Groundwater Exploration [6 Hours]

6.1 INTRODUCTION — WHAT IS GROUNDWATER EXPLORATION?

Groundwater exploration refers to the systematic investigation of subsurface conditions to locate, evaluate, and develop groundwater resources. It involves the application of geological, hydrological, and geophysical techniques to determine:

• The occurrence and distribution of groundwater
• The quantity of groundwater available for extraction
• The quality of groundwater for intended use
• The most suitable locations for well construction

6.2 OBJECTIVES OF GROUNDWATER EXPLORATION

The primary objectives of groundwater exploration are:

A. Determination of Groundwater Occurrence

• To identify the type, location, and extent of aquifers
• To determine whether the aquifer is confined, unconfined, or semi-confined
• To assess the geological formations and their water-bearing capacity

B. Assessment of Groundwater Quantity

• To estimate the yield of potential wells
• To determine aquifer parameters such as transmissivity (T) and storativity (S)
• To assess the sustainable yield for long-term extraction
• To evaluate the recharge potential and safe yield of the basin

C. Evaluation of Groundwater Quality

• To determine the chemical composition of groundwater
• To check suitability for drinking, irrigation, and industrial purposes
• To identify potential contamination sources and pathways

D. Selection of Well Sites

• To locate the most productive zones for drilling
• To determine the optimal depth and design of wells
• To minimize exploration and drilling costs

E. Groundwater Management

• To develop a groundwater management plan
• To monitor water levels and predict future trends
• To plan artificial recharge where overdraft is occurring

6.3 SURFACE INVESTIGATION METHODS

Surface investigation methods are preliminary, low-cost techniques applied before undertaking expensive subsurface exploration.

6.3.1 Geological Survey

Study of surface geological features to infer subsurface conditions:

• Lithological study (Study of rocks): Identification of rock types, their distribution, and water-bearing potential. Sand, gravel, limestone → store water well; clay, hard rock → store little water. Search for porous and permeable rock.
• Structural study: Study of folds, faults, joints, and fractures. Cracks, joints and faults allow water to move and collect. More cracks → more groundwater chance. Fracture zones in hard-rock terrain often indicate productive well sites.
• Study of weathering: Weathered zones contain more groundwater. A thick weathered layer indicates better water yield.
• Landform study (Geomorphology): Study of landforms such as alluvial fans, flood plains, river plains, valleys, buried channels, and paleochannels. River plains & valleys → good groundwater; hills & mountains → poor groundwater.
• Hydrogeological study: Estimates water table depth and groundwater movement direction. Water flows from recharge areas (where rain enters) to discharge areas (rivers, springs). Creates maps showing zones with high, medium, and low chances of finding water.
• Geological mapping: Prepare maps showing rock types, structure, weathered zones, landforms, and water features. Helps in identifying suitable sites for wells or boreholes and planning groundwater development.

6.3.2 Hydrogeological Survey

Study of existing water sources and their relationship to subsurface geology:

• Inventory of existing wells — depth, yield, water quality, and water level data
• Study of spring locations and discharge measurements
• Analysis of stream flow and base-flow separation
• Measurement and mapping of water table elevations

6.3.3 Aerial Photographs and Satellite Imagery

Used to identify geological structures, drainage patterns, and vegetation anomalies indicating groundwater presence:

• Lineaments (faults, fractures, joints)
• Drainage density and pattern
• Vegetation patterns (phreatophytes indicate shallow water table)
• Soil moisture variations

6.3.4 Hydrological Data Analysis

Analysis of rainfall data, evapotranspiration, surface runoff, and water balance to estimate recharge and groundwater availability in a basin.

6.4 SUBSURFACE INVESTIGATION METHODS

Subsurface methods involve direct probing of underground formations. More definitive but more expensive than surface methods.

6.4.1 Test Drilling (Exploratory Drilling)

Test drilling is the most reliable method of groundwater exploration. It involves drilling boreholes to:

• Determine the sequence and thickness of geological formations
• Identify the water-bearing zones and their depth
• Collect rock and water samples for analysis
• Measure static water levels
• Conduct in-situ permeability tests

Types of drilling methods include: rotary drilling, percussion (cable-tool) drilling, and auger drilling. The choice depends on the geological conditions, depth required, and available budget.

6.4.2 Geophysical Well Logging

Geophysical well logging (or borehole logging) involves lowering sensing instruments into a drilled borehole to measure physical properties of the surrounding formations. Common logging methods include:

• Electrical resistivity log: Measures the resistivity of formations to identify aquifers (high resistivity in clean sands, low in clays)
• Spontaneous potential (SP) log: Measures natural electrical potential differences to distinguish permeable from impermeable layers
• Gamma ray log: Measures natural radioactivity; clay and shale have high gamma readings while clean sands have low readings
• Caliper log: Measures borehole diameter; enlarged zones may indicate fractured or soft formations
• Neutron log: Measures hydrogen content to estimate formation porosity

6.4.3 Pumping Tests

Pumping tests (covered in detail in Chapter 5) are conducted by pumping water from a well at a controlled rate while monitoring the drawdown in the pumping well and nearby observation wells. This provides aquifer parameters (T and S) and verifies the productivity of the site.

6.5 GEOPHYSICAL METHODS — OVERVIEW

Geophysical methods are indirect techniques that measure physical properties of subsurface materials from the surface without drilling. Different geological formations have different physical properties (electrical resistivity, seismic velocity, density, magnetic susceptibility, etc.).

6.6 ELECTRICAL RESISTIVITY METHOD

6.6.1 Principle

The electrical resistivity method exploits the contrast between the insulating rock matrix and the conductive pore electrolytes. Different earth materials have different electrical resistivities. An electric current (I) is introduced into the ground through two current electrodes, and the resulting potential difference (ΔV) is measured between two potential electrodes.

From Ohm’s law, the resistance of a cylindrical conductor of length L and cross-sectional area A is:

where ρ is the resistivity — an intrinsic property of the material, independent of geometry.

The 4-Electrode Concept

A simple 2-electrode setup would measure contact resistance at the electrode–soil interface (which is very high), not the earth’s true resistivity. The solution is to use four electrodes with separate circuits — an outer pair (C₁, C₂) to inject current and an inner pair (P₁, P₂) to measure voltage. Since no current flows through the potential circuit, contact resistance errors are eliminated (Telford et al., 1990).

When current I is injected into a homogeneous half-space, it flows radially outward in hemispherical shells. The potential at distance r from a single electrode is:

In practice the subsurface is heterogeneous. What the instrument records is an apparent resistivity (ρ_a) — a volumetric weighted average of electrical properties over the region sampled:

where \( K \) is the geometric factor that depends only on the electrode arrangement (spacing and configuration).

6.6.2 Typical Resistivity Values

6.6.3 Electrode Configurations

The most commonly used electrode arrangements are:

(a) Wenner Array

Four electrodes are placed in a straight line at equal spacing ‘a’. The two outer electrodes (A, B) are current electrodes and the two inner electrodes (M, N) are potential electrodes.

(b) Schlumberger Array

The current electrodes (A, B) are placed much farther apart than the potential electrodes (M, N). The distance AB/2 = L and MN/2 = l, where L ≥ 5l.

6.6.4 Types of Resistivity Surveys

(a) Vertical Electrical Sounding (VES)

Also called electrical drilling or depth profiling. The electrode spacing is progressively increased while keeping the center point fixed. As the spacing increases, the current penetrates deeper, allowing measurement of resistivity at greater depths.

• Purpose: To determine variation of resistivity with depth at a single location
• Output: A plot of apparent resistivity (ρ_a) vs. electrode spacing (a or AB/2) on log-log paper
• Interpretation: The shape of the curve indicates the number and nature of subsurface layers

(b) Horizontal Profiling (Constant Separation Traversing — CST) (slide)

Also called Constant Separation Traversing (CST), the electrode spacing remains constant while the entire array is moved along a horizontal profile (traverse line). This detects lateral variations in resistivity at a constant depth. (slide)

• Purpose: To trace the lateral extent of aquifers, locate faults, detect buried channels
• Output: A plot of apparent resistivity along the traverse line

Field Calculation Format (Wenner Example)

Station: VES-1    Array: Wenner
a (m)    I (mA)    ΔV (mV)    K = 2πa    ρ_a = K·ΔV/I (Ω·m)
 1       100       320       6.28        20.1
 2       100       85        12.57       10.7
 5       100       21        31.42       6.6
10       100       18        62.83       11.3
20       100       30        125.66      37.7

The values of ρ_a vs. electrode spacing (a or AB/2) are plotted on log-log paper and the curve shape is matched against standard type curves to determine layer thicknesses and true resistivities.

6.6.5 Interpretation of Resistivity Data

For VES data, the apparent resistivity (ρ_a) vs. electrode spacing (AB/2) is plotted on log-log paper. The curve shape is compared with theoretical master curves (or computer models) to determine the number of layers, true resistivity and thickness of each layer, and depth to the aquifer.

Standard Curve Types (3-Layer Earth)

For a three-layer earth model, the VES curve shape defines the vertical succession of resistivity:

In alluvial environments (Terai, Kathmandu Valley), an aquifer typically appears as a conductive layer sandwiched between resistive layers (H-type curve — the “Sandwich Model”). Key resistivity guidelines:

6.6.6 Advantages and Limitations

6.7 SEISMIC REFRACTION METHOD

6.7.1 Principle

The seismic refraction method is based on the fact that seismic waves travel at different velocities through different geological materials. When a seismic wave generated at the surface (by an explosion or hammer blow) encounters a boundary between two layers with different densities, part of the wave is refracted along the boundary and some energy returns to the surface.

By measuring the travel time of seismic waves from the source to a series of geophones (receivers) placed at known distances, the depth to subsurface interfaces (such as the water table or bedrock surface) can be calculated.

6.7.2 Snell’s Law

Refraction of seismic waves at a layer boundary is governed by Snell’s law:

where \( i_1 \) and \( i_2 \) are the angles of incidence and refraction, and \( V_1 \) and \( V_2 \) are the seismic velocities in layers 1 and 2 respectively.

At the critical angle (\( i_c \)), the refracted wave travels along the interface:

6.7.3 Typical Seismic Velocities

6.7.4 Time-Distance Graph

The travel times of first arrivals at each geophone are plotted against the source-to-geophone distance. For a two-layer case:

• The first segment (shorter distances) represents the direct wave through the upper layer with slope = 1/V₁
• The second segment (longer distances, after the crossover distance) represents the refracted wave along the interface between layers, with slope = 1/V₂
• The crossover distance (x_c) is where the refracted wave arrives at the same time as the direct wave
• The critical distance (x_cr) is the minimum source-to-geophone distance at which the refracted wave is first observed — it is always less than the crossover distance (slide)

6.7.5 Depth Calculation (Two-Layer Case)

For a simple two-layer case (e.g., soil over bedrock), the depth to the interface (h) can be calculated using:

Method 1: Using Crossover Distance

Method 2: Using Intercept Time (Intercept Time Method) (slide)

where:

• \( x_c \) = crossover distance
• \( t_i \) = intercept time (refracted wave line extended back to the time axis)
• \( V_1 \) = velocity of Layer 1
• \( V_2 \) = velocity of Layer 2

Three-Layer Earth Model

For a three-layer case (e.g., topsoil / weathered rock / sound bedrock with \(V_3 > V_2 > V_1\)):

Depth to Interface 1 (between topsoil and weathered rock):

Depth to Interface 2 (time-stripping method — subtract travel time through first layer):

Application: Locating the Water Table

Seismic refraction is effective for locating the water table because P-wave velocity increases significantly in saturated soil compared to dry soil. Unsaturated topsoil has \(V_1 = 300\text{–}800\) m/s, while saturated soil has \(V_2 \approx 1400\text{–}1600\) m/s. If the refracted wave gives \(V_2 \approx 1500\) m/s, the interface corresponds to the water table.

6.7.6 Advantages and Limitations

6.7.7 Comparison: Electrical Resistivity vs. Seismic Refraction

6.9 WATER WITCHING (DIVINING)

Water witching (also called water divining, dowsing, or rhabdomancy) is a traditional, non-scientific method of locating underground water. A person (the “diviner”) walks over the ground holding a forked stick (usually of willow, hazel, or peach wood), a pair of L-shaped metal rods, or a pendulum. The device is claimed to move or react when passing over underground water.

How is it Practised?

• A Y-shaped (forked) stick is held with the fork ends pointing upward and the stem pointing horizontally forward
• As the diviner walks over an area, the stick supposedly dips downward over underground water
• Alternatively, two L-shaped metal rods held loosely in the hands are said to cross or diverge over water
• Some diviners use a pendulum that swings in a different pattern over water

Scientific Assessment

Water witching has no scientific basis. Controlled scientific studies have consistently shown that the success rate of water diviners is no better than random chance. The observed movement of the rod is attributed to the ideomotor effect — involuntary, subconscious muscle movements by the person holding the device.

Advantages (Why It Persists)

• Low cost: Requires only simple materials (forked sticks, metal rods)
• Ease of use: Simple to learn and apply; no technical training needed
• Speed: Does not require extensive setup time
• In many alluvial areas, groundwater is prevalent at shallow depths, so any random well location is likely to find water — reinforcing perceived “success”
• Deep cultural tradition in rural and suburban areas

Drawbacks

• Lack of scientific evidence; results no better than random chance
• Unreliable and imprecise results
• Risk of costly drilling errors
• Cannot discriminate water type, depth, or yield
• Professional geophysical surveys are always more reliable

6.10 PAST QUESTION BANK FOR CHAPTER 6

CHAPTER SUMMARY AND EXAM KEY POINTS