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article i nfo In this paper, the noise sources of an airborne electromagnetic frequency domain instrument used to measure sea-ice thickness are studied. The antennas are mounted on the wings of an aircraft. The paper presents real data examples showing that strong noise limited the accuracy of the thickness measurement to ± 0.5 m in the best case. Even drift cor­rection and frequency filtering did not reduce the noise to a level necessary for sea ice thickness measurements with an accuracy of 0.1 m. We show results of 3D finite element modeling of the coupling between transmitter and receiver coils and the aircraft, which indicate that wing flexure is the primary cause of the strong noise. Wing deflection angles below 5° relative to the fuselage are large enough to cause changes higher than the wanted signal from the seawater under the ice. Wing flexure noise can be divided into an inductive and geometric contribution, both of the same order. Most of the wing flexure signal appears on the inphase component only, hence the quadrature component should be taken for sea ice thickness retrievals when wing flexure is present even when the inphase produces a larger ocean sig­nal. Results also show that pitch and roll movements of the aircraft and electromagnetic coupling between seawater and aircraft can contribute significantly to the total noise. For flight heights of 30 m over the ocean these effects can change the sig­nal by about 10% or more. For highly quantitative measurements like sea-ice thickness all these effects must be taken into account. We conclude that a fixed wing electromagnetic instrument for the purpose of measure­ments in a centimeter scale must include instrumentation to measure the relative position of the antenna coils with an accuracy of 1/10 mm. Furthermore the antenna separation distance should be as large as possible in order to increase the measured ratio of secondary to primary magnetic field strength.

We present a large-scale study of the petrophysical relationship of resistivities obtained from densely sampledground-based and airborne transient electromagnetic surveys and lithological information from boreholes. Theoverriding aim of this study is to develop a framework for examining the resistivity-lithology relationship in astatistical manner and apply this framework to gain a better description of the large-scale resistivity structuresof the subsurface.In Denmark very large and extensive datasets are available through the national geophysical and borehole databases,GERDA and JUPITER respectively. In a 10 by 10 km grid, these data are compiled into histograms of resistivityversus lithology. To do this, the geophysical data are interpolated to the position of the boreholes, whichallows for a lithological categorization of the interpolated resistivity values, yielding different histograms for aset of desired lithological categories. By applying the proposed algorithm to all available boreholes and airborneand ground-based transient electromagnetic data we build nation-wide maps of the resistivity-lithology relationshipsin Denmark.The presented Resistivity Atlas reveals varying patterns in the large-scale resistivity-lithology relations, reflectinggeological details such as available source material for tills. The resistivity maps also reveal a clear ambiguity inthe resistivity values for different lithologies. The Resistivity Atlas is highly useful when geophysical data are tobe used for geological or hydrological modeling.

Abstract Influenced by the subduction of the Izanagi plate, the South China Block underwent explosive mineralization in the Mesozoic, and a series of important ore concentration areas and metallogenic belts, such as the Nanling metallogenic belt (NMB) and northeastern Jiangxi metallogenic Province (NJMP), were formed in the Early Jurassic. The NMB is composed of a series of non-ferrous metal deposits and rare earth element deposits. The ore-forming rock source of the NMB is granite that mainly originated crustal partial melting and mantle material rising in the Mesozoic. The NJMP consists of a series of copper, lead, zinc, gold and silver deposits associated with acid volcanic and intrusive rocks. The boundary fault zone between the Yangtze Block and the Jiangnan orogenic belt penetrates into the mantle. As certain mantle materials have participated in the ore-forming diagenesis, both the NMB and NJMP benefited from the crust-mantle mixed sources. To study the deep electrical structures and metallogenic types of the NMB and NJMP, 333 magnetotelluric (MT) stations in the eastern part of South China were employed to obtain a lithospheric 3-D electrical structure model. The results show a low-resistivity anomaly extending from the mantle to the crust beneath the NJMP, which is explained as a magma channel of ore-formed materials. The upper crust of the NJMP has high resistivity, and it has been affected by large regional faults, such as the northeastern Jiangxi fault. The low-resistivity bodies in the mantle and crust of the NMB are connected by small low-resistivity channels, which could be pathways of upwelling asthenosphereic materials. According to the 3-D electrical structure model, the deep upwelling asthenospheric materials beneath the NMB were blocked by the high-resistivity lower crust. The heat and metallogenic elements from the mantle upwelling may have triggered partial melting of the overlying crust and crust-mantle mixing, resulting in the formation of non-ferrous metal deposits such as tungsten, tin and rare earth elements. The low-resistivity anomaly zones of the NMB and the NJMP connected with upwelling low-resistivity deep anomalies may reflect shallow deposits.

Abstract We presented an innovative hydrogeophysical approach that allows numerical modeling of water flow in a variably saturated media. In our model, we approximated the subsurface by horizontally stratified porous media. The model output was a time varying water content profile. Then, we compared the water content provided by the model with the water content measurements carried out using the time-lapse Magnetic Resonance Sounding (MRS) method. Each MRS sounding provided a water content profile in the unsaturated zone down to twenty meters. The time shift between the profiles corresponded to the time lapse between individual MRS soundings. We minimized the discrepancy between the observed and the modeled MRS signals by varying hydraulic parameters of soil layers in the water flow model. For measuring and processing MRS data, we used NUMIS MRS instrument and SAMOVAR software. We carried out water flow modeling with HYDRUS-1D software. This paper reports our results and summarizes the limitations of the MRS method applied to water content measurements in the unsaturated zone.

The SISMAR marine seismic survey acquired over 3600 km of deep seismic reflection profiles on the Atlantic margin of Morocco, one of the oldest on earth. We present here a crustal scale cross section of the Moroccan margin off El Jadida. To penetrate below the salt layer, a nonconventional, low frequency seismic source was used in the >single bubble> mode (4805 in.3), and recorded by Ocean Bottom Instruments (hydrophones and seismometers) and a 4.5-km-long streamer that recorded multichannel seismic (MCS). This profile was also reshot with a conventional reflection seismic source (2369 in.3), in order to evaluate the differences with the single bubble source image. In this study, the single bubble mode source was the best source to image deep structures due to the strong energy produced in low frequency band. In the second part of this study, two velocity models were constructed in order to perform a pre-stack depth migration of the MCS section. The first velocity model was obtained by a depth-focussing error analysis of the MCS data and the second from modelling of the Ocean Botton Seismometer data located along the profile. A comparison of these two velocity models shows 10% higher mean P-wave velocities for the OBS velocity model, probably due to anisotropy. After reducing the OBS velocities by 10%, we show that there are no significant differences between the pre-stack depth migration images obtained by using MCS velocity analysis and the OBS velocity model. Using OBSs data is another way to provide a velocity model to perform pre-stack depth migration of MCS data We thank Dirk Klaeschen for his time and the European Community for financing the pre-stack depth migration performed at the large-scale facility, GEOMAR Seismic Processing Center (Kiel, Germany). [...] Support of GDR “Marges” is acknowledged This paper is part of the SISMAR project.-- 12 pages, 8 figures

During five austral summers, from 1994/95 until 1998/99, the Alfred Wegener Institute (AWI)carried out a large airborne radio echo sounding (RES) survey in Dronning Maud Land (DML),Antarctica. These ice thickness measurements are part of the AWI contribution to the pre-site survey for a deep ice core drill site in DML within the European Project for Ice Coringin Antarctica (EPICA). The survey encompasses more than 90,000 km RES profiles over DML and the adjacent coastal area, covering more than 1 million km^2. The lower boundary ofthe ice sheet could be determined area-wide. Internal horizons occuring in the upper two thirds of the ice column can also be traced for several hundred kilometers. This work presents the latest maps of the subglacial topography of the investigated area as well as of an internal horizon.

Abstract Spatially dense Airborne Electromagnetic Method (AEM) surveys are increasingly acquired to provide a background for creating 3D geological models. These datasets provide a high degree of detail that is very time-consuming and challenging to incorporate in manually-produced geological models. Automated modelling methods are therefore needed to make the modelling process more time-efficient. In this study we evaluate the results of two automated modelling methods against the results of a manually constructed geological model. The study area is characterized by a sequence of Quaternary glacial till deposits on top of Pre-Quaternary sand and clay deposits. The tested automatic methods are 1) an approach for clay fraction modelling, where borehole and AEM resistivity models are integrated through inversion, and 2) a stochastic approach based on transition probability indicator statistics. The most significant difference between the model results is the ability of the manually constructed geological model to contain stratigraphic information, contrary to the sand–clay models generated by the automated approaches. When all the model results are translated to binary models, they are generally in good agreement. The models have different strengths and limitations and the later application of the model should therefore be taken into consideration when choosing the modelling approach.

High-resolution marine seismic data acquisition and subsequent analyses are highly influenced by sea conditions, directly affecting data quality and interpretation. Traditional swell effect correction methods are effective in improving reflector continuity; however, they are less useful for enhancing travel time consistency at intersection points of crossing lines. To develop a robust swell-removal technique for a set of crossing lines multi-beam echo sounder (MBES) data and Chirp sub-bottom profiler (SBP) data were acquired. After generation of a time structure map of the sea-bottom converted from the final processed multi-beam data, a moving average was used to improve the event continuity of the sea-bottom reflection of the Chirp SBP data. Using the position of the Chirp SBP data, the difference between the travel time of the sea-bottom from the smoothed map and the original travel time of the sea-bottom is calculated as a static correction. The static correction method based on the MBES data was compared and verified using three different cases: (i) simple 2D swell effect correction on a line-by-line basis, (ii) comparing the swell corrections at the crossing positions of 2D lines acquired from different dates, and (iii) comparison of ties of intersection points between 2D lines after new swell correction applied. Although a simple 2D swell correction showed great enhancement of reflector continuity, only the full static correction using the newly proposed method using MBES data produced completely corrected reflection events especially at the crossing points of 2D lines.