Investigasi dan Interpretasi data Pendugaan keberadaan Airtanah Ir

Investigasi dan Interpretasi data Pendugaan keberadaan Airtanah Ir. Mohammad Sholichin MT. , Ph. D Website. www. water. lecture. ub. ac. id email : sholichin 67@gmail. com

Groundwater exploration Exploration of Groundwater 1. 1. 1 Surface exploration - “non-invasive" ways to map the subsurface. -less costly than subsurface investigations 1. Geologic methods 2. Remote Sensing 3. Surface Geophysical Methods (a) Electric Resistivity Method (b) Seismic Refraction Method (c) Seismic Reflection Method (d) Gravimetric Method (e) Magnetic Method (f) Electromagnetic Method (g) Ground Penetrating Radar and others

Groundwater exploration Exploration of Groundwater 1. 1. 2 Subsurface exploration 1. Test drilling geologic log drilling time log Water level measurement 2. Geophysical logging/borehole geophysics Resistivity logging Spontaneous potential logging Radiation logging Temperature logging Caliper Logging Fluid Conductivity logging Fluid velocity logging 3. Tracer tests and others

. 3 (a) Electric Resistivity Method ♦Electrical resistivity is the resistance of a volume of material to the flow of electrical current. ♦ current is introduced into the ground through a pair of current electrodes ♦ resulting potential difference is measured between another pair of potential electrodes ♦ Apparent resistivity is then calculated as: V is the measured Potential difference (in Volts) and I is the current introduced (in Amperes).

Electric Resistivity Method Wenner arrangement Schlumber configuration

The measured potential difference is a weighted value over a subsurface region controlled by the shape of the region, and yields an apparent resistivity over an unspecified depth. Vertical electrical Sounding (VES) Changing the spacing of electrodes changes the depth of penetration of the current. So it is possible to obtain field curve of apparent resistivity vs depth. For a single homogeneous, isotropic layer of infinite thickness, resistivity curve will be a straight line. True/actual resistivity - if formation is homogeneous and isotropic. Apparent resistivity if formation is anisotropic consisting of two or more layers of different materials.

Profiling or lateral traversing Electrode separation is kept constant for 2 to 3 values (say a=10 m, 15 m, 20 m) and the center of electrode spacing is moved from one station to another (grid points) to have the same constant electrode separations. Used to detect subsurface changes in horizontal direction. Wenner method is most convenient for this. Horizontal and vertical measurements allow to produce an iso-resistivity contour diagram.

Interpretation 1. Qualitative: from shape of the curves, decipher number of layers and resistivity relationships 2. Quantitative: (i) Analytical methods (mass curves or type curves)

Interpretation 1. Qualitative: from shape of the curves, decipher number of layers and resistivity relationships 2. Quantitative: (i) Analytical methods (mass curves or type curves) (ii) Empirical methods - Cumulative resistivity method - Inverse slope method Electrode spacing (a) Cumulative resistivity method (log scale) Semi-log paper Inverse slope method Electrode spacing, a

Usefulness valuable in determining -the depth and thickness of groundwater aquifers - the depth to groundwater, the depth and thickness of clay layers -in some cases the depth to bedrock, and accordingly limit the depth of well drilling. Resistivities vary over a wide range, depending on density, porosity, pore size and shape, water content and quality, and temperature.

Groundwater withdrawal exploration Groundwater Surface geophysical exploration methods Electric resistivity Surface geophysical methods In relatively porous formations, resistivity is controlled more by water content and quality within the formation than by the rock resistivity. For aquifers composed of unconsolidated materials, the resistivity decreases with the degree of saturation and the salinity of the groundwater. Clay minerals conduct electric current through their matrix, therefore, clayey formations tend to display lower resistivities than do permeable alluvial aquifers.

3 (b) Seismic refraction method based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy is provided by a source (hammer, weight drop or small explosive charge) located on the surface.

The seismic waves travel through the subsurface at a velocity dependent on the density of the soil/rock. When the seismic wave front encounters an interface where seismic velocity drastically increases, a portion of the wave critically refracts at the interface, traveling laterally along higher velocity layers. Due to compressional stresses along the interface boundary, a portion of the wave front returns to the surface A series of seismic receivers, geophones (right) are laid out along the survey line at regular intervals and receive the reflected wave energy.

Groundwater exploration Vo V 1 V 2 Surface geophysical methods Seismic refraction For horizontal two-layer case: For horizontal three-layer case: Data are recorded on a seismograph and later downloaded to computer for analysis of the first-arrival times to the geophones from each shot position. Travel-time versus distance graphs are then constructed and velocities calculated for the refractor layers Seismic refraction methods are effective in formations with definite boundaries between strata and where density increases with each successive lower layer.

q Travel time of waves depend on media (greatest in igneous, i. e. consolidated rocks, and least in unconsolidated rocks) q Seismic velocity increases markedly from unsaturaded to saturated zone. q The acoustic velocity of a medium saturated with water is greatly increased in comparison with velocities in the vadose zone. Thus, the refraction method is applicable in determining the depth to the water table in unconsolidated sediments.

SUBSURFACE EXPLORATION Groundwater exploration Subsurface geophysical methods Test drilling 1. 1. 2. 1 Test drilling -drilling small diameter holes that furnish information on substrata in a vertical line from the subsurface. - useful in (i) verifying other means of investigation (ii) confirm groundwater conditions prior to well drilling (iii) serving as observation wells for measuring groundwater levels and for conducting pumping tests. -If found fruitful, many a times the test holes are redrilled or enlarged to form pumping or production wells.

1. 1. 2. 1 Geologic Logging Groundwater withdrawal Groundwater exploration Subsurface geophysical methods Geologic logging constructed from the drill-cutting samples collected at frequent intervals during the drilling of a well. (samples are utilized for laboratory determinations of their hydrologic properties) Furnishes a description of the geologic character and the thickness of each stratum encountered as a function of depth, thereby enabling aquifers to be delineated. Preparation of proper geologic map may be difficult because of interference of drilling fluid with fine particles, and lack of proper knowledge to interpret the findings.

1. 1. 2. 1 (b) Drilling time log Groundwater exploration Subsurface geophysical methods Geologic logging - useful supplement to test drilling - consists of an accurate record of the time, in minutes and seconds, required to drill each unit of depth of the hole. - Because the texture of a stratum being penetrated largely governs the drilling rate, a drilling time log may be readily interpreted in terms of formation types and depths.

Groundwater withdrawal 1. 1. 2. 2 Geophysical logging/ Borehole geophysics What is Borehole Geophysical Logging? ØBorehole geophysical logging is a procedure to collect and transmit specific information about the geologic formations penetrated by a well by raising and lowering a set of probes or sondes that contain water-tight instruments in the well ØThe data collected can be used to determine general formation geology, fracture distribution, vertical borehole flow, and water-yielding capabilities. Groundwater exploration Subsurface geophysical methods Borhole geophysics

1. 1. 2. 2 Geophysical logging/ Borehole geophysics ØThe application of geophysical logging to groundwater hydrology lags far behind its comparable use in petroleum exploration. ØThe primary reason for this is cost. Most water wells are shallow, small-diameter holes for domestic water supply; logging costs would be relatively large and unnecessary. ØBut for deeper and more expensive wells, such as municipal, irrigation or injection purposes, logging can be economically justified in terms of improved well construction and performance. ØInterpretation of many geophysical logs is more of an art than a science

1. 1. 2. 2(a) Resistivity log Groundwater withdrawal Groundwater exploration Subsurface geophysical methods Resistivity logging

Factors that influence formation resistivity: (i)Nature of water (ii)Temperature of water (iii)Rock structure

ØMulti-electrode method is most commonly employed (minimizes the effects of drilling fluid and well diameter. ØSpacing between the elctrodes, and their arrangement, determine the radius of investigation. ØShort normal – records app. Resistivity of the mud-invaded zone (useful for locating boundaries of formations) ØLong-normal – records app. Resistivity beyond the invaded zone (useful for obtaining information on fluids in thick permeable formations) ØLateral – actual formation resistivity beyond the mud-invaded zone

Groundwater withdrawal Can be made only in the uncased portion of drill holes. Must also contain drilling mud or water Groundwater exploration Subsurface geophysical methods Resistivity logging Uses of Resistivity Logs interpretation and identification of rock types identification of the position of the water table determination of bed contacts and bed thickness determination of aquifer parameters evaluation of quality of formation water determination of depth of casing

1. 1. 2. 2(b) Spontaneous potential (SP) logging measures differences in the voltages of an electrode at the land surface and an electrode in the borehole potentials are primarily produced by electrochemical cells formed by the electric conductivity differences of drilling mud and groundwater where boundaries of permeable zones intersect a borehole. Groundwater exploration Subsurface geophysical methods Spontaneous potential logging

Deflections of the SP curve occurs due to the development of a liquid junction potential, i. e. potential difference across the junction from mud filtrate to formation water. The potential read for shales normally varies very little with depth. SP is measured relative to this base line zero called the shale line. . If water in permeable bed is more saline than drilling mud, SP is generally more –ve in the permeable bed than in the adjacent clay & vice versa. useful in determining water quality The right hand boundary generally indicates impermeable beds (e. g. clay, shale, and bedrock) Left-hand boundary indicates sand other permeable layers

Groundwater exploration Subsurface geophysical methods 1. 1. 2. 2(c) Radiation logging Generally two types: (i) measures the natural radioactivity (ii) detects radiation reflected from or induced in the formation from an artificial source 1. Natural-gamma logging 2. Gamma-gamma logging 3. Neutron logging

Groundwater exploration Subsurface geophysical methods Radiation logging Natural-Gamma logging all rocks emit natural gamma radiation originating from unstable isotopes (potassium, uranium, and thorium) Clayey formations (shale, clay) emit more rays than gravels and sands. Can be used to differentiate between sand, clay and gravel (this is identifying lithology, the primary application)

Groundwater exploration Subsurface geophysical methods Radiation logging Gamma-Gamma logging Gamma rays from a source in the probe (cobalt-60 or cesium -137) are scattered and diffused through formation. Part of the scattered rays re-enter the hole and are remeasured. The higher the bulk density of formation, the smaller the number of gamma-gamma rays that reach the detector. Primary applications: (i) identifying lithology (ii) measurement of bulk density and porosity of rocks.

Neutron logging Useful in determining the porosity of formations A fast neutron source is used to bombard the rock When any individual neutron collides with a hydrogen ion (of a water molecule), some of the neutron’s energy is lost and it slows down. A large number of slow neutrons, as recorded by a slow neutron counter, indicates a large number of fluid (i. e. high porosity) Results are influenced by hole size. Therefore, in large uncased holes, information on hole diameter is required for proper interpretation.

1. 1. 2. 2 (d) Temperature logging A vertical measurement of groundwater temperature in a well by a resistance thermometer Normally Temperature will increase according to geothermal gradients (roughly 3 o. C for each 100 m depth) Departures from this normal gradient may provide information on circulation (hydrologic Cycle) or geologic conditions in the well. Applications: (i) identify aquifers contributing water to a well. (ii) Provide data on the source of water (iii) identify rock types (iv) calculate fluid viscosity and specific conductivity from fluid resistivity logs (v) distinguish moving and stagnant water Abnormally cold water may indicate recharge from ground surface ( in deep well) Abnormally warm water may indicate water of deep-seated origin. Geothermal gradient is usually steeper in rocks with low permeability

1. 1. 2. 2 (e) Caliper Logging Provides a record of average hole diameter of a borehole Hole diameter will be equal to drilling bit when a hard sandstone is traversed. Diameter becomes larger for shales/clays as they become wet with mud fluid, slough off and cave into the hole. Applications: (i) identification of lithology and stratigraphic correlation (ii) Locating fractures and other rock openings (iii) Correcting other logs for hole-diameter effects

Groundwater withdrawal - Groundwater exploration - Well construction methods - Well design (to get optimum quantity of water economically from a given geological condition to meet the requirement for a particular scheme) - Well completion (placement and cementing of casing, screens) - Well development (increase specific capacity, obtain maximum economic well life)

TYPES OF WELLS AND METHODS OF CONSTRUCTION Factors important in choosing the type of wells §Quantity of water required §Economic consideration §Hydro-geologic consideration §Depth of water table

Well Construction Methods shallow depths; domestic wells; unconsolidated formations municipal, industrial domestic wells; Unconsolidated and consolidated formations Dug wells Bored wells Driven wells Jetted wells Percussion (Cable tool) method Hydraulic Rotary method Air Rotary method