By:

Muhammad Amar Gul

2012-MS-GS-01

Presented To:

Sir GMD Sohail

Department of Geological Engineering,

University of Engineering & Technology Lahore

ContentsIntroduction Principal InstrumentSurveying MethodsData Interpretation ApplicationsLimitations

Measures earth's response to electromagnetic signals transmitted by an induction coil.

Induction coil produces magnetic field The alternating magnetic field induces electric current in

the material under the ground, which in turn produces secondary magnetic field.

Intensity of Secondary magnetic field measured by sensor High-frequency signal shallow part of the ground, Deeper part of the earth responds to low-frequency

signal.

Introduction

Electric conductivity & magnetic susceptibility are calculated for each frequency.

Readings expressed in the conductivity units of

milli-ohms/meter

These Properties depend on following:

Nature of Rock

Water saturation

Salinity

The resultant maps are used for Nature of  underground rock formations Ground water & Ground Water Contamination Other Geological / environmental changes

The basic principle of EM surveying method is Electromagnetic induction.

Primary electromagnetic field generated by passing alternating current through a small coil.

The response of the ground is the generation of secondary electromagnetic fields .

The resultant fields detected by the alternating currents that they induce to flow in a receiver coil by the process of electromagnetic induction.

Differences between the transmitted and received electromagnetic fields reveal the presence of conductor and provide information on its geometry and electrical properties.

Principal

Commonly use a coil Consists on several hundred turns of copper wire wound on a

circular or rectangular frame typically between 0.5m and 1m across.

Coil configurations:

There are several possible transmitting - receiving coil configurations. These are:

Central loop (in-loop) Coincident loop Fixed loop Moving loop

Instrument

To obtain information about variations of conductivity (or its reciprocal, resistivity) with depth Frequency Domain Measures the electrical response of the subsurface at several frequencies Time Domain EM

Measures the electrical response of the subsurface to a pulsed wave at several time intervals after transmission, longer time intervals measure greater depths.

Electromagnetic Surveying Methods

Tilt Angle MethodVery Low Frequency (VLF) MethodAFMAG MethodGround Penetrating Radar(GPR)

Used for large-zone investigations

A set of coils is mounted and carried behind a small aeroplane or helicopter

In airborne EM systems both techniques, FDEM and TDEM, are present

Airborne EM systems

1st step: attempt to determine from the shape of the anomaly

a simple model 2nd step: to measure characteristics of the anomaly and

to plot these at the scale of the appropriate nomograms

From the nomogram and the shape of the anomaly, estimates generally can be made for quality of the conductor, depth to top of the conductor, conductor thickness, dip, strike, and strike length.

Interpretation of Data

Exploration of metallic ferrous mineral deposits Massive Sulfide mineral Deposits by AFMAG VLF is an effective reconnaissance geophysical tool

for mapping geoelectric features. They may includes

fault mapping, groundwater investigations, overburden mapping, contaminant mapping, mineral exploration

Applications

GPR method could be used for Engineering geology, Hydrogeology & Hydrology, Constructions & Civil engineering, Archeology & Environment protection

Versatile and efficient survey technique

But it suffers from several drawbacks such as Non-economic sources such as graphite, water-filled shear zones, bodies of water and man-made features wet clays and graphite-bearing rocks may screen the effects of deeper conductors

Penetrations not very great, Maximum penetration in ground surveys is 500m & 50m in

airborne survey.

Limitations