Geophysical investigations are essential methods for assessing subsurface details quickly, economically, and with minimal labor using simple and portable instruments. The document outlines various geophysical methods and their applications in understanding geology, locating mineral deposits, and assessing groundwater quality. It further discusses factors affecting groundwater quality, sources of pollution, and the implications of groundwater contamination on public health.

1. 2.1 Geophysical Investigations
• Geophysical investigations involve simple methods of study made
on the surface with the aim of ascertaining subsurface detail. This is
achieved by measuring certain physical properties and interpreting them
mainly in terms of subsurface geology.
Importance of Geophysical Investigations
• Geophysical methods are gaining importance very rapidly because
of their success in solving a vast variety of problems.
• These investigations are carried out quickly. This means large area
can be investigated in a reasonable short period and hence time is
saved.
• The geophysical instruments used in the field are simple, portable
and can be operated easily. This means fieldwork is not laborious.
• Since the work is carried out quickly and only physical
observations are made. Without the use of consumables (like
Chemicals), it is economical too.

2. • Different interferences to suit different purposes can be drawn
from the same field data, for example electric resistivity data can
be interpreted for knowing subsurface of rock types, geological
structures, groundwater conditions, ore deposits depth to the bed
rock, etc. Hence the investigations are multipurpose.
Applications of Geophysical Investigations
• Geophysical explorations are numerous, important and widely
varied.
• Investigations aimed in solving problems of regional geology.
• Investigations aimed at locating and estimating economically
important mineral deposits.
• Investigations aimed at locating and assessing groundwater
potential and its quality
• Investigations aimed at solving problems connected with geology.
Classification of Geophysical Methods
There are many kinds of geophysical methods of investigation.
These method are
• Gravimetric method
• Magnetic method
• Electrical method
• Seismic method
• Radiometric method
• Geothermal method
 Electrical Method
• Among the methods different geophysical Methods electrical
method are numerous and more versatile, They are more popular
because they are successful in dealing with a variety of problems
like groundwater studies, subsurface structure, and many others.
Controlling Properties
• In electromagnetic methods, electrical conductivity, magmatic
permeability and dielectric constant of subsurface bodies are the
relevant properties.
Principle
• Electric methods are based on the fact that the subsurface
formation, structures, ore deposits, etc. possess different

3. electrical properties. These differences are investigated suitably
and exploited to draw the necessary conclusion.
• Electrical resistivity methods, electromagnetic methods, self-
potential methods and induce polarization methods are the very
important categories of electrical methods.
•

4.  Electrical Resistivity Method
Principle
• The electrical resistivity's of subsurface formation vary from one
another if they are inhomogeneous and are studied with the help
of resistivity method. In the case of a resistive subsurface body,
current lines move away from it and in the case of a conductive
subsurface body, the current lines move towards it.
• Profiling and Sounding are two types of resistivity investigations.
Profiling is done to detect lateral changes in resistivity. This
throws light on the change in the subsurface lithology or structure
from place to place.
• Sounding is done to determine the vertical changes in resistivity.
In other words, this study reveals changes in lithology, etc. at a
particular place with increasing depth.
• All geological formations have a property called electrical
resistivity which determines the ease with which electric current
flows through them. This resistivity is expressed in the units of Ώm
ohms meter and is indicated by the symbol ᵖ

5. Electrical Resistivity Measurements

6. Resistivity method and measurement of Resistivity
• For the principle of the electrical resistivity method of exploration
and for measurement of resistivity. A high resistive overburden is
a disadvantage for resistivity studies. This is so because very little
current penetrates the ground which means that the investigation
of deeper layer is not possible.
Classification of Resistivity Methods
• The resistivity method are classified as profiling type, sounding
type, and potential type of methods.
• Profiling method is used for measurement of resistivity in lateral
direction. Sounding type in which measurement are made in
vertical direction. Potential methods are used in ore prospecting
and are of not of engineering relevance.
Seismic Methods
• Controlling Properties
• Elastic property differences in rocks is the controlling property.
Principle
• Seismic method of study is based on the principle that subsurface
rock formations bear different elastic properties. Because of this,
the velocities of propagation of seismic waves through the
subsurface layers of earth, suffer reflection or critical reflection
arrive at the surface of the earth where they are detected by
geophones. From the time taken by the waves to travel through
the subsurface formation and from the seismic wave velocities of
the media. It is possible to determine the depth of various elastic
boundaries.
• With the help of geophones fixed at suitable intervals on the
ground, the different seismic waves reaching the surface are
recorded and from the times of their arrival, time –distance curves
are constructed. The direct waves are the first to reach the
geophones placed between point and the distance beyond the
point is called the critical distance.

7. • Depending upon whether reflected waves or refracted waves are
used in the investigation, there are two types of methods, namely,
seismic reflection method and seismic refraction method.
• A geophone an amplifier and a galvanometer are the basic units
required for reflected or refracted wave registrations.
• Seismic refraction studies are effective for depths more than
100m but are not suitable for shallow exploration
• Refraction methods are employed for investigating depths from
close to the surface to several kilometer deep. These methods are
also followed for the investigation of deeper crust under seismic
studies.
• Shallow seismic refraction have found effective application in
investigating the suitability of foundation sites for civil engineering
structures.
• Seismic Refraction: the signal returns to the surface by refraction
at subsurface interfaces, and is recorded at distances much
greater than depth of investigation
• Seismic Reflection: the seismic signal is reflected back to the
surface at layer interfaces, and is
• recorded at distances less than depth of investigation

9. 2.2 Ground water quality
- Factors affecting ground water quality
• Sediment and natural organic materials
• Nutrients
• Bacteria
• Toxic substances
Sediment and Natural Organic Materials
• Sediment is defined as particles derived from soil, rock, or organic
matter that have been, or are being, transported by water or
wind. Natural organic materials include plant debris, and human
and animal wastes. The erosion of land surfaces and stream banks
produces sediment. Erosion occurs naturally, but human activities,
like farming, logging, or road construction can increase sediment
transport to and within streams. Sediment deposited in streams
can restrict navigation. Sediment can also increase the potential
for floods by decreasing reservoir storage and stream-channel
capacity. Suspended sediments contribute to the reduction of
water clarity and quality. Fine sediments can severely alter aquatic
communities by clogging fish gills and suffocating fish eggs and
aquatic insect larvae. Harmful materials such as heavy metals and
toxic chemicals can attach to sediments and move with them
down the stream system. Sediment is a major water quality issue
in most places.
Nutrients
• Nutrients are any organic or inorganic compound needed to
sustain life. Examples include carbon, nitrogen, phosphorus, and
potassium. Nutrients are contributed to waters from the
atmosphere, agricultural lands, golf courses, lawns, septic
systems, wastewater treatment plants, and factories. An excess
amount of nutrients in water can result in a disproportionate
amount of aquatic vegetation. The decomposition of this excess
vegetation can remove oxygen from water and cause fish and
other aquatic life to die. An overabundance of aquatic vegetation
can also interfere with recreation. High nitrate or ammonia
concentrations can impact drinking water or kill fish. Nitrate and
ammonia are forms of nitrogen.

10. Bacteria
• Some bacteria are disease-causing organisms that may be
delivered to surface water and groundwater by sewer overflows,
leaking septic tanks, and runoff from animal feedlots or
pastures. Some bacteria are a threat to humans, and indicator
organisms such as fecal coliform are used to determine their
presence. Indicator bacteria are found in great numbers in the
intestines of humans and other warmblooded animals. When
water tests confirm the presence of the indicator bacteria, the
water body may be contaminated by untreated sewage and other
more dangerous organisms may be present.
Toxic Substances
• In sufficient quantities, toxic substances, such as cleaning solvents,
pesticides, and certain metals, can cause sickness, genetic
disorders, and even kill organisms. Toxic chemicals can enter
waters through direct discharge from industry or by improper
disposal of industrial, mining, farm, and household wastes.
Contaminants contributed from industrial uses of water include
toxic substances produced from cleaning solvents, acids, and
alkalis. The over application of pesticides can result in the excess
entering waters through runoff to surface water and infiltration
into groundwaters.
• Even extremely low concentrations of some chemicals are
hazardous to humans and aquatic life. Toxic substances also can
affect an organism’s growth, metabolism, reproduction, or
behavior. The potential dangers of many toxic substances are only
now being recognized. Assessing the environmental dangers of
these substances has been enhanced as our ability to detect
smaller concentrations has improved and our understanding of
their effects on the environment has increased.

11. 2.3 Water quality requirements
• Water quality criteria for aquaculture systems have typically
considered parameters such as temperature, dissolved oxygen,
total gas pressure, ammonia, and nitrite. Many of the published
criteria are derived for environmental protection of a wide range
of species and life stages. These criteria may not be appropriate
for a single species and life stage, especially in commercial
applications. The value of a given water quality criterion may
depend strongly on the species, size, and culture objectives. In
water reuse systems, fine solids, refractory organics, surface-
active compounds, metals, and nitrate may become important.
The limiting factors in very high intensity reuse systems are not
entirely understood at this time. Development of more relevant
water quality criteria for reuse systems will require production-
scale trials. Separate water quality criteria for bio filter operation
are also needed.
• The following are important requirements of water for domestic
use :
•

12. 2.4 Groundwater quality degradation
• Groundwater pollution occurs when pollutants are released to the
ground and make their way down into groundwater. It can also
occur naturally due to the presence of a minor and unwanted
constituent, contaminant or impurity in the groundwater, in which
case it is more likely referred to as contamination rather than
pollution.
• The pollutant creates a contaminant plume within an aquifer.
Movement of water and dispersion within the aquifer spreads the
pollutant over a wider area. Its advancing boundary, often called a
plume edge, can intersect with groundwater wells or daylight into
surface water such as seeps and spring, making the water supplies
unsafe for humans and wildlife. The movement of the plume,
called a plume front, may be analyzed through a hydrological
transport model or groundwater model. Analysis of groundwater
pollution may focus on soil characteristics and site geology,
hydrogeology, hydrology, and the nature of the contaminants.
• Pollution can occur from on-site sanitation systems, landfills,
effluent from wastewater treatment plants, leaking sewers, petrol
filling stations or from over application of fertilizers in agriculture.
Pollution (or contamination) can also occur from naturally
occurring contaminants, such as arsenic or fluoride. Using polluted
groundwater causes hazards to public health through poisoning or
the spread of disease.
• Different mechanisms have influence on the transport of
pollutants, e.g. diffusion, adsorption, precipitation, decay, in the
groundwater. The interaction of groundwater contamination with
surface waters is analyzed by use of hydrology transport models.

13. 2.5 Reasons of groundwater quality degradation
Factors affecting ground water quality
 Principal sources and causes of pollution with regard to their
occurrence:-
a. Municipal sources and causes
b. Industrial sources and causes
c. Agricultural sources and causes
d. Miscellaneous sources and causes
1. Municipal sources and causes
a) Sewer leakage
 Sources: Poor workmanship, defective sewer pipe, breakage by
tree roots, ruptures from heavy loads, earthquakes, loss of
foundation support etc.
 Results: Introduce high concentrations of BOD,COD, nitrate,
organic chemicals, bacteria and heavy metals into groundwater
b) Liquid wastes
 Sources: Domestic wastes, Disposal wells industries, storm, runoff
etc.
 Results: Introduce bacteria, viruses, trace elements and heavy
metals, inorganic and organic chemicals etc.
c) Solid wastes
 Sources: Landfills
 Results:
 Leachate from landfills can pollute groundwater
 Leachate include iron manganese, nitrate, trace elements etc.
2. Industrial sources and causes
a) Liquid wastes
 Sources: Industrial waste water discharged into pits, ponds,
lagoons etc.
 Results: Introduction of hazardous and toxic industrial wastes into
the groundwater

14. b) Tank and pipeline leakage
 Sources: Gasoline stations, fuel oil tanks, petroleum and
petroleum products from industrial pipelines and tanks
 Results: Immiscible liquids like oil and petroleum, liquid
radioactive wastes etc. reaches the water table and pollutes the
groundwater
c) Mining activities
 Sources:-
 Coal, phosphate and uranium mines
 Stone, sand and gravel quarries
 Results: Low pH, increase in iron, aluminium and sulphate content
in the soil
d) Oil-field brines
 Sources:-
 Substantial discharges of wastewater in the form of brine
 Constituents of brine include sodium, calcium, ammonia, boron,
chloride, trace metals and high total dissolved solids
 Results: Groundwater become saline
3. Agricultural sources and causes
a) Irrigation Return Flows
 Sources: Irrigation return flow drains to surface channels or joins
the underlying water
 Results:-
 Increases salinity of groundwater
 Increases the amount of bicarbonate, sulphate, chlorides, nitrates
etc. in the groundwater
b) Animal wastes
 Sources: Wastes from slaughter houses
 Results:-
 The natural assimilative capacity of the soil become overtaxed
 Salts, organic loads and bacteria are transported into the soil
 Nitrate-nitrogen is the most important persistent pollutant that
may reach the water table

15. c) Fertilizers and Soil Amendments
 Sources:
 Leachate of phosphate and potassium fertilizers
 Leachate of soil amendments like lime, gypsum and sulphur
 Results: Increases salinity of soil
d) Pesticides, insecticides and herbicides
 Sources: Leachate of pesticides, insecticides and herbicides used
in agricultural fields
 Results: Causes serious consequences in relation to the portability
of water
4. Miscellaneous sources and causes
a) Urbanisation
Groundwater pollution can occur both in rural as well as urban
areas and is affected by differences in chemical composition,
biological and chemical reactions, density and distance from
discharge areas
b) Spills and Surface Discharges
 Causal activities includes leaks from pipes and valves, uncontrolled
waste disposal, intermittent dumping of fluids on ground, flushing
hazardous and flammable liquids into water etc.
 Washing aircraft with solvents and spills of fuel at airports can
form a layer of hydrocarbons floating on the water table.
c) Stockpiles
 Solid materials are frequently stockpiled near industrial plants,
construction site etc.
 Precipitation falling on unsheltered stockpiles causes leaching of
heavy metals, salts and other pollutants into the groundwater
d) Septic tanks and Cesspools
 Septic tank: A watertight basin intended to decompose the
domestic sewage and to
Discharge this into the biologically active zone
Of the soil mantle through a subsurface percolation system

16.  Cesspools: large buried chamber with porous walls designed to
receive and percolate raw sewage
e) Saline water intrusion
 Salt water may invade freshwater aquifers to create point or
diffuse pollution sources
g) Surface water
 Polluted surface water bodies that contribute to groundwater
recharge become sources of groundwater pollution