ASEG Extended Abstracts - ASEG2003 - 16th Geophysical Conference, 2003

Hydraulic connectivity between watercourses and aquifers can be rapidly imaged in great detail using a floating electrode array. Below watercourses, the salinity contrast between rising groundwater and sinking surface water is clear on electrical conductivity images. Interpretation is complicated by conductivity variations related to clay percentages and sediment porosity.

A new robust floating electrode array design (“Blue Eel”) using air filled layflat tubing addresses many of the problems that have previously prevented productive surveying of watercourses most of which contain many navigation hazards. A modified dipole-dipole array with exponentially spaced and sized receiver dipoles can provide optimal signal levels and depth resolution. Additional data from a GPS receiver and sonar depth profiler are important for managing the large volume of data acquired.

The data acquired can be presented as a vertical 2D inverted conductivity image with a water depth trace overlain. Because watercourses generally meander, this image is best wrapped like a ribbon along the track of the watercourse. If the ribbon is presented in orthographic projection, it can be overlain easily on airphotos and map layers.

This paper discusses a method to quantify fluid phase probabilities by integrating rock property trend models with seismic attribute maps.

Rock property trend models for the reservoir sandstone and surrounding shales, with quantified uncertainties were used to model the near and far angle seismic response.

Forward modelling of many realistic reservoir models was done in close cooperation with the petrophysicist and geologist. Brine, oil and gas filled reservoir sandstones were modelled. A statistical method was used to calculate fluid phase probability maps from calibrated fluid factor or near/far angle attribute maps.

These probability maps may be used to rank undrilled prospects, and as a part of risking the fluid phase for each prospect in the area under investigation.

Using two examples, we show good matches between field and 3D modelling data. The 3D models were small enough to reduce runtimes, yet incorporate sufficient complexity to model complex environments more accurately than plate or layered-earth based models.

Time and effort involved in this modelling, although greater than for simple models, is not excessive, and can easily be accomplished on a PC-class computer.

This abstract outlines the use of the transient electromagnetic (TEM) method for hydrogeophysical investigations. The TEM method has proven to be a strong tool for the delineation of water bearing sand and gravel layers.

We discuss newly developed TEM systems (PATEM and HiTEM) along with developments in data interpretation. Finally the TEM part of the Aarhus Survey is presented. This survey covers an area of more than 100 km² and about 6000 TEM soundings have been measured.

Forces internal and external to the petroleum industry have shaped the direction of the science of geophysics. New ideas and algorithms continue to have an impact, while lower computing costs enable processes that were not economic in the past.

The cost of storage, speed and bandwidth has been falling about on order of magnitude every five years. This technology disruption has provided significant benefits for the petroleum industry.

Computer gaming and medical imaging innovations have lowered the cost and improved the performance of visualization hardware and software. Parallel computing, faster networks and cheaper storage have led to advances in reservoir geophysics, seismic imaging, seismic acquisition and data management. Organizations that understand these trends and capitalize on them will be better prepared to compete in the future.

Amplitude versus offset (AVO) processing is typically based on a model of horizontal reflectors and common midpoint (CMP) gathers. Seismic prestack migration on constant offset sections has also been used to improve the signal to noise ratio (SNR) of the AVO effect on the migrated CMP gathers that are referred to as common reflection point (CRP) gathers.

The use of prestack migration in AVO measurements is aided by the scatterpoint concept that assumes reflectors can be composed of many scatterpoints (or reflecting elements) that are aligned along the reflector. The energy scattered by each point reconstructs to match the specula reflected energy. The energy from each scatterpoint forms a surface in the prestack volume that is often referred to as Cheops pyramid. The location of specula energy from a scatterpoint is identified by the tangential area between the reflected specula energy and the surface of Cheops pyramid. In addition, the incident and reflection angles from a scatterpoint may be superimposed on Cheops pyramid to identify any smearing that may occur. These principles apply to both horizontal and dipping layering.

Conventional constant offset prestack migration can map horizontal and dipping specula energy to corresponding CRP gathers for AVO analysis. This method, however, requires an accurate velocity model before AVO measurements can be obtained.

The equivalent offset method (EOM) of prestack migration forms prestack migration gathers that are referred to as common scatterpoint (CSP) gathers with no time shifting of the input data. This process uses minimal velocity information to rapidly form CSP gathers that that are also well suited for AVO analysis. After the gathers are formed, accurate velocities can be estimated and AVO measurements made.

The Kirchhoff migration algorithm is heuristically derived using the mathematics of least squares inversion and the concept of matched filters. These concepts are visualized with cartoon descriptions that describe inversion using linear algebra and time-varying deconvolution. The time varying wavelets are then replaced with diffractions, to form a diffraction matrix that is used to model seismic data. The product of the transpose of this diffraction matrix with the seismic data produces a band-limited inversion that is identical to a Kirchhoff migration. Simple modification to the diffraction matrix illustrate the use of variable velocities and constant offset prestack migration.

Successful implementation of a salt interception schemes (SIS) requires monitoring to determine locations where the scheme needs revision. Current monitoring methods involve near-surface water-salinity measurements, which are affected by water-flow displacement. A survey method that can determine the salinity of water contained in the top few metres of alluvial sediments immediately beneath the river would be a more accurate tool for SIS monitoring.

A fast sampling Transient EM technique is investigated as a potential tool for imaging the conductivity of the top 5m of sediment, and thus monitoring the Waikerie SIS in South Australia’s Riverland. A towed TEM array was used to collect 9km of data that shows resistive anomalies correlating with SIS production bores. The system has the advantage of being a small, manageable array and the short inversion time allows same-day interpretation.

The tau-p velocity imaging method, first developed for obtaining the velocity field from marine multichannel seismic data, has been applied to refracted waves from the regolith in regional seismic reflection surveys on land. The technique converts travel time picks from the refracted wavefield into two-dimensional velocity models, by transforming from time-offset into the tau-p domain. Each arrival is mapped individually, and the ‘true’ velocity and position of the ray turning point is obtained by considering reversed raypaths. Thus the data are transformed directly into a depth or two-way time image of the subsurface displayed in seismic velocity. The method is extremely fast and involves no interpretive steps or iteration. Ideal datasets contain the refracted wavefield sampled densely and equally in the shot and receiver domains. It was therefore decided to test the application of the method for mapping the velocity structure of a portion of regolith and to compare the results with those obtained using more conventional methods.

The area chosen for study was part of a regional seismic reflection line across the Lachlan River palaeo-valley in central NSW. The data set consisted of the first break picks for 240 channels with receivers spaced every 40 m and vibration points every 40 m. The velocity images were produced as both time and depth sections and compared with one and two layer refractor models from a refraction tomographic approach. A low velocity region on the image corresponds to the deepest part of the refractor model, interpreted as the thickest part of the palaeo-valley. Bedrock velocity variations are also mapped and agree with the changes along the (lowest) refractor velocity profile. While further tuning may be required for land work, the technique has the advantage that velocities can be directly imaged and potentially related to regolith physical properties.

Since the late nineties Cameco Australia Pty Ltd and Cogema Australia Pty Ltd have been involved in exploration for unconformity related uranium deposits in Arnhem Land Australia. During this time the exploration model has evolved from the initial Canadian-Athabasca based model. Physical property measurements and field tests have led to the current integrated exploration strategy that incorporates the disciplines of geology, geochemistry and geophysics.

Airborne radiometrics continues to be the primary tool for identifying surface uranium anomalies. However, other geophysical techniques are utilised in order to map basement lithologies, alteration and the depth of sandstone cover, which are also key exploration objectives. With these aims in mind airborne hyperspectral, magnetic and electromagnetic geophysical techniques have been extensively utilised as efficient methods for quickly evaluating large areas where rugged topography prevents effective use of ground techniques. The usefulness of these techniques is discussed utilising examples from Cameco’s King River project, located in northern Arnhem Land.

It is shown that .5 s sampling is viable for detailed airborne radiometrics, TEMPEST can be used to estimate the unconformity depth and Hyperspectral airborne surveys can be used instead of ground PBVIA measurements for detecting alteration within sandstone.

In both Arnhem Land and the Athabasca, exploration is now focusing on identifying uranium below thick sandstone cover sequences. The Australian challenge is to undertake this exploration without the strong graphitic conductors, which are closely related to the Athabasca unconformity uranium deposits.

The Magnetic Resonance Sounding method (MRS) has been used in the past years with success in various geological and geographical contexts for groundwater surveys. This method has indeed the ability of directly detecting the presence of water through the excitation of the hydrogen protons of water molecules.

The frequency to which the H protons react depends on the magnitude of the Earth magnetic field, while the intensity of the excitation determines the depth of investigation. The amplitude of the magnetic field generated in return by the water of a layer is proportional to the porosity of this layer, and the time constant of the relaxation curve is linked to the mean pore size of the material, that is to say tightly related to its permeability.

A loop laid on the surface of the ground is used for both transmitting the excitation pulse and measuring the response of the H protons. The linear relation between the measured signal and the layer porosity permits to interpret the ID sounding as soon as the readings have been collected in the field.

The main applications of this method concern the determination of the water level and of the total quantity of water available down to 100 to 150 m depths. Magnetic Resonance Soundings can also help to select the best place for drilling, to predict a yield using a calibration, and to determine the geometry of an aquifer layer for hydrogeological modelling

A set of field examples acquired in various countries (Africa, Asia, Europe) points out both the advantages and the limitations of this method and suggests the place it should take among other geophysical methods in the methodology of groundwater investigations.

Negative responses recorded during moving in-loop TEM surveys, where only positive data are expected, are a regular occurrence in TEM surveys. They are however generally poorly understood and little reported. In most cases, such negatives are ascribed to induced polarisation (IP) effects, but we suggest that there is at least one other cause of negative TEM response, one which is associated with very resistive terranes containing little if any polarisable material.

This paper presents some examples of this negative resistive response where there is little or no polarisable material present and where a source other than IP should be sought. A series of tests have been proposed to try and isolate the cause.

The western and north-western regions of Tasmania have recently been covered with airborne magnetic and radiometric surveys as part of the Western Tasmanian Regional Minerals Program.

This is the first consistent quality coverage of the area and a number of new and interesting features have been defined as well as much more structural and lithological detail than previously mapped. The data set is expected to be of considerable assistance to explorers in this highly mineralised and prospective area.T

This paper shows some examples of the data and the subsequent improvement in geological interpretation. One of the examples, Balfour, has also been flown with airborne electromagnetics and a presentation of that data forms the second WTRMP paper.

One of the main issues in the characterization of any reservoir is the ability to predict the effect of fluid properties on seismic characteristics. This effect is studied by modelling fractures as very thin and highly porous layers in a porous background. Elastic moduli of a porous rock permeated by a system of such fractures distributed periodically are obtained using the result of Norris for elastic properties of layered poroelastic media. When both pores and fractures are dry, such material is equivalent to a transversely isotropic elastic porous material with linear-slip interfaces. When saturated with a liquid this material exhibits significant attenuation and velocity dispersion due to wave induced fluid flow between pores and fractures. At low frequencies the material properties are equal to those obtained by anisotropic Gassmann theory applied to a porous material with linear-slip interfaces. At high frequencies the results are equivalent to those for fractures in a solid (non-porous) background. The characteristic frequency of the attenuation and dispersion depends on the background permeability, fluid viscosity, as well as fracture density and spacing.

Lithologies with high clay contents can have negative chargeability. This response is usually attributed to geometric effects in induced polarization and electromagnetic surveys. Negative chargeability has also been noted in laboratory studies, but has remained unexplained.

Traditional membrane polarisation models are of a series of membrane constrictions within a solid substrate, but this model produces only positive chargeability responses. Alternate soft clay polarisation model are considered, involving bulk sample ionic fluxes. These fluxes have been previously studied in electrokinetic dewatering and decontamination studies, but over longer time periods. Of particular note are the large pH gradients that form under an applied electric field. Relaxation of these gradients at the sample’s surface produces a potential in the opposite sign to the applied potential, and so produces negative chargeability.

Laboratory electrical impedance spectroscopy data is presented. The experimental data covers a variety of clay types at a variety of water contents. For pure clay samples, a relationship exists between negative chargeability and water content (as a percentage of liquid limit). This trend is not apparent in natural clays.

Field measurements of negative chargeability are presented. Logging data through a soft clay lithology is shown to discriminate between clay and sand layers.

Porous reservoirs with aligned fractures exhibit frequency dependent seismic anisotropy due to wave induced fluid flow between pores and fractures. We model this frequency dependent anisotropy by combining the low-frequency anisotropic Gassmann model with the Hudson et al. model for frequency dependent properties of a porous material with penny-shaped cracks.

The predictions of the anisotropic Gassmann model are compared with experimental measurements of elastic wave velocities as function of angle for a synthetic sample with aligned disc-like cracks. The properties of the host rock and fracture compliances are obtained from P, SV, and SH velocities measured on the dry sample. The dry fractured porous rock properties serve as input to anisotropic Gassmann fluid substitution model, which is used to compute the saturated rock properties. The stiffnesses of the saturated fractured porous rock are used to calculate the angular dependent compressional and shear wave velocities. The predicted velocities are then compared to the measured velocities. The agreement is reasonably good for both S-wave velocities, but P-wave anisotropy is overestimated by about 25%.

This discrepancy can be explained by the fact that the low frequency assumption of the anisotropic Gassmann model does not account for the fluid diffusion effects occurring at relatively high frequencies used in the experiment (100 KHz). A combination of the low frequency anisotropic Gassmann model with the Hudson et al. frequency dependency accounts for fluid diffusion effects and results in an excellent agreement for P- as well as S- waves.

Image retrieval systems provide an effective tool for signature mapping and retrieval that can be applied to magnetic images to assist with preliminary interpretation of large datasets. Image retrieval is currently a very active field of research, motivated by the significant increase in the size of digital image databases in a wide range of image-based fields. It has emerged as a powerful tool for searching and locating a desired image, or part-image, from a large image database. Locating discrete circular anomalies sought after when exploring for kimberlites is an example of a potential geophysical application.

A model for content-based magnetic image retrieval (CBMIR) using texture and shape descriptors has been developed. Region and boundary-based shape

information is extracted using various edge detection techniques, and texture content is derived using statistical and wavelet transform-based methods. The model has been incorporated into a Matlab-based system for image retrieval and results using an experimental magnetic database are presented. The system is interactive, allowing the users intentions to be incorporated into the retrieval results.

Tests an the experimental magnetic database, demonstrate that CBIR has the potential to be a powerful tool in magnetic image interpretation, as it has been in other image-based fields.

Ground-based electromagnetic and electrical surveys were conducted in areas of southeast Queensland in March 2001 to establish representative ground conductivity values down to depths of greater than 50 m for use in designing suitable airborne electromagnetic (AEM) surveys to be flown in those areas later that year. Once AEM data were available, a comparison between the results of the ground-based and AEM surveys was made.

High resolution definition of near-surface conductive structures was achieved with the combined use of DC resistivity and frequency-domain EM31 measurements, while transient electromagnetic (TEM) soundings provided reliable values of deeper conductivity.

A comparison of the results of ground-based and corresponding AEM data shows that there is broad agreement between the behaviour of the conductivity of the conductive layer detected by AEM and corresponding conductivity values derived from ground-based data. However, the models obtained from the two sets of data do not agree in detail, probably because the AEM inversions were constrained to have a resistive basement, whereas ground-based and down-hole results show that the basement is generally relatively conductive.