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Summary Understanding the top 100 m of the near subsurface using geophysical sensors has gained momentum over the last couple of decades but we are also in great need to better understand this region of the subsurface. In recent years, there has been several advancements in new geophysical sensors greatly pushed by new advanced electronic, powerful computers and integrated data processing algorithms. This has enabled us to move from single point 1D measurements to full 3D coverage. Specially EM sensors have become much more applicable for solving real world problems, thanks to the integration of data processing algorithms with instrumentation. One of the most recent and advanced tools that has created big difference in near-surface mapping is the tTEM system. Using the tTEM system one can achieve comprehensive continuous images of the sub surface’s electrical properties — extremely valuable information to map water flow pathways for many different applications. Also new geoelectrical sensors can be efficiently used to monitor the dynamic processes in the subsurface. The potential of TEM sensors can be fully exploited to monitor the groundwater table in water deficit regions. Machine learning algorithms are paving the way to handle the huge amount of data in real time.

Summary Locating safe drinking water sources in remote settings can be a challenging task, especially in regions where one has little to no pre-existing knowledge of local groundwater systems. In these cases, it is common that boreholes are drilled that fail to deliver on desperately needed water resources. Non-invasive geophysical approaches show great promise to address this problem — capable of informing conceptual models of local groundwater systems and helping guide siting of wells. In this work, we demonstrate the utility of a towed transient electromagnetic system for rapid cost-efficient mapping of groundwater systems in remote conditions. Results from two campaigns in Tanzania and South Africa are presented to highlight the value the system can bring to well-siting workflows.

Summary 3D transient electromagnetic (TEM) modelling and inversion has been continuously developed in the last decade, together with the undergoing rapid developments in computational resources. In this work we present an efficient and practical finite element 3D implementation, applicable to both airborne and ground-based TEM, developed with an octree-based hexahedral meshing that implements topography through a mesh deformation. This new implementation is compared to both 1D modelling and 3D finite-element tetrahedral modelling, in terms of accuracy, speed and memory requirement and inversion results on synthetic modelling. Furthermore, a verification of the topography implementation obtained comparing results with and without deformation is presented. Overall, the octree-based implementation is ~5x faster than tetrahedral simulation, while the memory cost in the forward modelling process is ~6x smaller.

Envelope detection is an integral step of the surface NMR data processing workflow to estimate these parameters. We present a new method for retrieving NMR signal envelopes using spectral analysis and the subsequent inversion scheme with the new data space. By exploiting the fact that the spectral magnitude at the Larmor frequency is proportional to the product of the initial amplitude and relaxation time, a high-SNR complex envelope can be extracted by Fourier transform for a number of sliding windows. However, SA estimated envelopes are weighted by the NMR relaxation time during processing and the envelopes have units of volt-seconds. We propose to modify the surface NMR forward model such that it predicts data directly in the voltage-time data space. Field data inversions are presented to demonstrate advantages of pairing the SA envelope detection scheme with a forward model that works in the voltage-time data space.

Summary We present new developments of the versatile towed transient electromagnetic system (tTEM). The system now meets the demand of near surface mapping down to 100m and is suitable for application not only ground-based but also in aquatic (rivers, lakes etc.) and snow environments. The receiver technology of the system has been upgraded and it has a 4 times lower noise level compared to previous system, increasing the depth of investigation down to 100 m. The latter two system are called FloaTEM and SnowTEM, respectively. FloaTEM surveys were conducted in several parts of United States including the Mississippi delta region for mapping and characterizing the alluvial aquifer system below the river beds. The results show that the FloaTEM system produces detailed subsurface resistivity images in quick time compared to other geophysical methods. We conducted over 300 km of survey along several rivers within a week in the Mississippi delta region. The SnowTEM system uses a 4 turn, 4 m x 4 m transmitter coil and a specially designed low noise receiver coil. It has been put on skies and is towed behind a snow scooter or a snow robot.

Summary This paper aims at providing more insight into the parameters that need to be modeled during inversion of Helicopter TEM data for accurate modeling in oil and gas exploration applications. We use 2 nominal oil and gas models and synthetic forward data to show in details the effect, both in data and in model space, of the low pass filters, which are present in any system, the waveform repetition, the gate integration, and the current turn on ramp. Results show how low pass filters, gate integration, turn on ramp and waveform repetition are parameters, like frame altitude, Tx Rx timing, ramp turn off and so on, that need to be taken into account and modeled correctly during inversion of HTEM data, in order to recover an accurate geoelectrical model. The shallow to intermediate parts of the model are heavily affected by the filters (and possible gate integration), the deeper parts by gate integration, shape of turn on ramp, and number of waveforms.

The Okavango Delta, Botswana, is one of the largest inland deltas on the planet and is unique in that surface water remains remarkably fresh despite evapotranspiration being the dominant water removal mechanism. To help understand the phenomena of salt sequestration, airborne transient electromagnetic TEM data with 2 km line spacing were acquired over the entire delta in 2007-2008. These data have been inverted using a quasi-2D least-squares linear optimization scheme to produce a 5-layer resistivity model to 250 m depth. Comparison of the model with available borehole data shows a strong correlation between resistivity and lithology and salinity. Further analysis should contribute significantly to understanding the hydrogeology of the delta.

Formålet med denne abstrakte er at give en kort beskrivelse af de væsentligste ideer i den danske nationale strategi for omfattende kortlægning af grundvandet resources.Emphasis vil blive lagt på en beskrivelse af de fordele ved at kombinere Erhvervelse af rumligt tætte geofysiske data, der dækker store områder med oplysninger fra et optimalt antal nye undersøgelse boringer, eksisterende boringer, logbøger og vandprøver for at få en integreret og detaljeret beskrivelse af grundvandsressourcerne og deres vulnerability.Development mere tid effektiv og luftbårne geofysiske data erhvervelse platforme (f.eks SkyTEM) har foretaget omfattende kortlægning af attraktive og økonomisk overkommelige i planlægning og forvaltning af grundvandsressourcer. Håndtering og optimeret anvendelse af enorme mængder af geofysiske data, som dækker store områder har også krævet en omfattende database, hvor data kan nemt gemmes, dokumenteres, udvundet, fortolket, rekombineret og genbruges en gang efter den anden. The aim of this abstract is to give a short description of the essential ideas of the Danish national strategy for large scale mapping of the groundwater resources.Emphasis will be put on a description of the advantages obtained by combining acquirement of spatially dense geophysical data covering large areas with information from an optimum number of new investigation boreholes, existing boreholes, logs and water samples to get an integrated and detailed description of the groundwater resources and their vulnerability.Development of more time efficient and airborne geophysical data acquisition platforms (e.g. SkyTEM) have made large-scale mapping attractive and affordable in the planning and administration of groundwater resources. The handling and optimized use of huge amounts of geophysical data covering large areas has also required a comprehensive database, where data can easily be stored, documented, extracted, interpreted, recombined and reused one time after the other.

We describe the challenges for the realization of a transient EM (TEM) system capable of measuring both the z- and x-components. This involves significant changes to instrumentation as well as to the processing and inversion algorithms. Geophysical mapping of groundwater resources using TEM involves both delineation of the aquifer structure and the aquifer protection. The demands for the instrumentation and the processing algorithms are quite different for the two tasks. Structural mapping requires large depth penetration and thereby a large transmitter moment while aquifer protection mapping requires early time measurements and extremely accurate determination of the system transfer function. Furthermore, the inversion software must be capable of modelling this system transfer function. For an airborne TEM system systematic analysis of the resolution capabilities of relatively thin low resistive layers in the upper 30 - 50 m of the subsurface has shown that the resolution is enhanced if not only the transient z-component but also the transient x-component of the induced fields in the Earth is measured and included in the inversion. The abstract will describe and discuss three major challenges related to the inclusion of the x-component in an airborne TEM system.

Resistivity imaging in combination with borehole information is a powerful tool for site investigation. We show that the combination of 1D laterally constrained inversion (1D-LCI) with the use of a priori information from borehole data and 2D smooth inversion adds significant value to the interpretation of continuous vertical electrical sounding (CVES) data. The ID-LCI offers an analysis of the resolution of the model parameters. This is helpful when evaluating the integrity of the model. Furthermore, with the 1D-LCI it is possible to constrain model parameters with a priori information, e.g. depth-to-layer interfaces, based on borehole information. We show that 2D smooth inversion resolves lateral changes well, while 1D-LCI results in well-defined horizontal layer interfaces. In geological environments where the lateral variations are not too pronounced, the 1D-LCI contributes to a geological interpretation of the resistivity measurements. Depths to layers can be interpreted with greater certainty than if using results from 2D smooth inversion only. The inclusion of a priori information in the inversion reveals further details and enhances the geological interpretation significantly.

A035 INTERPRETATION OF A HYDROGEOPHYSICAL SURVEY – DATA FROM THE HIGH- RESOLUTION SKYTEM SYSTEM Abstract 1 During the last decade approximately 50 000 ground-based TEM soundings have been carried out in Denmark for groundwater investigations. This number will increase in the future and therefore we have developed a new time-domain electromagnetic (TEM) helicopter system SkyTEM. In the development and design of the SkyTEM system it has been an unchangeable demand that the data quality of the SkyTEM system should be the same or better than the data quality of ground-based systems e.g. the Protem 47 system (40 x 40 m

A033 INTEGRATED INVERSION OF CVES AND TEM DATA USING LATERAL CONSTRAINTS Summary 1 We present a mutually and laterally constrained inversion between transient electromagnetic (TEM) and geoelectric (DC) data sets. Although both methods measure in some sense the electrical resistivity or conductivity of the subsurface they sample different volumes and have different sensitivities. The different sampling volumes and different sensitivities are exploited by the mutually and laterally constrained inversion algorithm combining the coinciding profile data sets. The output model incorporates the information from both profile data sets to obtain the optimum layered 1D models fitting both data types. All data

2D laterally constrained inversion (2D-LCI) is a new approach to interpretation of continuous resistivity data. The 2D-LCI enables a layered 2D inversion with sharp layer boundaries as compared to traditional smooth inversion types. However, the full 2D solution is quite time consuming when calculating the derivatives in the Jacobian matrix. To reduce the computation times we suggest using 1D derivatives, Broyden's update formula (Quasi- Newton) or combinations of Broyden's update formula and 1D derivatives together with the full 2D solution. The reduction in computation time is between 7 and 30 times dependent on the combination of approximations.

In this paper we present how a 2D least squares algorithm, Res2Dinv (Loke and Dahlin, 2002), and the 2D Lateral Constrained Inversion (2D-LCI) algorithm (Christiansen et al., 2003) can be applied in geotechnical site investigations. The 2D-LCI algorithm performs a parameterized 2D Laterally Constrained Inversion (Auken and Christiansen, 2003) on resistivity data.

In Denmark, 99% of the water supply is based on groundwater abstraction. Increasing problems with the groundwater quality, due to urban growth and agricultural contamination, have caused the Danish Minister of Environment to approve a plan for improving ground water protection.

In time domain electromagnetic, TDEM, it is a wide spread practice to model the current waveform (e.g. Asten, 1987), where as the effect of low pass filters (LP) in the receiver system is neglected. In this paper we show that when the TDEM method is used in hydrogeological and environmental investigations it is mandatory to include the filters in the forward and inverse modeling scheme to obtain avalid geological model for the survey site. IDEM systems is normally band-limited as input signals from amplitude modulated radio transmitters operating in the target bandwidth of the system often dominate the noise characteristic of the system over several decades in time. Band-limiting the input signals significantly improves the signal-to-noise ratio and thereby the depth of exploration. However, the LP filters distort the early and medium part of the decay curve due to damping of the high frequency content. The level of distortion depends on several factors such as cut-off frequency and subsurface resistivities. Therefore, the LP filters must be incorporated in the inversion scheme such that the calculated forward response is subject to the same filtering as the measured response. In this paper we (1) quantify the distortion of responses due to band-limitation for a number of typical resistivity models. The band-limitations are based on cut-off frequencies typically used in commercial instruments. (2) We demonstrate that distortion due to band-limitation can be modeled in the inversion scheme by including the LP filters in the forward response. This approach is tested on theoretical data.