PRESENTATION/LECTURE ON

GEOPHYSICAL METHODSSITE INVESTIGATION AND LABORATORY MEASUREMENTS

(CIV 8326)

ANDREW ABAH ONOJASPS/15/MCE/00023

M.ENG – CIVIL ENGINEERING DEPARTMENT OF CIVIL ENGINEERING

BAYERO UNIVERSITY, KANO

16TH June, 2016

• 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.

• 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.

• 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.

•

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

* 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:

• (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.

• 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.

* 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.

• 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.

• Fig: Electrical Resistivity Apparatus (TERAMETER)

• 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.

• 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.

• 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).

• Schematic of Electrical Resistivity

• 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.

• 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.

• Typical Result chart from Electrical Resistivity Investigation.

• 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.

• 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.

• Fig: Wenner Array

VES 1

VES 2

* 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.

• 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.

• Seismic Refraction on Site

Seismic refraction tomogram of a salt Hot colors correspond to higher velocities, i.e. salt. Cool colors delineate shale.

• Fig: Seismic Refraction Pattern

• 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.

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

• 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.

* TYPES OF ELECTROMAGNETIC CONDUCTIVITYi. Frequency Domain Electromagnetic (FDEM) ii. Time Domain Electromagnetic (TDEM) iii. Ground Penetration Radar (GPR)

LIMITATIONS OF GPRSite-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.

• 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.

• 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.

• Fig: Ground Penetrating Radar

• 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.

Fig: Gravimeter

* 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.

• 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.

• ARAN ROAD SURVEY VEHICLE

• 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.

• FRAME DIAGRAM OF THE ARAN SURVEY VEHICLE

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.

• 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 depthiii. Macrotexture depth, Mean Profile Depth

(MPD) and Estimated Texture Depth (ETD)

iv. Surface distress (Post) rating (Cracking, Potholes, etc.)

•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

* 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.

• Anaylzing Section in the ARAN Road Survey Vehicle

* 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.

Laser Roughness Measurement System

* 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.

Laser Rut Measurement System

• Part of the Processed Data.

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.

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.

END OF PRESENTATION THANK YOU FOR LISTENING