ASEG Extended Abstracts - ASEG2009 - 20th Geophysical Conference, 2009

To improve on earlier hydrogeological models and to better manage the River Murray floodplain environment and water resources from salinity and irrigation impacts, stakeholders in the New South Wales and Victorian Mallee worked with project scientists to identify hydrogeological parameters that might be collected from existing data sets to help resolve land and water management issues (Lawrie at al., 2008). Several areas where improved information were identified, including the extent and thickness of the river flush zone, salt stores in both the unsaturated and saturated zones, and the thickness, extent and depth to the top of the Blanchetown Clay and other clay units across the floodplain and its adjacent uplands. Previous AEM-based studies have demonstrated the potential for these data to provide high geospatial resolution of key elements of the hydrogeological system (Walker et al., 2004).

An increase in demand for bulk water storage free from evaporation loss has led to increased interest in groundwater. Full knowledge of groundwater recharge hotspots and flow pathways is needed for best management of both groundwater and surface water where both may interact. Electrical conductivity (EC) is usually the most appropriate property to use for economical imaging of recharge hotspots and groundwater flow pathways. Both galvanic and electromagnetic imaging techniques readily resolve recharge pathways due to the combined effect, on bulk electrical conductivity, of low clay content associated with recharge pathways and low salinity of surface water that is the source of the recharge. The key to viable recharge projects is provision of detailed information on the existing and potential recharge pathways; this can most effectively be provided using new specialized geophysical technology.

Improved knowledge of alluvial architecture is becoming crucial to investigators to assist in understanding the dynamics of large river systems like the Murray in southern Australia. Historically, information has been gathered by analysis of drill-hole information, which provides information about fine-scale vertical structure. This is often interpolated laterally over large areas with little consideration to the influence of depositional process, consequently with often poor results. Increasingly, airborne electromagnetic surveys (AEM) have been used to help fill in information gaps. Even this information is coarser than needed to define fine structure. This leaves a niche for high resolution ground geophysical surveys, both to validate the AEM, but also to fill in where other methods are too coarse.

For this study, we report the results from a coordinated geophysical and drilling program on highly salinised floodplains near Mildura, Victoria. Three accessible lines were chosen over recently acquired AEM surveys. A high-resolution electromagnetic survey and a low-frequency (25 MHz) ground penetrating radar survey (GPR) were run over each line. Where available, drillhole information was collected and compared with the geophysical data.

Comparison of the three data sets show good correlation between small to medium scale vertical and lateral variations in the geophysical data with observable aspects of the alluvial architecture. Much of the response is strongly correlated with the location, depth and salinity of local groundwater. The higher resolution ground-based techniques also assist in informing interpretation of the broader scale and coarser resolution AEM data.

An integrated geophysical strategy employing magnetic, electrical and refraction seismic methods was used to delineate geological contacts associated with an outlier in biotite gneiss and sandstones located near Tiruvuru, Andhra Pradesh, India. Generally these contacts are favorable for ground water occurrence and exploration. In this study, magnetic method used as a reconnaissance tool was found to be highly effective for delineating contacts and estimating the depths to the basement based on Hilbert transform analysis, Fourier spectral method followed by Geosoft modeling. Also the width of the outlier was established using the amplitude of the analytic signal of the magnetic anomalies. Refraction seismic studies proved to be useful in determining accurately the thickness of various layers. Certain low velocity pockets which are favorable to groundwater accumulation were also identified. Location of contacts was supported by vertical electrical soundings (VES) through pseudo sections; the depth to subsurface contact within the outlier was derived from geoelectrical sections. Reliability of interpretation is substantiated by correlating the signal with known geology and bore well data.

Magnetotelluric (MT) data have been frequently collected over the past few’ years in form of 2D and 3D surveys across much of the Gawler Craton. The increasing coverage of sites allows a regional analysis of the underlying resistivity distribution with the help of 2D and 3D inversion routines and helps constrain the delineation and nature of geological boundaries. Increasingly, the resistivity models have been combined with existing geological and geo chronological knowledge to analyse the tectonic evolution of the Gawler Craton.

The pitfall of large-scale regional analyses of MT data is the conductive influence of the ocean and thick sedimentary sequences on the MT responses. In the case of the Gawler Craton, the sediments, with resistivities of around 10Dm and thicknesses up to a few kilometers, contribute significantly to the inductive effect and can cause artefacts in the resistivity modeling. It is therefore essential to differentiate the inductive effect of the lithosphere and that of sediments and seawater.

We present a way of quantifying the sediment and ocean effects by means of 3D forward modeling of the electromagnetic fields associated with them. We show that such an analysis is highly beneficial to further modeling of lithospheric structures.

Geophysical "worming" was applied to potential field data over the Gawler Craton. "Worming" is a multi-scale edge analysis technique that can aid in identifying structural controls and depth to anomalies. A geological interpretation of the worming results was then undertaken; integrating drill-hole information, ground mapping and tectonic understanding with geophysical modelling to gain a better comprehension of the dominant structures present.

The "worming" process provides potential solutions for the lack of outcrop, particularly that which is representative of three-dimensional architecture. The latter is particularly important in understanding how terrains are juxtaposed or dissected tectonically, which in turn influences the style of any mineral system which may be present (for example, is a structure really likely to be associated with mantle-tapping fluids?). Moreover, correct identification of structural geometry and cross-cutting relationships allows a more confident assessment of fault kinematics and potential dilatancy. In particular, the degree of U-mineralisation in IOCG systems in the Gawler Craton may be dependent on the interconnectivity of fault plumbing in three dimensions to nearby uraniferous Mesoproterozoic granitoids.

The form of the Earth is much closer to a sphere than a flat slab, so why is gravity and magnetic modelling almost exclusively carried out within a flat Earth or Cartesian framework? The use of a Cartesian coordinate system can be attributed in part to the absence of alternatives in the commercial modelling and visualisation software packages and the need for additional computational resources if modelling is carried out in a spherical or ellipsoidal reference system. These are not sufficiently good reasons, however, to blindly continue with this practice.

Gravity modelling is a particularly widespread activity, being used for basic gravity data processing (e.g., Bouguer and terrain corrections – e.g., LaFehr, 1991a, 1991b, 1998; Talwani, 1998; Hinze et al, 2005), geodesy applications (e.g., geoid calculations), and geological interpretation. To what degree are the outcomes of these activities affected by the choice of Earth representation?

In the exploration geophysics community, there is a standard answer to the question "when do curvature effects become significant" which is "when the models are larger than 166.7 km in horizontal extent". This criterion can be traced back to Hayford and Bowie (1912). What was the basis for choosing this distance? Is this a criterion that can be applied to all modelling applications or just the original application of Hayford and Bowie (1912)?

Although there is general acknowledgment that the curvature of the Earth is important when performing gravity or magnetic modelling of long traverses or large regions, there are few studies of the errors involved if a Cartesian (or rectangular) coordinate reference system is used. Examples that demonstrate the magnitude of curvature effects on gravity response can be found in Hayford and Bowie (1912), Takin and Talwani (1966), Johnson and Litehiser (1972), Qureshi (1976), Mikuška et al. (2006), and Çavşak (2008).

In the following, results of simple calculations for a regular mesh of prismatic elements are presented to quantify the differences in vertical gravity and vertical gravity gradient response for equivalent representations of source elements in Cartesian or spherical coordinate reference systems. It is noted that outcomes for vertical gravity gradient response are the same as those that would be obtained for total magnetic intensity response (TMI). Although this work is based on models comprised of a regular array of prismatic mesh elements, the outcomes can be related to those that would be obtained for models based on arbitrary 3D polyhedra. There are clear extensions of methodology that would allow modellers to estimate the significance of curvature effects for any specific application. This will help to answer the question that is prompted by Figure 1, i.e., "How significant are the differences if I carry out gravity or magnetic modelling for a portion of the Earth using a Cartesian framework rather than a spherical framework?"

Geoscience Australia recently revised the corrections applied to the Australian National Gravity Database (ANGD) and switched from applying the simple Bouguer correction to the observed gravity values in its database to applying the more accurate gravity curvature (Bullard B) correction. This change is a straightforward procedure in the case of land-based gravity surveys. However, due to the inherent non-linearity of the Bullard B correction, the original formula for the gravity curvature correction is not applicable to observed data from gravity surveys, which involve layers of different materials, as is the case in marine or airborne gravity surveys. Here we present an extension of the closed-form solution for the Bullard B correction, which allows its proper application in any gravity survey setting. In particular, we present formulae to correctly apply the Bullard B correction to observed gravity data from airborne and marine gravity surveys.

The history of human civilisation shows a continuous trend towards urban life as the basis for social organisation. Over the last 50 years, this trend is accelerating with the proportion of the world urban population expected to reach 61% by 2030 (Godard, 2004). Maintaining quality of urban life and creating sustainable urban environments is one the most crucial current human challenges with continuing degradation of urban environments, increasing congestion and lack of green open space of critical concern. Going underground offers solutions to many of these problems.

The Epping to Chatswood Rail Line, in Sydney’s northern suburbs, is a local example of a major new shallow underground space development in an urban environment that will be completed during in 2009 (Figure 1). This case study demonstrates the benefits of applying geophysics integrated with geotechnics and hydrogeology to a critical river crossing section of this environmentally sensitive underground space development.

The Multichannel Analysis of Surface Waves, or MASW in short (Park, et al, 1999; Suto, 2007) analyses seismic data in the frequency-velocity domain and estimates the S-wave velocity structure under the seismic receiver array. Its application range varies, commonly from only a few metres to tens of metres, depending on the wavelengths of the surface waves used for analysis.

The output from an MASW survey and analysis is essentially a series of 1-dimensional S-wave velocity profiles, generating spatially discrete data points similar to borehole data. As the data are collected along a line and sampled at closely spaced intervals, it is common to present the data in the form of a 2-dimensional section of S-wave velocities along the survey line, rather than a 1-dimensional profile with depth. If an MASW survey consists of closely spaced survey lines, it is possible to present the output of the surveyed area as a 3- dimensional data set.

The dispersive characteristics of Rayleigh type surface waves were utilized to estimate shear wave velocity (Vs) profile followed by imaging the shallow subsurface granitic layers near Hyderabad India. The reliability of Multichannel Analysis of Surface Waves (MASW) depends on the accurate determination of phase velocities for horizontally traveling fundamental mode Rayleigh waves. Multichannel recording leads to effective identification and isolation of various factors of noise. Calculating the 1-D shear wave velocity (Vs) field from surface waves ensures high degree of accuracy irrespective of cultural noise. The main advantage of mapping the bed rock surface with shear wave velocity (Vs) is the insensitivity of MASW to velocity inversion besides being free from many constraints such as contrast in physical properties etc. Modeling of surface waves data results a shear wave velocity (Vs) of 250-750 m/sec covering the top soil to weathering and up to bedrock corresponding to a depth range of 10-30 m. A pair of selected set of results over granites are presented here as a case study highlighting the salient features of MASW.

The 2.06 Ga Bushveld Igneous Complex of South Africa is the world’s largest layered igneous intrusion. Situated to the north of the Johannesburg, it extends ~450 km from east to west and ~300 km north to south (Simmat et al., 2006) (Figure 1), and has a maximum thickness of ~8 km. It is estimated to contain over half the world’s Platinum Group Element (PGE) resources (Cawthom, 1999). The two PGE-bearing chromitite layers in the Bushveld Complex, the Merensky Reef and the UG2, occur within pyroxenites of the Critical Zone, show high lateral continuity and commonly dip at relatively shallow angles (2°-20°) towards the centre of the complex. These are overlain by a succession of norite and gabbronorite formations of the Main Zone, representing a more mafic phase of the intrusion.

The project area is the location of a planned new Anglo Platinum mine on the western limb of the Bushveld, south of the Pilanesberg Alkaline Complex, a later volcanic event (Figure 1). Major capital investment is required to sink the twin shafts, accessing the underground platinum reefs from ~700 m depth. Due to the proximity to the Pilanesberg, dykes and faults are pervasive throughout the deposit. Several dyke varieties occur and these differ in composition, texture and orientation, and consequently represent different geotechnical hazards. Dolerite dykes tend to be hard and competent with similar rock strength properties to that of the surrounding Bushveld rocks, and are not expected to represent significant geotechnical problems. Lamprophyre dykes, however, are often soft, weather easily when exposed and demonstrate significant contrast in rock strength properties to the Bushveld rocks.

The SkyTEM system is a helicopterborne TEM system (Sørensen and Auken, 2004) originally designed and developed for hydrogeophysical and environmental investigations. The aim was to develop an airborne system that would give the same resolution as conventional ground-based TEM soundings.

To deliver both early and late time data of good quality, the SkyTEM system can presently be programmed to deliver the combination of moments that is optimal for the survey aims: low moments at ~12kAm² where low current and fast transmitter turnoff provide early time data with good near-surface resolution, and high moments at 120-220 kAm² where a high current, more transmitter turns and lower base frequency provide late time data with good penetration. Repetition frequencies are chosen to optimally reject the power line frequency. In a sedimentary environment where small resistivity contrasts are significant, it is essential that the system produces data with high accuracy and that very early time gates are measured to provide good near-surface resolution. In its present configuration, the vertical resolution of the upper layers for the SkyTEM system is comparable to that of airborne frequency domain systems while the horizontal resolution is better due to less lateral filtering as the signal-to-noise ratio at early times is generally very good for TEM systems. The high moment provides good signal-to-noise ratio at late times. Because of its high lateral and vertical resolution, the SkyTEM system is increasingly used also in mineral prospecting where accurate definition of high conductivity anomalies is essential. The user-definable moments also makes it possible to include transmitter-off intervals so that background noise can be measured during flight. Before takeoff, a transmitter waveform calibration takes place.

The Chowilla floodplain in South Australia (Figure 1) is part of the Bookmark Biosphere Reserve with wetlands supporting a wide range of plants and animals, and has provided recreational opportunities to the general public (MDBC, 2006). However, the threats from saline groundwater and salt accumulation at the surface, combined with the overall low rainfall and the lack of natural flooding events in the recent past, have had a negative consequence for vegetation health, especially the river red gums. Information on the spatial patterns of groundwater salinity and the near surface (top 15 m) salt stores in the floodplain is integral to our understanding of floodplain hydrological processes that may be active in the Lower Murray Basin of Australia. This information can be used to help develop appropriate remedial activities for the management of ecologically important wetlands and in particular assist in the maintenance of a healthy vegetation community.

A shallow (depth < 20 m) layer of water, fresh, brackish or saline, covers millions of square kilometres of sediments and bedrock along the world’s coastlines, rivers, lakes, and lagoons. These geological units are extremely important, both environmentally and economically. Airborne electromagnetic (AEM) has been used with some success to obtain the bathymetry of shallow surface water ((Macnae et al., 2004; Vrbancich et ah, 2000; Vrbancich et ah, 2005). Some attempts have also been made to retrieve information about the sediments under the water bottom (Vrbancich et ah, 2000). The limited research that has been carried out so far tells us that we need to make improvements and further developments, both in hardware, as well as in data processing and modelling. This manuscript aims at making a contribution at the data inversion level, by applying constrained inversion methodology to different AEM datasets flown over water. In constrained inversion, adjacent model parameters are regularized through lateral constraints that allow information to flow from soundings that contain more information to those that contain less. We present results from constrained inversion (smooth and few layers) of a portion of SkyTEM survey (a helicopter borne time domain AEM system) flown in Denmark, and of a RESOLVE survey (a helicopter borne frequency domain AEM system) carried out along the Murray River in Australia. In both cases bird height was included as an inversion parameter, allowing the inversion to compensate for errors in laser altimeter readings over water.

Time domain airborne electromagnetic (AEM) data acquired from surveys over seawater in Australian coastal waters can be interpreted to obtain seawater depths (Vrbancich and Fullagar, 2007a; Wolfgram and Vrbancich, 2007; Vrbancich, 2009) and to identify the coarse features of bedrock topography (Vrbancich and Fullagar, 2007b; Vrbancich, 2009). A comparison of derived water depths in shallow areas (< 50 m) with known bathymetry has shown that submetre depth accuracies can be achieved but these accuracies are not maintained over the entire survey region. Furthermore, the quantitative interpretation of AEM data using ID inversion methods may require data rescaling (Vrbancich and Fullagar, 2007a; Vrbancich, 2009) and the measured seawater conductivity as a known parameter. The rescaling coefficients in these studies were obtained from the slope and intercept of linear fits between modelled and observed decays at representative sites (control points) with known water depths. These restrictions limit the potential of AEM for accurate bathymetric mapping.

A time domain helicopter AEM system (SeaTEM) is currently being developed for the Defence Science and Technology Organisation for shallow water bathymetric mapping. This system consists of a transmitter and receiver loop assembly mounted on a rigid structure referred to as a "bird" that is towed as a sling load below the helicopter. Instrument stability, calibration (Vrbancich and Fullagar, 2007a; Brodie and Sambridge, 2006; Davis and Macnae, 2008) and the ability to accurately track both the swaying motion (i.e. bird swing) and the altitude of the AEM sensor system over seawater during survey (Davis et al., 2006; Kratzer and Vrbancich, 2007) are issues that need to be addressed in order to develop AEM as a reliable and accurate bathymetry mapping technique. System calibration, self-response, transmitter current waveform, and altimetry were investigated and preliminary findings are reported in this paper.

Before going airborne, the response of SeaTEM instrumentation over seawater was studied in a controlled experiment designed to minimise the effect of bird swing and altimetry errors. For this purpose, the AEM system was floated above seawater using a circular ring, modified from structures used for open-sea fish farming, as the platform. This floating AEM system is referred to as the "sea-ring". Periodic EM measurements were made whilst the sea-ring was being towed at about 2 knots in areas of known water depth. A marine seismic survey provided independent estimates of sediment thickness. Sea-ring data was interpreted to appraise the accuracy of water depths and sediment thickness derived from AEM data and to identify calibration errors.

The AEM bathymetry method also has the potential to provide water depths in the surf zone, so that bathymetry can be used to measure water depths on approaches to beaches (Vrbancich, 2009). The use of LID.AR to estimate the sea surface topography in surf zone areas is under investigation (Vrbancich, unpublished) to support AEM bathymetric mapping. As well as mapping the topography, the LIDAR data also provides accurate altimetry which may be more reliable than using a laser altimeter over seawater.

Airborne and ground EM measurements were conducted over resistive granite to study the system self response. The AEM measurements involved flying over a closed loop of known electrical properties placed on the ground (Davis and Macnae, 2008). The response of the ground loop combined with the flight path can be used to predict the AEM system response. The AEM transmitter current waveform was also measured directly and indirectly from the ground loop data. Apart from the direct measurement of the transmitter current waveform, the results of these findings will be presented separately (Davis et al., 2009).

Within Otway Basin CO2 sequestration program, a small amount of CO2 is currently being injected into a depleted Naylor gas field, onshore Victoria. The reservoir is relatively deep (2 km) and complex with area extent of approximately 0.5 km² limiting the monitoring program to the application of seismic methods only. However the injection of CO2 into this heterogeneous reservoir, where residual gas saturation is present throughout most of the sand column, is expected to cause very subtle changes in elastic properties of the reservoir rock. Indeed initial approximate modelling of 4D seismic response showed that only 4-6 % change in the elastic parameters could be expected. Such small effect could be "lost" even through approximate fluid substitution methodology. Considering inherently low repeatability of land seismic it becomes even more important to accurately predict 4D seismic at this site. For that purpose we have investigated various methodologies that could increase the accuracy of the predicted changes in elastic properties of the reservoir rock. We derived a methodology for accurate prediction of elastic properties of the reservoir rock through calibration of the log and petrophysical data with core sample. The result showed core saturated velocities and log measurement agree with each other when the "effective" Kgrain is applied. It suggested that "effective" Kgrain could be used to represent the average mineralogy of the grains if we do not know the exact mineral composition making up the rock. However, comparative analysis and calibration of log measurement with core sample proved that accurate fluid substitution methodology at this site is hard to achieve without having dense core sample test results within the reservoir interval. In this paper, we present a methodology to derive elastic properties of the reservoir rock through calibration of the log and petrophysical data with core sample measurement.

We use the microtremor survey method to record ambient ground vibrations in Launceston, Tasmania. The presence of the ancient Tamar Valley, in-filled with soft sediments that vary rapidly in thickness from 0 to 250m over a few hundreds meters, is thought to induce a 2D resonance pattern, amplifying the surface motions over the valley and in Launceston. We use the spatially averaged coherency (SPAC) microtremor survey method to study its potential for characterising site effects across the Tamar Valley.

We present SPAC observations, recorded with increasing array size at 3 sites KPK (r1=28m), DBL (r1=50m) and RGB (r1=140m) in Launceston. Replacing the traditional spatial averaging of coherencies by temporal averaging, we compute coherencies with single pairs of sensors, oriented parallel (axial-coherency) and perpendicular (transverse-coherency) to the valley axis. Using multi-radii arrays, we analyse the impact of increasing radius on the spatial and temporal averaged coherencies recorded at all sites, to study SPAC capability to detect a 2D resonance pattern induced from the Tamar valley in Launceston.

Scintrex is in the final stages of the development of a borehole gravity meter, for mining and geotechnical applications, designed to log inside NQ drill rods to 2,000 m depth, using standard 4 conductor cable, with a sensitivity of better than 5 pgal, and operable in boreholes inclined from 30° to vertical. Ecole Polytechnique of Montreal has developed forward modelling software, as part of this project.

The first field test of the prototype probe was successfully conducted in December 2008 for Vale Inco in a deep borehole located in Norman township near Sudbury, Ontario. The results of this test show a large amplitude bipolar residual gravity anomaly, with the crossover at the location where the borehole intersected sulphides. Further analysis of the data is underway. A repeat log of the hole indicates that the Gravilog system has achieved operational specifications very close to its targets.

Field tests for the other sponsors are planned during the first half of 2009, with production surveys to follow during the second half of the year.

Gravity measurements inside boreholes provide evidence of density variations both in the immediate vicinity and at a distance from the hole. Scintrex’s development of a new borehole gravimeter will, for the first time, allow the application of gravity logging in typical mining and geotechnical boreholes.

Primary applications of the Gravilog system in mining include the sensing and mass-estimates of massive sulphide bodies, either intersected by or remote from the hole; and accurate bulk density measurements of formations intersected by the hole.

Nind, Chris J. M.

Position: President & CEO

Affiliation: Scintrex Ltd.

Email: [email protected]

Chris Nind obtained a BSc, Mathematics, from Queen’s University in Canada and joined Geoterrex Ltd as a geophysicist in 1977. At Geoterrex, he worked in the ground, processing and airborne departments. From 1990 to 1994, he managed Geoterrex’ airborne geophysics department in Australia. In 1994, he moved to Dighem Surveys in Toronto. From 2000 to 2004, he was the Regional Manager, Americas, for Fugro Airborne Surveys. In mid-2004, he joined LaCoste & Romberg-Scintrex as President & CEO. His background includes many gravity surveys using L&R Model G gravimeters. His interest in gravity continues at Scintrex in Toronto, Canada, which builds the CG-5 gravimeter, and at Scintrex’s sister company, Micro-g LaCoste in Denver, USA, which builds absolute, airborne, marine and monitoring gravimeters.