ASEG Extended Abstracts - ASEG2012 - 22nd Geophysical Conference, 2012

Magnetite and hematite are common iron-oxides, being found in sedimentary, metamorphic and igneous environments and being associated with a wide variety of deposits styles, including orogenic gold, iron-oxide copper-gold and iron-ore deposits. While the magnetic and mass properties of magnetite and hematite have been comprehensively studied, there is relatively limited published information on their electrical properties, although anecdotally, it would appear that many geophysicists have encountered the situation in which their ‘highly prospective’ EM or IP anomaly has turned out to be the result of barren magnetite, and/or hematite.

In 1994, Emerson and Yang extensively studied the electrical properties of magnetite-rich rocks as part of AMIRA project P416. Eight sponsor companies contributed a variety of samples for laboratory measurements of mass, magnetic, galvanic electrical, electromagnetic and induced polarisation properties. A petrological study was also carried out. The electrical properties of hematite have been similarly investigated on behalf of individual companies.

This work has demonstrated that sulphide-free, magnetite- and/or hematite-rich rocks can be moderate to good conductors and also exhibit a measurable IP response. And in some cases, electrical anisotropy may be significant. The electrical behaviour of magnetite and hematite is related to factors such as quantity, grain size and texture and their electrical response can be considerably enhanced by relatively small amounts of sulphides, such as chalcopyrite. Field examples are presented confirming laboratory observations.

Variable-depth streamer acquisition is a broadband streamer acquisition technique where the depth profile of the streamer is optimized in order to ensure receiver ghost diversity, which in turn allows the deconvolution of the residual ghost at the imaging stage, pre-stack or post-stack. This technique allows the streamers to be towed at an average depth of several tens of meters, which combined with the use of solid streamers, ensures the raw data has an exceptionally good signal-to-noise ratio, especially at low frequencies. This variable-depth streamer acquisition and processing has been field-tested on a variety of locations, achieving bandwidth up to 6 octaves (2.5Hz -160 Hz). This broad bandwidth translates into improved results for the acoustic impedance inversion. The lack of low frequencies in conventional seismic data means that a low frequency model must be incorporated in the inversion process, obtained by interpolating low-passed filtered impedance logs between well locations. With variable-depth streamer data, high-resolution NMO-derived seismic velocities are used to define the low frequency model in a range 0-5Hz, while the reflectivity provides information from 2.5Hz. Variable-depth streamer data thus have the potential to fill the usual gap between the high frequencies of the seismic velocities and the low frequencies of the reflectivity, the 2.5-5 Hz octave being the overlapping zone. Pre-stack elastic inversion has also been performed, providing both impedance and Vp/Vs sections, proving the feasibility of pre-stack deghosting of variable-depth streamer data.

Basically, ground-roll noise causes signal quality information decrement in land seismic data. Ground-roll is the main type of Rayleigh-wave that is characterized with low velocity, low frequency and high amplitude. Separating them by using frequency filtering, like (f,k) filtering, causes signal distortion due to overlapping signal with ground-roll in time and frequency domain. To solve this problem, we proposed a redesigned Optimum Wiener filter method. The basis of the proposed method is estimating ground-roll in contaminated data by using a reference noise trace, sweep signal. By Wiener filtering, the reference noise is then adjusted to match the ground-roll noise in contaminated data. The adjusted reference noise then subtracted from the seismic trace to obtain the signal. In this research, we also give objective analysis to find the best parameter in maximizing the obtained result whereas the frequency interval is likely the most influenced parameter in Wiener filtering. The obtained results show that Wiener filtering can suppress ground-roll without disturbing the signal while the signal is overlap with ground-roll.

The Kauring Gravity Gradiometry Test site offers a unique opportunity to understand the information content of various types of gravity gradient measurements and the required processing and interpretation techniques. In this paper, we utilize the measured ground vertical gravity data to simulate realistic airborne gravity gradiometry data from several current and future systems using an equivalent-source technique. We then invert these simulated data for the underlying density contrast and evaluate the performance of these inversions. The study demonstrates the effectiveness of equivalent-source based method for data conversion in the presence of scattered observations. The major density features recovered from the optimally converted gradient data are highly consistent with those from the original ground gravity.

Full Tensor Gravity Gradiometry (FTG) data measures the 5 independent tensor components, providing us with a measurement of the complete gravity field; standard visualisation techniques do not always allow us to extract the best from this dataset.

Tensor axis realignment allows us to visualise the data in an alternative reference frame, rather than the typical East-North-Down system used for gravity gradiometry which is chosen because it conforms to mapping conventions rather than for any geological reason.

With axis realignment, the coordinates are rotated about the z-axis, so we are effectively visualising the potential field from a different orientation. In doing this we highlight different geological trends which were not so prominently visualised in the typical END coordinate frame.

By using tensor axis realignment we are able to draw out further geological information from FTG data and hence produce a better interpretation of the dataset. Tensor axis realignment provides a way to visualise the threedimensionality of FTG data and gain more geological insight into the survey area.

Surface seismic is fundamental information for reservoir mapping and in-fill drilling decisions. A good seismic is critical to successful development of a field. However, at times, the surface seismic is of degraded quality due to various reasons which makes the mapping and interpretation of the reservoir layers difficult and uncertain. Near-surface conditions such as topography, weathering layer adversely affect the surface seismic data acquisition & processing. In Bunyu field in Indonesia, problem is further compounded by the presence of near-surface coal seams which renders seismic to be of poor quality. In order to optimize the field development and boost recovery, enhanced seismic is seen as a fundamental requirement to make informed in-fill drilling decisions.

Crosswell seismic provides high resolution imaging of the subsurface at the reservoir scale. The technology is used for delineating complex structure. It is not affected by the surface conditions as the transmitter and receivers are deployed downhole in separate wells, providing velocity tomogram and reflection imaging between the wells.

Bunyu Field in East Kalimantan is on production since 1950 with declining production over the years. It is primarily a discontinuous sand-shale sequence with shallow coal layers. The existing 3D seismic is of poor quality which makes reservoir interpretation very uncertain. It is difficult to map these channel sands due to poor seismic. Crosswell seismic was thus acquired to complement the 3D seismic information for informed in-fill drilling decisions and field optimization.

Crosswell seismic has provided a dramatic improvement in seismic imaging even at large interwell distances and will be helpful in delineating reservoir and ultimately aid in better in-fill drilling

The marine controlled source electromagnetic method has developed during the last decade for direct hydrocarbon indication. Marine controlled source electromagnetic software is still in its infancy with only a small number of open source algorithms and even fewer integrated software environments. We have developed an open source software package to encourage the development and use of the marine controlled source electromagnetic method in both industry and educational institutions. The software was written in Java and was made to perform interactive real-time synthetic modeling for varying earth models or survey parameters.

Understanding the impact of bathymetry can be a critical in application of the marine controlled source electromagnetic methods. Electromagnetic field strength and direction can be affected by small changes in water column depth. The bathymetry, the air-water interface and sub-surface resistivity variations will all contribute to any single electromagnetic measurement. In such complex geo-electric settings a deeper understanding of how and where each feature is expressed in the electromagnetic response is required. We compute the electromagnetic fields for a model with complex bathymetry and a hydrocarbon target. We compute the response with and without hydrocarbon to investigate the effect of bathymetry. A 2.5D finite element algorithm was used to forward model the MCSEM response. The interaction of the electromagnetic fields with the target and the bathymetry can be appreciated by viewing electric and magnetic streamlines. A key benefit of using streamlines is that they quickly show where the electric or magnetic fields would be most strongly altered by the target or alternatively by bathymetry. This aids both interpretation and survey design.

Geophysical data processing is a highly quantitative field that involves modelling, inversion and visualization. In most cases a geophysical experiment is conducted to collect data that are sensitive to a particular physical property of the earth. The data is processed and inverted to generate an earth model of the physical property in question. To better understand the structure of the earth, different experiments are conducted using a variety of imaging modalities. For example, from seismic, gravity and electromagnetic experiments we may obtain information about the earth's elastic, density and conductivity characteristics. Usually the data of each experiment are inverted separately to generate an ensemble of earth models. However, since the inversion process of each geophysical modality is typically carried out independently, most inversion algorithms do not utilize the information obtained through other modalities.

In this research we propose to jointly invert the data obtained by two physical experiments since the information contained in each model can be used to correct the other model. In many of the cases the two models share the important structures, therefore, edges occur in the same locations. In order to exploit this information, we propose using a level set formulation of the problems. Assuming that both models take two known discrete values, we can then use a single level set function for both models together. This can be later extended to multi-level set functions and with unknown values. By using this formulation we are able to improve inversion results of both problems.

Bayesian Markov chain Monte Carlo (MCMC) algorithms are introduced for the analysis of one- and two-dimensional airborne frequency-domain electromagnetic datasets. Substantial information about parameter uncertainty, non-uniqueness, correlation, and depth of investigation are revealed from the MCMC analysis that cannot be obtained using traditional least-squares methods.

In the one-dimensional analysis, a trans-dimensional algorithm allows the number of layers to be unknown, implicitly favouring models with fewer layers. Assessment of data errors and systematic instrumentation errors can also be incorporated. An example from western Nebraska shows that the MCMC analysis reveals important details about the subsurface that are not identified using a single ‘best-fit’ model.

A geostatistical facies-based parameterization is introduced in order to reduce the number of underlying parameters for the two-dimensional MCMC analysis. This parameterization naturally incorporates lateral constraints in the proposed models, which is important for efficiently sampling the model space. A trans-dimensional component can be optionally incorporated in the two-dimensional algorithm by allowing the number of facies to vary, but with models that contain fewer facies implicitly favoured.

Airborne electromagnetic (AEM) surveys provide densely sampled data over large areas (typically several hundred sq. km) that cannot be covered effectively using ground-based methods. AEM data are inverted to estimate the three-dimensional distribution of electrical resistivity structures from shallow depths to several hundred meters. These models convey unparalleled details that are used to make inferences about hydrogeologic properties and processes at the watershed and local scale. This information is being used in groundwater models that are critical to water management decisions, to better understand geologic frameworks, and to improve climate change models. The U.S. Geological Survey (USGS) has been engaged in the application of AEM to many watershed and local scale groundwater projects within United States. We present the results of several frequency- and time-domain AEM surveys acquired by the USGS that have been used for mapping alluvial valleys, buried glacial aquifers, fault-bounded basins, and understanding permafrost distributions.

Modem data acquisition technology in airbome electromagnetics (AEM) produces huge data sets which cover areas of considerable extend. Both the large size of the domain of interest, in generai a three-dimensional volume, and the large number of transmitters and receivers pose challenges to any type of modelling or inversion software. Solution of the inverse problem requires repeated solves of the forward problem. The time to solve one forward problem in turn scales linearly with the number of transmitters. In this paper we examine stochastic optimization techniques for the solution of the inverse problem which essentially allow us to work with small subsets of transmitters/receivers and, thus, reduce the computational load significantly.

Aquifer influx and pressure depletion are key variables during the production and development of a natural gas field. To obtain an understanding of how aquifer influx and pressure depletion varies across the reservoir, remote geophysical monitoring techniques are commonly used, particularly in offshore environments where well data is geographically sparse. The seafloor time-lapse gravity technique is a candidate technique for remote reservoir monitoring of water influx into producing gas fields in the Northern Carnarvon Basin.

We have developed a method to quickly assess the sensitivity of time lapse gravity measurements to water influx or pressure depletion using a vertical cylinder model for gas reservoirs. In strong water-drive gas reservoirs, a field-wide height change in the gas-water contact greater than 5m may produce a detectable gravity response depending on the reservoir depth and rock quality. In depletion-drive gas reservoirs, large pressure changes between 6MPa (~870psia) throughout the reservoir can produce a detectable response. Applying this technique to Carnarvon Basin gas fields where the primary reservoir is the Mungaroo Formation suggests that gravity monitoring of production related changes may be feasible but needs to be assessed on a field-by-field basis.

The method employed is both flexible and practical. It can be used in a range of applications, and provides a quick assessment of the feasibility of time-lapse monitoring of subsurface density changes.

Standard processing of AGRS data is based on the well defined one-dimensional theoretical and empirical facts from gamma-ray physics and experience. In many cases it is quite satisfactory approach giving good new information for geology. However there are situations when 1D conditions are violated at least in one of essential parameter – flight altitudes, rugged topography, abrupt changes in source contents, even aircraft speed (because processing is mostly in time). These deviations might lead to wrong results. Here different approach to processing is suggested, which has some attractive features: it explicitly uses 2D model of topography and ground sources and implements the processing of AGRS data as solution of 2D inverse problem which seems pretty natural for processing the data acquired along the flight lines; it does not uses powerful smoothing of data, it shows spatial resolution – and does not lose the details arbitrary. Eventually it clearly shows that inverse problem of AGRS has no unique solution, that is silently implied by standard processing.

Magnetic field data forms the most detailed and comprehensive geophysical coverage of Australia but at present there are challenges in reliable interpretation of that data. One major concern is the recognition and correct treatment of remanent magnetization which can result in mislocation of drill targets. To both reduce hazards of incorrectly interpreting magnetic field data and also to derive new geological information we have developed a web-delivered database of Australian magnetic anomalies recognised as being at least in part due to remanent magnetization. The database is linked with databases of model studies, magnetic moment analyses (MMA) and palaeomagnetic and rock magnetic investigations. These linked databases presented in the context of magnetic field imagery provide geoscientists with a rapid means of recognising the expression of remanent magnetization and of accessing information about that magnetization to assist in their interpretations. The databases have been lightly populated and can be accessed with a web utility. Community involvement is sought to further populate the databases with information currently distributed through geological survey, company and academic records.

Airborne electromagnetic methods have been used for manganese exploration in parts of the Pilbara since 2002 when the first Hoistem survey was flown for Pilbara Manganese. A XTEM survey was commissioned by Montezuma Mining to test the effectiveness of the technique in a new manganese province near Kumarina, WA.

A series of test lines was flown over areas of known manganese ore deposits and revealed a successful correlation between the electromagnetic conductors and the known manganese zones. In addition the test lines revealed new zones of conductivity where traces of manganese had been found but not yet drilled. Based on the preliminary results the complete Butcherbird tenement was flown in December 2010.

The survey successfully mapped the extent of the known manganese zones and assisted in the identification and mapping of several new targets which were subsequently drilled and found to bear manganese oxide. The shape of the Electromagnetic (EM) anomaly maps the resource outline and the strength of the EM signal correlates strongly with the grade of the resource.

The cost of land seismic surveys is largely reliant on the total acquisition time. If the total acquisition time itself is limited then we may need to compromise the parameters used for the survey and thus the quality of the data acquired may be reduced. To overcome the productivity limitations associated with conventional Vibroseis acquisition, simultaneous shooting, where fleets of vibrators sweep at different locations at the same time, has been employed. Such methods typically require that the number of sweeps acquired at each source position is equal to the number of fleets, thus reducing any increase in productivity. Despite the use of such methods the data may still contain significant levels of noise contamination.

In this study we describe a new technique for generating sets of pseudorandom sweeps with cross-correlation attributes tailored to reduce interference noise. By using both synthetic and real data examples we show that such sweeps can significantly reduce the interference noise inherent in simultaneous shooting and thus have the potential to dramatically increase Vibroseis productivity without significantly affecting data quality.

Seismic reflection surveys with targets at depths of 0-100m have been widely reported in the published literature. There are many case studies and theoretical discussions of methodology and technique. However, there exist no significant modelling studies of shallow seismic methodology.

In this study we have used viscoelastic finite-difference modelling to generate shot records that can be processed using a real world sequence. This approach has allowed investigation of a range of problematic issues relating to very-shallow reflection. Perhaps the most fundamental shallow-reflection problem relates to the extraction of signal from beneath coherent noise. Our modelling provides a clear demonstration of the stringent acquisition parameters that are needed if this noise is to be removed using conventional f-k domain processing tools.

Concepts investigated in the modelling are further explored via an engineering scale hammer seismic survey, which successfully images reflectors in the range 0-50m, using both pre-critical and post-critical reflections.

An innovative time-lapse borehole seismic project for onshore CO2 injection monitoring was conducted in 2010 for ZeroGen in Australia. The goal of the seismic project was to investigate borehole seismic methods to monitor the effects of supercritical CO2 injection in tight, saline reservoir rocks. Results from processing data acquired before and after injection show that a time-lapse, pseudo-4D VSP approach detected likely changes in subsurface acoustic behaviour. A comprehensive borehole seismic program, consisting of two rig source VSPs, an offset VSP, and four Walkaway VSP surveys, was acquired pre-CO2 Injection in the vertical well and then repeated post-injection. Data were acquired using an eight-shuttle VSI seismic tool down-hole and a vibroseis mini-buggy as source. Data processing focused on achieving a repeatable workflow to ensure that time-lapse effects can be effectively analysed. A near surface basalt layer significantly affected both measured time and recorded signatures. This required special handling of surface statics and of source signature variations. The pre- and post injection survey results were analysed for acoustic changes related to the injection program. A time-lapse change in acoustic response in the injection interval was observed on all available borehole seismic datasets. The processed offset and Walkaway VSP images also provided estimates of lateral extent and orientation of these anomalies. Detection of time lapse changes in such a small injection test requires an investigation into the underlying causes such as fluid and pressure effects and the possibility that such techniques might be used to monitor pressure evolution in similar injection sites.