We present a multi-resolution approach for 3-D magnetotelluric forward modelling. Our approach is motivated by the fact that fine-grid resolution is typically required at shallow levels to adequately represent near surface inhomogeneities, topography and bathymetry, while a much coarser grid may be adequate at depth where the diffusively propagating electromagnetic fields are much smoother. With a conventional structured finite difference grid, the fine discretization required to adequately represent rapid variations near the surface is continued to all depths, resulting in higher computational costs. Increasing the computational efficiency of the forward modelling is especially important for solving regularized inversion problems. We implement a multi-resolution finite difference scheme that allows us to decrease the horizontal grid resolution with depth, as is done with vertical discretization. In our implementation, the multi-resolution grid is represented as a vertical stack of subgrids, with each subgrid being a standard Cartesian tensor product staggered grid. Thus, our approach is similar to the octree discretization previously used for electromagnetic modelling, but simpler in that we allow refinement only with depth. The major difficulty arose in deriving the forward modelling operators on interfaces between adjacent subgrids. We considered three ways of handling the interface layers and suggest a preferable one, which results in similar accuracy as the staggered grid solution, while retaining the symmetry of coefficient matrix. A comparison between multi-resolution and staggered solvers for various models shows that multi-resolution approach improves on computational efficiency without compromising the accuracy of the solution.

Summary The DESMEX Project aims to develop semi-airborne systems for mineral exploration down to depths of about 1 km. The main system components have been tested and improved over last two years. After several preliminary test surveys, the main experiment was conducted in October 2017. Different layouts for the ground based survey set up were tested and evaluated. Large parts of the acquired data have been processed and first results are ready for display. Parallel to these works, petrographic studies have been performed in order to be able to complement the geophysical and geological data needed for a combined 3D-model of the main DESMEX survey region.

s Session 1 SESSIONS DESCRIPTION Session 1. Instrumentation, sources and data processing This session solicits contributions on EM data processing, source field analysis, instrumentation, and field practice. We welcome contributions on all aspects of data processing, including theory and practical applications. Development and application of time lapse observations (monitoring) of transfer functions and other parameters as well as new approaches to estimation of transfer functions and their uncertainty can be presented here. We also solicit studies on source fields, whether controlled or uncontrolled. This includes characterization of external source spatial structure and temporal variability, novel (distributed) transmitter configurations, and effects of finite spatial-scale sources (natural and anthropogenic) on transfer functions. Contributions on new instrumentation and field practice and related methodological developments are also appropriate for this session. Studies of sensor fidelity either by noise level analysis or by field comparison with other sensors are appropriate. Sensors can include ohmic or capacitively coupled electrodes, magnetometers, and any other instruments whose inter-comparison with EM data demonstrates new insights. Conveners: Pierre Wawrzyniak, Karl Kappler, Maxim Smirnov Abstract, 24th EM Induction Workshop, Helsingør, Denmark, August 12-19, 2018 24th EM Induction Workshop, Helsingør, Denmark, August 12-19, 2018 1 / 1 A Compact Ocean Bottom ElectroMagnetic Field Receiver& Seismometer Kai Chen1, Li Zhou2, Ming Deng3, Zhongliang Wu4and Xianhu Luo5 1 China University of Geosciences (Beijing), ck@cugb.edu.cn 2 China University of Geosciences (Beijing), zhouli100083@163.com 3 China University of Geosciences (Beijing), dengming@cugb.edu.cn 4 Guangzhou Marine Geological Survey, 52936891@qq.com 5 Guangzhou Marine Geological Survey, Luoxh@163.com

Summary We present a new semi-airborne frequency domain electromagnetic system being developed within the DESMEX project. In the system, the high-moment transmitter is positioned on the ground and the receivers (induction coil and fluxgate magnetometers) are installed in a helicopter-towed bird. The major difficulty is to overcome the problem of motion noise and motion-induced voltages, due to the pendulum-like behaviour of the bird. For this purpose, we developed a processing scheme which corrects data for motion related noise. Specifically, for processing in frequency domain we utilize only free-of-motion-noise frequencies. The initial design of the system was developed and tested in several flight campaigns. In the current paper, we present a first 3D inversion of the data acquired during the experiment in Schleiz, Germany. In the model the conductive anomalies which we interpret as alum shales, are embedded within generally resistive Cambrian basement. A comparison with the 2D electrical resistivity tomography model shows that our model represents the same resistivity structures. The experiment also proved that our system allows us to cover an area of around 36 square km during one flight (3 hours) resulting in penetration depth of 1–1.5km.

Summary A novel semi-airborne frequency domain system is developed within the DESMEX project. The system is designed to achieve a penetration depth of about 1km with an areal coverage of about 6×6km2 per source installation on the ground. Here, we report on first results achieved with the new system. Horizontal electric dipole transmitters were used to inject time-varying electrical currents into the ground, and both a three-axis induction coil set up (Metronix MFS-11e) and a fluxgate (Bartington MAG-03) were installed on a helicopter-towed platform. We measure the induced magnetic field in the air in a frequency range from 1Hz to 10kHz. Additionally recorded attitude data can be used for correction of motion noise and for in-flight calibration to determine orthogonality and scaling errors in the fluxgate data as well as orientation errors of all sensors within the bird. In a first flight test the corrected fluxgate data shows a good signal-to-noise-ratio in the range 10ߝ400Hz with a noise level generally below 50pT/√Hz. The coils yield superior noise levels of less than 1pT/√Hz at frequencies higher than 400Hz. Frequency-domain response functions between the magnetic field and the source current are found to be consistent with subsurface conductivity structures.

Summary We present a new multi-resolution (MR) approach to 3D electromagnetic (EM) modelling. Initially, the approach was developed for plain wave assumption. Current work is a further extension of the MR solver for controlled-source applications. Modelling with controlled-source requires the finer grid discretization in the near source location to accurately model the rapidly varying EM fields in the vicinity of the source. On the other hand, the EM field propagates in a diffusive manner and can be sufficiently well described on a grid that becomes gradually coarser with depth. We implement a MR finite-difference scheme that allows us to decrease the horizontal grid resolution with depth. MR approach, therefore, provides a means to significantly decrease the number of degrees of freedom and hence improve on computational efficiency without significantly compromising the accuracy of the solution. In MR implementation, the full grid is represented as a vertical stack of sub-grids, each of which is a standard staggered grid. Therefore, operators and functions already developed for a simpler standard structured grid can be applied for each sub-grid. The major difficulty lies in discretizing the operators around interfaces between varying cell sizes.

New magnetotelluric (MT) data in north-west Fennoscandia were acquired within the framework of the project "Magnetotellurics in the Scandes" (MaSca). The project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia. An array of 59 synchronous long period and 220 broad-band MT sites was occupied in the summers of 2011 to 2013. We estimated MT transfer functions in the period range from 0.003 to 10(5) s.The Q-function multi-site multi-frequency analysis and the phase tensor were used to estimate strike and dimensionality of MT data. Dimensionality and strike analyses indicate generally 2-D behaviour of the data with 3-D effects at some sites and period bands. In this paper we present 2-D inversion of the data, 3-D inversion models are shown in the parallel paper. We choose to invert the determinant of the impedance tensor to mitigate 3-D effects in the data on our 2-D models. Seven crustal-scale and four lithospheric-scale 2-D models are presented. The resistive regions are images of the Archaean and Proterozoic basement in the east and thin Caledonian nappes in the west. The middle and lower crust of the Svecofennian province is conductive. The southern end of the Kittila Greenstone Belt is seen in the models as a strong upper to middle crustal conductor. In the Caledonides, the highly conductive alum shales are observed along the Caledonian Thrust Front. The thickest lithosphere is in the Palaeoproterozioc Svecofennian Domain, not in the Archaean. The thickness of the lithosphere is around 200 km in the north and 300 km in the south-west. (c) 2015 Elsevier B.V. All rights reserved.

New magnetotelluric (MT) data in north-west Fennoscandia were acquired within the framework of the project "Magnetotellurics in the Scandes" (MaSca). The project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia. An array of 59 simultaneous long period and 220 broad-band MT sites were occupied in the summers of 2011 to 2013.The 3-D inversion of the MaSca data was obtained using the ModEM 3-D code. The full impedance and tipper data were used for the inversion. The rocks of Archaean and Proterozoic basement towards east and the Caledonian nappes towards west are modelled as resistive structures. In the central and southern parts, the whole crust is resistive and reflects the Trans-Scandinavian Igneous Belt granitoids. The middle to lower crust of the Svecofennian province is conductive. An uppermost crustal conductor is revealed in the Skelleftea Ore District. The south end of the Kittila Greenstone Belt is seen in the models as a strong upper to middle crustal conductor. In the Caledonides, the highly conductive alum shales are observed along the Caledonian Thrust Front. A map of the crustal conductance for the north-west Fennoscandian Shield is presented. (c) 2015 Elsevier B.V. All rights reserved.

We use data from two magnetotelluric profiles, ToSca10 and ToSca'09, over the Scandinavian Mountains to study the crustal structure in southern Norway. The profiles cross the major tectonic structures of the Caledonian orogen as well as the western margin of the Precambrian Baltica. Dimensionality and strike analyses indicate generally 3-D behavior of the data. However, the majority of the used data distinguishes a preferable strike direction, which is supported by the geology of the region. Hence, we employ 2-D inversion and choose to invert the determinant of the impedance tensor to mitigate 3-D effects in the data on our 2-D models.Magnetotelluric data from both profiles are inverted using a damped least squares solution based on a singular value decomposition. We improved the solution by defining the inverse model covariance matrix through gradient or Laplacian smoothing operators.The two-dimensional inversion models of the ToSca'09 and ToSca'10 field data from southern Norway derived from the damped least squares scheme with the Laplacian inverse model covariance matrix are presented. Resistive rocks, extending to the surface, image the autochthonous Southwest Scandinavian Domain and the allochthonous Western Gneiss Region. Near-surface conductors, which are located between the resistive Caledonian nappes and Precambrian basement, delineate highly conductive shallow-sea sediments, so called alum shales. They exhibit a decollement along which the Caledonian nappes were overthrust. A deeper, upper to midcrustal conducting layer in the Southwest Scandinavian Domain may depict the remnants of closed ocean basins formed during the accretions and collisions of various Sveconorwegian terranes. In ToSca'10, the Caledonian nappes, the conducting alum shales and the deeper conductor are terminated in the west by the Faltungsgraben shear complex which represents a crustal scale boundary between the Western Gneiss Region in the west and the Southwest Scandinavian Domain in the east. (C) 2014 Elsevier B.V. All rights reserved.