TECHNIQUES OF FINDING GROUND WATER TABLE

• Groundwater is the water located beneath the earth's surface in soil pore spaces and in the fractures of rock formations.

• The depth at which soil pore spaces or fractures and voids in rocks become completely saturated with water is called the water table.

Introduction

Determination of Groundwater

• There are various techniques of finding ground water. Some of these are:

i. Electrical Resistivity Methodii.Seismic-Refraction Methodiii.Test Drilling Methodiv.Remote Sensing Methodv.Water Divining

Electrical Resistivity Method• Most used method in Groundwater exploration.• It is based on the theory that “when an electric

current travels through a wire, it experiences a resistance (R) that is length proportional to the length of the wire (L) and inversely proportional to the cross sectional area (A) of the wire.”

• Resistance is a variable that depends on the intrinsic property of solid and fluid bodies called resistivity.

• Due to the presence of dissolved salts, Groundwater is ionically conductive and allows electric currents to flow into the ground.

• Measuring the ground resistivity gives the possibility to identify the presence of water.

• Following properties must be considered:i. A Hard Rock without pores or fracture and

a Dry Sand without water or clay are very resistive

• A porous or fractured rock bearing free water has a resistivity which depends on the resistivity of the water and on the porosity of the rock.

• An impermeable clay layer, which has bound water, has a low resistivity

• Mineral ore-bodies (iron, sulphides, etc.) have very low resistivities due to their electronic conduction.

• A current is transmitted with two electrodes, while the potential created on the surface by the circulation of this current into the ground is measured with two other electrodes.

• After scanning the data of electrical potential applied & its drop measured at different locations, the possibility of groundwater occurrence can be assessed.

Seismic Refraction• It is a geophysical principle (see refraction) governed

by Snell's Law. Used in the fields of engineering geology, geotechnical engineering and exploration geophysics, seismic refraction traverses (seismic lines) are performed using a seismograph(s) and/or geophone(s), in an array and an energy source. The seismic refraction method utilizes the refraction of seismic waves on geologic layers and rock/soil units in order to characterize the subsurface geologic conditions and geologic structure

• The methods depend on the fact that seismic waves have differing velocities in different types of soil (or rock): in addition, the waves are refracted when they cross the boundary between different types (or conditions) of soil or rock. The methods enable the general soil types and the approximate depth to strata boundaries, or to bedrock, to be determined.

Propagating seismic waves (bottom) and related travel time diagram (top) of the direct (blue) and the first refracted phase (green)

Seismic Refraction (Cont..)• Generally the Seismic method utilizes the

propagation of waves through the earth. This propagation depends upon the elastic properties of the rocks. The size and shape of a solid body can be changed by applying force. The body tends to return to its original condition when the external forces are removed. Similarly, a fluid resists changes in sizes (volume) but not changes in shape. This property of resisting changes in size or shape and of returning to the undeformed condition when the external forces are removed is called elasticity.

Uses• Mapping bedrock topography  • Determining the depth of gravel, sand or

clay deposits  • Delineating perched water tables  • Determining the depth to the water table  • Detecting subsurface caverns  • Estimating rippability  • Detecting shallow faults and fracture zones

 • Detecting large boulders

Advantages– Simple layout– Low manpower requirements– Limited Equipment Requirements– Rapid data reduction and analysis

(computer not needed)– Easy interpretation

Disadvantages– Relatively large energy input required– Relatively long layout (10 times depth)– Limited number of model layers– Limited velocity differences– Limited interface geometry (assume

smooth)

Test Drilling Method• This method is surest way of

finding the possibility of the availability of groundwater.

• Although it is costly and time taking.

• Drilling is done up to a depth of about 100 m and sample are collected to record the log of the hole.

Test Drilling Method (contd….)

• The test drill reveals the character, depth and thickness of the various strata , water table and water quality.

• This method is normally used in a flat country.

• The test holes also serves as observation wells during pumping test.

Test Drilling Method (contd….) • This was used to obtain detailed

subsurface information about rock types, fracture zone(s), rock fabrics, and hydrologic characteristics of water-bearing zones.

• Aquifer thickness

• Geological formations(geological features of the earth).

Test Drilling Method (cont..) • Hydrogeological properties (porosity,

permeability, major structural features such as unconformities).

Advantages

• Can reach great depths.

• Can penetrate wide range of rock types.

• Can produce high yielding wells.

Remote sensing is a technology for sampling electromagnetic radiation to acquire and read non-immediate geospatial data from which to pull info more or less features and objects on his Earths land surface, seas, and air.

REMOTE SENSING METHOD

What is remote sensing used for???

Remote sensing is a method for getting information about of different objects on the planet, without any physical contacts with it.

Image Source: cimss.ssec.wisc.edu

Advantages of Remote Sensing• Provides a view for the large region• Offers Geo-referenced information

and digital information• Most of the remote sensors operate

in every season, every day, every time and even in real tough weather

Application of Remote Sensing (Cont..)

Urbanization & Transportation•Urban planning•Roads network and transportation planning•City expansion•City boundaries by time•Wetland delineation

Image source: www.ldeo.columbia.edu

Image source: www.geospectra.net

AgricultureThe application of remote sensing in agriculture include: - Soil sensing- Farm classification - Farm condition assessment - Agriculture estimation - Mapping of farm and agricultural land characteristics - Mapping of land management practices - Compliance monitoring

Wageningen UR 2002

Application of Remote Sensing (cont..)

Dowsing or Divining • Many people believe that dowsing is a valid method

for finding groundwater, but most geologists and hydrologists disapprove of the practice.

What Is Dowsing or divining or water witching?• Dowsing,” “water witching,” “divining,” and “doodle-

bugging” are all names for the practice of locating groundwater by walking the surface of a property while holding a forked stick, a pair of L-shaped rods, a pendulum, or another tool that responds when the person moves above a location that will yield an adequate flow of water to a drilled well. 

What Is Dowsing or divining or water witching?

A person using a forked-stick dowsing rod in a field. The dowser walks through the field with the dowsing rod. When he walks over a location that has the potential of yielding water, the dowsing rod will rotate in his hands and point toward the ground. Many dowsers prefer forked sticks made from willow, peach, or witch hazel wood.

What Is Dowsing or divining or water witching?• People who practice dowsing believe that

groundwater moves in subsurface seams, veins, or streams that must be intersected by the drill to produce an adequate flow of water. They believe that locations where this water is present are surrounded by forces that will produce a response in their tools. Forked sticks held in front of a dowser will be deflected toward the ground, a pair of L-shaped rods held lightly in the dowser's hands will cross one another, and a pendulum suspended on a string will deflect from vertical as the dowser moves over a good location. 

The Basic Principle• Choosing a forked or Y-shaped twig

from a tree such as a mango tree or using metal rods.

• Positioning the twig or rods between the fingers so as to amplify the sensations felt and seeing if they twitch and go down (or cross) at the expected place.

• Criss-crossing the area in order to determine the most promising areas.

There Are Different Types of Divining Rods• The Y-shaped or V-shaped twig

- Metal rods- Parallel rods- The Hartmann lobe- The Lecher antenna

When the dowser uses metal rods, he holds them parallel between his fingers. When he comes close to a place under which there is water, the rods get closer together and end up crossing over one another where a groundwater source is significant. This can be tested and proves successful with many people, but the findings are imprecise and do not indicate the size of the aquifer.

Metal Rods (cont..)

 Why Do Landowners Hire Dowsers?• Drilling a water well can cost thousands of

dollars. It is a major investment that many landowners are hesitant to make without professional consultation. They want to be sure that the well is drilled in a location where it will produce water of adequate quantity and quality. This is why many people hire a dowser. They want to drill a successful well, close to their house, where the cost of installing water lines and an electrical conduit will be minimal and where a drilling rig can be easily driven.