A high-resolution aeromagnetic field test in Friuli: towards developing remote location of buried ferro-metallic bodies

High Resolution AeroMagnetic surveys (HRAM) are a novel tool experimented in several countries for volcano and earthquake hazard re-assessment, ground water exploration and mitigation, hazardous waste site characterization and accurate location of buried ferrous objects (drums, UXO, pipelines). The improvements achieved by HRAM stem from lower terrain clearance coupled with accurately positioned, real-time differential navigation on closely spaced flight grids. In field cultural noise filtering, advanced data processing, imaging and improved interpretation techniques enhance data information content. Development of HRAM approaches might also contribute to mitigate environmental hazards present throughout the Italian territory. Hence an HRAM field test was performed in July 2000 in Friuli, North-Eastern Italy to assess the capabilities and limitations of HRAM over a buried pipeline and a domestic waste site. A Cesium magnetometer in towed bird configuration was used on two separate grids. Profile line spacing was 50-100 m and bird nominal ground clearance was set to 50 m. Microlevelled total field magnetic anomaly data forms the basis for subsequent advanced processing products including 3D analytic signal, maximum horizontal gradient of pseudo-gravity and 3D Euler Deconvolution. The magnetic signatures we detected and enhanced over the environmental test site area in Friuli are also compared with similar but more extensive HRAM signatures recently observed in other countries

Enhanced images and new models of the Wilkes Subglacial Basin help constrain the variability in geological boundary conditions for the East Antarctic Ice Sheet

The Wilkes Subglacial Basin (WSB) is a huge tectonic feature formed by Cenozoic lithospheric flexure coupled with Mesozoic to Cenozoic extension localised in sub-basins (Paxman et al., 2019, JGR). The deep northern WSB underlies the catchments of the Matusevich, Cook, Ninnis and Mertz glaciers that are largely marine-based, which renders them more vulnerable to past and predicted future ocean and climate warming. Here we present airborne radar and enhanced magnetic and gravity views of the northern WSB that help unveil the spatial variability in geological boundary conditions for this key sector of the East Antarctic Ice Sheet (EAIS). Residual gravity anomalies obtained by stripping out Moho effects were compared with aeromagnetic anomaly images to glean new perspectives into intra-crustal features. Depth to magnetic and gravity source estimates were then used to help derive the first combined 2D forward models for the region. We first examine a model crossing the northern WSB extending from the Matusevich Glacier to the deep Cook Basins. The model reveals a major crustal boundary along the eastern margin of the WSB interpreted as separating the Ross Orogen from a composite Precambrian Wilkes Terrane buried beneath Devonian to Jurassic sediments and early Cambrian metasediments. By analogy with the better understood Rennick Graben in northern Victoria Land, the Cook basins are interpreted as glacially over deepened grabens. The Cook basins clearly play a major role in EAIS dynamics, as they steer fast glacial flow deep into the interior of East Antarctica where they connect to the Central Basins. Our new model across these basins shows that the inferred Precambrian basement is both shallower and of more felsic bulk composition compared to the Cook basins. This fundamental difference in basement depth, bulk composition and thickness of sedimentary cover is likely to exert major influences on geothermal heat variability in this key sector of the EAIS

Geophysical imaging unveils the largest pull-apart basin in East Antarctica

West Antarctica hosts one of the largest continental rift systems on Earth, the West Antarctic Rift System (WARS) that forms the lithospheric cradle for the West Antarctic Ice Sheet. The WARS is known to have experienced several stages of extension starting with distributed/wide mode extension in the Cretaceous, followed by narrower mode and variably oblique extension in the Cenozoic, the latter potentially triggered by the onset of oceanic seafloor spreading in the Adare Basin (Davey et al., 2016, GRL). However, the extent and impact of Cenozoic extension and transtension within the Transantarctic Mountains sector of East Antarctica is much less well understood. Here we present results from a new project (REGGAE) that by analysing aeromagnetic, aerogravity and land-gravity and bedrock topography images and models provides key new geophysical constraints on the form, extent and kinematics of the largest Cenozoic pull-apart basin recognised so far in East Antarctica, the Rennick Graben (RG). Potential field imaging reveals the extent of part of a Jurassic tholeiitic Large Igneous Province preserved within the RG and helps delineate the inherited structural architecture of the underlying Ross-age basement in northern Victoria Land, including highly magnetic arc basement in the northern Wilson Terrane and the subglacial extent of a thrust fault belt located between the western flank of the RG and the eastern margin of Wilkes Subglacial Basin (WSB). We show that the RG is a major composite right-lateral pull-part basin that extends from the Oates Coast to the Southern Cross Mountains crustal block and propose that it is kinematically connected with both the western edge of the WARS and the eastern margin of the WSB. More cryptic evidence for an earlier phase of left-lateral strike slip deformation is also emerging from our recent geological field work in the study region and relatively subtle offsets in aeromagnetic anomaly patterns. Our findings suggest that the RG is part of a distributed region of the continental lithosphere in East Antarctica that was preferentially deformed in response to Cenozoic transtensional stresses that likely also facilitated propagation of accelerated oceanic transform faulting in the adjacent oceanic lithosphere located between southeastern Australia and Tasmania

Geophysical imaging of the Luhoi geothermal field, Tanzania

The Luhoi prospect is a coastal basin located within Rufiji Trough along the passive continental margin of western Indian Ocean of Tanzania, a sector extending south of the termination of the eastern branch of the African Rift System. The structural pattern is dominated by tectonic features belonging to the WNW-ESE Tagalala Trend and to the NE-SW Selous Trend that have been active until recent times. The thermal manifestations are mostly located along a WNW-ESE direction flowingfollowing the Ruhoi River, in the south-western sector of the focal study area. The Wingoyongo Hill, located in the north-eastern sector of the focal area, forms a morphological high where emissions of H2S and bituminous staining were observed. Here, an old well intersected 800 m of quartz sandstone with minor intercalations of siltstone and claystone (Kipatimu Series, Lower Cretaceous). Magnetotelluric (MT), time-domain electromagnetic (TDEM) and gravimetric geophysical surveys were carried as part of a geoscientific study funded by the Ministry of Foreign Affairs of Iceland through Icelandic International Development Agency (ICEIDA) and the Nordic Development Fund (NDF). The goal of the survey was to advance the state of knowledge of the Luhoi geothermal prospect, in order to elaborate the conceptual model of the field. The survey area extends over a surface of approximately 75 km2, designed to include the Luhoi hot springs and the Wingoyongo fumaroles. The gravimetric data set is composed of 124 measurements collected on a regular grid at a nominal spatial sampling of 800 m, and 16 more to infer the regional field. The residual Bouguer anomaly map shows an elongated gravimetric high trending NE-SW with values up to 3 mGal, surrounded by gravimetric lows up to -2 mGal. Forward and inverse 2D/3D models image an asymmetrical horst like structure trending NE-SW. Both the thermal manifestations and the Wingoyongo Hill are aligned along the NW flank of the horst. Since the Wingoyongo well intersected sandstones for about 800 m, the horst like structure is interpreted as made by the Kipatimu sandstones. The two depressions bordering the horst like structure are filled with lower density materials, likely siltstones, claystones and/or mudstones, with an estimated maximum thickness of 1.1 km. The MT and TDEM data were acquired at 76 locations, with a nominal spatial sampling of 750 m. The static shift effect has been corrected by TDEM/MT phases joint inversion. MT impedances and tippers have been estimated by means of the remote reference technique with robust processing methods coupled with a coherence rejection scheme. Resistivity 3D inversion reveals two conductive anomalies coincident with the low-density sedimentary rocks bordering the horst structure. A clear updoming of resistivity marks the NW flank of the horst and it is interpreted as due to a combined effect of different alteration, lithology and fluid content and to reflect the main upflow of the geothermal system

Magnetic Base Station Deceptions, a magnetovariational analysis along the Ligurian Sea coast, Italy

Reliability of high resolution airborne and shipborne magnetic surveys depends on accurate removal of temporal variations from the recorded total magnetic field intensity data. At mid latitudes, one or a few base stations are typically located within or near the survey area and are used to monitor and remove time dependent variations. These are usually assumed to be of external origin and uniform throughout the survey area. Here we investigate the influence on the magnetic base station correction of the time varying magnetic field variations generated by internal telluric currents flowing in anomalous regional 2D/3D conductivity structures. The study is based on the statistical analysis of a data set collected by four magnetovariational stations installed in northwestern Italy. The variometer stations were evenly placed with a spacing of about 60 km along a profile roughly parallel to the coastline. They recorded the geomagnetic field from the beginning to the end of April 2005, with a sampling rate of 0.33 Hz. Cross-correlation and coherence analysis applied to a subset of 125 five hours long magnetic events indicates that, for periods longer than 400 s, there is an high correlation between the horizontal magnetic field components at the different stations. This indicates spatial uniformity of the source field and of the induced currents in the 1D Earth. Additionally, the pattern of the induction arrows, estimated from single site transfer functions, reveals a clear electromagnetic signature of the Sestri-Voltaggio line, interpreted as a major regional tectonic boundary. Induced telluric currents flowing through this 2D/3D electrical conductivity discontinuity affect mainly the vertical magnetic component at the closer locations. By comparing this component at near (32 km) and far (70 km) stations, we have found that the mean value of the power spectra ratio, due to the electromagnetic induced field, is about 1.8 in the frequency band ranging from 2.5×10?3 to 5.5×10?5 Hz. This energy, folded in the spatial domain of an hypothetical survey in this region produces unwanted noise in the dataset. Considering a fifth of nyquist frequency the optimal tie-line spacing to assure complete noise removal would be 1 km and 15 km for a rover speed of 6 knots (marine magnetic survey) and 100 knots (aeromagnetic survey) respectively. Similar power spectra analysis can be applied elsewhere to optimise tie-line spacing for levelling and filtering parameters utlilised for microlevelling

Three-dimensional geophysical modelling of Kiejo-Mbaka geothermal field, Tanzania

The Kiejo-Mbaka geothermal field is located close to the eastern margin of the Karonga Rift Basin and is part of the Rungwe volcanic province where the EARS splits up into its Western and Eastern branches in southern Tanzania. The area is characterised by a Precambrian gneiss metamorphic basement complex, outcropping along the NW-trending, SW-dipping Mbaka fault. Geothermal manifestations mainly consist of hot springs, flowing close to the Mbaka fault. An integrated geophysical survey was carried out over the Kiejo-Mbaka geothermal field by TGDC (Tanzania Geothermal Development Company), under the supervision of ELC-Electroconsult (Italy). The campaign included 76 Magnetotelluric (MT) and Transient Electromagnetic (TEM) soundings and 133 gravity measurements; a dense station grid allowed for a detailed geophysical 2D and 3D modelling. Two and 3D gravity modelling indicate that the positive residual Bouguer anomaly can be explained by a high density (3 g/cm3) body, constituting the gneiss basement, elongating NW-SE. NE and SW of it, lower density layers (2.5 g/cm3) are observable; the attitude of their bottoms is compatible with the Mbaka fault direction and the Livingstone fault trend (NNW). We found that 3D MT inversion was the only tool giving a reliable resistivity imaging in the Mbaka prospect. From the final 3D MT model, a very resistive body (>2000 Ohm m) deepening toward SE is visible; this body represent the gneiss basement, and the surfaces delimiting it are associated with the Mbaka fault and the Livingstone fault trend. Three conductive zones (less than 10 Ohm m) have been identified: two of them affect the Mbaka fault footwall, NE of the resistive basement, while another one is located beneath the plain, SE of it. This latter zone shows a thickness of about 1 km. It is apparent that the low-density regions well correspond with the high-conductivity zones imaged by the MT 3D inversion. The integrated geophysical interpretation then leads to two possible geological scenarios: these regions can be constituted by (post-rift) sediments (possibly affected by low-T geothermal alteration) or by intensively fractured and low-T altered basement; however, we stress that the possible geothermal alteration is not necessarily related to the present-day geothermal activity, and caution should be taken in result interpretation

Interpretation of Thermal Response Tests in Borehole Heat Exchangers Affected by Advection

ABSTRACT We focus on the treatment of thermal response test data when both advection and short-period changes of surface temperature occur. We used a moving line source model to simulate temperature-time signals under an advective thermal regime. The subsurface thermal conductivity, the Darcy velocity and the borehole thermal resistance were inferred by means of an optimisation procedure. In case of Darcy velocity lower than 10 -7 m s -1 , the underground thermal conductivity is comparable to that obtained by means of the infinite line source model, which assumes a purely conductive thermal regime. The optimisation analysis was finally applied to real thermal response test data. The temperature-time curves were filtered to remove the disturbing spectral components associated with a non-optimal thermostatic behaviour of the apparatus. This produced reliable estimates of thermal and hydraulic parameters. An independent method based on the analysis of temperature-depth logs was also used to validate the inferred groundwater flow. INTRODUCTION The thermal power that can be extracted with borehole heat exchangers (BHE) depends mainly on the thermal properties of the underground, and in particular, on thermal conductivity. Laboratory measurements of thermo-physical properties can be unfeasible, as core samples are often unavailable in boreholes. Thus, in-situ tests are routinely used to determine subsurface and borehole thermal properties. Tests record the underground temperature variation with time due to a constant heat that is injected (or extracted) by means of a carrier fluid into a borehole acting as a heat exchanger. The most commonly used model to analyse temperature-time curves obtained from these tests is the infinite line source (ILS). If some conditions are fulfilled, this model can give rapid and appropriate estimations of thermal parameters. On the other hand, several flaws can often affect the data interpretation. Some of them are related to the model assumptions, which imply a purely conductive heat transfer regime, a homogeneous medium, no vertical heat-flow and infinite length of the borehole. Others have to do with the difficulty in the proper thermal insulation of the test equipment, and consequently with the oscillations of the carrier fluid temperature due to surface air temperature changes, which generally produce a periodic offset in the recorded temperature-time curves. In this paper, we discuss the treatment of the thermal response test (TRT) data when both advection caused by groundwater flow and periodic changes of air surface temperature occur. An approach based on the moving line source (MLS) model is tested with simulations of temperaturetime signals obtained under different hypothesis of thermal and hydraulic conditions. Then, the same procedure is applied to real TRT data to estimate thermal conductivity, borehole thermal resistance and Darcy velocity. The magnitude of the inferred groundwater flow is finally checked by means of an independent method based on the analysis of the undisturbed temperaturedepth records

Focused Inversion of Gravimetric and Magnetotelluric Data for Geothermal Investigations

Focused inversion techniques may be applied to geophysical data inversion in order to image complex structures in the subsoil. These algorithms may image complex \u201cblocky\u201d structures giving useful information in geothermal exploration that may be smoothed out by standard inversion algorithms that use stabilizer that penalize sharp transitions. We have tested the modified total variation and the maximum gradient support stabilizers in the inversion of synthetic and field magnetotelluric and gravimetric data. The gravimetric data from the Luhoi geothermal prospect have been used to map the sharp density transition between the sandstone and the overlying claystone layers. The resulting horst structure imaged in 2D and 3D models by the maximum gradient support stabilizer solution allow to trace the main fault system that drives the up-flow of hydrothermal waters. The 1D magnetotelluric \u201cblocky\u201d models with lateral constrain (pseudo-3D) image the lithological contact between the claystone and sandstone far from the horst area and reveal resistivity variations in the claystone layer associated with sand lenses. In the horst area, resistivity models image hydrothermal alteration affecting the sandstone layer

Integrated geophysical imaging of the Aluto-Langano geothermal field (Ethiopia)

The Aluto-Langano geothermal system is located in the central part of the Main Ethiopian Rift, one of the world\u2019s most tectonically active areas, where continental rifting has been occurring since several Ma and has yielded widespread volcanism and enhanced geothermal gradient. The geothermal system is associated to the Mt Aluto Volcanic Complex, located along the eastern margin of the rift and related to the Wonji Fault Belt, constituted by Quaternary NNE-SSW en-echelon faults. These structures are younger than the NE-SW border faults of the central Main Ethiopian Rift and were originated by a stress field oblique to the rift direction. This peculiar tectonism yielded local intense rock fracturing that may favour the development of geothermal reservoirs. In this paper, we present the results of an integrated geophysical survey carried out in 2015 over an area of about 200 km2 covering the Mt Aluto Volcanic Complex. The geophysical campaign included 162 coincident magnetotelluric and time domain electromagnetic soundings, and 207 gravity stations, partially located in the sedimentary plain surrounding the volcanic complex. Three-dimensional inversion of the full MT static-corrected tensor and geomagnetic tipper was performed in the 338-0.001 Hz band. Gravity data processing comprised digital enhancement of the residual Bouguer anomaly and 2D-3D inverse modelling. The geophysical results were compared to direct observations of stratigraphy, rock alteration and temperature available from the several deep wells drilled in the area. The magnetotelluric results imaged a low-resistivity layer which appears well correlated with the mixed alteration layer found in the wells and can be interpreted as a low-temperature clay cap. The clay-cap bottom depth is well corresponds to a change of thermal gradient. The clay cap is discontinuous, and in the central area of the volcanic complex is characterised by a dome-shape structure likely related to isotherm rising. The propilitic alteration layer, pinpointed as the 80-Ohm-m isosurface, shows two dome-shape highs. The first is NNE-trending, and may be interpreted as an upflow zone along a fault of the Wonji belt. Two productive wells are located along the borders of this area, as well as the alignements of fumaroles and altered grounds. The second is linked to a wide resistive area, located at shallow depth, where no clay cap was detected. It could be interpreted as a fossil high-temperature alteration zone reaching shallow depths, and it is associated to several fumaroles. Modeling of 2D/3D gravity data shows that the anomalies are due to shallow density variations likely related to lithology. The deep lateral variations due to structural lineaments inferred from well stratigraphy have no detectable signature. However, the trend analysis performed on the residual Bouguer anomaly (via horizontal and tilt derivative computations), allowed to identify five lineaments. Three of them exhibit NNE-SSW strike, corresponding to the Wonji Fault Belt Trend, whereas two have NNW-SSE strike, corresponding to the Red Sea Rift trend, which in this area is of minor evidence. The signature of shallow structures is then indicative of major regional structures. One of the lineaments marks the presence of a major fumarolic zone