In hardrocks that cover about 20% of the Earth's surface, it is difficult to locate steady sources for groundwater due to inadequate understanding of the fracture networks. A comprehensive knowledge ...of fracture distribution at the regional scale is necessary to delineate sustainable aquifers and manage them efficiently. The resistivity maps derived from the airborne electromagnetic (AEM) survey over the Ankasandra watershed in Karnataka, India, reveal sharp and deep zones of low formation resistivity, which indicate groundwater-bearing zones. It is found that some of these zones are hydrogeologically connected through fracture networks resulting in augmented yield. AEM results in combination with an in-depth understanding of the geological structures successfully map these groundwater-saturated fracture networks (or hydrogeological lineaments) that we term as 'Hydrolins'. As groundwater occurrence is generally associated with lineaments, we analyzed the drilling and geophysical logs from 21 wells within a 380 sq.km area to study the relationships of various lineaments with 'Hydrolins', particularly in respect of their groundwater potential. AEM results, though calibrated and correlated with a limited number of well data, revealed a threshold groundwater horizon (TGWH), found to be at 80 m depth for Ankasandra watershed, beyond which a strong correlation exists between the depth of a well and its yield. While the TGWH may differ for different watersheds, the approach presented here can be readily adopted to map sustainable groundwater sources in hardrocks worldwide.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Core Ideas
We introduce a new geophysical imaging method to assess managed aquifer recharge sites.
This method provides high‐resolution 3D imaging of the subsurface down to a minimum depth of 50 m.
...We introduce Resistivity Distribution Plot as a new approach to assign a saturated–unsaturated boundary.
In many places around the world, much attention is focused on managed aquifer recharge (MAR) because of reduced groundwater levels due to droughts. To assess the suitability of a site for MAR, detailed three‐dimensional (3D) information about the subsurface materials and their hydraulic properties is needed. In areas where the groundwater level is at an intermediate depth (e.g., 20–40 m), such information is needed from the ground surface down to a minimum depth of ∼50 m. To achieve this goal, we used a new geophysical imaging system: a towed time‐domain electromagnetic system that is efficient for acquiring data at a significantly improved resolution and a scale needed for MAR. During a 2‐d period, we acquired ∼92 line‐kilometers of data in one almond Prunus dulcis (Mill.) D.A. Webb grove, one pistachio (Pistacia vera L.) grove, one open field, and two active recharge basins in the Tulare Irrigation District in the Central Valley of California. At each site, a detailed 3D resistivity model with a resolution down to the 10‐ by 10‐m scale is presented in terms of resistivity distribution plots, which are then used to assign a saturated–unsaturated boundary. In addition, we used a resistivity–lithology transform to interpret the resistivity models and create lithology maps at each site. We used this information to assess the suitability of each site for MAR.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Increasingly, electromagnetic induction methods (EMI) are being used within the area of archaeological prospecting for mapping soil structures or for studying paleo-landscapes. Recent hardware ...developments have made fast data acquisition, combined with precise positioning, possible, thus providing interesting possibilities for archaeological prospecting. However, it is commonly assumed that the instrument operates in what is referred to as Low Induction Number, or LIN. Here, we detail the problems of the approximations while discussing a best practice for EMI measurements, data processing, and inversion for understanding a paleo-landscape at an Iron Age human bone depositional site (Alken Enge) in Denmark. On synthetic as well as field data we show that soil mapping based on EMI instruments can be improved by applying data processing methodologies from adjacent scientific fields. Data from a 10 hectare study site was collected with a line spacing of 1-4 m, resulting in roughly 13,000 processed soundings, which were inverted with a full non-linear algorithm. The models had higher dynamic range in the retrieved resistivity values, as well as sharper contrasts between structural elements than we could obtain by looking at data alone. We show that the pre-excavation EMI mapping facilitated an archaeological prospecting where traditional trenching could be replaced by a few test pits at selected sites, hereby increasing the chance of finding human bones. In a general context we show that (1) dedicated processing of EMI data is necessary to remove coupling from anthropogenic structures (fences, phone cables, paved roads, etc.), and (2) that carrying out a dedicated full non-linear inversion with spatial coherency constraints improves the accuracy of resistivities and structures over using the data as they are or using the Low Induction Number (LIN) approximation.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
On 30 September 2017, an Air France Airbus A380-800 suffered a failure of its fourth engine while over Greenland. This failure resulted in the loss of the engine fan hub, fan blades and surrounding ...structure. An initial search recovered 30 pieces of light debris, but the primary part of interest, a ~220 kg titanium fan hub, was not recovered because it had a different fall trajectory than the light debris, impacted into the ice-sheet's snow surface, and was quickly covered by drifting snow. Here we describe the methods used for the detection of the fan hub and details of the field campaigns. The search area included two crevasse fields of at least 50 snow-covered crevasses 1 to ~30 m wide with similar snow bridge thicknesses. After 21 months and six campaigns, using airborne synthetic aperture radar, ground-penetrating radar, transient electromagnetics and an autonomous vehicle to survey the crevasse fields, the fan hub was found within ~1 m of a crevasse at a depth of ~3.3 to 4 m and was excavated with shovels, chain saws, an electric winch, sleds and a gasoline heater, by workers using fall-arrest systems.
Geophysical modelling and data inversion are important tools for interpreting the physical properties of Earth's subsurface. Solving the inverse problem involves several computational steps and is ...generally a time consuming task. Artificial neural networks have the potential to speed up large computations. Such networks provide the means to model the relationship between the inputs and outputs without needing to know the physical model of the underlying problem. There are two main aspects that affect the performance of neural networks: optimization of network architecture and pre-processing of data. In this article, we investigate several traditional pre-processing techniques including the min-max scaling, z-score scaling, and the logarithmic transform scaling, and propose some novel data pre-processing approaches for the 1-D forward modelling of time-domain electromagnetic data based on signal characteristics. We evaluate the performance of the conventional and the proposed pre-processing methods against a 3% relative error metric, which corresponds to the typical data uncertainty, to show that forward data pre-processing has significant effect on the performance of neural networks. The proposed gate-wise min-max scaling achieves the best performance with 96% of gates within a 3% relative error, while the commonly used logarithmic transform results only in 75% of gates within a 3% relative error. We provide insights into how various pre-processing methods affect the performance of these networks and recommend optimal pre-processing strategies that may be used where similar data content is encountered to achieve superior performance. Finally, we show the effect of forward modelling accuracy in inverse modelling.
This study presents a novel expansion of the clay-fraction (CF)/resistivity clustering method aiming at developing realizations of subsurface structures based on multiple point statistics (MPS). The ...CF-resistivity clustering method is used to define a data driven training image (TI) for MPS simulations. By combining this TI with uncertainty estimates obtained from correlation between the resistivity models and the unique categories in the TI, subsurface realizations are generated honoring geophysical and lithological data. The generated subsurface realizations were calibrated in a steady state groundwater model. Forecasts of well catchment zones were derived based on two wells located in areas with different levels of structural uncertainty. The catchment probability maps derived from the structural realizations were compared with the well catchment forecasted by a deterministic subsurface structure, and we are able to capture this catchment within the estimated uncertainties. We believe that this study is the first to combine MPS methods with a complete data driven workflow going directly from lithological and geophysical data to realizations of the subsurface structures. The main benefits of this is that it is data driven, fast, reproducible, and transparent.
The fidelity of magnetic resonance sounding signals is often severely degraded by noise, primarily electrical interference from powerline harmonics and short electromagnetic discharges. In many ...circumstances, the noise originates from multiple sources. We show that noise cancelling can be improved if the multiple origins of noise are taken into account. In particular, a method is developed where powerline harmonics are efficiently removed through a model-based approach. Subsequently, standard multichannel Wiener filtering can be used to provide a further noise reduction. The performance of the method depends on the distribution of noise on the particular site of measurement. Simulations on synthetic signals embedded in real noise recordings show that the combined approach can improve the signal-to-noise ratio with an accompanying improvement in retrieval of model parameters.
The transient electromagnetic (TEM) method has been used extensively for hydrogeophysical exploration in Denmark for the past decade. Innovative instrumentation combined with multi-dimensional ...modelling and interpretational insights based on experience gained through numerous case studies have proven to be a successful strategy. In the case study reported here, the combination revealed an unknown and unexpected buried valley complex. Drill hole data were in good agreement with estimates of both the bearings and depths of valleys defined by the TEM surveys. The Pulled Array Transient Electromagnetic (PATEM) system was built to provide high data density for increased lateral resolution. A High moment Transient Electromagnetic (HiTEM) system was developed for delineation of aquifers to depths up to 300 m. Because both of these systems provide high data density, data quality can be assessed as part of the interpretational strategy. When acquiring TEM measurements in areas as densely populated as the Danish countryside, precautions must be taken to minimize coupling between the TEM system and man-made conductors. Modelling the slope of the flanks of buried valleys has challenged the adequacy of the one-dimensional (1-D) assumption for inversion of TEM data. The study shows that for a valley structure in a low-resistive layer, the 1-D assumption is sufficient to track the presence of rather steep slopes. For a valley structure in a high-resistive layer, however, the insensitivity of the TEM method to resistors makes it difficult to determine a slope with a 1-D inversion, and only the overall structure is defined.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
One of the primary shortcomings of the surface nuclear magnetic resonance (NMR) freeinduction decay (FID) measurement is the uncertainty surrounding which mechanism controls the signal’s time ...dependence. Ideally, the FID-estimated relaxation time T$*\atop{2}$ that describes the signal’s decay carries an intimate link to the geometry of the pore space. In this limit the parameter T$*\atop{2}$ is closely linked to a related parameter T2, which is more closely linked to pore-geometry. If T$*\atop{2}$ T2 the FID can provide valuable insight into relative pore-size and can be used to make quantitative permeability estimates. However, given only FID measurements it is difficult to determine whether T$*\atop{2}$ is linked to pore geometry or whether it has been strongly influenced by background magnetic field inhomogeneity. If the link between an observed T$*\atop{2}$ and the underlying T2 could be further constrained the utility of the standard surface NMR FID measurement would be greatly improved. We hypothesize that an approach employing an updated surface NMR forward model that solves the full Bloch equations with appropriately weighted relaxation terms can be used to help constrain the T$*\atop{2}$ –T2 relationship. Weighting the relaxation terms requires estimating the poorly constrained parameters T2 and T1; to deal with this uncertainty we propose to conduct a parameter search involving multiple inversions that employ a suite of forward models each describing a distinct but plausible T$*\atop{2}$ –T2 relationship. We hypothesize that forward models given poor T2 estimates will produce poor data fits when using the complex-inversion, while forward models given reliable T2 estimates will produce satisfactory data fits. By examining the data fits produced by the suite of plausible forward models, the likely T$*\atop{2}$ –T2 can be constrained by identifying the range of T2 estimates that produce reliable data fits. Synthetic and field results are presented to investigate the feasibility of the proposed technique.
The Southern Ocean receives limited liquid surface water input from the Antarctic continent. It has been speculated, however, that significant liquid water may flow from beneath the Antarctic Ice ...Sheet, and that this subglacial flow carries that water along with dissolved nutrients to the coast. The delivery of solutes, particularly limiting nutrients like bioavailable iron, to the Southern Ocean may contribute to ecosystem processes including primary productivity. Using a helicopter-borne time domain electromagnetic survey along the coastal margins of the McMurdo Dry Valleys region of Southern Victoria Land, Antarctica, we detected subsurface connections between inland lakes, aquifers, and subglacial waters. These waters, which appear as electrically conductive anomalies, are saline and may contain high concentrations of biologically important ions, including iron and silica. Local hydraulic gradients may drive these waters to the coast, where we postulate they emerge as submarine groundwater discharge. This high latitude groundwater system, imaged regionally in the McMurdo Dry Valleys, may be representative of a broader system of Antarctic submarine groundwater discharge that fertilizes the Southern Ocean. In total, it has the potential to deliver tens of gigagrams of bioavailable Fe and Si to the coastal zone.
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