M.ENG PRESENTATION LECTURE ON GEOPHYSICAL METHODS BY A. A. ONOJA. BAYERO UNIVERSITY, KANO

1. PRESENTATION/LECTURE
ON
GEOPHYSICAL METHODS
SITE INVESTIGATION AND LABORATORY MEASUREMENTS
(CIV 8326)
ANDREW ABAH ONOJA
SPS/15/MCE/00023
M.ENG – CIVIL ENGINEERING
DEPARTMENT OF CIVIL ENGINEERING
BAYERO UNIVERSITY, KANO
16TH June, 2016

2. • INTRODUCTION
• Geophysics is the application of the principles of
physics to the study of the Earth. The Earth is
comprised of materials that have different
physical properties. Clay and dolerite, for
example, have different densities, acoustic
velocities, elastic moduli, electrical
conductivities, magnetic susceptibilities, and
dielectric constants. Geophysical instruments
are designed to map spatial variations in the
physical properties of the Earth.

3. • Exploration in geophysics is an applied branch of
geophysics, which uses physical methods (such as
seismic, gravitational, magnetic, electrical and
electromagnetic) at the surface of the Earth to measure
the physical properties of the subsurface, along with the
anomalies in those properties.
• The science of geophysics applies the principles of
physics to the study of the Earth. Geophysical
investigations of the interior of the Earth involve taking
measurements at or near the Earth’s surface that are
influenced by the internal distribution of physical
properties. Analysis of these measurements can reveal
how the physical properties of the Earth’s interior vary
vertically and laterally.

4. • Geophysical methods are investigative techniques that
directly or indirectly measure the presence of resources or
material behaviour concealed within the earth’s subsurface
as a result of geologic processes or human disturbances.
• The methods describe the techniques used to collect
subsurface information related to the physical properties of
earth material (Technos, 2004). The techniques are useful in
the following areas:
• To detect subsurface contrasts, including mass-density
relationships, ionic or electrical potentials, magnetic
susceptibilities, and elemental decay.
• To identify the location of archeological resources and lead
to their identification.
• To investigate buried prehistoric and historic structures and
artifacts.

5. GEOPHYSICAL METHODS
Electrical
Resistivity
Methods
Seismic
Methods
Gravity
Geophysical
Method
Electromagnetic
Method
Geothermal
Method
Resistivity
Profiling
Resistivity
Sounding
Refraction
Reflection
Frequency
Domain EM
2D-Resistivity
imaging
Time Domain
EM
Ground
Penetrating
Radar
Radiometric
Surveys
Micro-Gravity

6. * Geophysical methods are generally non-intrusive
and can be employed quickly to collect subsurface
data. When performed properly and utilized early in
the site characterization process, the methods can
provide valuable information for placing monitoring
wells and borings. They can be used later in the
investigation to confirm and improve site
characterization.
* Measurements are taken at or near the surface
and are classified by the physical property being
measured. When selecting a geophysical method,
the following should be completed:

7. • (1) Define the objective of the investigation.
• (2) Review site-specific geology.
• (3) Determine if cultural features are present that may
interfere with the instrument(s).
• (4) Determine site access.
• (5) Consult with a person with expertise in geophysical
data reduction and interpretation.
• (6) Determine cost.

8. • Geophysical exploration may be used with advantage to
locate boundaries between different elements of the
subsoil as these procedures are based on the fact that
the gravitational, magnetic, electrical, radioactive or
elastic properties of the different elements of the subsoil
may be different.
• Differences in the gravitational, magnetic and radioactive
properties of deposits near the surface of the earth are
seldom large enough to permit the use of these
properties in exploration work for civil engineering
projects. However, the resistivity method based on the
electrical properties and the seismic refraction methods
based on the elastic properties of the deposits have been
used widely in large civil engineering projects.

9. * Electrical Resistivity as a Geophysical Method
• Electrical resistivity uses electrical resistance (poor
conductivity) properties to identify buried cultural
resources.
• A highly refined electrical resistivity survey may be
the most revealing geophysical technique, but it is
expensive to perform because it requires a high
number of readings per unit area. Resistivity experts
interpret electrical resistivity patterns to identify the
presence of nearly all forms of constructed features,
such as foundations, paths, and roads. The
technique can also reveal compacted soils, indicative
of a former pathway, and disturbed soils, such as
those found at burial sites and cultivated fields.

10. • Electrical resistivity is useful for measuring
depth to bedrock and is often performed
before GPR in geophysical surveys involving
multiple techniques. Depth to bedrock
measurements is useful in calibrating GPR
equipment. Electrical resistivity uses current
electrodes to introduce into the soil, an
electrical current of known amplitude
(amps) and frequency (volts), and potential
electrodes with an ohmmeter to measure
resistance changes in the soil, vertically and
horizontally.

11. • Fig: Electrical Resistivity Apparatus (TERAMETER)

12. • The field between the electrodes is distributed
only near the surface when the electrodes
spacing is close but the electrical flux flows
deeper when the electrodes are further apart.
The flux will crowd into the more conductive
layers and will rarefy in the more resistive
layers.
• The potential at the surface will reflect these
path differences and will provide a data set for
which an electrical profile model of the
subsurface can be calculated.

13. • It is important to note that not all geophysical methods are
appropriate for groundwater exploration. The principal
methods used in groundwater investigations include Electrical
Resistivity (ER), Electromagnetics (EM), and Nuclear Magnetic
resonance (NMR). The limitations associated with these
methods have prompted hydrogeologists to use more than one
method to collect accurate data in groundwater exploration
(Revil et al., 2012).
• The most popular methods used in hydrogeological
applications are ER and EM because of the close relationship
between electrical conductivity and the physical properties of
aquifers, i.e. conductance and resistance. Thus Resistivity and
Electromagnetic methods are usually coupled in groundwater
investigations for optimum results.

14. • CONDUCTING THE TEST.
• Resistivity Profiling and Soundings
• Measurements of vertical changes in resistivity are called
“soundings” and measurements of horizontal changes in resistivity
are called “profiling.” The technique requires at least three
individuals to move two current electrodes and two potential
electrodes along a survey grid.
• Measurement of ground resistivity involves passing an electrical
current into the ground using a pair of steel or copper electrodes
and measuring the resulting potential difference within the
subsurface using a second pair of electrodes. These are normally
placed between the current electrodes. Typically, current (I) is
induced between paired electrodes (C1, C2). The potential
difference (ΔV) between paired voltmeter electrodes P1 and P2 is
measured. Apparent resistivity (Δa) is then calculated (based on I,
ΔV, electrode spacing).

15. • Schematic of Electrical Resistivity

16. • Resistivity soundings involve gradually
increasing the spacing between the
current/potential electrodes (or both) in
order to increase the depth of investigation.
• The resistance data collected in this way are
converted to apparent resistivity readings
that can then be modelled to provide
information on the thickness of individual
resistivity layers within the subsurface.

17. • Along survey gridlines, changes in resistance readings are
used to create “contour maps” of soil resistivity. On the
map, concentric contours emanating from a location (called
a “spot elevation”) represent material of lowest
conductivity. Because soil conductivity is directly related to
the presence of water, locations measuring the greatest
resistance will have a lower soil-water content.
• Electrical resistivity tests should be performed in more than
one season with varying soil-water conditions. In some
geologic conditions the native soil may have a lower water
content and therefore higher resistivity than buried cultural
resources. Because resistivity is directly related to
permeability, degree of saturation, and the chemical nature
of entrapped fluids, prior knowledge of indigenous geologic
conditions is requisite to accurately interpret resistivity
data.

18. • Typical Result chart from Electrical Resistivity Investigation.

19. • 2D resistivity imaging
• Another form of electrical resistivity technique is, 2D resistivity
imaging. This is a fully automated technique that uses a linear
array of up to or beyond 72 electrodes connected by a multicore
cable. The current and potential electrode pairs are switched
automatically using a laptop computer and control module
connected to a ground resistivity meter (that provides the output
current). In this way a profile of resistivity against depth
('pseudo-section') is built up along the survey line. Data is
collected by automatically profiling along the line at different
electrode separations. The computer initially keeps the spacing
between the electrodes fixed and moves the pairs along the line
until the last electrode is reached. The spacing is then increased
by the minimum electrode separation (the physical distance
between electrodes which remains fixed throughout the survey)
and the process repeated in order to provide an increased depth
of investigation.

20. • The raw data is initially converted to
apparent resistivity values using a
geometric factor that is determined by the
type of electrode configuration used. Many
2D resistivity imaging surveys are carried
out using the Wenner Array. In this
configuration the spacing between each
electrode is identical. Once converted the
data is modelled using finite element and
least squares inversion methods in order to
calculate a true resistivity versus depth
pseudo-section.

21. • Fig: Wenner Array

22. VES 1
VES 2

23. * SEISMIC METHODS
* The seismic method measures the response of seismic (sound)
waves that are input into the earth and then refract along or reflect
off subsurface soil and rock boundaries. The seismic source is
usually a sledgehammer blow to a metal plate on the ground, a
larger weight drop, or an explosion. The earth response is measured
by sensors called geophones, which measure ground motion. Two
basic methods of seismic exploration are used refraction and
reflection. These methods determine geological structure and rock
velocities by either refracting or reflecting waves off boundaries
between rock units with different seismic velocities or impedance.
* Seismic techniques are commonly used to determine site geology,
stratigraphy, and rock quality. These techniques provide detailed
information about subsurface layering and rock geo-mechanical
properties using seismic acoustical waves.

24. • CONDUCTING SEISMIC TEST
• Seismic Refraction:
• The seismic refraction method measures head waves that are refracted along
geologic formations below the earth's surface. Refractions generally occur
along the top of the water table and the uppermost bedrock formation. A plot
of the arrival time of the first seismic wave to each geophone gives
information about the depth and location of these geologic horizons. This
information is plotted in a cross section that shows the depth to the water
table and to the first bedrock layer.
• The method is based on the measurement of the travel time of seismic waves
refracted at the interfaces between subsurface layers of different velocity.
Seismic energy is provided by a source (S) located on the surface. The energy
radiates out from the shot point, either travelling directly through the upper
layer (direct arrivals), or travelling down to and then laterally along higher
velocity layers (L1) as refracted arrivals (R1, R2, etc.) before returning to the
surface. This energy is detected on the surface using a linear array of
GEOPHONES. Observation of the travel-times of the refracted signals provides
information on the depth profile of the refractor.

25. • Seismic Refraction on Site
Seismic refraction
tomogram of a salt
Hot colors
correspond to
higher velocities,
i.e. salt. Cool
colors delineate
shale.

26. • Fig: Seismic Refraction Pattern

27. • Seismic Reflection:
• The reflection method measures the time necessary
for a sound impulse to travel from the source,
bounce off a geologic boundary, and return to the
surface at a geophone. The reflection from a
geologic horizon is similar to an echo off a cliff face.
• Seismic Reflection follows the law of mirror images
– angle of reflection from a surface is equal to the
angle of incidence. Shots are fired, in turn, at each
of the geophone positions and active geophones are
progressively added ahead of the shots, and taken
up from behind the shots, in a roll-along fashion.

28. • Fig: Seismic reflection plot of a Fault. Indicated is the main
normal fault and an associated antithetic fault.

29. • ELECTROMAGNETIC CONDUCTIVITY
• Electro-magnetic conductivity, also called EM, is used to
detect and differentiate metallic artifacts buried near
the earth’s surface. The technique locates near-surface
cultural features (structures, compaction, excavation,
and habitation sites) by their various water saturations
(their conductivity). A conductivity measurement is the
reciprocal of resistivity.
• The EM method is also very sensitive to metal. Thus, the
location of buried metal objects, such as drums or
pipes, can be mapped with this technique.

30. * TYPES OF ELECTROMAGNETIC CONDUCTIVITY
i. Frequency Domain Electromagnetic (FDEM)
ii. Time Domain Electromagnetic (TDEM)
iii. Ground Penetration Radar (GPR)
LIMITATIONS OF GPR
Site-specific conditions may limit the success of
GPR in geophysical surveys. The presence of
highly conductive clay soils in proportions of 10
percent or more is probably the greatest limiting
factor affecting radar signals. Highly conductive
soil conditions result in the attenuation of
electromagnetic energy, a reduction in signal
velocity, and a decrease in depth of signal
penetration.

31. • HOW TO CONDUCT THE ELECTRO-MAGNETIC TEST
• Two individuals are required to perform the
technique, but the conductivity instrument
can be moved from station to station by one
operator. Resistivity requires a crew of at least
three to move and place electrodes in the
ground along a survey line.
• Electromagnetic conductivity uses a non-
surface contacting radio transmitter and
receiver. The transmitter induces an
electromagnetic field in the earth, causing an
electrical current to flow.

32. • The electrical current generates a secondary
magnetic field that causes the flow of an
electrical current signal in the receiver. The
receiver signal is measured for conductivity by a
voltmeter incorporated in the EM instrument.
The voltmeter is calibrated to measure the soil as
having a homogeneous level of conductivity. It is
assumed that buried cultural resources cause
anomalies in the homogenous level of
conductivity detected along survey lines. Large
fluctuations in conductivity are indications of
highly conductive subsurface materials, such as
buried metallic artifacts.

33. • Fig: Ground Penetrating Radar

34. • GRAVITY GEOPHYSICAL METHODS
• State-of-the-art gravity meters can sense differences in the
acceleration (pull) of gravity to one part in one billion.
Measurements taken at the Earth’s surface express the acceleration
of gravity of the total mass of the Earth but because of their high
sensitivity; the instruments can detect mass variations in the crustal
geology.
• Microgravity profiling is a passive technique that involves highly
accurate measurement of relative changes in the Earth's gravitational
field. Measurements are made using a gravity meter, which comprises a
highly sensitive temperature stabilized spring balance. Subtle changes
in gravity result from variations in the density of materials within the
subsurface and the method can therefore be used to successfully locate
voids or buried features such as underground storage tanks. The effects
of tidal and instrument drift that would otherwise mask any subtle
anomalies are overcome by repeat readings at a fixed base station
throughout the survey. Accurate topographic levelling is carried out at
each station in order to correct for the effects of terrain.

35. Fig: Gravimeter

36. * GEOTHERMAL METHOD
* Radiometric Surveys
* Radiometric surveys involve the measurement of
gamma radiation resulting from natural radioactive
sources. Instruments are available to measure either total
count or provide spectral information on individual
elements such as uranium, thorium and potassium in
order to identify specific sources of radiation. Modern
multispectral meters capable of measuring up to 256
channels are being increasingly used in environmental
mapping. Radiometric measurements are primarily used
in mineral exploration but can also be applied to the
detection of faults, location of caves and for mapping
contamination.

37. • RECENT DEVELOPMENTS FOR HIGHWAY ENGINEERS
• AUTOMATIC ROAD ANALYZER (ARAN) – FUGRO INC.
• The ARAN is a network of tightly integrated
subsystems that synchronously collect
accurate and reliable data for roadway
infrastructure management applications.

38. • ARAN ROAD SURVEY VEHICLE

39. • Description
• Automatic Road Analyser (ARAN) is one of the most
advanced platforms available for collecting pavement
condition and road asset data, providing you with a
safe, accurate, reliable and cost effective
understanding of the condition of your infrastructure.
• The ARAN system adds new technology to the
Australasian market with advancements on the
currently available systems, particularly in the area of
locational accuracy and repeatability. The system is a
modular solution that can be reconfigured to meet
the specific data collection needs of users.

40. • FRAME DIAGRAM OF THE ARAN SURVEY VEHICLE

41. Laser Roughness Measurement System
The ARAN system enables:
• Complete roadside inventories extracted from specially
calibrated digital video log images. Inventories
containing type, location, condition, measurements,
unique identifiers, etc. Data output can be formatted
for subsequent import into a GIS or road asset
management software environment.

42. • Data Generated
• The integrated ARAN system enables us to collect
the following data sets in a single pass i.e. visual
and laser simultaneously:
i. Longitudinal profile and roughness (IRI,
NRM and HATI)
ii. Transverse profile and rut depth
iii. Macrotexture depth, Mean Profile Depth
(MPD) and Estimated Texture Depth
(ETD)
iv. Surface distress (Post) rating (Cracking,
Potholes, etc.)

43. •Characteristics:
• * High definition digital images
• * Measure transverse profiles up to 4m wide
• * Multiple data sets collected in a single pass
• * High precision positioning system
• * Curvature, gradient, crossfall and slope data
• * Road profile data capture
• * Texture data

44. * Digital Video
* The ARAN is fitted with HDTV cameras
which capture Right-of-Way images allowing
you to virtually view the road from the
comfort and safety of your office.
* The ARAN platform offers a variety of video
logging options to suit every need and
budget. Correlate images with road condition
data and geometry information to get the
complete picture for efficient asset
management and decision making.

45. • Anaylzing Section in the ARAN Road Survey Vehicle

46. * Laser Roughness Measurement System
* The Laser SDP is a longitudinal profile measurement
system that provides road profile data capture and real-
time roughness index calculations using a combination
of high-speed lasers and accelerometers.
* The Laser SDP samples at 12.5 mm intervals and
measures bumps as short as 100 mm at variable speeds
up to 100 km/h without loss of accuracy (Type 1
Profiler). 64kHz lasers are used to define mean profile
depth, which can in turn be used to determine the
Estimated Texture Depth or Equivalent Sand Patch
Texture Depth. Based on the South Dakoda Profiler
(SDP), it is accepted as a Class A device under ASTM950.

47. Laser Roughness Measurement System

48. * Laser Rut Measurement System
* The laser rut measurement system (LRMS) is a
vehicle mounted subsystem that uses dual scanning
lasers to accurately measure transverse profiles up to
4m wide.
* The transverse profile is measured in order to
calculate the depth of roadway rutting. By measuring
the complete profile instead of just the ruts, the effect
of vehicle wander on measured rut values is
eliminated. The LRMS uses two synchronized, laser-
based devices to measure the transverse profile of a
4m lane width, with a lateral resolution of
approximately 1,280 points – this is in contrast to
existing systems whic h only measure up to 15 points.

49. Laser Rut Measurement System

50. • Part of the Processed Data.

51.  CONCLUSION:
* Conclusively, successful implementation of a geophysical
survey depends on the following:
*A comprehensive survey design that specifies the set of
techniques chosen for a survey (multiple techniques are
requisite for a thorough site investigation), the order in
which the techniques are implemented, the size and
location of the survey grid applied, and the compatibility of
the techniques with the site (that is, compatible with
geology and physical access).
*An experienced geophysicist contractor who is skilled in
multiple geophysical methods and knowledgeable about
the physical and historic context of the survey and the
nature of the expected results.

52. Possible limitations of geophysical surveys include the following:
Geophysical surveys are equipment-intensive and may be
expensive to conduct.
Geophysical survey equipment cannot distinguish between cultural
and geologic anomalies.
Geophysical survey techniques are limited to near-surface
detection. There are limits to the depth and scale of resolution.
Geophysical survey equipment may not detect subtle contrasts or
weak signals. If the contrast between the sought-after
archeological material and incubating soil is small, detection is
hindered.
Erroneous readings may occur as a result of distortion from nearby
cultural entities with physical or electromagnetic properties, such
as subterranean utilities, powerlines, metal fences, transmission
towers, buildings, roads, railroads, aircraft, and two-way radios.

53. END OF PRESENTATION
THANK YOU FOR LISTENING