Exploration for Groundwater | Geology Optional Notes for UPSC PDF Download

Groundwater Exploration

• Groundwater exploration is a common task undertaken by hydrogeologists or engineers to locate available groundwater resources.
• Finding the position of groundwater is a challenging endeavor that necessitates a fundamental comprehension of its presence in the subsurface geological structure.
• Exploration methods for groundwater can be categorized into direct and indirect approaches.
• Direct methods often involve test drilling, which is costly and not feasible for every individual due to its expense.
• Over the past two centuries, numerous techniques have been developed to facilitate groundwater exploration, falling into surface and sub-surface methods.

Surface Methods

• Surface methods are simple to use and require basic tools like maps, reports, field measurements, and lab data interpretations.
• Types of surface methods:
  • Esoteric Methods
  • Geomorphologic Methods
  • Geological & Structural Methods
  • Soil and Micro-Biological Methods
  • Remote Sensing Techniques
  • Surface Geophysical Methods

Subsurface Methods

• Subsurface methods involve Test Drilling & Borehole Geophysical Logging techniques, which are more costly than surface methods.
• Subsurface methods are used in large-scale government projects for detailed investigations.
  • Esoteric methods

Esoteric Methods

Esoteric methods are ancient practices like water divining or dowsing, where natural objects are used to locate groundwater sources.

Water Witching

Water witching, also known as water dowsing, is a traditional method involving a forked stick to find water sources.

Geomorphological Methods

• Surface drainage is a reflection of the land's topography, influenced by the underlying rocks.
• Groundwater flow often aligns with surface drainages, which can be impacted by underlying geological structures.
• Areas where streams meet at lower elevations are promising for groundwater presence.
• Various landforms are created by geological processes, some of which contain permeable layers conducive to groundwater. Examples include river-borne alluvial terraces, floodplains, abandoned channel deposits, glacial formations, and more.
• Geomorphological studies help identify potential groundwater-rich landforms like alluvial fans, beach ridges, sand dunes, and marshy areas.

Study of Landforms

• Landforms can indicate the presence of permeable layers. Locations like modern alluvial terraces, floodplains, and glacial deposits are favorable for groundwater exploration.
• Features such as drift-filled valleys, sand dunes overlying sandy sediments, and proximity to lakes and streams are promising signs for groundwater prospecting.

Topography and Drainage

• Physiographic methods assess surface topography and drainage patterns to identify potential groundwater sources.
• Areas where surface streams converge in small watersheds are good for groundwater collection, as hydraulic gradients typically follow topographic slopes.

Drainage Density of Stream Network

• Drainage density, the ratio of stream length to watershed area, indicates groundwater potential. Lower drainage density suggests higher groundwater potential.
• Higher drainage density, caused by more streams, leads to increased runoff and potentially lower groundwater potential.

Geological Methods

• Geologic investigations involve analyzing topographic maps, aerial photographs, and geologic records to understand groundwater dynamics.
• Field reconnaissance and evaluation of hydrologic data aid in assessing factors like groundwater recharge, discharge, and quality.
• Geological features such as joints, faults, and lineaments can significantly influence surface drainage and groundwater movement.

Groundwater Exploration Techniques

Zones for Groundwater Exploration

• Zones with high potential for groundwater exploration act as pathways for groundwater movement.
• Structural methods involve identifying contact points between permeable and impermeable strata, such as valleys, hillsides, and dykes.
• For instance, springs at the base of hillsides indicate groundwater presence, while analyzing dykes can help determine groundwater flow directions.

Well-Inventory Method

• The well-inventory method involves analyzing well-cuttings and inner surfaces of wells to understand subsurface geology, water level fluctuations, and recovery rates after pumping.
• By studying multiple wells in an area, researchers can assess regional groundwater potential and identify promising exploration zones.

Soil and Micro-Biological Methods

• Geo-botanical indicators, such as unusual vegetation growth patterns and presence of specific plant species, offer clues about groundwater locations.
• For example, the growth of termite mounds or alignment of trees can signal shallow groundwater, while certain plant species indicate saline groundwater.

Moist Depressions and Seepages

• Moist depressions, marshy areas, and seepages are indicative of groundwater presence, especially when associated with salt precipitates or specific vegetation types.
• Features like depression springs and artesian springs provide valuable information about water table levels and groundwater availability in different terrains.

Geophysical Methods Overview

• What is Geophysical Methods?
  • Geophysical methods refer to techniques used on the earth's surface to explore groundwater resources by observing physical parameters like density, velocity, conductivity, resistivity, magnetism, electromagnetic, and radioactive phenomena.
• Purpose of Geophysical Investigations
  • Geophysical investigations aim to detect differences or anomalies in physical properties within the earth's crust such as density, magnetism, elasticity, and electrical resistivity.
  • The main goal is to locate potential zones for groundwater exploitation by identifying indirect indicators.
• Common Geophysical Methods
  • Electrical, Seismic, Gravity, and Magnetic methods are commonly used for solving hydrogeology problems.
• Gravity Method
  • The gravity method involves using gravimeters to measure density differences on the earth's surface, indicating underlying geologic structures.
  • It is widely used for finding mineral resources and groundwater in sedimentary terrain.
• Magnetic Method
  • Magnetometers are used in the magnetic method to detect and map magnetic fields of the earth.
  • While not directly related to groundwater, this method can provide information on geologic structures relevant to groundwater studies.
• Seismic Method
  • Seismic methods include seismic refraction and reflection techniques.
  • Seismic waves are used to study subsurface zones like fractures, fissures, faults, and lineaments.
  • Seismic reflection methods offer information on geologic structure deep below the surface, while seismic refraction methods focus on shallower depths relevant to groundwater studies.

Seismic Velocities Analysis

• A fundamental comprehension of seismic velocities is crucial for identifying geological materials like alluvium or bedrock.
• Seismic velocities increase notably in coarse alluvial areas from unsaturated to saturated zones.
• Seismic methods can map the depth to the water table with approximately 10% accuracy in uniform geological conditions.
• Changes in seismic velocities are influenced by alterations in the elastic properties of formations.
• Distinct contrasts in these properties allow for clearer identification of formations and their boundaries.

Electrical Resistivity Method

• Electrical surveys aim to determine subsurface resistivity distribution by conducting measurements on the ground surface.
• These measurements help estimate the true resistivity of the subsurface, which is linked to geological factors such as mineral content, porosity, and water saturation in rocks.
• Electrical resistivity surveys have been utilized for hydrogeological, mining, geotechnical, and environmental investigations.
• Various geophysical methods are based on electrical properties, with resistivity measurements typically involving current injection and voltage measurement using different electrode arrays.

Vertical Electrical Sounding (VES)

• VES is employed to assess resistivity changes with depth and is suitable for horizontally layered grounds with minimal lateral variation.
• Apparent resistivity values are obtained using resistivity meters, which provide resistance values that are then converted to apparent resistivity.
• Interpreting true subsurface resistivity involves complex relationships and often necessitates inversion of apparent resistivity data using specialized software.
• Data interpretation usually assumes a horizontally layered subsurface structure for analysis.

Electrical Profiling

• Electrical profiling involves the measurement and interpretation of subsurface resistivity variations.
• This method provides insights into the resistivity characteristics of the ground at different depths.

Resistivity Survey Techniques:

• Profiling Method:
  • Electrode spacing is fixed, and the array moves along a straight line.
  • Provides information on lateral subsurface resistivity changes but not vertical changes.
  • Interpretation of profiling survey data is mainly qualitative.
• Limitations of Resistivity Sounding Method:
  • Commonly encounters horizontal changes in subsurface resistivity.
  • May lack accuracy in complex geologies with rapid resistivity changes over short distances.
• Conversion of Resistivity Data:
  • Understanding typical resistivity values for different subsurface materials and survey area geology is crucial.
  • Resistivity surveys offer insights into subsurface resistivity distribution.

Aquifer Characteristics:

• Resistivity Values for Various Materials:
  • < 20 Ω-m: Indicates aquifer with chloride ion concentration of 250 ppm (e.g., fine sand & limestone).
  • Porosity significantly influences resistivity.
  • Pore fluid conductivity is impacted by water quality and lithology.
• Types of Aquifers Based on Resistivity:
  • Brackish aquifer: Moderate salinity.
  • Freshwater aquifer: Low salinity, suitable for drinking water.
  • Clay aquifer: Low permeability, affects water movement.
  • Oil-bearing sands: Contains oil deposits.
• Geological Materials and Resistivity:
  • Crystalline rocks: Granite, igneous rocks, compact sandstones.
  • Consolidated sedimentary rocks: Slates, shale, limestone.
  • Unconsolidated sedimentary rocks: Clays, sands, alluvium.
  • Surface soils: Upper layer of the Earth's crust.
• Electrical Resistivity of Rocks and Soils:
  • Igneous and Metamorphic Rocks: These types of rocks typically exhibit high resistivity values, which are influenced by the degree of fracturing and the presence of groundwater in fractures.
  • Sedimentary Rocks: In contrast, sedimentary rocks, being more porous with higher water content, generally have lower resistivity values compared to igneous and metamorphic rocks.
  • Soils: Wet soils and fresh groundwater demonstrate even lower resistivity values, with clayey soil typically having lower resistivity than sandy soil.
  • Factors Affecting Resistivity: The resistivity of rocks and soils depends on various factors like porosity, water saturation, and salt concentration, leading to an overlap in resistivity values among different classes of rocks and soils.
• Groundwater Resistivity:
  • Variability: Groundwater resistivity ranges from 10 to 100 ohm-m, depending on the dissolved salt concentration, with seawater having a low resistivity of about 0.2 ohm-m due to its high salt content.
  • Application: The resistivity method is effectively used to map saline and fresh water interfaces in coastal areas due to these distinctive resistivity values.
• Wenner Array:
  • Functionality: The Wenner array, known for its sensitivity to vertical subsurface resistivity changes, has a moderate depth of investigation but is less sensitive to horizontal changes.
  • Advantages: It offers a strong signal strength compared to other arrays, making it suitable for surveys conducted in noisy environments.
• Schlumberger Array:
  • Configuration: The Schlumberger array comprises current electrodes (A, B) and potential electrodes (M, N) and is used for resistivity calculations in homogeneous and isotropic mediums.
  • Equations and Calculations: Resistivity calculations in the Schlumberger array involve specific equations to determine resistivity values based on potential differences and geometric factors.
• Dipole-Dipole Array:
  • Application: Widely utilized in resistivity surveys, the dipole-dipole array is known for its sensitivity to horizontal resistivity changes while being less sensitive to vertical variations.
  • Characteristics: The spacing between current and potential electrodes influences the array's ability to map horizontal resistivity changes effectively.

Geophysical Surveying Techniques

Vertical Electrical Sounding (VES)

• Vertical structures like dykes and cavities can be well mapped using VES, but it is less effective for horizontal structures such as sills or sedimentary layers.
• Interpreting VES data involves two main steps: initial interpretation through curve-matching methods and a more detailed interpretation using computer programs.
• From these interpretations, layer-wise resistivities and thicknesses can be determined to create spatial variation maps highlighting low resistivity areas and aquifer horizons.

Electromagnetic Method

• Electromagnetism involves creating a magnetic field through current flow in a conductor.
• An electromagnet is formed by coiling a current-carrying conductor around a core material like iron, concentrating the magnetic field in the core.
• The strength of the magnetic field is directly proportional to the current flowing through the conductor.
• There are two methods: Passive (using natural signals) and Active (using artificial sources).

Principles of EM Surveying

• An alternating current (AC) passed through a wire coil generates an EM field above and below the ground.
• If there is conductive material underground, the magnetic component of the EM wave induces eddy currents in the conductor.
• The receiver detects both primary and secondary EM fields, providing information about subsurface conductivity and geometry.
• The apparent conductivity measured reflects the average conductivity of ground layers near the instrument to a certain depth, influenced by various factors like coil spacing and operating frequency.

General Principles of Electromagnetic (EM) Operation

• Two Methods of EM Surveys

  • The Time-Domain Electromagnetic (TDEM) surveys involve measurements over time using electromagnetic induction to generate responses from sub-surface features.
  • In TDEM, a time-varying magnetic field is created by terminating a steady current in a cable loop, inducing eddy currents in conductive underground materials.
  • Decay of these eddy currents reflects the materials' conductive properties, measured by a receiver coil on the surface.
  • Frequency-Domain (FDEM) EM surveys relate to measurements at different frequencies, with a transmitter producing a continuous EM field and secondary field determined by nulling the primary field.
  • In TDEM, the primary field is applied in pulses then switched off for measurement, often using a large coil on the ground and a small moving coil.

Geophysical Logging Techniques

• Definition and Evolution of Geophysical Logging

  • Geophysical logging involves recording measurements or observations in boreholes to gather specific information about penetrated geological formations.
  • These records help determine lithology, structural distribution, fluid flow, and water-yielding potential of formations.
  • Geophysical logging has evolved since 1927 when the first electric log was run by the Schlumberger brothers.
  • In India, geophysical logging of water wells began in 1953 by the Geological Survey of India (GSI).

• Types of Logging Techniques

  • Logging techniques can be categorized into those utilizing natural sources and those using controlled sources.
  • These techniques measure physical parameters across subsurface formations to provide a continuous record of parameters like lithology, porosity, and fluid quality.
  • Properties such as electrical conductivity, magnetic susceptibility, radioactivity, and velocity are utilized in interpreting the data.

• Purposes and Applications of Well Logging

  • Well logging helps identify formations, determine fluid salinity, and aids in stratigraphic correlation, detecting bed boundaries, and porous zones.
  • It is crucial for water well design, construction, and studying sea water intrusion in coastal aquifers.

• Types of Logging Methods

  • Electric logging involves measuring electrical resistivity and Self-Potential (SP).
  • Radioactive logging uses gamma ray and neutron logs.
  • Induction logging and sonic logging are also common methods in geophysical logging.

Geophysical Methods in Hydrogeology

Electric Well Logging

• Fluid logging includes temperature, fluid resistivity, flow meter, and tracer logging.
• Caliper logging measures borehole diameter.
• Electric well logging records electrical resistance/resistivity and SP in formations through a drill borehole.
• SP logging records potential drop between borehole electrode and a surface reference electrode, useful for deciphering saline water and clay zones.
• Resistivity logs are crucial for groundwater and mineral exploration.

Photogeology

• Photogeology involves analyzing earth features through aerial photographs.
• Aerial photographs provide insights into physiographic features, rock types, mineralized zones, water resources, vegetation, cultivation zones, and urbanization.
• Remote sensing technology aids in rapid development.
• Photogeology distinguishes between rock and soil types, indicating permeability and groundwater recharge areas.
• Aerial photos reveal fracture patterns in rocks, aiding in identifying potential groundwater zones.

Remote Sensing Techniques

• Remote sensing acquires Earth surface information without direct contact.
• It involves sensing and recording energy, analyzing it for various applications.
• Remote sensing plays a crucial role in hydrology, water resources, mineral exploration, and environmental monitoring.
• Applications include delineating groundwater potential zones, identifying recharge sites, and analyzing future recharge locations.

Applications of Remote Sensing

• Satellite data products offer varied spectra for interpreting high-resolution images to identify groundwater potential zones.
• Thematic layers are prepared based on hydrogeomorphic units, land use, rock types, and structures.
• Hydrogeological conditions are influenced by lithology, geomorphology, and structural features.

Groundwater Exploration Methods

• Geological, hydrogeological, and geophysical techniques are used to identify groundwater potential zones.

Utilization of Satellite Data

• Satellite data, along with limited field verification, is crucial in identifying priority zones, often represented as hydrogeomorphic units.

Role of Remote Sensing

• Remote sensing aids in mapping the distribution of hydrogeomorphic units, providing valuable insights for groundwater exploration.

Importance of Interpretation

• Interpreting satellite images and aerial photographs is essential for understanding geological, structural, and hydrogeological conditions favorable for groundwater occurrence.

Challenges in Groundwater Exploration

• Groundwater exploration is challenging due to the hidden nature of the resource, necessitating the use of indirect methods to locate potential sites.

Key Factors for Success

• Success in groundwater targeting depends on experience in recognizing geological, structural, and hydrogeological conditions conducive to groundwater presence.

Role of Modern Tools

• Modern tools such as remote sensing and aerial photography offer valuable spatial data, aiding in rapid understanding of geological features and informed decision-making.

Conclusion

• Groundwater exploration involves a combination of traditional methods and modern technologies to identify potential zones effectively.

By combining various geological, hydrogeological, and geophysical methods, experts can pinpoint areas with high groundwater potential. Satellite data, coupled with on-ground verification, plays a crucial role in prioritizing exploration efforts. Remote sensing techniques help map out hydrogeomorphic units, providing a detailed distribution of these features across the landscape. Interpreting satellite images and aerial photographs is vital in understanding the geological and hydrogeological conditions conducive to groundwater presence.Groundwater exploration is inherently complex due to the hidden nature of this vital resource. Success in this endeavor hinges on the ability to interpret geological structures, understand hydrogeological conditions, and utilize modern tools like remote sensing for efficient decision-making. By leveraging these tools and methods, researchers and practitioners can enhance their understanding of groundwater dynamics and optimize exploration strategies.