EXPLORING FOR FAVORABLE GROUNDWATER CONDITIONS IN HARDROCK ENVIRONMENTS BY RESISTIVITY IMAGING METHODS: SYNTHETIC SIMULATION APPROACH AND CASE STUDY EXAMPLE

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EXPLORING FOR FAVORABLE GROUNDWATER CONDITIONS IN HARDROCK ENVIRONMENTS BY RESISTIVITY IMAGING METHODS: SYNTHETIC SIMULATION APPROACH AND CASE STUDY EXAMPLE
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   International Conference on Earth Sciences and Electronics – 2002 (ICESE – 2002) EXPLORING FOR FAVORABLE GROUNDWATER CONDITIONS IN HARDROCK ENVIRONMENTS BY RESISTIVITY IMAGING METHODS: SYNTHETIC SIMULATION APPROACH AND CASE STUDY EXAMPLE ∗   Ioannis F. Louis 1 , Filippos I. Louis 2  and Alexia Grambas 3   1  Geophysics & Geothermic Division, Geology Department, University of Athens, Panepistimiopolis, Ilissia, Athens 15784, Greece.  jlouis@geol.uoa.gr   2  Geophysics & Geothermic Division, Geology Department, University of Athens, Panepistimiopolis, Ilissia, Athens 15784, Greece. flouis@geol.uoa.gr   3  Department of Geoinformatics, National Technical University of Athens, Greece. alexia@central.ntua.gr  ABSTRACT Synthetic simulation investigations and case study examples were used to explore the capabilities of the resistivity imaging method to detect fractures and or fractured zones, regarded as aquifers, in hardrock environments. Synthetic simulation indicated that fractures of high vertical  penetration in the hardrock basement, when they are covered by overburden and their thickness is comparable to their depth of burial, produce responses greater than any measurement noise leading in that way to relatively high-resolution resistivity images. However, as the thickness of the overburden increases and the vertical extent of fractures is reduced, the inverted resistivity images loss in resolution. Synthetic outcomes were confirmed by demonstrating a case study example where field geophysical experiments were conducted in the context of investigating the hydrogeological conditions of the greater area of Mandoudi, Euboea Island Greece. Favorable areas were located where the intense fracturing of the  basement rocks has produced extensive or local thickening of overburden material . INTRODUCTION Aquifers in fractured rocks are generally considered of minor importance compared to those in primary porous media, on which the attention has mainly been focused up until now. In fact, the amount of groundwater available in fractures is generally limited, at least in arid and semi-arid regions. Nevertheless, this type of aquifer is a primary source of water for man in vast areas throughout the world, where aquifers with primary porosity are practically non-existent and surface waters are ephemeral (Barrocu, 2002). The subject also appears of certain significance in view of the fact that fractured rocks are not to be regarded only as aquifers according to the traditional meaning of the word, as they are being considered more and more as the ideal medium for storage of radioactive materials and pollutants. Furthermore, groundwater may represent serious problems in civil and mining engineering, both from technical and economical point of view. Fractures in a geologic medium can greatly influence its hydrogeological characteristics. They can increase the hydraulic conductivity of an otherwise impermeable rock or soil by orders of magnitude in the dominant fracture directions. Therefore knowledge of the presence, extent, intensity, and direction of fractures is desirable for any hydraulic engineering project. The resistivity imaging technique can detect vertical and lateral electrical resistivity variations related to fracture presence and intensity. ∗    Journal of Electrical & Electronics Engineering, Special Issue October 2002, 1-14. 1   International Conference on Earth Sciences and Electronics – 2002 (ICESE – 2002) The purpose of this study is to explore the capabilities of the resistivity imaging method for the detection of fractures and/ or fractured zones buried below an overburden by using synthetic simulation methods, and demonstrating case study examples from experiments in hardrock environments. SIGNIFICANCE OF FRACTURES IN HARDROCK AQUIFER DEVELOPMENT In hardrock areas, the geological structure normally encountered is characterized  by the existence of a hardrock basement overlain by a weathered overburden of variable thickness. Hydrogeologically, the weathered material, which constitutes the overburden, has high porosity and contains a significant amount of water, and, at the same time, it  presents low permeability due to its relatively high clay content (Barker, 2001). The  bedrock, on the other hand, is fresh but frequently fractured, presenting high  permeability, but as fractures do not constitute a significant volume of the rock, fractured basement has a low porosity. For this reason a good borehole, providing long term high yields, is one, which penetrates a large thickness of weathered overburden, which acts as a reservoir, and one which additionally intersects fractures in the underlying bedrock, where the fractures provide the rapid transport mechanism. Boreholes which intersect fractures, but which are not overlain by thick saturated weathered material, cannot be expected to provide high yields in the long term. Boreholes which penetrate saturated weathered material but which find no fractures in the bedrock are likely to provide sufficient yield for a hand pump only. The short-term success of borehole siting is clearly dependent on the fractures in the bedrock that the borehole intersects. However, once the bedrock is covered by any thickness of weathering, the fractures are notoriously difficult to find and geophysics  provides no direct solution to the problem. It can easily be demonstrated that, although fractures of a few centimeters thickness, which may be very important hydrogeologically, cannot normally be located by geophysics once they are buried  below a few meters of overburden, it is possible to use geophysics to increase the chances of intersecting fractures while drilling. Besides the problem of locating aquifers in hardrock terrains, the city and other potential appropriators in the area still face the  problem of defining the lateral extent of these aquifers so that the resources could be used in the most cost-efficient manner. Resistivity imaging methods, apart from helping to delineate the maximum thickness of the weathered overburden, locating in this way the appropriate site for borehole siting, can also help to define the lateral extent of these aquifers. Figure 1 show how, closely spaced joints and fractures facilitate the downward migration of water that then causes increased weathering (Barker, 2001). 2   International Conference on Earth Sciences and Electronics – 2002 (ICESE – 2002) Figure 1 . Stages in the evolution of weathered basement (After Herbert et al, 2001) The important feature is that, where fractures are present, the bedrock is expected to be more strongly weathered to a greater depth than where it is unfractured. For this reason, it is best to drill at points where the bedrock reaches its greater depth, since greater depth ensures a thick reservoir of water in the overburden. This knowledge has led to the application of combinations of geophysical methods, normally very low frequency electromagnetic profiling (VLF), followed by resistivity sounding. Resistivity sounding is attractive as it is one of the cheapest geophysical techniques to employ and the measurements can normally be interpreted with low cost manual methods. In last decade, with the advent of high memory and low cost portable computers, more effective imaging techniques can be considered. SYNTHETIC SIMULATION APPROACH Synthetic modeling methods were used to model the electrical response of complex subsurface structures using finite element or finite differences modeling schemes. In this case, the software package RES2DMODE was used, which is a version of the srcinal finite difference code of Dey and Morisson (1966) modified by Loke and Barker (1996). This software was used to model fracture zones of various widths and depths of cover to demonstrate the limits of resolution of the resistivity imaging technique. 3   International Conference on Earth Sciences and Electronics – 2002 (ICESE – 2002) The synthetic models represent single fractures and fractured zones in an ophiolite basement rock underlying a conducting overburden. Resistivity values of 30 Ohm-m, 50 Ohm-m and 250 Ohm-m were used to represent three different portions of the models. 50 Ohm-m was chosen to represent the top layer of overburden material. 30 Ohm-m was chosen to represent a fracture in the basement rock. The 250 Ohm-m value was chosen to represent the resistivity of the peridotites basement rock as it was obtained from the in situ measurements over peridotite outcrops. The single fracture model of Figure 2a would probably produce a very high yield of water if intersected by a borehole. However, when covered by 15m of overburden, the response computed for this model (Fig. 2b) does not clearly show any evidence of the fracture. Figure 2 . Synthetic modelling of a single fracture. The next step was to invert the response and to get an image of improved resolution. 3% Gaussian noise was added both to background and target models to demonstrate that the inversion scheme is reasonably robust and will work in an environment with unsystematic geologic or instrumental noise. The RES2DINV software which was used for the inversions is based on the smoothness – constrain least squares method and basically tries to reduce the difference between the calculated and 4   International Conference on Earth Sciences and Electronics – 2002 (ICESE – 2002) measured apparent resistivity values with respect to some smoothness constraints such as the complexity of a model. The resulting inversion (Fig. 2d) was compared with the srcinal input model but there was no evidence of the structure again since the magnitude of the measurement errors is greater than the response. Increasing the size of the fracture has little effect until it reaches a thickness that is greater than the depth of its top (Fig. 3a). Then the response, shown in the bordered area of figure 3b, is clear since it becomes greater than any measurement noise, which might be expected. In this case, the inverted resistivity image (Fig. 3d) clearly depicts the fracture zone improving in this way the resolution considerably. Figure 3 . When the fracture thickness becomes greater than the depth of burial the inverted image improves the resolution considerably. Figure 4 shows a model of fractured hardrock where the fractures have caused increased weathering. The model’s response (Fig. 4b) now clearly shows the drop in the  bedrock. The resultant resistivity image (Fig. 4d) depicts the fracture zone exactly in the same position with the srcinal input model, but this will only be effective where the observed response of the earth is greater than the magnitude of the measurement errors. 5
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