Borehole Logging Data Research Articles

Evaluasi Sistem Pengendapan Uranium Pada Batuan Sedimen Formasi Sibolga, Tapanuli Tengah

ABSTRAKUranium di alam dapat terbentuk dalam berbagai tipe cebakan, sesuai dengan sumber, proses, dan lingkungan pengendapannya. Keterdapatan uranium di Sibolga pada batuan sedimen Formasi Sibolga merupakan suatu potensi yang layak untuk dikembangkan tetapi hingga saat ini belum diketahui pola pengendapan dan proses mineralisasi uranium tersebut. Penelitian bertujuan untuk mengetahui pola sebaran batuan dan keterdapatan anomali kadar uranium berdasarkan data geologi, radiometri permukaan, dan data log bor untuk mengetahui proses pengendapan batuan dan mineralisasi uranium. Keterdapatan mineralisasi berdasarkan data log bor tersebar dari satuan konglomerat alas (Kgl 1), satuan batupasir 1 (Bp 1), satuan konglomerat 2 (Kgl 2), dan satuan batupasir 2 (Bp 2) dengan ketebalan dan sebaran semakin ke atas semakin menipis. Sebaran mineralisasi pada bagian timur pada satuan batuan konglomerat 1 lebih didominasi oleh mineral detrital hasil pengendapan epigenetik berupa monasit yang terbentuk pada saat pembentukan granit sebagai batuan sumber. Pada satuan batuan di atasnya mineralisasi berbentuk pola alur (channel) yang berarah timur laut-barat daya, yang terbentuk secara syn-genetic dengan mineral berupa uraninite, carnotite, dan coffinite. Pengendapan batuan Formasi Sibolga berasal dari timur ke arah barat dan pengendapan uranium terjadi akibat perbedaan kondisi lingkungan pengendapan dari oksidasi di bagian timur menjadi lebih reduktif di bagian barat daya. Peningkatan kandungan material organik yang cukup tinggi pada lingkungan pengendapan bagian barat daya menyebabkan lingkungan pengendapan dalam kondisi reduksi.ABSTRACTUranium in nature formed in various deposit type, depends on its sources, process, and depositional environments. Uranium occurrence in Sibolga, hosted in sedimentary rocks of Sibolga Formation, is properly potential to develop; nevertheless, the depositional pattern and uranium mineralization process so far had not been recognized. The research aim is to determine the rock distribution patterns and the existence of uranium grade anomalies based on surface geology and borehole log data. Mineralization occurrences from borehole log data distributed from basalt conglomerate unit (Kgl 1), sandstone 1 unit (Bp 1), conglomerate 2 unit (Kgl 2), and sandstone 2 unit (Bp 2) with their distribution and thickness are thinning to the top. Mineralization distribution in the eastern area, mainly on Kgl 1 unit, dominated by detritus materials from epi-genetic depositional in the form of monazite which is formed along with the formation of granite as its source rock. Meanwhile, mineralization on the upper rocks units formed a channel pattern trending northeast-southwest, which formed in syn-genetic process consist of uraninite, carnotite, and coffinite. Sibolga Formation deposition originated from east to west and uranium deposit formed because of the differences of depositional environment from oxidation in the east to the more reductive in the southwest. The increasing of organic materials in southwest basin caused the reduction condition of depositional environment.

2D probabilistic prediction of sparsely measured earth properties constrained by geophysical imaging fully accounting for tomographic reconstruction ambiguity

Many hydrological, environmental, or engineering exploration tasks require predicting spatially continuous scenarios of sparsely measured borehole logging data. We present a methodology to probabilistically predict such scenarios constrained by ill-posed geophysical tomography. Our approach allows for transducing tomographic reconstruction ambiguity into the probabilistic prediction of spatially continuous target parameter scenarios. It is even applicable to data sets where petrophysical relations in the survey area are non-unique, i.e., different facies related petrophysical relations may be present. We employ static two-layer artificial neural networks (ANNs) for prediction and additionally evaluate, whether the training performance of the ANNs can be used to rank geophysical tomograms, which are mathematically equal reconstructions of physical parameter distributions in the ground. We illustrate our methodology using a realistic synthetic database for maximal control about the prediction performance and ranking potential of the approach. For doing so, we try to link geophysical radar and seismic tomography as input parameters to porosity of the ground as target parameter of ANN. However, the approach is flexible and can cope with any combination of geophysical tomograms and hydrologic, environmental or engineering target parameters. Ranking of equivalent geophysical tomograms based on additional borehole logging data is found to be generally possible, but risks remain that the ranking based on the ANN training performance does not fully coincide with the closeness of geophysical tomograms to ground truth. Since geophysical field data sets do usually not offer control options similar to those used in our synthetic database, we do not recommend the utilization of recurrent ANNs to learn weights for the individual geophysical tomograms used in the prediction procedure.

Sedimentary facies analysis using borehole log data from turbidite successions

Sedimentary facies analysis using borehole log data from turbidite successions

Improved estimation of hydraulic conductivity by combining stochastically simulated hydrofacies with geophysical data.

Hydraulic conductivity is a major parameter affecting the output accuracy of groundwater flow and transport models. The most commonly used semi-empirical formula for estimating conductivity is Kozeny-Carman equation. However, this method alone does not work well with heterogeneous strata. Two important parameters, grain size and porosity, often show spatial variations at different scales. This study proposes a method for estimating conductivity distributions by combining a stochastic hydrofacies model with geophysical methods. The Markov chain model with transition probability matrix was adopted to re-construct structures of hydrofacies for deriving spatial deposit information. The geophysical and hydro-chemical data were used to estimate the porosity distribution through the Archie’s law. Results show that the stochastic simulated hydrofacies model reflects the sedimentary features with an average model accuracy of 78% in comparison with borehole log data in the Chaobai alluvial fan. The estimated conductivity is reasonable and of the same order of magnitude of the outcomes of the pumping tests. The conductivity distribution is consistent with the sedimentary distributions. This study provides more reliable spatial distributions of the hydraulic parameters for further numerical modeling.

Geo-Spatial Mapping of the Northern Bushveld Rustenburg Layered Suite (RLS) in South Africa

The focus of this paper is on determination of the geometry and stratigraphic contact pattern of the Rustenburg Layered Suite (RLS) in the Northern Bushveld Complex area using available borehole data and trend surface analysis technique. This technique was used to analyse over one hundred borehole log data in the Northern Bushveld Complex in order to describe the geometric pattern and trends of the RLS rocks. The results demonstrate the usefulness of this technique in identifying structural features. Regional trends of each of the stratigraphic units reveal the presence of regional structures that were not obvious at the surface. This first part of the paper focused on the Northern Bushveld Complex, while the second and the third part focused on the eastern and western Bushveld limbs respectively.

Groundwater flow modeling in an intermontane basin

Groundwater model has become a commonly used tool to perform various tasks. Geological, hydrogeological and geophysical data is required for constructing 3D hydrogeological framework models. Most of the time, it is realised that there is lack of sufficient data to build a groundwater model. The present work has been achieved after systematic data collection and hydrogeological study of Nara Basin, west Japan. Groundwater has been widely exploited for drinking water supply as well as for recreation purpose as Thermal Springs in the Nara Basin. There are hundreds of wells drilled in unconsolidated sediments and some tens of deep wells encountering the fractured basement. When water is exploited from a groundwater basin, it is necessary to understand properly the groundwater flow in different aquifer zones in the basin. Hydrostratigraphic units in the unconsolidated sediments overlying the basement rocks were established by using the borehole log data. In order to have understanding of the three dimensional configuration of these units, fence diagram was constructed. The geological and hydrogeological information were used to develop a conceptual model which was further calibrated and an acceptable model was obtained. The model was validated by comparing the observed and simulated heads and discharge.

시추 및 물리탐사를 이용한 석회암 공동의 분포 규모 분석

본 연구에서는 석회암 공동이 분포하는 지역에서 발생한 지반침하 현상의 원인을 파악하고 공동의 분포 위치 및 규모를 분석하기 위하여 지표지질조사, 시추조사, 전기비저항, 토모그래피 탐사 등을 실시하였다. 지표지질조사 및 시추조사 결과에 의하면 연구지역에서 석회암은 방해석의 함량에 따라 고함유대와 저함유대가 교호하며 분포한다. 시추조사 결과를 중심으로 전기비저항 및 탄성파 토모그래피 탐사 결과와 비교분석한 결과에 의하면 대부분의 공동은 지반침하가 발생한 지점을 중심으로 심도 7~14 m의 토사층과 암반의 경계부 주변에 집중되어 분포하고 있다. 또한 공동의 규모는 558~835 ㎥로 소규모이며, 침하의 형태는 사질토 지반에서 흔히 발생하는 Suffosion sinkhole에 속한다. Geological mapping, borehole drilling, electrical resistivity, and seismic tomography surveys were conducted in order to map underground cavities and better understand the mechanisms driving subsidence in a limestone region in Korea. Limestone outcrops in the study area generally alternate between calcite-rich and calcite-poor rock. The results reveal that in areas experiencing subsidence, cavities occur mainly around soil-rock boundaries at depths of 7~14 m. These results are based on comparative analyses of electrical resistivity, seismic tomography, and borehole logging data. The volumes of the cavities are relatively small in a range of 558~835 ㎥ and they have a shape typical of suffosion sinkholes, which are typically found where sandy soils overlie bedrock cavities.

3D seismic imaging of the Lalor volcanogenic massive sulphide deposit, Manitoba, Canada

ABSTRACTA three‐component three‐dimensional seismic data set was acquired over the Lalor volcanogenic massive sulphide deposit near Snow Lake, Manitoba, Canada, to assess the reflectivity of the ore and further validate the potential of three‐dimensional reflection seismic methods for deep mineral exploration. The Lalor deposit was chosen as a test site as it provided an intact, well‐characterized 25‐Mt‐deep ore deposit with a rich catalogue of geological and geophysical data, as well as extensive drill‐core and drill‐hole geophysical and geological logs. An analysis of physical rock properties from borehole logging data indicates that massive sulphides associated with the zinc‐rich zones could produce prominent reflections, whereas acoustic impedances of zones with disseminated gold do not sufficiently differ from the impedances of the host rocks to produce reflections. The interpretation of the seismic data is constrained with a detailed three‐dimensional lithofacies model constructed from the categorical kriging of 15 lithological units identified in borehole intersections. Processing of the seismic data included prestack dip‐moveout and poststack time migration. Final images reveal some strong reflections associated with the zinc‐rich massive sulphide zones. The most prominent reflection results from the constructive interference of thin and closely spaced massive sulphide zones and felsic–mafic volcanic rock contacts above and below the mineralization. Contacts between felsic and mafic volcanic rocks, including those that were hydrothermally altered and subsequently metamorphosed, produced prominent and continuous reflections that are used to map the main architecture of the footwall rocks. At depth, a series of continuous and conformable reflections indicate the general geometry of the volcanic sequences in the area of the three‐dimensional seismic survey.

"Sedimentary facies analysis" of turbidite succession based on borehole-log data

"Sedimentary facies analysis" of turbidite succession based on borehole-log data

Stability assessment of stope sequence scenarios in a diminishing ore pillar

Numerical modelling is a vital tool in the mining industry and is widely used for planning purposes as well as for examining phenomena on a mine-wide scale. In this paper, a comprehensive rock mechanics study is conducted for the planned mining sequence within an orebody that leads to the formation of a diminishing ore pillar at the Vale Garson Mine in Sudbury, Ontario. A mine-wide numerical model is first constructed in FLAC3D with input parameters derived from laboratory tests and borehole log data, and is then calibrated based on in-situ stress measurements. The planned mining sequence scenario, comprising mining fronts advancing from the eastern and western sides of the orebody, is examined based on a combined numerical-volumetric analysis. A qualitative assessment of “ore at risk” is first made based on two instability indicators; the brittle shear ratio (BSR) and major principal stress (σ1). In addition, two other sequence scenarios are examined in which mining is accelerated in the east. A quantitative comparison of the results is then made for the three options using the same instability indicators. Based on the optimum sequence scenario, an assessment is conducted of effects emanating from variations in constitutive models and the presence of a shear zone. In the final phase, the BSR and the presence of tensile conditions are used as instability indicators to study the volume of “ore at risk” in stopes adjacent to an opening during a mining stage. It is shown that the proposed methodology, while simple, can nevertheless be a powerful tool for the planning and validation of stope extraction sequence scenarios, and can be used in conjunction with multiple stability indicators.

The significance of hydrothermal alteration zones for the mechanical behavior of a geothermal reservoir

The occurrence of hydrothermally altered zones is a commonly observed phenomenon in brittle rock. The dissolution and transformation of primary minerals and the precipitation of secondary minerals affect rocks in terms of mechanics, stress conditions, and induced seismicity. The present study investigates commonly observed phenomena of hydrothermal alteration and observations at the geothermal site of Soultz-sous-Forêts, which are related to the occurrence of hydrothermal alteration. Geomechanical observations at Soultz are interpreted on the basis of synthetic clay content logs, which are created from borehole logging data, and which identify clay in hydrothermally altered zones. It is shown that hydrothermal alteration results in a reduction of the frictional strength of the reservoir rock. Weak zones can act as stress-decoupling horizons, which locally perturb the stress field and affect the evolution of the microseismic cloud. For the first time, it is shown on a reservoir scale that large magnitude seismic events are restricted to unaltered granites, whereas in clay zones, only small magnitudes are observed. It is demonstrated that clay-rich zones foster the occurrence of aseismic movements on fractures. Secondary mineral precipitation during hydrothermal alteration has a great effect on the geomechanical properties of a geothermal reservoir. The identification of such zones is a first step towards understanding the relation between alteration and mechanical processes inside a reservoir and can help in reducing induced seismicity during hydraulic stimulation of a reservoir.

Influence of total organic carbon deposition on the inventory of gas hydrate in the Indian continental margins

Total organic carbon (TOC) content of marine sediments represents residual carbon, originally derived from terrestrial and marine sources, which has survived seafloor and shallow subseafloor diagenesis. Ultimately, its preservation below the sulfate reduction zone in marine sediments drives methanogenesis. Within the gas hydrate stability zone (GHSZ), methane production along continental margins can supersaturate pore fluids and lead to the formation of gas hydrate. In this paper we examine the inventory and sources of TOC in sediments collected from four regions within the GHSZ along the Indian continental margins. The recovered sediments vary in age from Oligocene to recent. Mean TOC abundance is greatest in the Krishna–Godavari (K–G) Basin and decreases progressively to the Mahanadi basin, Andaman wedge, and Kerala–Konkan (K–K) Basin. This decrease in TOC is matched by a progressive increase in biogenic CaCO3 and increasing distance from terrestrial sources of organic matter and lithogenic materials. Organic carbon sources inferred from C/N and δ13CTOC range from terrestrial (K–G Basin) to mixed marine and terrestrial (Mahanadi Basin), to marine dominant (Andaman wedge and K–K Basin). In the K–G Basin, variation in the bulk δ13CTOC is consistent with changes in C3 and C4 vegetation driven by monsoon variability on glacial-interglacial timescales, whereas in the Mahanadi Basin a shift in the δ13CTOC likely reflects the onset of C4 plant deposition in the Late Miocene. A large shift the δ13CTOC in the K–K basin is consistent with a change from C3 to C4 dominated plants during the middle Miocene. We observe a close relationship between TOC content and gas hydrate saturation, but consider the role of sedimentation rates on the preservation of TOC in the zone of methanogenesis and advective flow of methane from depth. Although TOC contents are sufficient for in situ methanogenesis at all the sites where gas hydrates were observed or inferred from proxy data, seismic, borehole log, pressure core, and gas composition data coupled with relatively high observed gas hydrate saturations suggest that advective gas transport may also play a role in the saturation of methane and the formation of gas hydrates in these regions. Although TOC content may be a first order indicator for gas hydrate potential, the structural and stratigraphic geologic environment along a margin will most likely dictate where the greatest gas hydrate saturations will occur.

KURT 2단계 건설부지에 대한 암석역학모델 설정

KURT 2단계 구간에 대한 암반역학 모델을 설정하기 위하여 대상지역의 암반을 지질특성과 절리발달정도에 따라 총 6개의 암반 단위체로 구분하였다. 연구지역 암반은 대부분 화강암 그룹인 G1, G2, G3 단위체로 이루어져 있으며, 관입암 그룹인 D1, D3 단위체가 소규모로 분포한다. 또한 단층파쇄대로 이루어진 불량한 암반인 F3 단위체가 KURT 2단계 구간의 입구를 가로지르는 형태로 분포한다. 암반의 상태를 파악하기 위하여 시추조사 자료를 바탕으로 RMR, Q-system, RMi 등 3종류의 암반분류법으로 암반을 분류하였고, 선행 연구들에서 제안된 다양한 경험식과 암반분류 결과를 이용하여 암반의 변형계수, 강도, 점착력, 마찰각 등의 지반정수를 계산하였다. 최종적으로 각 암반 단위체에 대한 대표 암반분류 값과 대표 지반정수 값들을 산정하여 연구지역의 암반역학 모델을 설정하였다. 이 연구에서 설정된 암반역학 모델은 KURT 2단계 구간에 대한 설계와 안정성 예측 연구에 중요한 자료로 활용될 수 있을 것으로 판단된다. Rock masses at the site for the <TEX>$2^{nd}$</TEX> step construction of the KAERI Underground Research Tunnel (KURT) are divided into six units to establish a rock mechanics model that is dependent on the geological characteristics and degree of joint development. The site primarily consists of three granitic units (G1, G2, and G3), two dykes (D1 and D3), and a fault zone of poor rock mass quality (F3). The F3 unit crosses the tunnel at the beginning of the site of <TEX>$2^{nd}$</TEX> step construction. The rock masses of each unit are classified by RMR (Rock Mass Rating), Q-system, and RMi (Rock Mass Index), all based on borehole logging data. The deformation modulus, rock mass strength, cohesion, and friction angle for each unit are calculated using established empirical relationships. The representative rock mass classification and geotechnical parameters for the rock mass units are established, and a rock mechanics model for the site is proposed, which will be useful in the design and stability analysis of the <TEX>$2^{nd}$</TEX> step construction of KURT.

Formation mechanism of large subsidence sinkholes in the Lar valley in Iran

Large suffusion and cover-collapse sinkholes, of up to 100 m in diameter, developed in the Lar valley in Iran. The sinkholes formed through overburden several hundred metres thick comprising coarse- and fine-grained deposits. The formation mechanism for these large sinkholes was studied based on the topography of the bedrock and on deep borehole log data. It could be determined that the large cover-collapse sinkholes are located above permeable bedrock berms and that they formed through a layer of fine-grained, cohesive soil that was both underlain and overlain by coarse-grained soil layers. The data analysis showed that suffusion from the coarse-grained layer at the bedrock and subsequent erosion of these particles through permeable zones in the bedrock to the karst channels caused settlement of these coarse-grained layers and the formation of open caverns above them, at the interface with the fine-grained layer. With continuing erosion, the caverns widened until the limit equilibrium of the fine-grained layer was reached and then collapsed to form a depression with a horizontal bottom surface, termed a cover-collapse sinkhole. An analytical model was developed for this mechanism, considering cohesive and non-cohesive soil layers. This analytical model confirmed that, for the given composition of the overburden, cover-collapse sinkholes of 80–100 m in diameter can be expected. The paper also analysed the formation mechanism of the suffusion sinkholes and it was shown that their formation took place through coarse-grained, non-cohesive formations, in this case alluvium and slope talus. Finally, it was analysed whether the formation of new cover-collapse and suffusion sinkholes after the rehabilitation of the reservoir leakage is likely.

Phase-based dispersion analysis for acoustic array borehole logging data.

A phase-based dispersion analysis method for velocity (slowness) extraction from guided waves recorded by an acoustic borehole logging tool in a geological formation is presented. The technique consists of acquiring waveforms from an array of receivers distributed along the tool and constructing the dispersion characteristic by processing in the frequency domain and exploiting phase information to measure the travel time for each frequency component. The approach is nonparametric and completely data-driven and provides high resolution estimates that do not rely on velocity guesses or assumptions regarding the type of modes. Results are free of the aliases and spurious modes which are characteristic of some prior approaches. Examples of dispersion estimation curves are presented using synthesized flexural waves and field data from wireline dipole sonic tools; results are compared with those from the weighted spectral semblance (WSS) and amplitude and phase slowness estimation (APES) methods to demonstrate the effectiveness and utility of the proposed method.

Joint Vp and Vs tomography of Taiwan: Implications for subduction-collision orogeny

The Taiwan orogen sitting at the pre-rifted Eurasian margin and bracketed by two sub-orthogonal subduction systems is subject to complicated geotectonics, motivating numerous seismic tomographic studies in local and regional scale. Most of them obtained P-wave velocities (Vp), or with Vp/Vs ratio jointly, but few were for S-wave velocities (Vs). With unprecedented amount of S-wave data, in this study a new set of well-tuned Vp, Vs, and Vp/Vs models were determined by an elaborate joint-inversion scheme, integrating the data of P- and S-wave travel times, S–P times, and the borehole logging data (for near-surface correction) into one system. This allowed us to revisit the subduction-collision process in detail by comparing Vp, Vs, and Vp/Vs ratio simultaneously. With enhanced slab imaging of Philippine Sea Plate (PSP) under northeast Taiwan, we are able to discriminate the plausible missing Luzon forearc basement along the plate boundary and propose a “skateboarding” edge model, in which the westernmost PSP rides on the east-subducted forearc basement and subducts northward underneath the Eurasian lithosphere. The underlain forearc basement along with the PSP subduction then results in the anomalous shallow double seismic zone observed only near Taiwan Island. Meanwhile, at around 121°E the north-subducting PSP seems dipping west to collide with the deep Eurasian lithosphere, shaping an irregular corner wedge structure. In addition, we evaluate our model with previous ones by waveform modeling approach and show the current performance of travel-time tomography in Taiwan region.

Transdimensional change-point modeling as a tool to investigate uncertainty in applied geophysical inference: An example using borehole geophysical logs

Recently developed methods for inferring abrupt changes in data series enable such change points in time or space to be identified, and also allow us to estimate noise levels of the observed data. The inferred probability distributions of these parameters provide insights into the capacity of the observed data to constrain the geophysical analysis and hence the magnitudes, and likely sources, of uncertainty. We carry out a change-point analysis of sections of four borehole geophysical logs (density, neutron absorption, sonic interval time, and electrical resistivity) using transdimensional Bayesian Markov chain Monte Carlo to sample a model parameter space. The output is an ensemble of values which approximate the posterior distribution of model parameters. We compare the modeled change points, borehole log parameters, and the variance of the noise distribution of each log with the observed lithology classes down the borehole to make an appraisal of the uncertainty characteristics inherent in the data. Our two examples, one with well-defined lithology changes and one with more subtle contrasts, show quantitatively the nature of the lithology contrasts for which the geophysical borehole log data will produce a detectable response in terms of inferred change points. We highlight the different components of variation in the observed data: due to the geologic process (dominant lithology changes) that we hope to be able to infer, geologic noise due to variability within each lithology, and analytical noise due to the measurement process. This inference process will be a practical addition to the analytical tool box for borehole and other geophysical data series. It reveals the level of uncertainties in the relationships between the data and the observed lithologies and would be of great use in planning and interpreting the results of subsequent routine processing.

Reconstructed Holocene sea level curve from the Akkeshi barrier system in eastern Hokkaido, Japan

An active barrier system is present in Akkeshi Bay and a lagoonal area along the Pacific coast of eastern Hokkaido, Japan. This rare feature consists of a so-called lagoon (the Akkeshi-ko lagoon), a flood tidal delta, a barrier, and a tidal inlet that leads into the outer sea of Akkeshi Bay and the Pacific Ocean. Large oyster reefs were living on the flood tidal delta until their sudden extinction in the early 1980s. The processes involved in the formation of this barrier system during a postglacial transgression period are not well understood. Here, we use sedimentological, paleontological, and radiocarbon dating methods to analyze Holocene drillcore obtained from the Akkeshi fishing port in 2009 and borehole log data previously obtained from public works around the Akkeshi lowlands. The data obtained from both drillcore and borehole logs indicates that: (1) postglacial transgression occurred in this area some 11,400 years ago and reached 50 m below the present sea water level, with the barrier system in the study area established 8800 years ago. (2) The preservation of this barrier system is due to the stable sea level in this area. The sea level has been constant for the past 5500 years, and has led to the development of oyster reefs on the tidal flood delta. (3) The current topographically distinctive barrier system has been affected by sudden uplifts associated with giant earthquakes. These events occur every few hundred years, with the barrier undergoing prolonged but relatively rapid post-earthquake subsidence at about 1 cm/year, as observed after a giant 17th century earthquake.

Electrical Resistivity Imaging of Suspected Seepage Channels in an Earthen Dam in Zaria, North-Western Nigeria

To determine and map the subsurface conditions of a dam, a 2D electrical resistivity tomography study was carried out within the two flanks of Zaria dam at Shika. This was done to ascertain if the variations in the volume of water content in the dam is due to an anomalous seepage beneath the subsurface or seasonal effects. On the basis of the interpretation of the acquired data, various zones of relatively uniform resistivity values were mapped and identified. The first zone is characterized by moderate resistivity values of 150 - 600 ohm-m. It represents unsaturated topsoil with thicknesses varying from 1 - 4.5 m. The second (intermediate depth) resistivity zone, with values ranging from 5 - 100 ohm-m and thickness varying from 3.5 - 10 m, represents a silt clay layer with high moisture content. The third resistivity zone represents fairly weathered granite and is characterized by relatively high resistivity values ranging from 700 - 6000 ohm-m. The available borehole log data correlated well with the pseudo-sections in relation to the obtained resistivity values and depth. Zones of relatively low resistivity within the bedrock are interpreted to represent potential seepage pathways. Hence, this geophysical method can be successfully used to delineate and map these seepage pathways within the subsurface of the earth dam.

Modelling climate change effects on a Dutch coastal groundwater system using airborne electromagnetic measurements

Abstract. The forecast of climate change effects on the groundwater system in coastal areas is of key importance for policy makers. The Dutch water system has been deeply studied because of its complex system of low-lying areas, dunes, land won to the sea and dikes, but nowadays large efforts are still being done to find out the best techniques to describe complex fresh-brackish-saline groundwater dynamic systems. In this paper, we describe a methodology consisting of high-resolution airborne electromagnetic (EM) measurements used in a 3-D variable-density transient groundwater model for a coastal area in the Netherlands. We used the airborne EM measurements in combination with borehole-logging data, electrical conductivity cone penetration tests and groundwater samples to create a 3-D fresh-brackish-saline groundwater distribution of the study area. The EM measurements proved to be an improvement compared to older techniques and provided quality input for the model. With the help of the built 3-D variable-density groundwater model, we removed the remaining inaccuracies of the 3-D chloride field and predicted the effects of three climate scenarios on the groundwater and surface water system. Results showed significant changes in the groundwater system, and gave direction for future water policy. Future research should provide more insight in the improvement of data collection for fresh-brackish-saline groundwater systems as it is of high importance to further improve the quality of the model.