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Lao PDR is a poorly developed country, with a large rural population which relies heavily on agricultural production. The Vientiane Plain is one of the most important and largest agricultural produ...

Surface Nuclear Magnetic Resonance (NMR) is a non-invasive geophysical technique providing the ability to image and investigate aquifer properties. In order to produce reliable images and interpretations of subsurface properties accurate modelling of the underlying physics is required. In magnetic environments, where the background magnetic field varies spatially, challenges can arise that lead to difficulty accurately modelling the excitation process and interpreting the signal's time dependence. We demonstrate using field data collected in the Anangu Pitjantjatjara Yankunytjatjara (APY) Lands of South Australia that neglecting the influence of the magnetic environment can significantly alter the final images and interpretation of the subsurface structure and properties.

The Aboriginal population of the Anangu Pitjantjatjara Yankunytjatjara (APY) lands in South Australia is dependent on groundwater for nearly all water needs. In that region, placement of wells in productive aquifers of appropriate water quality is challenging because of lack of hydrologic data and variable aquifer properties. It is desirable to have an improved ability to identify and evaluate groundwater resources in this remote region with cost-effective methods that make minimal impact on the environment. A project supported by the Society of Exploration Geophysicists program Geoscientists Without Borders tested a combined geophysical approach with airborne and ground-based data sets to locate a potential aquifer, confirm water content, and estimate the subsurface extent of the water-bearing zone. This hydrogeophysical approach was an effective means for exploration and evaluation of groundwater resources in APY lands generally, and it characterized a specific aquifer as a case study.

SUMMARY Artesian mound springs occur along the south-western edge of the Great Artesian Basin, in northern South Australia, but their underground structure and relationship to faulting is not well understood. We have performed geophysical surveys over three different systems using a range of techniques: early-time TEM, self-potential, and magnetotellurics. The self-potential data contains a local response due to specific spring vents, and also a broader stronger response due to laterally extensive upwelling in the lower part of the Bulldog Shale, at depths of approximately 100 m. Modelling of TEM and magnetotelluric data show that the confining Bulldog Shale, which is generally very conductive, contains resistive areas underneath springs and spring complexes which are believed to be related to spring-related carbonate deposition. Magnetotelluric modelling in particular indicates that anisotropic resistivity in the form of vertical sheets at a depth of 100 to 200 m, can explain the observations more readily than a conductive 2D feature, suggesting that the structures underlying the springs are sets of closely-spaced faults. The orientation of this anisotropy matches the regional NNW/SSE orientation of spring complexes.

All resistivity/induced polarisation (IP) data are contaminated to varying degrees by inductive coupling. Spectral IP (SIP) data sets are usually more affected than conventional IP data sets as data are collected at higher frequencies, where coupling is stronger. For SIP surveys interpretation is based on examination of the phase response over the entire frequency range so it is necessary to use relatively sophisticated methods to remove coupling. Unfortunately, most of the methods described in the literature are complicated to implement, or are not described well enough to be practical. Pelton et al. suggest a method for decoupling based on the commonly used Cole-Cole equation that is relatively simple to implement using a Monte Carlo examination of the solution space. In this approach the Cole-Cole response is separated into two terms, the ground response and the inductive response. For each data spectrum, a random number generator is used to determine a set of test values for each the seven parameters in the extended Cole-Cole equation. The values for each of the seven variables are limited to â??reasonableâ?? levels based on initial testing of the solution space. The randomly generated Cole-Cole parameters are then used to generate magnitude and phase responses at the frequencies used for the survey. These are compared with the field data and the fit is calculated. This is run 1 million times for each data point and the best fit chosen from the complete run. Results from this study are encouraging, but the method requires refinement.

A number of tools have been developed to help understand the processes of salinisation at work along the Murray River in Southern Australia. Four techniques that have been used to help investigators either directly measure the salt load entering the river, or to image the distribution of conductivities under the river are examined here. They include Run-of-River surveys (ROR), in-stream towed NanoTEM, in-stream towed Resistivity, and Helicopter EM (specifically using the RESOLVE FDHEM system). Each technique has strengths and weaknesses related to its mode of operation and the approach adopted in field data collection. Run-of-River samples the water salinity directly and then attempts to estimate river salt load and source location. It provides a direct measure of the salt entering the river but a) only provides salt load information and b) generally only provides information on a kilometre scale. The other three techniques are all geophysical techniques that do not directly inform the investigator about salt loads in the river, but provide information about conductivity distributions in the sediments under the river, which then may be related to salt loads. Each of the geophysical techniques sample the instream environment at three to 20 metre intervals, and provide information from near the river surface to depths of between 10 and 40 metres below the surface. Data may be displayed as depth sections, or as contoured depth slices prepared to examine different levels beneath the river bottom.

As part of an ongoing commitment to the reduction of salt accession to the River Murray in South Australia, the South Australian Department for Water, Land and Biodiversity Conservation (DWLBC) is engaged in a series of investigations to determine the feasibility of the construction of salt interception schemes (SIS). An integrated multi-disciplinary approach has been adopted to develop an understanding of the hydrogeological processes, which is essential for effective scheme design. To assist in identifying areas of salt accession, a river-borne NanoTEM survey was conducted in September 2003. Previous work at the Waikerie SIS highlighted the success of the river-borne Transient Electro-Magnetic (TEM) technique. These systems can provide valuable information and understanding of salt accession to the River Murray. Traditional methods, such as river EC measurements and Run of River (RoR) EC surveys, have revealed that the Loxton and Bookpurnong reaches of the river receive substantial salt loads. These techniques provide limited spatial resolution, whereas the river-borne TEM has exceptional spatial resolution with soundings every 5 to 10 metres. A total of 80 km of data over a 37 km stretch of the River Murray was collected near the townships of Loxton and Bookpurnong. The survey results correlate well with recent RoR data, sediment core ground-truthing and groundwater model flow budget results.

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.

Australian Water Environments (AWE) has been engaged by the Bookpurnong - Lock 4 Environmental Association (BL4EA) to investigate the interception of saline groundwater along the River Murray downstream of Berri, South Australia (Figure 1). Groundwater discharge contributes approximately 70 tonnes per day of salt to this reach of the river. Salt loads are predicted to rise to more than 200 tonnes per day over the next 30 years. The proposed Environmental Enhancement Scheme (EES) comprises a series of shallow pumping wells constructed primarily on the floodplain to intercept saline groundwater before it reaches the river. Intercepted groundwater will be pumped to the Noora Disposal Basin located some 25 km inland. A detailed understanding of the hydrogeological environment is essential to the design and effective operation of the EES and similar groundwater interception schemes. In March 2001, Zonge Engineering conducted a fast-sampling TEM (time-domain EM) traverse to investigate changes in sub-surface resistivity across the highland area adjacent the River Murray and the floodplain within the riverine trench. The highland is planted with citrus and has been irrigated since the 1960?s. The shallow depth to saline groundwater within the Monoman Formation aquifer has seriously degraded the health of native vegetation health across the floodplain. In May 2002, twelve shallow wells were constructed along a narrow stretch of floodplain located approximately one kilometre downstream of the first traverse. Accurate formation samples were collected and logged. Zonge Engineering conducted a second fast-sampling TEM survey in June 2002 along a line parallel to the trial borefield to investigate the correlation between TEM data and variations in hydrogeology identified during drilling. This paper reports the results of the two land-based geophysical surveys and comments on the utility of fast-sampling TEM as a tool for mapping subsurface hydrogeology across the highland and floodplain at Bookpurnong.

Anomalous conductive responses that could not be related to obvious geological sources were observed in field data from a CSAMT survey in the Cobar area, New South Wales. Repeat measurements over the anomalous stations were made, yielding a substantially different resistivity response. Approximate positioning of the E-field dipole stations due to the presence of thick scrub was believed to be one possible source of error contributing to the differing responses. Subsequent test measurements in which the E-field dipole angle was varied by 10 degrees east and west of the correct direction demonstrated that such an orientation change resulted in resistivity variations of up to 50% from the true value. This is significantly larger than expected from simple theory, which suggests that the results from location errors up to 10 degrees from correct should only be a few percent. This large variation observed in the Cobar area is believed to be due to a strong geo-electrical anisotropy within the steeply dipping and highly cleaved meta-sedimentary rocks of the Cobar Basin. It was concluded that relatively small errors in receiver orientation could have a large effect on the magnitude of the received signal, which can potentially generate `false' anomalies in the derived resistivity pseudosections and inversions. It is possible that such location errors are commonly made when a standard GPS is used to locate stations on a grid. Based on the experience from the Cobar area, accurate station positioning is clearly a pre-requisite for reliable electrical surveys in any geological terrain. This paper presents field examples demonstrating the potential errors, and the results of applying more-accurate positioning techniques. Recommended methods to gauge the magnitude of the potential errors in any survey area are proposed.

Most irrigation channels in Australia are earth lined and leak water to the surrounding terrain. The cost of lost water can be high in economic and environmental costs. The solution is to completely line the canals or replace them with pipes, both expensive options. An alternative is to focus on areas of highest seepage. In this study we trialed a dipole-dipole resistivity array towed behind a dinghy in the canal. The approach followed trials using electromagnetic techniques in previous years that showed conductivity changes below and around the canals in part reflected seepage. We inverted the resistivity results to sections and statistically analysed the data from various depths in the ground by correlating the resistivity against pondage seepage results. Our conclusion was that the resistivity worked best where there was a diffuse seepage from the canal. The greatest effect was around the watertable. Where seepage rates were low there was no correlation with resistivity. The correlations improved with increasing seepage particularly where the inverted sections indicated the seepage rates were uniform along the channel.

New ways of energy production through the use of coal seam gas plays and geothermal hot dry rock and hot sedimentary aquifer systems pose challenges in identifying and monitoring fluid in the subsurface. We propose the use of the magnetotelluric (MT) method to image static and dynamic fluid distributions in the subsurface exhausting the contrast in electrical conductivity between resistive host rock and conductive fluid-filled, porous rock. Base line MT measurements provide reference transfer functions and inverse models to characterise the electrical conductivity distribute on which is linked with bore hole and other geophysical data to obtain knowledge about fluid distribution at depth. The reference models are used to accurately forward model fluid injection or extraction temporally and spatially. This work shows results from fluid injections at a hot dry rock system at Paralana, South Australia, and its applicability to other geothermal and coal seam gas systems.

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.