4.8 Borehole logging in hydrogeology

Transcription

1 4.8 Borehole logging in hydrogeology 4.8 Borehole logging in hydrogeology Borehole logging is the process of measuring physical, chemical, and structural properties of penetrated geological formations usually using logging tools that are lowered into a borehole on a wireline cable. Geophysical downhole logging provides in situ information about the physical properties of the rock strata and groundwater. In a hole with limited core recovery or the drill cuttings or samples are of questionable origin, the depth of even significant lithological changes can be uncertain. The geophysical logs give a continuous depth record of formation properties, which clearly identifies the lithological changes with an uncertainty of a few centimetres. This information is of great importance in geological modeling, and borehole logging is therefore often used together with seismic profiles to construct subsurface reservoir models. Since the first geophysical borehole logs were made more than seventy years ago, a number of probes have been developed to measure nearly every possible physical parameter in a borehole. In contrast to measurements in deep boreholes, those in shallow boreholes, normally less than 100 m deep, are often made in unconsolidated sediments. The equipment, field application, and interpretation methods have been described, for example, by Hallenburg (1987, 1992), Keys (1990), Yearsley & Crowder (1990) and Samworth (1992). The following applications of geophysical well logging methods in shallow boreholes have been dealt with by a large number of publications over the last several years: geological investigations (Doveton & Prensky 1992), determination of physical properties (Hearst et al. 2000), hydrogeological investigations (Repsold 1989, Mares et al. 1994, Jorgensen & Petricola 1995, Kobr et al. 2005), coastal aquifers and fresh/salt water boundaries (Buckley et al. 2001, Hwang et al. 2004), and environmental investigations (Taylor et al. 1990, Keys 1997, Krammer 1997). This chapter concentrates on the use of geophysical well logging methods for hydrogeological investigations in shallow boreholes and wells. Table summarizes basic information on the most important and widely applied tools in hydrogeology. Table 4.8.1: Geophysical logging methods, required borehole conditions and objectives Log type Specific log Borehole Conditions Information Nuclear Gamma-ray Spectral gamma-ray Gamma-gamma (density) Neutron-neutron (porosity) Open and cased holes with or without fluid Open holes with fluid Electrical Electromagnetical Self-potential Resistivity Focused Resistivity Induction Susceptibility Open or screened holes with fluid Open and pvc cased holes with or without fluid Lithology, density, porosity, calibration of surface geophysics Lithology, calibration of surface geophysics, location of pvc screens Lithology, saline waters Acoustical Sonic Open holes with fluids Lithology (porosity) Physical Caliper Open and cased holes Borehole diameter with or without fluid Optical Borehole camera Optical borehole televiewer Open and cased holes with clear water Flow Impeller flowmeter Heat pulse flowmeter Open and cased holes with fluid Fluid Water quality Open and cased holes with fluid Casing or borehole condition, caving, slope and aspect of fractures and layers Vertical water movement in the borehole EC, temperature, ph, O 2, NO 3, Eh, total gas pressure 107

2 THOMAS WONIK & KLAUS HINSBY Basics and resolution of geophysical logging methods In order to obtain the maximum benefit from geophysical logging methods it is necessary to understand the basic principles of the many tools commonly used and the most effective log analysis techniques. The different tools are not named according to any particular system: Some are named on the basis of the parameter measured, others according to the principle by which the measurement is made, and still others on the basis of the geometry of the probe or the trade name. Further information about the individual methods is given, for example, by Rider (1996), Keys (1997), Zscherpe & Steinbrecher (1997) and Fricke & Schön (1999). Prensky (2002) describes recent developments in logging technology. In the following we describe the logging tools, which we consider the most important and widely used in the field of hydrogeology Radioactivity logging methods Passive radioactivity logging methods measure the natural gamma-radiation. A review of all radioactivity logging methods is given by Meyers (1992). The natural gamma-radiation measured with a gamma-ray logging tool (GR) is from the natural 40 K in the ground and the isotopes of the uranium and thorium decay series. These isotopes occur naturally in clay, making it possible to distinguish between sand and clay layers and to estimate the clay content. Uranium ores are characterised by sharp peaks and high counting rates in the GR log. A spectral gamma-ray probe (GRS) permits a distinction between different minerals containing radio-isotopes by measuring the proportion of the gamma-radiation from the 40 K, 238 U and 232 Th decay series. This is done with a scintillation detector (e.g., bismuth-germanium oxide). After calibration with a standard, the concentrations of K, U, and Th can be given in percent or ppm. In order to compare measurements made with differently designed probes, the calibration is done in API (American Petroleum Institute) units according to international convention. An API unit is 1/200 of the difference in measured activity of a layer with low radioactivity and a layer with a higher radioactivity in the reference well on the campus of the University of Houston. Gamma-ray logs can be made in dry and cased boreholes. The attenuating influence of the casing can be corrected for. The vertical resolution of a gamma-ray log depends mostly on the length of the scintillation crystal, normally about 20 cm. The lateral distance of detection is about cm. Ninety percent of the measured gamma-radiation is from this interval. At the bottom of a density probe (GGD, GGL) there is a gamma-ray source (usually 137 Cs); at the top of the probe there is a gamma-ray detector (e.g., a NaI scintillation counter), shielded from direct radiation from the source below by a lead cylinder. The gamma-radiation emitted by the source is scattered by the atoms of the surrounding rock partially adsorbed, depending on the density of the rock (Compton Effect). Some of the scattered radiation is deflected back to the detector and recorded. Depending on the distance between the source and the detector, the detection distance is 5 10 cm. The vertical resolution (20 60 cm) also depends on the source-detector spacing. A neutron-neutron tool (NN or INN) contains a neutron source and a neutron detector. The neutron source is either a neutron-emitting radioactive isotope, e.g., californium-252, (NN) or a neutron generator (INN). In an INN tool, an accelerated deuterium beam is directed at a tritium target to produced neutrons with energy of about 14 MeV. There are between 10 and 20 hits per second. These fast neutrons loose energy when they collide with the nuclei of the atoms of the surrounding rock and are registered by the detector as thermal and/or epithermal neutrons. Because the energy transfer is the most effective when the neutrons collide with hydrogen nuclei, which have the same mass, the counting rate is inversely proportional to the water content and porosity of the rock. The counting rate is calibrated with a material with a known porosity and expressed as neutron porosity. The depth of investigation varies from about 10 cm in watersaturated, porous rock to as much as 100 cm in dry rock. The vertical resolution of an NN or INN tool is about 50 cm. 108

3 4.8 Borehole logging in hydrogeology Electrical methods Electrical well logs can be made only within water or drilling fluid in a well. They are used in open holes to determine the electrical resistivity of the rock, which together with other physical parameters can be used to derive a lithological log for the borehole (Maute 1992, Spies 1996). A self-potential tool (SP) measures the natural electrical potential between an electrode at the ground surface and an electrode in a drillingfluid-filled borehole. This natural potential is caused by electrochemical processes occurring between different fluids (in this case the drilling fluid and the groundwater). In conventional resistivity logging (electric log EL and microlog ML), the resistivity of the rock is measured using a four-electrode array, analogous to DC resistivity surveys at the ground surface (Sect. 4.5). A constant current is introduced into the rock between two current electrodes in the logging tool. The potential measured between two other electrodes (potential electrodes) is proportional to the electrical resistivity of the rock. The tools used for conventional resistivity logging have the following lengths (potential electrode spacings): small (L = cm), large (L = cm), 16 inch (L = 40 cm), and 64 inch (L = 160 cm). The measured value is called the "apparent resistivity" and is dependent on the size of the borehole, the adjacent rock and the overlying and underlying rock. The "true" resistivities of the rock can be derived from the apparent resistivities using master curves. The measured resistivity logs are symmetric. The curve deviates strongly from the true value when the layer thickness h is less than 5L and a reverse curve when h/l < 1. The depth of investigation and vertical resolution are determined by the length L of the tool. The depth of investigation and vertical resolution are inversely proportional. The ratio of the rock resistivity R t to the resistivity of the drilling fluid R m also influences the results. Useable results are obtained when R t /R m > 1. A laterolog (focused electrolog, FEL) is made with a tool that uses additional electrodes, called guard or bucking electrodes, to focus the current to nearly right angles to the logging tool, considerably increasing the lateral investigation depth and vertical resolution over those of the conventional logging method. Thin layers (down to 20 cm) can be detected with this tool. In a dual laterolog tool (DLL), two focusing systems are used with different investigation depths. The system with the smaller penetration depth mainly measures the rock next to the borehole that has been disturbed by the drilling and has been penetrated by the drilling fluid (invaded zone). The system with the greater penetration depth is influenced mainly by the undisturbed rock further from the borehole. In a microlaterologging tool (MLL), the electrodes have a small spacing and are pressed against the borehole wall during the measurement, so that there is no drilling fluid between the tool and the rock. The vertical resolution is about 5 cm and the investigation depth is as much as 10 cm. Laterologging tools can be used to detect damage to plastic casing in a well Electromagnetic methods Electromagnetic well logging methods can be used in both dry wells and those containing water or drilling fluid. In contrast to electrical methods, these methods can be used in boreholes with plastic casing. The parameter electrical conductivity can be determined using an induction tool. This parameter can be used for lithological classification of the rock sequence. Induction tools (IL) are used to determine electrical conductivity/resistivity of the rock surrounding a borehole and for identification of salt water in the formation. A transmitter coil generates an alternating magnetic field around the borehole, which in turn induces electrical eddy currents that are proportional to the conductivity of the rock. An induction tool usually contains two coil systems with different coil spacings and thus different investigation depths. Coil systems with several transmitter and receiver coils (5 6) are used to focus the field to minimize the influence of the borehole itself on the recorded signal. The investigation depth depends on the conductivity of the rock and is cm for a dual induction log. The vertical resolution is about 150 cm for low conductivities and about 50 cm for higher conductivities. If the differences in conductivity are large, the resistivity curve recorded by focused systems overshoots the proper value when the logging tool passes 109

4 THOMAS WONIK & KLAUS HINSBY the layer interface. In highly conductive rock, the signal is attenuated, due to the skin effect. Induction logging is particularly suitable for dry boreholes and those with plastic casing. The same principles are used by a susceptibility tool (SUSZ, MAL). Besides magnetic markers in a borehole, metal parts that have been lost in a borehole can also be found Acoustic methods An acoustic tool or sonic tool (AL) is used to determine the velocity of sound in the rock surrounding the borehole. Acoustic well logging tools contain one or two ultrasound generators and several receivers in a linear array. The value for the ultrasound traveltime is an average for the distance between the transmitter and the receiver(s). Acoustic well logs can be made only within water or drilling fluid in a well. They are used in open holes for lithological classification of the rock sequence, as well as for detecting joint and fracture zones. If the velocity in the rock matrix is known, the porosity of the rock can be calculated. The depth of investigation is 1 2 cm, the vertical resolution 60 cm. By increasing the distance between the ultrasound generator and receiver to 200 cm, the investigation depth is increased and the vertical resolution decreased. The main aspects of the acoustic method are discussed by Paillet et al. (1992) Optical methods An image of the borehole wall can be obtained directly using a video camera (OPT), permitting a qualitative assessment of the borehole wall or casing. The depths of screens in the casing can be checked and problems with the casing can be determined. Together with direction and distance measuring systems, the size of penetrated cavities can be determined (Gochioco et al. 2002). A video camera can be used only in dry boreholes or in clear water; visibility also depends on the lens and available light Methods for determining the properties of drilling fluids (fluid logs) The temperature (TEMP) and electrical resistivity (SAL, for salinity) of the drilling fluid are usually measured together with a single logging tool. A thermister is used to register the temperature. The electrodes of the SAL probe are in a minifour-point configuration with relatively small electrode spacings of about 50 mm. To avoid eddies in the drilling fluid, this is usually the first well log to be carried out in a borehole or well. Electrical resistivity cannot be measured above the groundwater table, and therefore it is easy to determine when the probe enters the water or drilling fluid. The salinity of the water can be determined from the SAL readings. The tool is calibrated in the laboratory with a standard salt solution. The data from a SAL probe are often used to correct the values obtained with the electrical logging tool (Sect ). The combination of the TEMP and SAL logs provides an indication of vertical movement of the water in the well. The logs often show irregularities at depths at which inflow or efflux of water occurs in borehole with no casing, as well as where leaks in the casing of a well occur. If the temperature of the surrounding rock undisturbed by the drilling process is needed, it is necessary to wait until the temperature has returned to the natural state. A rule of thumb is to wait as long as time taken by the drilling. Calculations for the equilibration time have been published by Bullard (1947). A multi-parameter probe (MIL) measures ph, oxygen concentration, nitrate, total gas pressure, and redox potential in groundwater. It can be used to monitor water quality (e.g., contamination) and inflow of different water types in open holes or wells with long screens covering aquifer sections with different groundwater chemistry and age. The vertical flow velocity of water in a borehole is measured with a flowmeter (FLOW). The water flowing through the meter turns a propeller, whose revolutions are recorded. Movement of the water caused by movement of the tool is corrected for during processing of the data. The measurement is strongly influenced by the diameter of the borehole. If used in open boreholes it should therefore always be 110

5 4.8 Borehole logging in hydrogeology accompanied with a caliper log (see ). The tool can be used only in clear water, because the high viscosity and suspended matter in drilling mud prevents proper functioning. The pumping rate or artesian flow rate must be adequately high to determine the level(s) at which there is inflow of groundwater and its contribution to the total amount of water pumped. A pump must generally be installed in the well before the FLOW measurements are made. In some cases caused by artesian flow or flow between separate aquifers with different hydraulic pressures velocities are high enough to be recorded by a flowmeter. An electromagnetic flowmeter is being increasingly used (Molz & Young 1993). The heat pulse flowmeter may be used in cases were very low vertical flow velocities have to be measured or identified e.g. through leaks in a casing. The heat pulse flowmeter can identify upwards or downwards flow in a well by shortly heating up a small microtoaster between two sensors above and below the toaster. The heat pulse created moves either upwards or downwards with the moving water and is recognized by one of the two sensors. If there is no flow the upper sensor will record the heat pulse after some time (e.g. after more than 30 seconds), since the heat pulse will move upwards. The flowmeter is run as a stationary log at distinct levels in the well, e.g., in sections where leakage in casings or cross flow between separate aquifers are suspected. The turbidity of the water is measured with a photometric probe (PMT). With this tool, the amount of fine-grained material washed into the well, even if it cannot be seen with the naked eye, can be determined. By adding a dye tracer, the groundwater flow rate can also be determined. A sample collecting tool (SAMP) is used to take groundwater samples from a given depth, keeping it at the original pressure. Samples of about 1 litre are collected in a hermetically sealed cylinder. The sampling tool is lowered to the specified depth, a valve is opened, allowing water to flow into the cylinder, and the valve is closed again so that when the tool is raised again the sample remains at the pressure at the sampling depth Methods for determining borehole properties Caliper logging tools (CAL) have one to four arms with springs to hold the ends of the arms against the borehole walls. Caliper data are used for cementation and installation of casing, as well as the plugging of boreholes and wells. They are also used for the evaluation of the data from nearly all other logging tools in open boreholes Field techniques Main components of logging equipment are: surface unit, winch and cable, and logging tools. The surface unit controls the measurements, including the movement of the probe in the borehole; it provides the energy supply to the probe and records, displays, and stores the data. The energy is supplied through the cable, which also conveys the electrical signals between the probe and the surface unit. The depth at which a measurement is made is given by a gauge on the winch. Two types of logging equipment are used for shallow boreholes, differing mainly with respect to the type of winch. Portable equipment is needed for rough terrain, with a winch with about 300 m of single-core cable. For deeper holes, it is best to use a truck or van in which the surface unit and a winch with about 1000 m of cable are installed. Three kinds of logging tools are used: (a) the probe is lowered free in the hole, (b) the probe is centred in the hole by centralizer, and (c) the sensors in the tool are pressed against the borehole wall. The utility of geophysical well logs strongly depends on the well construction, casing, drilling fluid, borehole diameter and whether the borehole is drilled into consolidated or unconsolidated rock. The cost of well logging is determined primarily by how long it takes: Not only the actual logging time, but also the idle time waiting for the borehole to come to equilibrium costs money. One method of saving time is to use multiparameter tools, with which several parameters can be measured at the same time. For this purpose, several tools are lowered in "strings". It 111

6 THOMAS WONIK & KLAUS HINSBY must be taken into consideration, however, that not all of the parameters can be measured down to the bottom of the borehole. Another cost factor is the rate at which the measurements can be made. The logging engineer will choose a logging speed to minimize acquisition time while maintaining an acceptable resolution and signal-to-noise ratio. The best data are obtained at the slowest speeds. Consequently, it is recommended to run the tool as slowly as possible. Typical logging speeds are between 1 and 10 m/min. The time needed for the actual logging can thus be calculated for a particular borehole and problem. A rule of thumb says that the total time needed is the actual logging time times 2. This factor takes into consideration the time needed to lower the tool to the bottom of the borehole before the logging is started, the time necessary to wait for temperature equilibrium of the electronic equipment (or crystals), and the time needed for setting up and dismantling of the equipment (tripod, diverter pulleys, laying out of cables, etc.). There are additional costs for mobilization and demobilization and for the surveying to determine the coordinates and elevation of the borehole. The well logging can be carried out by one geophysical engineer and one assistant. One geophysicist and possibly one assistant are needed for the processing, interpretation, and reporting. The processing will require a PC with the appropriate software and a plotter. The cost of equipment and services tends to be much lower than for petroleum logging. The cost of logging is typically a small percentage of the cost of drilling a borehole. It is important to select the proper sequence in which the measurements are made. The following criteria should be taken into consideration: If temperature is to be measured, it should be measured first so that the drilling fluid is not first mixed by the lowering and raising of the other logging tools. Also, a temperature logging tool is relatively inexpensive, and its loss in the case of difficult conditions in the borehole would not be as serious as would be the case if one of the more "sophisticated", expensive tools were lost. If there is the danger of caving of the borehole walls, the type of tool (free-hanging or wall contact) and the cost of the tool should be taken into consideration when the sequence of measurements is selected. Because it can be used for many purposes in both open and cased holes, a gamma-ray log is particularly important and is often one of the first measurements. The safety measures required when radioactive sources are used must also be taken into consideration. Most of the standard density devices use GBq ( mci) sources, which must be used with a controlled area and by an operator licensed for work with radioactive materials Quality assurance Normally, one value every ten centimetres is stored for each logging method. Field plots are prepared for the standard methods for immediate delivery to the client. The header of the field plot contains the essential data for the borehole and the logging method. The depth profiles are plotted at the scale agreed upon and labelled. A plot containing the curves for more than one method is called a composite log. Sections of a log are repeated only allow to prove that there is no drift or artefacts in the measurements. Repeatability, however, is not sufficient to demonstrate the quality of a well log. A number of factors influence the quality of logging data (Theys 1991). A major factor is the quality of the equipment the detectors and logging tools. This means an important decision about the quality of the logging data is made when the logging tool is purchased. The quality of the data is also affected by how carefully the sensors are calibrated. The measured values are influenced not only by the rock, but also by the geometry of the borehole, the drilling fluid, the well completion, and the position of the tool in the hole (centring). The correction terms for these factors are called 112

7 4.8 Borehole logging in hydrogeology environmental corrections (caliper corrections, stand-off corrections, and corrections for mud properties). The complexity of the environmental factors that affect log sensors has led to the development of numerous empirical curves for determining their influence on the measured geophysical parameters (e.g. Schlumberger Educational Services 1989, Western-Atlas International 1985). Only when the environmental corrections have been applied, are data obtained that really represent the properties of the rock. The quality of these corrections has a direct influence on the quality of the data and so on the determination of rock properties. If the measurements are made in several passes through the borehole ("runs"), the "zero depth" must be the same for each run. To take possible deviations in the starting depth from run to run into consideration, as well as any stretching of the cable, a gamma-ray detector is included in each tool so that it is possible to correlate depths of the different logs. the borehole diameter and the physical properties of the content of the borehole the ratio of the borehole diameter to the diameter of the tool and the position of the tool in the borehole the tool design (detector size, electrode spacing, transmitter-receiver spacing, radioactive source-detector spacing). Thus, each tool has a characteristic depth resolution and an average radial depth of investigation under the given conditions Interpretation of borehole logging data focused on hydrogeological problems Possible applications of borehole logging data in the field of hydrogeology are: The logging speed depends on the logging method and has a large influence on the quality of the data. Typical logging speeds are between 1 and 10 m/min. The sampling interval also influences data quality. It is typically 5 10 cm, with a vertical resolution of about 20 cm. In total, the quality of the data and information content of geophysical well logs are dependent on the measurement conditions, the investigation interval, and the depth resolution. The measurement conditions are determined primarily by the drilling method, the borehole geometry (diameter, borehole wall, caving), the content of the borehole (water, air, drilling fluid) and its physical properties (density, electrical resistivity, ph, neutron braking and absorption properties), the properties and size of the infiltrated zone (open borehole), and the type and properties of the well completion (fully cased, screens, annulus filling, gravel, clay, cement). The well logging methods which are based on different physical principles together with a sensor configuration adapted to the measurement conditions, can be used to obtain data from a limited, irregular rock volume. The vertical and radial extent of this volume is influenced by the following factors: (1) Lithological classification and documentation of the strata penetrated by a borehole on the basis of physical rock parameters (2) Correlation of the strata penetrated by a borehole with neighbouring boreholes (3) Determination of parameter values (e.g., electrical conductivity, seismic velocity) needed for the geological modelling of geophysical survey data obtained at the ground surface (4) Determination of hydrogeological and geotechnical parameter values from geophysical parameters (e.g., clay content, porosity, degree of saturation with water) (5) Determination of water influx and efflux in wells (6) Inspection of well completion conditions, including hydraulic efficiency (vertical flow in and around well casings) 113

8 THOMAS WONIK & KLAUS HINSBY (7) In situ determination of physico-chemical parameter values of groundwater (e.g. amount of dissolved ions in groundwater and important geochemical parameters such as ph, O 2, NO 3, Eh); pollution analyses or analyses of environmental tracers for groundwater dating (8) Identification of saline waters in the formation around a well. (1) The primary objective of geophysical well logging is to obtain geological information from the data. Characteristic physical rock parameters, especially several in combination, can be used to determine or confirm the lithology of the rocks penetrated by the borehole. The most useful parameters for this are gamma-radiation, electrical resistivity, acoustic velocity, and caliper. Increasing the number of parameters measured either increases the accuracy of the interpretation due to redundancy or can be used to determine further constituents of the rock. A plot of the typical responses of a set of hypothetical geophysical logs to different rock types is useful for deriving lithological profiles. This is shown for sedimentary rocks in Figure An example of a lithological log derived from well logs made in unconsolidated rock at the Schöneiche test site near Berlin is shown in Figure The borehole was drilled with a nominal diameter of 180 mm using drilling mud. The resistivity and neutron-neutron logs indicate thick layers of sand at m and m in the borehole. The gamma-ray, resistivity, and density logs indicate layers of silt at m and m. The susceptibility log and microlaterolog have a vertical resolution in the centimetre range. Interpretation of the well logs provides, for example, the thicknesses of homogenous layers for modelling. In addition to the identification of aquifers and low permeable layers, the lithological logs often aid decisions about the completion of the well. Fig : Typical geophysical downhole logs for a sequence of sedimentary rocks (Repsold 1989). SP: selfpotential log, ES: electric log 16- and 64-inch normal, FEL: focused electric log, IEL: induction log, GR: gamma-ray log, D: gamma-gamma-density log. 114

9 4.8 Borehole logging in hydrogeology Fig : Simplified lithology derived from borehole logs in unconsolidated sediment at Schöneiche in Brandenburg, Germany. The following logs were made in the open borehole (Zscherpe & Steinbrecher 1997): caliper (CAL) in mm; gamma-ray (GR) in API unit; electrical resistivity 16" normal (EL.N 16), 64" normal (EL.N 64), 25 cm normal (EL.KN), 100 cm normal (EL.GN); microlaterolog (MLL); gamma-gamma density (GG.D) in g/cm 3 = 10 3 kg m -3 ; neutron-neutron (NN) (calibrated with respect to water (WU=water units); and magnetic susceptibility (MAL) in relative units. S: sand; U: silt, Mg: marl. 115

10 THOMAS WONIK & KLAUS HINSBY (2) The borehole logs reflect the lithological units in the boreholes. Therefore, a correlation of the geophysical well logs of neighbouring boreholes is possible. The correlation distance depends on the lateral homogeneity of the strata. (3) Measurements of physical parameters in boreholes can be very helpful for the interpretation of surface geophysical surveys. Especially the parameters resistivity and seismic velocities are of great importance. Therefore, for the geological modelling based on geophysical survey data downhole logs can provide essential information. (4) Many geoscientific problems, particularly hydrogeological ones, can be solved by deriving petrophysical properties from well logs. Analysis of geophysical well logs requires knowledge of petrophysics (e.g., Schön 1996). Because rocks normally consist of a number of different minerals, their physical properties are determined by the proportions and properties of these minerals, their distribution in the rock and how they are bound. If the rock contains fluids (either liquids or gases), the properties of the rock are also influenced by the properties and distribution of the fluids within the rock. Normally, the properties of the rock matrix and the contents of the pores and fractures usually differ considerably. The number of geological models can be considerably reduced by quantitative data for parameters such as clay content, density, porosity, permeability, and storativity. The acoustic logging tool (Sect ) can be used to determine porosity. Assumptions about the lithology and fluid properties based on local knowledge or other measurements have to be made to estimate porosity. The first attempts to derive permeability values from well logs were made in the 1950s using resistivity data (Sect ). A qualitative permeability index can be indirectly calculated from gamma-ray, conductivity, and acoustic logs. Spectral gamma-ray logs (Sect ) can be used to distinguish between different clayey sediments on the basis of their U, Th, and K content, providing indirect information about their contaminant retention capacity. Acoustic logs (Sect ) via the compression modulus provide indirect information about the storage coefficient of the rock. (5) Horizons from which there is inflow into or outflow from the well are determined by measuring vertical flow in the well with a flowmeter (Sect ). These measurements also permit determination of the proportion of the total yield contributed by each horizon. Temperature and drilling mud resistivity measurements can be used also for this purpose. Other methods (e.g., PMT, Sect ) using tracers allow determination of groundwater flow rates. (6) Well logs can be used to check the condition of a well. Damage to PVC casing, for example, can be easily determined via focused electrologs (laterologs) (Sect ). Caliper logs (Sect ) and videocameras (Sect ) can also be used to check the condition of a well. Several parameters are logged to check the annulus fill, e.g., the depth and condition of the clay barriers. For this purpose, natural gamma-ray logs (Sect ) provide the best information. Electromagnetic methods (Sect ) have proved to be useful for identifying metal scrap forced into the rock during drilling. An example of well logs for checking the condition of a groundwater observation well is shown in Figure The well was drilled with a diameter of 150 mm and reamed out to 250 mm. Well completion was carried out using several different materials to fill the annulus for testing and calibration purposes. The well has a 5" diameter PVC casing with screens at depths of m and m. The following materials were used for the annulus fill: Dämmer- Suspension, Quellon, gravel, clay spheres, drill cuttings, Witterschlicker Clay, Compactonit, and a cement-bentonitemonazite-sand mixture. 116

11 4.8 Borehole logging in hydrogeology Fig : Borehole logs testing the well completion of a groundwater observation well: The different materials used for filling the annulus are indicated by the following logs (Zscherpe & Steinbrecher 1997): GR (gamma log in API units), MAL (magnetic susceptibility in relative units), GG.L with 0.15 and 0.35 m source-detector spacings (gamma-gamma log in cps), NN (neutron-neutron log in cps) 117

12 THOMAS WONIK & KLAUS HINSBY Fig : Freshwater/salt water interface in the Cuxhaven borehole in northern Germany in conductivity logs (DDL, see Sect ); the electrical conductivity of the drilling fluid was 0.33 S/m; DDLs: shallow dual laterolog; DLLd: deep dual laterolog; GR: gamma-ray log. The well logs provide an unambiguous determination of the annulus fill. The Quellon swelling clay can be clearly identified in the gamma-ray and susceptibility logs. The Compactonit swelling clay ( m) can be identified in the neutron-neutron, gammagamma, and susceptibility logs. The limits of the Witterschlicker Clay can be identified in the gamma-gamma and neutron-neutron logs. The gravel can be recognized in the neutron-neutron log together with gamma-gamma log. (7) The physical and chemical properties of groundwater can be determined by taking samples at specific depths (Sect ) or by measuring temperature, electrical resistivity, ph, redox potential, oxygen concentration and degree of saturation (Sect ). Determination of the fresh water/salt water boundary is an important application of drilling mud resistivity logging. This cannot be done, however, immediately after the drilling of the well or if a saline drilling mud was used. In such cases, it may be possible to measure the electrical resistivity/conductivity of the rock (Sects and ). The logging tools used for this 118 purpose have a relatively large lateral depth of penetration of about 100 cm and the data thus also reflect the electrical properties of the pore water in the sediment. A DLL log (Sect ) made in a borehole about 5 km from the coast near Cuxhaven is shown in Figure The electrical conductivity of the drilling fluid was 0.33 S/m. A transition zone separating fresh water and salt water is recognizable between 50 and 60 m in the DLL logs. Contamination of the groundwater can be easily detected if the electrical resistivity is changed by the contaminant. This is the case for brine and wastes from the metal processing industry. Organic substances do not ionize sufficiently to change the resistivity of the water, especially at low concentrations. Chemical and optoelectro sensors can be used to determine concentrations of dissolved substances (e.g., nitrate). Such sensors are becoming increasingly important for identifying and delimiting contamination plumes originating from old landfills and other contaminated sites. Well logs can be used for monitoring water quality in a well or well field.

13 4.8 Borehole logging in hydrogeology (8) The borehole logs of CAT_LAF07 (Figure 4.8.5) and GUEN 5374 (Figure 4.8.6) in the Cuxhaven area in Northern Germany show saline waters in and around the wells. In the borehole CAT_LAF07 the presence of saline (brackish) waters in both sand (0 46 m) and clay (46 69 m) sediments around and in the borehole is obvious. The electrical conductivity log of the water column in the borehole (SAL) can be used to calculate the NaCl equivalent content of the water (Western Atlas International 1985). The sandy and clayey parts of the borehole show NaCl equivalents of 1500 and 11,000 ppm, respectively. These values are considerably higher than the EU guideline value of 250 ppm chloride. The logging curve has a red fill where the logs indicate higher salinities than the guideline value. This seems also to be the case for the lower part (34 46 m) of the main aquifer at this site. The upper 20 m of the main aquifer (14 34 m below surface) contains fresh water. A sudden significant jump in the fluid conductivity is observed at about 35 m depth. This may indicate a leakage in the plastic casing of the well, as freshwater seems to enter the borehole at this depth. This also imply that the water samples collected from the well may be a mixture of saline waters from the screen interval at m depth and fresh water from around 35 m below surface. However, the contribution from the leak could be insignificant. measured for these two parameters at the upper 1/3 rd of the screen ( m), indicating that the major part of the water is pumped from this section Summary and conclusion Borehole logging is an important and strong tool in hydrogeology, e.g., for constructing regional or local geological models of groundwater reservoirs (quite similar to the construction of models for hydrocarbon reservoirs) and for evaluation of borehole condition and water producing fractures at a detailed scale. Borehole logging is the most certain and exact way to locate lithology changes, hence geophysical logging is important in combination with, e.g., seismic investigations and other surface geophysical measurements for geological modelling. Further, it is very important in combination with groundwater sampling from, e.g., monitoring wells, when delineating salt water intrusion or pollution plumes. The results and interpretation of hydrochemical groundwater analyses may be strongly misleading if sound knowledge about sampling intervals, borehole conditions and hydrochemical evolution is insufficient. Therefore, borehole logging should be considered in all management and research studies of the quantity and quality of groundwater. The logs measured in borehole GUEN 5374 also show the influence of saline waters both in the formation and in the borehole, but at a considerably lower concentration level. The groundwater pumped from the well is fresh although the fluid conductivity of the lowermost 2/3 rd of the screen (inside the borehole) indicates elevated NaCL equivalent contents of about 500 ppm. There is no indication of elevated chloride concentrations in the formation at this level, and this leads to the conclusion that the elevated chloride concentrations is drawn up into the screen from below the well. Comparison of the fluid conductivity logs run before and after pumping confirms that this must be the case. Logging of the changes in the fluid conductivity and temperature during pumping did not reveal any general trends. The conductivity and temperature show quite stable values of 90 ms/m and o C. This corresponds to the values 119

14 THOMAS WONIK & KLAUS HINSBY Fig : Geophysical logs in the borehole CAT_LAF07 in the Cuxhaven area. Column 1 is elevation in m, column 2 shows the location of the water table (near 0 m elevation) and the screen location at bottom. The log traces from left to right are (1) natural gamma-ray indicating clay contents in sediments (2) induction log (IL) (3) fluid electrical conductivity (SAL). Log traces 2 and 3 indicate the salinity in the formation around the borehole and in the borehole, respectively. Log trace (4) is the fluid temperature in the well. The areas with red fill indicate elevated salinity (salt contents) with chloride possibly above the drinking water standard (250 mg/l). Fig : Geophysical logs in the borehole GUEN_5374 in the Cuxhaven area. Column 1 is elevation [m], column 2 shows the location of the water table (near 0 m bsl) and the screen location at bottom. The log traces from left to right are (1) natural gamma-ray (2) induction log (IL) (3) fluid temperature (blue) and salinity (black/red). Red fills indicate elevated salinity (salt contents). Note that water from the upper part of the screen has chloride contents below the drinking water standard, while the lower part probably has concentrations above the drinking water standard (increasing downwards). 120

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16 THOMAS WONIK & KLAUS HINSBY Schön J (1996): Physical Properties of Rocks: Fundamentals and Principles of Petrophysics. Elsevier. Spies BR (1996): Electrical and Electromagnetic Borehole Measurements: A Review. Surveys in Geophysics 17: Taylor K, Hess J, Wheatcraft S (1990): Evaluation of Selected Borehole Geophysical Methods for Hazardous Waste Site Investigations and Monitoring. Final Report, United States Environmental Protection Agency, EPA/600/4-90/029, Las Vegas, Nevada. Western Atlas International (1985): Atlas Log Interpretation Charts. Houston TX. Yearsley LE, Crowder RE (1990): State-of-the-art Borehole Geophysics Applied to Hydrology. Canadian/American Conference on Hydrology, Sept , Zscherpe G, Steinbrecher D (1997): Bohrlochgeophysik. In: Knödel K, Krummel H, Lange G (Eds.): Handbuch zur Erkundung des Untergrundes von Deponien und Altlasten, 3, Geophysik, Springer-Verlag; Berlin: Theys PP (1991): Log Data Acquisition and Quality Control. Éditions Technip, Paris. 122