerful demonstration is made that tight 3D spatial sampling must be the foundation for all high resolution seismic acquisition.

3D PP full-azimuth, full-offset surveys are being processed to determine the azimuthal variation in AVO (AVOA) and in velocities, among other quantities. A map display of the 3-dimensional number that quantifies the AVOA – the large AVO gradient, azimuth of the large AVO gradient, and azimuthal variation in the AVO gradient – tells the viewer about the contrasts in the shear wave birefringence across the boundary. The shear-wave birefringence is usually taken as proportional to the fracture density. Two types of additional information are needed to assess what the P-wave AVOA is telling us: 1) the azimuthal interval velocity information above and below the reflector; and 2) anisotropic AVOA modeling to depict various geologic scenarios likely present at given locations.

Evaluation of coalbed methane plays, prospects, and properties is challenging work. Complex and computationally intensive, it employs a considerable diversity and volume of data, and involves a lot of interpretive analysis and mapping. The sources of data can also be a bit typical and there will also be need to combine and display the data in unusual ways. Many geologic interpretation software packages on the market today were designed specifically to handle data and workflows in conventional reservoirs. Programs that are limited, inflexible, or lack versatility can therefore be severely challenged by the differences encountered in coalbed methane projects. GeoGraphix software is not limited or inflexible and is of considerable help to the geologist and engineer in handling and interpreting a large diversity and volume of data in CBM plays.

The primary objective in this CBM evaluation will be to lay-out a standard workflow for reservoir characterization, resources assessment, reserves estimation, and new well location selection on CBM properties using GeoGraphix software. Although the approach and techniques presented in this paper are very useful for many CBM plays, generalizing about what works or doesn’t work in CBM plays is dangerous because the keys to making each a success are different. So, even though the workflow will cover many of the tasks that need to be done for any CBM play, it is very probable that this procedure will need to be adjusted and customized to the specific play.

A secondary objective of this paper will be to demonstrate practical application of fairly high-level CBM scientific theory in practical, real-world project settings. The main body of the paper will focus on the step-wise process of “How-To”, with only some explanation. But there will be distinct need, at times, for some technical discussion of the “Why” in various sections. To keep the paper flowing, we will refer the reader to Appendices located at the end of the paper for discussion of the deeper scientific detail in each section.

Also, it is beyond the scope of this paper to instruct the reader on exactly how to run the GeoGraphix software, and some working level of knowledge of it is assumed. However, we will present a procedure-based approach here that will describe not only the steps that need to be taken but also will advise the reader on how to accomplish those steps, in general, with the program.

Airborne electromagnetics (AEM) has been used on a test basis for bathymetric sounding, with both fixed- and rotary-winged aircraft systems. Applications are usefully restricted to shallow water (up to a few tens of metres), and the systems can be used through ice. Most processing methods to date have, however, involved extensive post-processing of data before bathymetric information can be extracted, with lag times of the order of months.

We have investigated a number of methods for rapid bathymetric depth estimation that could be used in realtime with AEM data acquisition. The fastest methods are all based on an initial rapid transform of data to conductance-depth sections. Once in conductance-depth space, it is possible to estimate depth from the maximum conductance encountered at each reading if the conductivity of seawater is known, and if the sea floor can be assumed to be electrically resistive.

Alternatively, it is possible to attempt to solve for both seawater conductivity and depth independently using a thick-layer approximation such as that developed by Singer and Green, but this is a slower and potentially unstable process, with the potential advantage that seafloor conductivity can be assessed. AEM and bathymetric sounding data have been used to assess the validity of the approximate methods. The data assessed has included the helicopter based Dighem data collected over Sydney Harbour and fixed-wing Geotem data collected over and near Geographe Bay, Western Australia.

Experimentation with sub-audio magnetic (SAM) survey parameters over a recently discovered Archaean mesothermal gold deposit demonstrates that this technology can be effective for identifying conductive, mineralised structures and regolith features at high resolution to a depth of 100 m, as long as the transmitter electrodes are placed sub-parallel to the strike direction of features to be detected. The horizontal total field magnetometric resistivity (TFMMR) response produced by current channelling during SAM surveying is shown to have a very similar pattern to gradient array apparent resistivity results using 50 m dipole spacing or less. However, SAM data are recorded using a magnetic sensor, thus avoiding electrical contact with the ground, and 2 m along-line sample density provides much greater resolution. TFMMR anomaly trends were found to correspond to gold-mineralised shears, and dips of these shears estimated using gravity modelling methods agree with drilling results. The main ore zone forms an elongated, shallow plunging pod of weakly chargeable ore that shows up in gradient array and dipole-dipole IP surveys. SAM total field magnetometric induced polarisation (TFMMIP) was ineffective at imaging this ore zone using surface transmitter electrodes and 1 Hz transmitter frequency. However, the TFMMIP response of the main ore zone was later imaged in great detail by placing transmitter electrodes down boreholes into a shallow part of the ore body, and then down-plunge 700 m to the south. SAM surveying using optimal survey parameters identified shallow conductive shears and deeper chargeability anomalies within the main ore zone that correlated to economic gold mineralisation, and the SAM results helped to target resource definition drilling.

The Euler method of automating depth to source from potential field data has undergone resurgence in popularity, with several new extensions to the method developed. Perhaps the most revolutionary of these provides a solution of the structural index as part of the inversion process. Previously, structural index was a required input parameter. Many solutions are generated with the Euler method, and care is used to select the most appropriate. In some cases, accuracies of depth estimates within ±15% of depth of actual source below acquisition height are achieved.

Examples of Euler depth results from across three representative areas of Australia are used to demonstrate the utility of the method. Data are presented by several methods, including full 3D visualisation, which allows the solutions to be integrated with other data and inversion results. A depth-to-basement map of the Springvale 1:250 000 map sheet area in Queensland has previously been generated using the Naudy technique, and those results provide a useful comparison with depths derived using the Euler method. A small area in the eastern Yilgarn, Western Australia, contains an elongated sedimentary basin overlying shallow basement cross-cut by numerous dykes and faults. The magnetic expression of basement can be traced from near-surface to a few hundred metres depth under basin cover. This is a good test of the method for providing estimates along the incline from outcrop to increasing thickness of cover. Basement to the eastern Gawler Craton (Olympic sub-domain) lies under several hundred metres of cover, and Euler depth estimates for this area are also examined.