Aaron Davis

Measuring AEM waveforms with a ground loop

Measuring a transmitter-current waveform provides critical data unavailable for some airborne electromagnetic (AEM) systems yet needed to model AEM data quantitatively. We developed a novel experimental method of measuring an airborne transmitter waveform by monitoring the current induced in a closed, multiturn, insulated ground loop of known inductance L and resistance R . The transmitter waveform of five different time-domain systems is deconvolved from the measured ground-loop response when excited by the primary electromagnetic field of the AEM system. In general, our measurements agree well with contractor-described transmitter current waveforms, although crucial differences exist between our deconvolved waveforms and those described in the literature. Using the pulse-per-second feature of a GPS antenna, the ground loop can monitor the frequency drift of a frequency-domain system. The ground loop behaves like a lossy electric-field antenna when the resistance closing the ground loop is too large. Thi...

First evidence of detecting surface nuclear magnetic resonance signals using a compact B-field sensor

The noninvasive detection and characterization of subsurface aquifer structures demands geophysical techniques. Surface nuclear magnetic resonance (SNMR) is the only technique that is directly sensitive to hydrogen protons and, therefore, allows for unambiguous detection of subsurface water. Traditionally, SNMR utilizes large surface coils for both transmitting excitation pulses and recording the groundwater response. Recorded data are thus a voltage induced by the time derivative of the secondary magnetic field. For the first time, we demonstrate that the secondary magnetic field in a SNMR experiment can be directly detected using a superconducting quantum interference device magnetometer. Conducting measurements at a test site in Germany, we demonstrate not only the ability to detect SNMR signals on the order of femtoTesla but also we are able to satisfy the observed data by inverse modeling. This is expected to open up completely new applications for this exciting technology.

Derivative analysis for layer selection of geophysical borehole logs

Analysis of geophysical borehole data can often be hampered by too much information and noise in the trace leading to subjective interpretation of layer boundaries. Wavelet analysis of borehole data has provided an effective way of mitigating noise and delineating relevant boundaries. We extend wavelet analysis by providing a complete set of code and functions that will objectively block a geophysical trace based on a derivative operator algorithm that searches for inflection points in the bore log. Layer boundaries detected from the operator output are traced back to a zero-width operator so that boundaries are consistently and objectively detected. Layers are then classified based on importance and analysis is completed by selecting either total number of layers, a portion of the total number of layers, selection of minimum layer thickness, or layers detected by a specified minimum operator width. We demonstrate the effectiveness of the layer blocking technique by applying it to a case study for alluvial aquifer detection in the Gascoyne River area of Western Australia.

6th International Conference in Airborne Electromagnetics (AEM 2013)

This special issue of Exploration Geophysics comprises papers from the 6th International Conference in Airborne Electromagnetics (AEM 2013) held in South Africa, and showcases the latest ideas and advancements in the discipline of airborne electromagnetic geophysics.

Characterisation of a Coastal Aquifer System in the Eyre Peninsula, South Australia, Using Nuclear Magnetic Resonance Methods

The coastal aquifers of the Uley Basin, which are the most important source of potable groundwater for the Eyre Peninsula, consist of unconfined Quaternary limestone overlying Tertiary clays and sandstones. Despite its importance, elements of the connectivity and total water resource basin remain relatively poorly understood. To address this, hydrogeophysical methods have been employed to better characterise the aquifer systems present. Interpretation of airborne electromagnetic data provided evidence for the delineation of the base of the Quaternary (limestone) aquifer and a basement low in the southwest corner of the South Uley Groundwater Lens, where there is a limited number of lithological bores or groundwater wells. The basement low, adjacent to the coast, suggests a preferential groundwater flow path and a possible connection between the Basin aquifers and the Southern Ocean.

Near-surface geophysics for informed water-management decisions in the Aṉangu Pitjantjatjara Yankunytjatjara (APY) lands of South Australia

The Aboriginal population of the Aṉangu 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.

Derivative analysis of geophysical borehole traces

We present a derivative analysis method that automatically detects and selects layers in any geophysical borehole trace. Using a wavelet analysis, we delineate relevant boundaries from inflection points. This allows for the automatic, objective detection of layers. Our software classifies layers based on importance in the geophysical data, and allows a user to select blocked layers based on total number of layers detected, a portion of the total layers, minimum layer thickness or the number of layers detected using a minimum operator width. We demonstrate the effectiveness of the layer blocking technique with some field examples in Western Australia and New South Wales for aquifer detection and soil classification.

The application of AEM to mapping sea-water intrusion at La Grange, WA

We describe interpretation of an AEM survey around the La Grange allocation area, WA. This survey was designed to map aquifer bounds and the sea water intrusion, and then to assess groundwater in the region, and to facilitate planning water use. The simple, stratified nature of sediments of the western onshore Canning Basin allowed us to use blocky layered earth models and we found that five-layer models were the most parsimonious. After deriving surfaces representing the top of the Jarlemai siltstone and the top of the sea water ingress, we were able to effectively characterise the spatial characteristics of the sea water intrusion. We found that in places, sea water intruded 40 km inland, and could be found at a depth of over 250 m.

Near-surface geophysics for informed water-management decisions in the Anangu Pitjantjatjara Yankunytjatjara (APY) lands of South Australia

The Anangu Pitjantjatjara Yankunytjatjara (APY) Lands of South Australia is an arid environment and the population relies largely on groundwater resources for potable water and agricultural needs. Historically, locating productive wells in the region has been hit-and-miss and even if a water source was found, the quality may be unreliable. In this project, we seek to improve the water security in the APY lands by demonstrating that surface Nuclear Magnetic Resonance (NMR) and Time-Domain Electromagnetic (TEM) geophysical measurements are able to map local aquifers and quantify ground water resources, thereby optimizing site selection for potential future wells. Surface NMR is directly sensitive to water and TEM measurements detecting the electrical conductivity structure and able to image the subsurface over large areas - all entirely non-invasively and with minimal risk of disturbing sites of importance to the local Aboriginals.

Surface NMR to Image Aquifer Properties in a Magnetic Subsurface

Surface Nuclear Magnetic Resonance (NMR) is a noninvasive 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.