Marc Descloitres

Small glaciers disappearing in the tropical Andes: a case-study in Bolivia: Glaciar Chacaltaya (16o S)

Glaciar Chacaltaya is an easily accessible glacier located close to La Paz, Bolivia. Since 1991, information has been collected about the evolution of this glacier since the Little Ice Age, with a focus on the last six decades. The data considered in this study are monthly mass-balance measurements, yearly mappings of the surface topography and a map of the glacier bed given by ground-penetrating radar survey. A drastic shrinkage of ice has been observed since the early 1980s, with a mean deficit about 1 m a -1 w.e. From 1992 to 1998, the glacier lost 40% of its average thickness and two-thirds of its total volume, and the surface area was reduced by >40%. With a mean estimated equilibrium-line altitude lying above its upper reach, the glacier has been continuously exposed to a dominant ablation on the whole surface area. If the recent climatic conditions continue, a complete extinction of this glacier in the next 15 years can be expected. Glaciar Chacaltaya is representative of the glaciers of the Bolivian eastern cordilleras, 80% of which are small glaciers ( <0.5 km2). A probable extinction of these glaciers in the near future could seriously affect the hydrological regime and the water resources of the high-elevation basins.

Application of integrated magnetic resonance sounding and resistivity methods for borehole implementation. A case study in Cambodia

Abstract A geophysical survey was conducted in Cambodia to measure the contribution of geophysics to a running drilling programme. The geology of the area (the province of Siem Reap) mainly consists of recent heterogeneous sediments of sand, silt and clay. The thickness of this formation ranges from 20 to 100 m and lies on Jurassic to Tertiary rocks. Surveys were done with direct current methods [one-dimensional vertical electrical sounding (1D VES), and two-dimensional (2D) electrical imaging], time domain electromagnetic (TDEM) sounding and proton magnetic resonance sounding (MRS). To validate the geophysical results, boreholes were drilled and tested with electrical logs and pumping tests. We found that: (1) The resistivity methods (VES, 2D electrical imaging and TDEM) are very sensitive to the groundwater electrical conductivity which is highly heterogeneous within the province. A preliminary relationship between measured groundwater conductivity and aquifer resistivity is proposed. (2) The MRS gives accurate information on groundwater occurrence for the 5- to 60-m-deep layers. A preliminary relationship between MRS data (aquifer transmissivity estimated from MRS field measurements) and hydrodynamic parameters (aquifer local transmissivity and borehole relative specific capacity estimated from borehole pumping tests) is proposed. (3) The resistivity methods and MRS are complementary, and a joint use is recommended. (4) At the survey scale, the borehole success rate was improved from 56% to 90% by the use of geophysics. Crossing the technical and cost analyses, we propose a geophysical methodology to implement boreholes in the province of Siem Reap. This methodology could both increase the borehole success rate and save money at the programme scale. It consists of the use of: 1. MRS, TDEM and electrical methods jointly where the borehole success rate is less than 30%. 2. MRS and TDEM jointly where the borehole success rate ranges from 30% to 50%. 3. Electrical methods (VES and 2D electrical imaging) as standard methods.

Three-dimensional magnetic resonance imaging for groundwater

The surface nuclear magnetic resonance method (SNMR) is an established geophysical tool routinely used for investigating one-dimensional (1D) and sometimes 2D subsurface water-saturated formations. We have expanded the tool by developing a 3D application. 3D-SNMR is a large-scale method that allows magnetic resonance imaging of groundwater down to about 80?m. Similar to most surface geophysical methods, 3D-SNMR has limited resolution, but it is effective for investigating water-saturated geological formations larger than several tens of meters. Because the performance of the method depends on variable survey conditions, we cannot estimate it in general. For demonstration purposes, we present an example of numerical modeling under fixed conditions. Results show that under certain conditions it is possible to detect a water volume as small as 500?m3 and the detection threshold depends on the ambient electromagnetic noise magnitude and on the location of the target volume relative to the SNMR loops. The 3D-SNMR method was used to investigate accumulated water within the T?te Rousse glacier (French Alps). Inversion of the field measurements made it possible to locate the principal reservoir in the central part of the glacier and estimate the volume of accumulated water. These results were verified by 20 boreholes installed after the 3D-SNMR results were obtained and by pumping water out of the glacier. Very good correspondence between the 3D-SNMR and borehole results was observed.

Improvement of electrical resistivity tomography for leachate injection monitoring

Leachate recirculation is a key process in the scope of operating municipal waste landfills as bioreactors, which aims to increase the moisture content to optimize the biodegradation in landfills. Given that liquid flows exhibit a complex behaviour in very heterogeneous porous media, in situ monitoring methods are required. Surface time-lapse electrical resistivity tomography (ERT) is usually proposed. Using numerical modelling with typical 2D and 3D injection plume patterns and 2D and 3D inversion codes, we show that wrong changes of resistivity can be calculated at depth if standard parameters are used for time-lapse ERT inversion. Major artefacts typically exhibit significant increases of resistivity (more than +30%) which can be misinterpreted as gas migration within the waste. In order to eliminate these artefacts, we tested an advanced time-lapse ERT procedure that includes (i) two advanced inversion tools and (ii) two alternative array geometries. The first advanced tool uses invariant regions in the model. The second advanced tool uses an inversion with a minimum length constraint. The alternative arrays focus on (i) a pole-dipole array (2D case), and (ii) a star array (3D case). The results show that these two advanced inversion tools and the two alternative arrays remove almost completely the artefacts within +/-5% both for 2D and 3D situations. As a field application, time-lapse ERT is applied using the star array during a 3D leachate injection in a non-hazardous municipal waste landfill. To evaluate the robustness of the two advanced tools, a synthetic model including both true decrease and increase of resistivity is built. The advanced time-lapse ERT procedure eliminates unwanted artefacts, while keeping a satisfactory image of true resistivity variations. This study demonstrates that significant and robust improvements can be obtained for time-lapse ERT monitoring of leachate recirculation in waste landfills.

Characterization of crystalline basement aquifers with MRS: comparison with boreholes and pumping tests data in Burkina Faso

In the Sahelian region of Burkina Faso (Western Africa), groundwater resources are scarce. The hydrogeological context is mainly crystalline basement aquifers that often present a challenge to hydrogeologists when investigating their exploration and management. A magnetic resonance sounding (MRS) survey was conducted to evaluate the ability of the method to answer the following main questions encountered by hydrogeologists in this hard-rock context: Where is the groundwater? How deep and how thick are the water-bearing formations? What are the reserves of groundwater? What is the productivity of the aquifer? MRS measurements were implemented around recent boreholes drilled both in the weathered and in the fissured-fractured units of the reservoirs. In order to evaluate the MRS method, MRS results are compared with borehole and pumping test data. The depths and thicknesses of the saturated aquifers encountered by the boreholes are compared with those estimated by MRS. The T1decay-time constant of the magnetic resonance signal is used for calculating the storativity and transmissivity estimators from geophysical data. These MRS hydrogeological estimators are compared with the local transmissivity and storativity of the aquifer, estimated from pumping test results. The main conclusions of the comparison between the 13 MRS results and the borehole data are: The depths and thicknesses of the saturated alterites are accurately described by the MRS results, and the mean differences with the borehole data are ±12% and ±17%, respectively. The storativity estimated from MRS data is not reliable. The proposed estimators need to be confirmed with larger data sets, and further research needs to be conducted on this matter. The transmissivity can be accurately estimated from MRS data after calibration with pumping test results. The mean difference between MRS and pumping test results is ±41%. The main limiting factors of MRS applied in hard-rock areas are the 1D approximation in a highly heterogeneous context, the screen effect that causes deep weathered-fissured reservoirs to be poorly resolved when topped by shallow alterites reservoirs, and the suppression principle that causes deep narrow fractures to be undetectable. MRS is a useful tool to characterize the saturated alterites and the weathered-fissured zones of aquifers in a crystalline rock context. With knowledge of its limitations, its use within the framework of hydrogeological strategy is promising, both for borehole implementation and for groundwater reserve evaluation.

Improvement in TDEM sounding interpretation in presence of induced polarization. A case study in resistive rocks of the Fogo volcano, Cape Verde Islands

Abstract A Time Domain Electromagnetic (TDEM) survey was carried out in and around the caldera of the Fogo volcano, Cape Verde Islands, to detect the low resistive structures that could be related to groundwater. A sign reversal in the sounding curves was encountered in central-loop measurements for the soundings located in the centre of the caldera along three main radial profiles. The negative transients are recorded in the early channels between 6.8 and 37 μs. Negative values in an early time transient is an unusual field observation, and consequently the first step was to check the data to ascertain their accuracy and quality. In the second step, three-dimensional (3D) effects are evaluated and ruled out in this zone, while an Induced Polarization (IP) phenomenon is observed using Direct Current (DC) sounding measurements. In the third step, the IP effect is called upon to explain the TDEM distortions; a Cole–Cole dispersive conductivity is found to be adequate to fit the field data. However, the more relevant one-dimensional (1D) model is recovered when both central-loop and offset-loop data are jointly taken into account, thus indicating that an effect of dispersive conductivity is necessary to explain the field data. The 1D electrical structure exhibits four layers, with decreasing resistivity with depth. Only the first layer is polarizable and its Cole–Cole parameters are m =0.85, c =0.8 and τ =0.02 ms for chargeability, frequency dependence and time constant, respectively. However, the Cole–Cole parameters deduced from TDEM forward modelling remain different from those deduced from DC/IP sounding. In this volcanic setting, this IP effect may be caused by the presence of small grains of magnetite and/or by the granularity of effusive products (lapillis). As a conclusion, it is shown that a modelling using different TDEM data sets is essential to recover the electrical structure of this area.

Contribution of 3-D time-lapse ERT to the study of leachate recirculation in a landfill.

Leachate recirculation is a key process in the operation of municipal waste landfills as bioreactors. It aims at increasing the moisture content to optimise the biodegradation. Because waste is a very heterogeneous and anisotropic porous media, the geometry of the leachate plume recirculation is difficult to delineate from the surface at the scale of the bioreactor site. In this study, 3-D time-lapse electrical resistivity tomography (ERT) was used to obtain useful information for understanding leachate recirculation hydrodynamics. The ERT inversion methodology and the electrode arrays were optimised using numerical modelling simulating a 3-D leachate injection scenario. Time-lapse ERT was subsequently applied at the field scale during an experimental injection. We compared ERT images with injected volumes to evaluate the sensitivity of time-lapse ERT to delineate the plume migration. The results show that time-lapse ERT can accomplish the following: (i) accurately locate the injection plume, delineating its depth and lateral extension; (ii) be used to estimate some hydraulic properties of waste.

The location of infinite electrodes in pole-pole electrical surveys: consequences for 2D imaging

Abstract In 2D-multielectrode electrical surveys using the pole–pole array, the distance to `infinite electrodes is actually finite. As a matter of fact, the available cable length generally imposes a poor approximation of theoretical location of these electrodes at infinity. This study shows that in most of the cases, the resulting apparent resistivity pseudosection is strongly distorted. Numerical simulation validated by field test also shows that a particular finite array provides results that are as close as possible to the ones of the ideal pole–pole array. This is achieved when two conditions that are weaker than an infinite location are fulfilled: (i) the `infinite electrodes are placed symmetrically on both sides of the in-line electrodes with a spread angle of 30° and (ii) the length of `infinite lines is at least 20 times the greatest distance between in-line electrodes. The electrical 2D image obtained with this enhanced array is the least distorted one with respect to the pole–pole image. The apparent resistivities are generally underestimated, but this deviation is almost homogeneous. Though the shift cannot be determined a priori, the interpretation of such an image with direct or inverse software designed for pole–pole data provides an accurate interpretation of the ground geometry.

Study of infiltration in a Sahelian gully erosion area using time-lapse resistivity mapping

Abstract Observing that concentrated runoff destroys indurate and impermeable surface horizons to form gullies on Sahelian slopes, we investigated whether these gullies are preferential places for deep infiltration and groundwater recharge processes. The primary aim of this study is to determine if resistivity mapping is an appropriate method to use for locating recharge zones from the surface. The study area, in northern Burkina Faso, is a typical (1 ha) gully erosion area located at the outlet of an 82-ha catchment with solonetz soils and a crystalline basement. Taking advantage of a long dry season followed by a short rainy season, we made use of a time-lapse approach to carry out electrical resistivity mapping and monitor apparent resistivity variations that occurred in the soils during the rainy season, between June and September. We made nine apparent resistivity maps in the year 2000 and two in January and March 2001. To monitor expected infiltration and percolation to depths of 5 m or more, we laid out Wenner array profiles with an inter-electrode spacing of 5 m. The time-lapse mapping was also controlled with: (i) neutron probe measurements; (ii) resistivity measurements on outcrops during infiltration tests; (iii) electrical resistivity logging in auger holes. Geophysical results showed that the apparent resistivity parameter can either decrease (typical case) or increase (unexpected case) after a rain. Neutron probe measurements indicated that infiltration varies within a few decimeters even at the centre of the main gully. Using one dimensional (1D) modelling based on resistivity variations monitored during infiltration tests, we concluded that apparent resistivity variations are linked to the presence of carbonate in the soils. When soluble carbonates are present, the resistivity of the infiltrated layer varies from 220 Ω m (dry state) to less than 5 Ω m (wet state), bringing about a decrease in apparent resistivity value for the 5m spacing. In the absence of carbonate, resistivity varies from 1500 to 180 Ω m, but produces an increase of the apparent resistivity value for the same spacing. Consequently, we found time-lapse apparent resistivity mapping to be an efficient way to delineate certain soil properties. It also provided additional information about punctual observations. However, our results have led us to conclude that the 5-m inter-electrode spacing is too large to monitor this type of shallow infiltration phenomenon and that the effect of temperature on resistivity should be considered when comparing maps over the period of a few months. Furthermore, this type of survey should be controlled using electrical loggings in auger holes, or electrical soundings in order to get a better understanding of in-depth resistivity variations. Finally, this survey indicated that deep infiltration processes are not occurring below the gully situated on the slope. Further studies are required downstream to identify the location of groundwater recharge in Sahelian crystalline contexts.

Water storage changes as a marker for base flow generation processes in a tropical humid basement catchment (Benin): Insights from hybrid gravimetry

In basement catchments of subhumid West Africa, base flow is the main component of annual nstreamflow. However, the important heterogeneity of lithology hinders the understanding of base flow ngeneration processes. Since these processes are linked with water storage changes (WSCs) across the ncatchment, we propose the use of hybrid gravity data in addition to neutron probe-derived water content nand water levels to monitor spatiotemporal WSC of a typical crystalline basement headwater catchment n(16 ha) in Benin. WSC behaviors are shown to provide insights into hydrological processes in terms of water nredistribution toward the catchment outlet. Hybrid gravimetry produces gravity change observations from ntime-lapse microgravity surveys coupled with gravity changes monitored at a base station using a superconducting ngravimeter and/or an absolute gravimeter. A dense microgravity campaign (70 surveys of 14 stations) ncovering three contrasted years was set up with a rigorous protocol, leading to low uncertainties (<2.5 lGal) non station gravity determinations (with respect to the network reference station). Empirical orthogonal function nanalyses of both gravity changes and WSCs from neutron probe data show similar spatial patterns in the nseasonal signal. Areas where storage and water table show a capping behavior (when data reach a plateau nduring the wet season), suggesting threshold-governed fast subsurface redistribution, are identified. This nobserved storage dynamics, together with geological structures investigated by electrical resistivity tomography nand drill log analysis, make it possible to derive a conceptual model for the catchment hydrology.