Combined electrical and electromagnetic imaging of hot fluids within fractured rock in rugged Himalayan terrain

Integrated electrical resistance tomography (ERT) and short-offset transient electromagnetic (TEM) measurements were carried out to investigate a geothermal area in the Main Central Thrust (MCT) zone of Garhwal Himalayan region, India. The study area is located around Helang on either side of Alaknanda River and it is dotted with hot water springs with water temperature of 45°–55 °C emerging at the surface.To assess the geothermal potential and its lateral and vertical extension in and around the hot water springs in the study area, 7 ERT profiles and 21 TEM stations on 7 profiles were established around the hot water spring and at far distant locations. The 2D inversion of ERT data indicates a low resistivity (< 50 Ωm) zone in the vicinity of hot springs, which appears to be associated with an underground water channel through the fractured rock. The bedrock resistivity is very high (> 1000 Ωm) whereas the resistivity of the weathered near surface soil at a far distant location from the hot spring is low (< 100 Ωm) again. A common feature of all TEM data is the sign reversal observed at roughly 10 μs. The consistent sign reversal in all TEM data indicates the existence of the multi-dimensionality of the geoelectrical structure. Therefore, the TEM data were treated by using the SLDM (Spectral Lanczos Decomposition Method) 2D/3D forward modeling code based on the finite difference algorithm. The resistivity structure obtained from ERT data was used as an input for the modeling of TEM data. Based on the joint analysis of the ERT and TEM data it can be inferred that geothermal anomalies associated with the hot spring in the MCT zone are a local feature appearing as a low resistivity zone (< 50 Ωm) at shallow depth (< 100 m) in the vicinity of the hot spring region.     

Resistivity imaging of Pleistocene alluvial aquifers in a contractional tectonic setting: A case history from the Po plain (Northern Italy)

In this work we present the hydrogeophysical imaging of a key sector of the Quaternary Po foreland basin (northern Italy), focussing on the reconstruction of clastic aquifers and aquitards in a complex tectono-sedimentary subsurface architecture. The study area includes the relic reliefs of Casalpusterlengo and Zorlesco, two smooth morphological features involving uplifted and gently folded Pleistocene marine to alluvial sediments, plausibly linked to the buried Northern Apennines thrust and fold belt. The geophysical data include 35 Direct Current Vertical Electrical Soundings collected over a 37 km² wide area, acquired with Schlumberger array and maximum half-spacing of 500 m. 1-D resistivity-depth profiles were computed for each VES. An integrated hydrostratigraphic approach was applied, to constrain the interpretation of the geophysical data along several cross-sections, including the comparison of resistivity soundings to stratigraphic logs, borehole electric logs and the pore-water properties.The resistivity interfaces, traceable with the same laterally continuous vertical polarity, were used to develop an electrostratigraphic model in order to portray the stacking of electrostratigraphic units down to 200 m below ground surface. Their vertical associations show a general upward increase of electrical resistivity. This assemblage mimics the regional coarsening upwards depositional trend, from the conductive units of the Plio-Pleistocene marine-to-transitional depositional systems to the resistive units of the Middle–Late Pleistocene fluvial and alluvial plain depositional systems. Middle Pleistocene depositional systems host an alternation of North-dipping, high-to-intermediate permeability aquifer systems (70–180 Ωm, thickness of 5–70 m) separated by low permeability aquitards (20–50 Ωm, thickness up to 40 m). These units pinch out against the Casalpusterlengo and Zorlesco relic reliefs, where they cover the uplifted and folded regional aquitard (20–50 Ωm) formed by Pliocene-Lower Pleistocene clays to sandy silts with gravel lenses in agreement with borehole data. In the deepest part of the local stratigraphy, a broad low-resistivity anomaly (< 10 Ωm) was clearly mapped through the study area. By comparison with electrical borehole logs in deep oil-wells, it could be interpreted as the fresh–saltwater interface due to the presence of connate waters and brines hosted by the marine-to-transitional shales.     

Characterization of aquifers in the Vientiane Basin,Laos, using Magnetic Resonance Sounding and Vertical Electrical Sounding

The aim of this study is to define and characterize water bearing geological formation and to test the possibility of using geophysical techniques to determine the hydrogeological parameters in three areas in the Vientiane basin, Laos. The investigated areas are part of the Khorat Plateau where halite is naturally occurring at depths as shallow as 50 m in the Thangon Formation. Magnetic Resonance Sounding (MRS) has been used in combination with Vertical Electrical Sounding (VES) in different geological environments. In total, 46 sites have been investigated and the MRS and VES recognized the stratigraphic unit N₂Q_1–3, consisting of alluvial unconsolidated sediments, as the main water bearing unit. The aquifer thickness varies usually between 10 and 40 m and the depth to the main aquifer range from 5 to 15 m. The free water content is here up to 30%, and the decay times vary between 100 and 400 ms, suggesting a mean pore size equivalent to fine sand to gravel. The resistivity is highly variable, but usually around 10–1500 Ω-m, except for some sites in areas 1 and 2, where the aquifer is of low resistivity, probably related to salt water. Hydraulic and storage-related parameters such as transmissivity, hydraulic column, have been estimated from the MRS. The MRS together with VES has been shown to be a useful and important tool for identifying and distinguishing freshwater from possible salt-affected water as well as the salt-related clay layer of the Thangon Formation. This clay layer is characterized by very low free water content and a resistivity lower than 5 Ω-m and can be found in all 3 areas at depths from 15 to 50 m.     

Resistivity monitoring of the tephra barrier at Crater Lake,Mount Ruapehu,New Zealand

The eruptions of Mt Ruapehu in the North Island of New Zealand in 1995 and 1996 caused a tephra barrier to be formed across the outlet of Crater Lake. By 2005 seepage from the refilled lake into the barrier raised the possibility of an eventual collapse of the barrier, releasing a catastrophic lahar down the mountain.As part of an extensive monitoring programme of the tephra barrier, direct current (dc) resistivity surveys were carried out on a number of lines along and across it in order to test whether the extent of the seepage could be measured (and monitored) by geophysical means. Two dimensional inversion of measured apparent resistivity data showed that between the initial measurements, made in January 2005, and February 2006, there was a gradual decrease in resistivity above the old outlet from ~ 50–60 Ωm to ~ 30 Ωm. This gave the first indication that lake water was seeping into the barrier. Between October and December 2006 there was a rapid rise in lake level to only 2 m below the top of the barrier, and a further resistivity survey in January 2007 showed that there had been a further decrease in resistivity throughout the entire barrier with values dropping to < 10 Ωm. The extent of this low resistivity indicated that the barrier was now saturated. At this stage lake water was penetrating the barrier and starting to cause erosion on its downstream side. Catastrophic collapse occurred on 18 March 2007, accompanied by a lahar in the Whangaehu river valley.Subsequent forward 3D numerical modelling of the resistivity structure of the barrier has confirmed that the observed changes in measured resistivity were directly related to the progress of seepage of lake water into the barrier.     

Carbon remineralization in the Amazon–Guianas tropical mobile mudbelt: A sedimentary incinerator

The Amazon River spawns a vast mobile mudbelt extending ∼1600 km from the equator to the Orinoco delta. Deposits along the Amazon–Guianas coastline are characterized by some of the highest C_org remineralization rates reported for estuarine, deltaic, or shelf deposits, however, paradoxically, except where stabilized by mangroves or intertidal algal mats, they are usually suboxic and nonsulfidic. A combination of tides, wind-driven waves, and coastal currents forms massive fluid muds and mobile surface sediment layers ∼0.5–2 m thick which are dynamically refluxed and frequently reoxidized. Overall, the seabed functions as a periodically mixed batch reactor, efficiently remineralizing organic matter in a gigantic sedimentary incinerator of global importance. Amazon River material entering the head of this dynamic dispersal system carries an initial terrestrial sedimentary C_org loading of ∼ 0.7 mg C m⁻² particle surface area. Total C_org loading is lowered to ∼ 0.2 mg C m⁻² in the proximal delta topset, ∼60–70% of which remains of terrestrial origin. Loading decreases further to 0.12–0.14 mg C m⁻² (∼60% terrestrial) in mudbanks ∼600 km downdrift along French Guiana, values comparable to those found in the oligotrophic deepsea. DOC/ΣCO₂ ratios in pore waters of French Guiana mudbanks indicate that >90% of metabolized organic substrates are completely oxidized. Within the Amazon delta topset at the head of the dispersal system, both terrestrial and marine organic matter contribute substantially to early diagenetic remineralization, although reactive marine substrate dominates (∼60–70%). The conditional rate constant for terrestrial C_org in the delta topset is ∼0.2 a⁻¹. As sedimentary C_org is depleted during transit, marine sources become virtually the exclusive substrate for remineralization except very near the mangrove shoreline. The δ¹³C and Δ¹⁴C values of pore water ΣCO₂ in mudbanks demonstrate that the primary source of remineralized organic matter within ∼1 km of shore is a small quantity of bomb signature marine plankton (+80‰). Thus, fresh marine organic material is constantly entrained into mobile deposits and increasingly drives early diagenetic reactions along the transit path. Relatively refractory terrestrial C_org is lost more slowly but steadily during sedimentary refluxing and suboxic diagenesis. Amazon Fan deposits formed during low sea level stand largely bypassed this suboxic sedimentary incinerator and stored material with up to ∼3X the modern high stand inner shelf C_org load (Keil et al., 1997b. Proceedings of the Ocean Drilling Program, Scientific Results. Vol. 155. pp. 531–537). Sedimentary dynamics, including frequency and magnitude of remobilization, and the nature of dispersal systems are clearly key controls on diagenetic processes, biogeochemical cycling, and global C storage along the continental margins.     

Combining Electrical Resistivity Tomography and Ground Penetrating Radar to study geological structuring of karst Unsaturated Zone

This paper highlights the efficiency and complementarity of a light package of geophysical techniques to study the structure of karst Unsaturated Zone (UZ) in typical Mediterranean environment where soil cover is thin or absent. Both selected techniques, 2D Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT), are widely used in environmental studies and their application is accessible for a lot of scientists/engineers. However, GPR or ERT alone is not able to provide an enhanced characterization of geological features in karst media. In the present study, GPR results supply a near surface high resolution imaging and thus can provide relevant geological information such as stratifications and fractures. Despite the quality of the results GPR's investigation depth remains limited to around 12 m. Apparent and inverted resistivity provided by ERT surveys shows strong lateral and vertical variations. These variations can inform about general geological structuring and feature orientation. ERT is able to prospect down to 40 m but it's a low resolution integrative technique. In the study area the investigated limestone is a commonly electrical resistive formation (more than 2000 Ω.m). However deeper than 5–7 m, the ERT profiles reveal several zones of moderate resistivity (around 900 Ω.m). In these zones a stratification change corresponding to slanted bedding is clearly identified by GPR results. The combination of both GPR and ERT results can allow a well-established geological interpretation. These moderate resistivity zones with slanted beddings can explain the presence of a perennial water flow point 35 m below the surface of the studied site within the underground gallery of the Low-Noise Underground Laboratory (LSBB).     

Electrical resistivity characterization and defect detection on a geosynthetic clay liner (GCL) on an experimental site

In this paper we analyze the onsite characterization of a geosynthetic clay liner (GCL) that serves to ensure the impermeability of a landfill cap by DC electrical methods. The imaging of the GCL geoelectrical properties is a challenging problem because it is a very thin (between 4 and 7 mm thick) and resistive layer (from 100,000 to 2,000,000 Ω·m) depending on meteorological conditions and aging. We compare results obtained using electrical resistivity tomography (ERT) using two different kinds of arrays (dipole–dipole DD and Wenner–Schlumberger) on an experimental site with engineered defects. To confirm these results and to find the real onsite GCL resistivity we have performed sampling of the posterior distribution of this parameter using vertical electrical sounding (VES) inversions. Different VES methods were extracted from ERT with DD array and converted into a Schlumberger array.As a main conclusion the dipole–dipole array provides a better resistivity resolution of the defects than the Wenner–Schlumberger array. On ERT images, the defect detection seems to be impossible if the GCL has very high resistivity, as it happened when it was put in place. Taking into account the equivalence rules, the inversions are in both cases (ERT and VES) compatible. The GCL resistivity estimated from PSO (particle swarm optimization) varies from 3.0 10⁵ to 1.10⁶ Ω·m depending on saturation conditions during the twenty first months of its placing. Then, the resistivity dropped to 4.10⁴–9.10⁴ Ω·m, indicating a probable chemical damage of the GCL due to aging. Finally the fact that the VES inversions are solved via PSO sampling allows for the detection of a very thin and resistive layer and opens the possibility of performing micro VES surveys along the landfill to detect possible GCL defects.     

A 3D resistivity model derived from the transient electromagnetic data observed on the Araba fault,Jordan

72 inloop transient electromagnetic soundings were carried out on two 2 km long profiles perpendicular and two 1 km and two 500 m long profiles parallel to the strike direction of the Araba fault in Jordan which is the southern part of the Dead Sea transform fault indicating the boundary between the African and Arabian continental plates. The distance between the stations was on average 50 m.The late time apparent resistivities derived from the induced voltages show clear differences between the stations located at the eastern and at the western part of the Araba fault. The fault appears as a boundary between the resistive western (ca. 100 Ωm) and the conductive eastern part (ca. 10 Ωm) of the survey area. On profiles parallel to the strike late time apparent resistivities were almost constant as well in the time dependence as in lateral extension at different stations, indicating a 2D resistivity structure of the investigated area.After having been processed, the data were interpreted by conventional 1D Occam and Marquardt inversion. The study using 2D synthetic model data showed, however, that 1D inversions of stations close to the fault resulted in fictitious layers in the subsurface thus producing large interpretation errors. Therefore, the data were interpreted by a 2D forward resistivity modeling which was then extended to a 3D resistivity model. This 3D model explains satisfactorily the time dependences of the observed transients at nearly all stations.     

Enhancing hardrock seismic images: Reprocessing of high resolution seismic reflection data from Vihanti,Finland

Seismic reflection data were acquired across volcanic hosted massive sulfides (VHMS) of Vihanti in order to improve the understanding of the regional geological setting. Commercially processed seismic data from Vihanti are of good quality, but reprocessing can be used to extract additional information about geological structures. Especially, careful velocity analysis influences the quality of seismic images. Differentiating reflections caused by fractures from those caused by lithological contacts is very important for exploration and geological modeling. Reflections from fracture zones known from drilling stack with lower velocity (~ 5100 m/s) compared to typical stacking velocities of the Vihanti area (> 5500 m/s). The reprocessing also indicated that fracture zones are better imaged with low frequencies due to the better overall continuity of the fault zones at scales of hundreds of meters rather than at shorter seismic wavelengths.In full stacks, long offset data can mask structure close to the surface. More detailed seismic images of the shallow subsurface emerged by preferentially stacking short offset data wherever acquisition and processing lines lay close together and were nearly straight. Long offset data remains valuable for imaging deeper structures as well as dipping reflectors. Cross-dip-analysis revealed a bright diffractor located near the base of the Vihanti volcanic basin at 1.5 km depth. The seismic data allow a geological interpretation in which the Vihanti structure has developed through significant thrust faulting and displacement of the lithological contacts. Gentle folds that were formed prior to faulting are visible as undulating reflectivity in seismic sections. The reprocessed seismic section indicates a potential deep extension of the ore-hosting altered volcanic and calc-silicate rocks previously unexplored.     

Combined three-dimensional electric and seismic tomography study on the Åknes rockslide in western Norway

We present a combined 3-D geoelectric and seismic tomography study conducted on the large Åknes rockslide in western Norway. Movements on the slope are strongly influenced by water infiltration, such that the hydrogeological regime is considered as a critical factor affecting the slope stability. The aim of our combined geophysical study was to identify and visualize the main shallow tension fractures and to determine their effect on hydraulic processes by comparing the geophysical results with information from borehole logging and tracer tests. To resolve the complex subsurface conditions of the highly fractured rock mass, a three-dimensional set-up was chosen for our seismic survey. To map the water distribution within the rock mass, a pattern of nine intersecting 2-D geoelectric profiles covered the complete unstable slope. Six of them that crossed the seismic survey area were considered as a single data set in a 3-D inversion. For both methods, smoothing-constraint inversion algorithms were used, and the forward calculations and parameterizations were based on unstructured triangular meshes. A pair of parallel shallow low-velocity anomalies (< 1400 m/s) observed in the final seismic tomogram was immediately underlain by two anomalies with resistivities <13 kΩm in the resistivity tomogram. In combination with borehole logging results, the low-velocity and resistivity anomalies could be associated with the drained and water-filled part of the tension fractures, respectively. There were indications from impeller flowmeter measurements and tracer tests that such tension fractures intersected several other water-filled fractures and were responsible for distinct changes of the main groundwater flow paths.     

Investigation of groundwater resources using controlled-source radio magnetotellurics (CSRMT) in glacial deposits in Heby,Sweden

We have combined tensor radio magnetotelluric- (RMT, 15–250 kHz) and controlled source tensor magnetotelluric (CSTMT, 1–12 kHz) data for the mapping of aquifers in gravel formations lying in between crystalline bedrock and clay rich sediments in the Heby area some 40 km west of Uppsala in Sweden. The estimated transfer functions, the impedance tensor and the tipper vector generally satisfy 1D or 2D necessary conditions except for the lowest CSTMT frequencies where near field effects become more dominant.The data measured from 8 profiles were inverted with the Rebocc code of Siripunvaraporn and Egbert (2000) assuming plane wave conditions. This meant that only 12 frequencies in the range of 4–180 kHz could be used. The four lowest frequencies of CSTMT in the range of 1–2.8 kHz were excluded because of source effects. Data from all profiles were inverted with a starting model of 100 Ω-m and a relative error floor of 0.02 on apparent resistivity, corresponding to less than 1^° on phase. Tipper vectors are generally small except when source effects become dominant in the lowest frequencies of CSTMT and were therefore not used for inversion. Comparing with models derived from vertical electrical soundings, refraction and reflection seismic data as well as ground truth from exploration wells assessed the reliability of the deep part of the models. Furthermore we carried out a non-linear resolution analysis to better quantify the depth extent of the aquifers.The inverted models from the Heby area show well the thickness variations of glacial deposits overlying crystalline bedrock. Generally, the upper 20 m of the models have resistivities below 40 Ω-m, taken to represent clay rich formations. Below the clay layer resistivities increase to about 40–400 Ω-m, interpreted to represent sand/gravel formations with a maximum thickness of about 40 m and a width of several hundred metres. This is a potential aquifer that extends in approximately N–S direction for some kilometres.     

Three-dimensional electrical resistivity model of a nuclear waste disposal site

A three-dimensional (3D) modeling study was completed on a very large electrical resistivity survey conducted at a nuclear waste site in eastern Washington. The acquisition included 47 pole–pole two-dimensional (2D) resistivity profiles collected along parallel and orthogonal lines over an area of 850 m × 570 m. The data were geo-referenced and inverted using EarthImager3D (EI3D). EI3D runs on a Microsoft 32-bit operating system (e.g. WIN-2K, XP) with a maximum usable memory of 2 GB. The memory limits the size of the domain for the inversion model to 200 m × 200 m, based on the survey electrode density. Therefore, a series of increasing overlapping models were run to evaluate the effectiveness of dividing the survey area into smaller subdomains. The results of the smaller subdomains were compared to the inversion results of a single domain over a larger area using an upgraded form of EI3D that incorporates multi-processing capabilities and 32 GB of RAM memory. The contours from the smaller subdomains showed discontinuity at the boundaries between the adjacent models, which do not match the hydrogeologic expectations given the nature of disposal at the site. At several boundaries, the contours of the low resistivity areas close, leaving the appearance of disconnected plumes or open contours at boundaries are not met with a continuance of the low resistivity plume into the adjacent subdomain. The model results of the single large domain show a continuous monolithic plume within the central and western portion of the site, directly beneath the elongated trenches. It is recommended that where possible, the domain not be subdivided, but instead include as much of the domain as possible given the memory of available computing resources.     

Distributions of polychlorinated biphenyls in the Daliao River estuary of Liaodong Bay,Bohai Sea (China)

The distributions of 41 polychlorinated biphenyls (PCBs) were determined in the aqueous phase, suspended particulate matter (SPM), and sediment of the Daliao River estuary in Liaodong Bay, Bohai Sea (China). The total PCB concentrations ranged from 5.51 to 40.28 ng L⁻¹ in the surface water, from 6.78 to 66.55 ng L⁻¹ dry weight in the SPM, and from 0.83 to 7.29 ng g⁻¹ dry weight in the sediment. The PCB concentrations in water, SPM, and sediment were moderate relative to those reported for other estuary and marine systems around the world. Sedimentary PCB concentrations decreased offshore due to the active deposition of laterally transported river-borne particles. The predominance of the highly chlorinated congeners for the water, SPM, and sediment samples are an indication of either a lack of degradation or the presence of nearby or recent releases into the environment.     

The impact of floods and storms on the acoustic reflectivity of the inner continental shelf: A modeling assessment

Flood deposition and storm reworking of sediments on the inner shelf can change the mixture of grain sizes on the seabed and thus its porosity, bulk density, bulk compressional velocity and reflectivity. Whether these changes are significant enough to be detectable by repeat sub-bottom sonar surveys, however, is uncertain. Here the question is addressed through numerical modeling. Episodic flooding of a large versus small river over the course of a century are modeled with HYDROTREND using the drainage basin characteristics of the Po and Pescara Rivers (respectively). A similarly long stochastic record of storms offshore of both rivers is simulated from the statistics of a long-term mooring recording of waves in the western Adriatic Sea. These time series are then input to the stratigraphic model SEDFLUX2D, which simulates flood deposition and storm reworking on the inner shelf beyond the river mouths. Finally, annual changes in seabed reflectivity across these shelf regions are computed from bulk densities output by SEDFLUX2D and compressional sound speeds computed from mean seafloor grain size using the analytical model of Buckingham [1997. Theory of acoustic attenuation, dispersion, and pulse propagation in unconsolidated granular materials including marine sediments. Journal of the Acoustical Society of America 102, 2579–2596; 1998. Theory of compressional and shear waves in fluidlike marine sediments. Journal of the Acoustical Society of America 103, 288–299; 2000. Wave propagation, stress relaxation, and grain-tograin shearing in saturated, unconsolidated marine sediments. Journal of the Acoustical Society of America 108, 2796–2815]. The modeling predicts reflectivities that change from <12 dB for sands on the innermost shelf to >9 dB for muds farther offshore, values that agree with reflectivity measurements for these sediment types. On local scales of ∼100 m, however, maximum changes in reflectivity are <0.5 dB. So are most annual changes in reflectivity over all water depths modeled (i.e., 0–35 m). Given that signal differences need to be ⩾2–3 dB to be resolved, the results suggest that grain-size induced changes in reflectivity caused by floods and storms will rarely be detectable by most current sub-bottom sonars.     

Bioavailability of heavy metals in water and sediments from a typical Mediterranean Bay (Málaga Bay,Region of Andalucía,Southern Spain)

Concentrations of heavy metals were measured in sediment and water from Málaga Bay (South Spain). In the later twentieth century, cities such as Málaga, have suffered the impact of mass summer tourism. The ancient industrial activities, and the actual urbanization and coastal development, recreation and tourism, wastewaters treatment facilities, have been sources of marine pollution. In sediments, Ni was the most disturbing metal because Ni concentrations exceeded the effects range low (ERL), concentration at which toxicity could start to be observed in 85% of the samples analyzed. The metal bioavailability decreased in the order: Cd > Ni > Pb > Cu > Cr. In the sea water samples, Cd and Pb were the most disturbing metals because they exceeded the continuous criteria concentration (CCC) of US EPA in a 22.5% and 10.0% of the samples, respectively. Statistical analyses (ANOVA, PCA, CA) were performed.     

Characterization of recharge through complex vadose zone of a granitic aquifer by time-lapse electrical resistivity tomography

The vadose zone is the main region controlling water movement from the land surface to the aquifer and has a very complex structure. The use of non-invasive or minimally invasive geophysical methods especially electrical resistivity imaging is a cost-effective approach adapted for long-term monitoring of the vadose zone. The main aim of this work is to know the fractures in the vadose zone, of granitic terrene, through which the recharge or preferred path recharge to the aquifer takes place and thus to relate moisture and electrical resistivity. Time lapse electrical resistivity tomography (TLERT) experiment was carried out in the vadose zone of granitic terrene at the Indian Geophysical Research Institute, Hyderabad along two profiles to a depth of 18 m and 13 m each. The profiles are 300 m apart. Piezometric, rainfall and soil moisture data were recorded to correlate with changes in the rainfall recharge. These TLERT difference images showed that the conductivity distribution was consistent with the recharge occurring along the minor fractures. We mapped the fractures in hard rock or granites to see the effect of the recharge on resistivity variation and estimation of moisture content. These fractures act as the preferred pathways for the recharge to take place. A good correlation between the soil moisture and resistivity is established in the vadose zone of granitic aquifer. Since the vadose zone exhibits extremely high variability, both in space and time, the surface geophysical investigations such as TLERT have been a simple and useful method to characterize the vadose zone, which would not have been possible with the point measurements alone. The analyses of the pseudosection with time indicate clearly that the assumption of the piston flow of the moisture front is not valid in hard rocks. The outcome of this study may provide some indirect parameters to the well known Richard's equation in studying the unsaturated zone.     

Geoelectrical,strain and tilt investigations of hydrological processes at the broadband Geodynamical Observatory Moxa,Germany

The Geodynamic Observatory Moxa, located in Thuringia/Germany, is dedicated to studies of temporal deformations of the earth's crust and of variations of the gravity field. One of the essential issues with respect to these investigations is the reduction of the hydrological impact on the data of the gravimeters, strainmeters and tiltmeters. In order to optimise the reductions, we investigated the changes in the hydrological conditions in the woody mountain slope above the observatory with time-lapse electrical resistivity tomography (ERT), and analysed the strain and tilt measurements for prominent signatures of pore pressure induced subsurface deformations.Here we present the results for two profiles – parallel and perpendicular to the slope – measured with ERT during 33 campaigns between June 2007 and April 2010. Resistivity changes and variations of apparent soil moisture, inferred from ERT sections, were found to primarily occur in the first two metres of the subsurface. These variations can be related to subsurface flow in the upper two metres induced by precipitation events and snowmelts. Trees close to the profiles only show a minimum impact on the resistivity and soil moisture changes.Furthermore, systematic hydrologically induced deformations can be observed in hodographs of strain and tilt measurements for large precipitation events (> 80 mm) and snowmelts. In the strain data a short-term (< 3 days) dilatational signal is found with an amplitude of 20 nstrain to 60 nstrain and a long-term (> 7 days) compressional signal between 40 nstrain and 180 nstrain. The preferential N–S direction of long-term deformational signals (> 1 week) is also observed in the tilt data. The direction of tilt changes (25 nrad–120 nrad) is nearly parallel to the drainage direction of the nearby Silberleite creek indicating variations of pore pressure gradients during hydrological events.The results of these hydrological studies at the Geodynamic Observatory Moxa can be used for removing the time dependent hydrological signal in strain and tilt data and, thus, better correction algorithms for hydrological impacts can be developed to enhance the value of the data for geodynamic studies.     

Analytical solution for enhanced recharge around a bedrock exposure caused by deep-aquifer dewatering through a variable thickness aquitard

In this study an analytical solution was developed to predict steady radially-symmetric percolation rates from an aquifer underlain by a variable thickness aquitard. The solutions consider an aquitard with constant thickness and with radial-symmetrically increasing thickness outward from the center. The solution was used to predict the percolation rate from a peat layer around a bedrock outcrop in the James Bay Lowland near the De Beers Victor diamond mine. In this case the marine sediment layer limited the direct connection between the peat layer and the bedrock as an aquitard. Our zero order solution with constant marine sediment thickness showed the best fit to the steady state water level data of June 2012. It was found that the enhanced recharge around bioherms (i.e., at rates greater than the regional average of 0.7 mm/day) will only occur in marine sediments less than 4.3 m thick, for extreme depressurization of 30 m.     

Nutrient inputs from submarine groundwater discharge on the Santiago reef flat,Bolinao, Northwestern Philippines

Submarine groundwater discharge (SGD) on the reef flat of Bolinao, Pangasinan (Philippines) was mapped using electrical resistivity, ²²²Rn, and nutrient concentration measurements. Nitrate levels as high as 126 μM, or 1–2 orders of magnitude higher than ambient concentrations, were measured in some areas of the reef flat. Nutrient fluxes were higher during the wet season (May–October) than the dry season (November–April). Dissolved inorganic nitrogen (DIN = NO₃ + NO₂ + NH₄) and soluble reactive phosphorus (SRP) fluxes during the wet season were 4.4 and 0.2 mmoles m⁻² d⁻¹, respectively. With the increase population size and anthropogenic activities in Bolinao, an enhancement of SGD-derived nitrogen levels is likely. This could lead to eutrophic conditions in the otherwise oligotrophic waters surrounding the Santiago reef flat.     

Bottom topography,recent sedimentation and water volume of the Cerro Prieto Dam,NE Mexico

The Cerro Prieto dam, a relatively small water reservoir in NE Mexico, is one of the main resources of potable water for Monterrey, a city with a population of about four million inhabitants. A high resolution seismic study using non-linear parametric sub-bottom echo-sounder SES-2000 was carried out in this water reservoir. High resolution acoustic data interpretation shows that the thickness of recent sediments due to siltation of the reservoir reaches 3.5-4.0 m. It shows a high recent sedimentation rate (1-2 up to 14 cm/ year). Based on the echo-sounder data, the first bathymetric map and a digital model of recent sediment thickness were designed. A significant (5-12%) difference between the volume capacity value used by National Commission of Water (CNA) and acoustic survey results was revealed. Differences between the CNA and acoustic data indicate storage losses from 12-17 up to 30 million cubic meters. The results obtained through study such as this one, could be useful to improve a water resources management.