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What are Geophysical Methods of sub-surface Investigations?

These are non-destructive methods (techniques) used to provide spatialinformation on soils roc! and hydrological and environmental conditions

("udhu M# Soil Mechanics and Foundations, $%&& 'rd dition)#

Geophysical methods are used in geotechnical investigations to evaluate and

predict the sites behavior in a seimic event and for to assess its suitability

for the construction of earth borne engineering structures#

Methods of sub-surface geophysical investigation

Electromagnetic wave methods

Ground *enetrating +adar (G*+ or georadar)

This technique is used primarily for the imaging of soils and groundstructures using high frequency electromagetic ,aves ,hich aretransmitted into the ground# n antenna is used to transmitt andrecieve radar inpluses produced by a generator# The returning pulsesare then processed to develop images of the soil pro.le# /hanges inthe dielectric of the soil (permittivity) re0ect changes in the sub-surface enviroment#

s such the main purposes for the G*+ are for soil imaging and thelocation buried ob1ects and is not suitable for highly conductive ,et

clays and silts# The G*+ survey provide a quic! imaging of thesubsurface conditions leaving everything virtually unchanged andundisturbed# It is particulary useful for de.ning sub-soil strataunderground tan!s buried pipes cables as ,ell as to characterisearchealogical sites before soil borings probes or e2cavation# 3nelimitation is that the G*+ is limited to shallo, depts of only about threeto si2 meters#

3ther electromagnetic ,ave methods include4

1. electical resistivity (+)

2.electromagnetic conductivity (M)

3. magnetometer surveys (M5)

4. +esistivity pie6ocone (+/*Tu)

Seismic Surveys

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5eismic surveys are achieved by measuring the magnitude and traveltimes of seismic ,aves as they penetrate subsurface materials#/ompression ,aves (* ,aves) are mainly used in the ,ave propagationand since its velocity changes through di7erent materials densitiesand elastic properties of geological materials can be determined# Inaddition the ,ave energy transmitted encounters re0ection refraction

and di7raction as it passes bet,een the boundaries of t,o elasticmedia# This information is used for futher geotecnical characteri6ation#

The technique sometimes use a device called a Geophone (motionsensitive transducers) ,hich are strategically positioned to convertground motion to electric signals# This is used ,hen a high resolutionsoil pro.le is required especially at large depths# 8or shallo, depthsthe ground is pounded by a sledgehammer to generate seimic ,aves#

2. Electrical Resistvity

+esistivity is fundamental electrical property of geomaterials and can

be used to evaluate soil types and variations of pore 0uid (e#g# ground,ater and conductivity) and changes in subsurface media# 5oilsresistivity is measure in ohm-centimeters (ohm-cm) and is thereciprocal of electrical conductivity# In general soil resistivity increases,ith soil grain si6e and decreases ,ith a decrease in soil moisture#

The method involves inserting four electrodes in the ground equallyspaced and applying an / current to the outer t,o electrodes# The/ voltage bet,een the inner t,o electrodes are then measured# Thisinformation is computed to determine corresponding resistivities#+esistivities of ,et .ne-grained soils are much lo,er than those of ,etcoarse-grained soils# subsurface resistivity pro.le of a soil is formedby ma!ing successive measurements at several electrode spacings atone location#

3ther methods of subsurface geophysical investigations include4

3. Mechanical ,aves