Field Practicals of Geo-Physical Exploration

8/7/2019 Field Practicals of Geo-Physical Exploration

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2011

Praticals of Geophysical Exploration

Department of Geological Engineering

BUIITEMS Quetta.

1/17/2011

Study Tour To GSP Quetta.

Submitted by:Adnan Ali 019

Adnan Khan 011

Muhammad Iqbal 021

Ikram ullah Khan 012

Fawad Ali 040

Riasat Ali 041

Submitted to: Sir Shoukat Parvaiz


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Acknowledgement

Our Sincere thanks to Dean Faculty of Engineering Dr. syed AbidHussain, Chairman Faculty of Geological Engineering Engr Kamran Zafar, Sir Shoukat Parvaiz

and Engr Shabir Ahmad For Arranging Practicals of Geophysical Exploration in Geological

Survey of Pakistan (GSP) Headquarter Quetta


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Introductory Presentation

Director General presentation

(Director general of GSP presenting introductory presentation)

On Arrival at GSP Quetta, we were warmly welcomed by the GSP Staff. Then an Introductory

presentation was given by Director General of GSP , He Shows his height of pleasure of our

interests in Geological survey of Pakistan. He gave General overview about Geophysical

Techniques and Future Scope of Geophysics. He emphasize on understanding these techniques

and their applications for future field works.

Day 01


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Explaining various methods of GeophysicalExploration

Mr Saeed Works as a Geophysict at GSP. First of all he welcome us and thanked Director

General of GSP For General Overview. He further carried out the presentation in detail.

First he presented Various Geophysical techniques namely;

yElectrical resistivity Survey (ERS)yGravity SurveyyMagnetic SurveyyGround Penetrating Radar (GPR)ySeismic SurveyyBore hole logging

The above techniques were discussed Theoretically i.e performing techniques, Calculations,

Error Corrections,

Data collecting

sheets, etc on

Particular day and

he assured us to

perform practical

of the above

techniques on

next day.

( Mr. Saeed presenting the General overview of various method of geophysical

Exploration)


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Field Practicals &

Discussions

The Second day of Study tour Starts with Practicals Session i-e performing of geophysicalTechniques Practically. All the setup of the practicals along with instruments was already

prepared by the team of Geological Survey of Pakistan (GSP). All the practicals and their

theoretical discussions i-e performing Methods, Data Acquisitions, Error Corrections, Field

Precautions, Instrument safety Precautions are discussed below;

Day 02


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1.Electrical Resistivity Survey (ERS):-1.1Equipments:

Current Source, Potential Electrodes, Current Electrodes, Terrameter,Cables, Measuring Tapes.

1.2 Working Principle:

Observation of electric fields caused by current introduced intothe ground as a means of studying earth resistivity in geophysical exploration. Resistivity is theproperty of a material that resists the

flow of electrical current.Electricalresistance surveys work on the

principle that anomalies beneath theground can be detected by differences

in their resistance to the flow of anelectrical current. These surveys

measure the distortion of an inducedelectrical field caused by something in

the subsurface, in our case it is thearcheology or cultural feature (Clark

2000). Because an electrical current isinjected into the ground to generate an

electrical field, this type of survey is anactive remote sensing technique.

1.4Explanation: (Schlemberger configuration)

To cause a charge to flow, a voltage must be applied. Voltage is also

referred to as potential difference (a measure of the energy used to move the charge). As the

voltage is applied and the current flows, a resistance is encountered to the movement of the

charge. The resistance is dependent on the physical characteristics of the medium in which the

charge flows. These three quantities (current, voltage, and resistance) are related by Ohms law

where resistance is measured in Ohms (), voltage in volts (V), and current in amperes or amps

(A).

Voltage

Resistance = -----------------, orR= ------

Current


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Resistance can then be converted to resistivity () where V/L is the change of voltage withdistance in the direction of current flow and J is the current density in the medium in which

charge is flowing.

V/L

= ------------,

J

The basic measure of resistivity is the Ohm-meter or Ohm centimeter. The inverse of resistivity(1/) is conductivity.

In resistance and resistivity surveys, an electrical current is injected into the ground to

generate an electric field in the subsurface. If the sediments are completely uniform, there will beno contrast in the electrical data and the resulting map will be featureless. However, when the

archaeological feature (or geological feature) differs from the sediments in various properties,then the induced electrical field is no longer uniform. The resistance either increases or

decreases. The differences in the electrical properties or contrast combined with the size anddepth of archaeological features produces a record that can be mapped

The resistance to the flow of electrical current in sediments and soils depends on several

variables. The most important variables are soil moisture and soluble salts (mobile ions), but the

most important factor is the soil moisture content. Other significant variables to resistanceinclude soil permeability and temperature. Seldom is there a one-to-one correspondence betweenan individual variable and the resultant resistance data. On the contrary, these variables show

wide spatial variation depending on environmental conditions. Therefore, the resistivity ofdifferent archaeological sites changes accordingly. Because no two archaeological sites possess

the same subsurface properties, the resistivity data from different archaeological sites will varyas well. It is entirely possible that a feature that is easily found by resistivity survey in one

location may be imperceptible in another.

Resistivity and resistance surveys are dependent on the underlying sediments, which

themselves differ in their resistant values. Loams have the lowest resistivity and crystalline rocks

the highest. Conductivity is the inverse of resistivity and the higher the resistivity, the lower theconductivity. For example, clays are low in resistance but high in conductivity due to factors ofmoisture retention, permeability and ion content, whereas crystalline rocks are virtually

nonconductive. Thinking in terms of conductivity along with resistance may be useful especiallyfor beginners in the field of remote sensing.

It should be apparent that knowing as much as possible about the matrix of the

archaeological site and the types of features that might be encountered is necessary for accurate


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data interpretation. Geologic strata produce strong electrical resistance contrasts, which couldeffectively wash out the lower contrasting archaeological features. Archaeological features that

typically produce resistance maxima are buried walls and features that restrict the flow of ions.Ditches and pits that were later filled, even with the same sediments or soil from the surrounding

area, will result in resistivity minima. Lower resistivity may result from loosely packed fill,

which is more permeable, retains more moisture, and has organic matter creating more ions

1.5 Wenner and Schlumberger

When doing resistivity sounding surveys, one of two survey types is most commonly used. Forboth of these surveys, electrodes are distributed along a line, centered about a midpoint that is

considered the location of the sounding. The simplest in terms of the geometry of electrodeplacement is referred to as a Wennersurvey. The most time-effective in terms of field work is

referred to as a Schlumbergersurvey. For a Wenner survey, the two current electrodes (green)and the two potential electrodes (red) are placed in line with each other, equidistant from one

another, and centered on some location as shown below.

The apparent resistivity computed from measurements of voltage, V, and current, i, is given by

the relatively simple equation shown above. This equation is nothing more than the apparentresistivity expression shown previously with the electrode distances fixed to a. To generate a plot

of apparent resistivity versus electrode spacing, from which we could interpret the resistivityvariation with depth, we would have to compute apparent resistivity for a variety of electrode

spacings, a. That is, after making a measurement we would have to move all four electrodes tonew positions.


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For a Schlumberger survey, the two current electrodes (green) and the two potential electrodes(red) are still placed in line with other, centered on some location, but the potential and current

electrodes are not placed equidistant from one another.

The current electrodes are at equal distances from the center of the sounding, s. The potentialelectrodes are also at equal distances from the center of the sounding, but this distance, a/2, is

much less than the distance s. Most of the interpretation software available assumes that thepotential electrode spacing is negligible compared to the current electrode spacing. In practice,

this is usually interpreted as meaning that a must be less than 2s/5.

In principle, this implies that we could set a to be less than 2s/5 for the smallest value ofs wewill use in the survey and never move the potential electrodes again. In practice, however, as the

current electrodes are moved outward, the potential difference between the two potentialelectrodes gets smaller. Eventually this difference becomes smaller than our voltmeter is capable

of reading, and we will need to increase a to increase the potential difference we are attemptingto measure.

1.6 Data collection

Survey usually involves walking with the instrumentalong closely spaced parallel traverses, taking readings at regular intervals. In most

cases, the area to be surveyed is staked into a series of square or rectangular survey

"grids" (terminology can vary). With the corners of the grids as known reference

points, the instrument operator uses tapes or marked ropes as a guide when

collecting data. In this way, positioning error can be kept to within a few


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centimeters for high-resolution mapping. Early surveys recorded readings by hand,

but computer controlled data logging and storage are now the norm

RepeatedDCResistivityObservations - Schlumberger Array

AB/2 (m) Number of Readingsrho (ohm.m)

(Group 1)

rho (ohm.m)

(Group 2)

rho (ohm.m)

(Group 3)

rho (ohm.m)

(Group 4)

0.25 1 350.18 350.12 350.13 350.19

0.5 1 349.99 349.98 349.63 349.52

0.75 1 349.08 349.46 349.75 350.04

1.0 1 349.49 350.22 349.13 350.00

1.25 1 348.03 349.71 349.16 348.32

1.75 1 347.02 347.10 347.75 348.85

2.0 1 345.56 345.38 343.40 345.79

2.25 1 343.71 343.92 342.70 342.49

2.5 1 343.61 337.98 341.59 338.74

5.0 1 301.23 295.02 296.94 296.04

7.5 1 236.33 238.67 244.05 239.90

10.0 1 202.46 208.05 208.71 191.27

12.5 1 188.32 174.77 167.77 177.83

15.0 1 173.49 148.82 154.22 162.53

17.5 1 147.17 158.11 146.56 163.20

20.0 1 144.70 164.38 176.71 168.20

22.5 1 166.55 155.96 160.93 168.92


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25.0 1 189.31 185.72 159.45 179.18

1.7 Procedure

y The first column in this data set is the half-spacing of the current electrodes, AB/2, inmeters. The second through fifth columns represent the apparent resistivities determinedby each group from a single voltage reading at the specified electrode spacing. A

Schlumberger spread was used to collect these observations. Clicking on the highlightedarea will create a file with the name resgrp.daton your machine. *

y Import resgrp.datinto your favorite spreadsheet.y Using the

graphicalfeatures of your

spreadsheet, plotthe apparent

resistivity versuselectrode

spacing for eachgroup on a Log-

Logplot. This isa plot that has as

its axes the Logof the electrode

spacing versusthe Log of the

apparentresistivity.

y For the listedelectrode

spacings,

(ERS Configuration)

y compute the average apparent resistivity and the standard deviation of the apparentresistivities observed by the four groups.y On a Log-Log plot, plot the average resistivity versus electrode spacing, and on a Log-

Linear plot, plot the standard deviation of the measured apparent resistivities versuselectrode spacing.

y If the errors associated with each voltage reading are randomly distributed, we can reducethe standard deviation of the apparent resistivity observations by making repeated voltage

readings at each electrode position and averaging the repeated observations. For noise


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that is randomly distributed, the standard deviation of the average observation is reducedby the square root of the number of readings made at each electrode position. How many

repeated readings must be collected at each electrode spacing to reduce the standarddeviation of the average at each spacing to 1 ohm.m? **

y Briefly discuss your results.

1.8 Advantages and Disadvantages of Wenner and SchlumbergerArrays

The following table lists some of the strengths and weaknesses of Schlumberger and Wenner

sounding methods.

Schlumberger Wenner

Advantage Disadvantage Advantage Disadvantage

Need to move the

two potential

electrodes only for

most readings. This

can significantly

decrease the time

required to acquirea sounding.

All four electrodes, two current

and two potential must be

moved to acquire each reading.

Because the potential

electrode spacing is

small compared to the

current electrode

spacing, for large

current electrode

spacings very sensitive

voltmeters are required.

Potential

electrode spacing

increases as

current electrode

spacing increases.

Less sensitive

voltmeters are

required.

Because the

potential electrodes

remain in fixed

location, the effects

of near-surface

lateral variations in

resistivity are

reduced.

Because all electrodes are

moved for each reading, this

method can be more

susceptible to near-surface,

lateral, variations in resistivity.

These near-surface lateral

variations could potentially be

misinterpreted in terms of

depth variations in resistivity.

In general,

interpretations based on

DC soundings will be

limited to simple,

horizontally layered

structures.

In general, interpretations

based on DC soundings will be

limited to simple, horizontally

layered structures.


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2.Gravity Survey:

2.1Equipment:

Gravimeter

2.2 Working Principle Along withExplanation:

A gravimeter is simply a veryprecise weighing machine used to find the weight of a certain lump of metal or other material at

a series of stations distributed over the area being surveyed. Since the weight of an object is itsresponse to the Earth's gravitational attraction, this weight will be slightly affected by the nature

of subsurface materials at the place of measurement. It will be slightly larger, for example, atstations where the subsurface material is of higher density or where dense material comes closer

to the surface. The changes in weight are so small that the weighing machine must be capable ofdetecting changes of the order of one part in ten million.

Gravimeters must not only resolve small variations in the gravity field, but must also remainstable over a large

range of values andenvironmental

conditions. It istherefore required

that practicalinstruments must be

highly precise,portable, robust,

simple to use andrelatively

inexpensive. All

gravity meters haveinherent limitations,however, and

survey objectivesmust be consistent

with the particularequipment

deployed. There areGravimeter used for gravity survey


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clear distinctions between making absolute measurements of gravity and measuring relativegravity, or variation from place to place.

Historically, absolute measurements of gravity have provided key data for global mapping(geodetic) studies and tie points for linking independent exploration surveys. Absolute

determinations of gravity are now relatively rare, but can still be made using a variety of devices

based on the timing of a free-falling weight. Other methods, viz., the periodicity of a fixedpendulum can be used to make absolute measurements of gravity.

There is little benefit these days in obtaining absolute determinations of gravity. Detailed

exploration surveys almost always use relative determinations based on a simple spring balance.The operating principle is simple. A small change in gravity varies the force acting on a constant

mass suspended from a spring. The external force required to hold the mass in its null positionprovides a measure of the gravity at the station, relative to other stations.

2.3 Types of Gravity Meters

There are three main classes of gravity measuring instruments:

1. Pendulums - where the period of the pendulum is inversely proportional to g

2. Sensitive spring balances - where the spring extension is proportional to g

3. Free falling bodies where the time of free fall over a fixed distance is proportional to g

Within each class there are several variants. The spring balances are relative instruments, which

means that they can only be used to measure the difference in gravity between two or morepoints. Pendulums can be used for relative and absolute measurements by calculating the ratio of

periods measured at two points or the exact period at a particular point. The falling body classmeasures the absolute gravity.

1. Pendulums

The pendulum method of measuring gravity was used all over the world up to the middle of the

20th

Century and was the basis for the 1930 Potsdam Gravity Datum. By the time pendulummeasurements were phased out in the 1950s the instruments had become quite sophisticated with

vacuum chambers, knife edge quartz pivots and precision chronometers. Mechanicalimperfections and wear of the pivot were the limiting factors in the accuracy of this class of

apparatus.

2. Spring Gravimeters

The practical implementation of a simple spring mechanism for gravity measurements is far fromtrivial. The basic requirement for resolution exceeding one part in ten million demands pure

materials and precise engineering. Relative dimensions must remain stable through a range oftemperatures and pressures. Springs made of silica glass have become standard, along with

suitable suspension frames. However, steel springs are also used. Two types of springgravimeters are therefore in common use:


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There have been various designs for this type of instrument; some prominent examples are theJILAG, FG5 and A10 'portable' meter.

2.4 Positioning equipment

The dramatic improvement in the accuracy of gravity surveying over the last 20 years has beendue to the revolution in positioning technology. Positions and particularly heights had been the

limiting factors in calculating accurate gravity anomalies. Positioning for the first reconnaissancegravity surveys was done without any equipment, it relied on the observer marking a spot on a

map or aerial photograph. Theodolites were used for positioning the more detailed gravitysurveys.

2.5 Global positioning system receivers

The introduction of the Global Positioning System (GPS) in the late 1980s enabled gravity to

take its place as a precision tool in mapping the fine detail of crustal structure. The GPS receivermonitors time encoded signals being broadcast by a constellation of GPS satellites orbiting theEarth, from 4 or more of these signals the position of the receiver can be calculated in reference

to the centre of the Geoid. The position values are referred to as the geocentric coordinates. Thegeocentric geoid differs from the local geoid so local coordinates have to be obtained by

applying a geoid transformation and/or by tying the network to local spatial control points.

2.6 Pressure based height instruments

Atmospheric pressure decreases with altitude, so pressure measurements can be used to calculateelevation. A rough estimate of the pressure decrease is 1 millibar for each 8.7 metre increase in

altitude. Theoretically it is possible to calculate an absolute height above sea level if one assumesa standard atmosphere; however atmospheric conditions are constantly changing due to

movement of pressure systems and daily heating cycles (diurnals). Reasonably accurate heightdifferences can be measured in a local area (within the same pressure regime as the base) if base

pressure variations are recorded, the weather pattern is stable and repeat readings are made at thebase and selected field stations during the loop. Particular problems occur if a pressure front

travels through the area during a gravity survey as the base pressure may be out of step with thefield pressure during the transit. Pressure measuring apparatus that have been used in gravity

surveys are altimeters, precision micro-barometers and digital barometers.

2.7 Factors that Affect the Gravitational Acceleration

Temporal Based Variations

y Instrument Drifty Tides


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y A Correction Strategy for Instrument Drift and Tidesy Tidal and Drift Corrections: A Field Procedurey Tidal and Drift Corrections: Data Reduction

Spatial Based Variations

y Latitude Dependent Changes in Gravitational Accelerationy Correcting for Latitude Dependent Changesy Variation in Gravitational Acceleration Due to Changes in Elevationy Accounting forElevation Variations: The Free-Air Correctiony Variations in Gravity Due to Excess Massy Correcting forExcess Mass: The Bouguer Slab Correctiony Variations in Gravity Due to Nearby Topography

2.8 Local/Regional Gravity Anomaly SeparationExample

As an example of estimating the regional anomaly from the recorded data and

isolating the local anomaly with this estimate consider using a moving average operator. Withthis technique, an estimate of the regional gravity anomaly at some point along a profile is

determined by averaging the recorded gravity values at several nearby points. The number ofpoints over which the average is calculated is referred to as the length of the operator and is

chosen by the data processor. Averaging gravity values over several observation points enhancesthe long-wavelength contributions to the recorded gravity field while suppressing the shorter-

wavelength contributions. Consider the sample gravity data shown below.

(Terrain Corrected Bougher Anomaly)

Moving averages can be computed across this data set. To do this the data processor chooses thelength of the moving average operator. That is, the processor decides to compute the average

over 3, 5, 7, 15, or 51 adjacent points. As you would expect, the resulting estimate of the regionalgravity anomaly, and thus the local gravity anomaly, is critically dependent on this choice.


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Shown below are two estimates of the regional gravity anomaly using moving average operatorsof lengths 15 and 35.

(Moving Averages)

Depending on the features of the gravity profile the processor wishes to extract, either of theseoperators may be appropriate. If we believe, for example, the gravity peak located at a distanceof about 30 on the profile is a feature related to a local gravity anomaly, notice that the 15 length

operator is not long enough. The average using this operator length almost tracks the raw data,thus when we subtract the averages from the raw data to isolate the local gravity anomaly the

resulting value will be near zero. The 35 length operator, on the other hand, is long enough toaverage out the anomaly of interest, thus isolating it when we subtract the moving average

estimate of the regional from the raw observations.

The residual gravity estimates computed for each moving average operator are shown below.

(Local Gravity Anomaly Estimates)

As expected, few interpretable anomalies exist after applying the 15 point operator. The peak at a

distance of 30 has been greatly reduced in amplitude and other short-wavelength anomalies

apparent in the original data have been effectively removed. Using the 35 length operator, the

peak at a distance of 30 has been successfully isolated and other short-wavelength anomalies

have been enhanced. Data processors and interpreters are free to choose the operator length they


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wish to apply to the data. This choice is based solely on the features they believe represent the

local anomalies of interest. Thus, separation of the regional from the local gravity field is an

interpretive process.

3.Magnetic Survey:3.1 Introduction

The magnetic method is a very popular and inexpensive approach for near-surface metal

detection. Engineering and environmental site characterization projects often begin with amagnetometer survey as a means of rapidly providing a layer of information on where

utilities and other buried concerns are located The principal of operation is quite simple.When a ferrous material is placed within theEarth's magnetic field, it develops an

induced magnetic field. The induced field is superimposed on the Earth's field at thatlocation creating a magnetic anomaly. Detection depends on the amount of magnetic

material present and its distance from the sensor. The anomalies are typically presentedon colour contour maps.

Common uses of magnetometers include:

3.1.locating buried tanks and drums3.2.fault studies3.3.mineral exploration3.4.geothermal exploration3.5.mapping buried utilities, pipelines3.6.buried foundations, fire pits for archaeological studies

In geothermal application the main objective of the magnetic study is to contribute with

information about the relationship among the geothermal activity, the tectonic andstratigraphy of the area by means of the anomalies interpretation of the underground

rocks magnetic properties (Escobar, 2005). Most of the rocks are not magnetic; however,certain types of rocks contain enough minerals to originate significant magnetic

anomalies. The data interpretation that reflects differences in local abundance ofmagnetization is especially useful to locate faults and geologic contacts (Blakely, 1995).

The magnetic anomalies can be originated from a series of changes in lithology, variationsin the magnetized bodies thickness, faulting, pleats and topographical relief. A significant

quantity of information can leave a qualitative revision of the residual magnetic anomalies

map of the total magnetic field. In this sense, we can say that the value of the survey doesnot finish with the first interpretation, but rather it increases as more geology is known.

It is more important, at the beginning, to detect the presence of a fault orintrusive body, than to determine their form or depth. Although, in some magnetic risings,

such determination cannot be made in a unique manner, the magnetic data has been usefulbecause the intrusive is more magnetic than the underlying lava flows. The faulting

creates spaces so that the warm fluids displace and therefore alter the guest rocks. The


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hydrothermal system temperature and the oxygen volatility will determine the quantity ofpresent loadstone in the area of faults and therefore, their magnetic response.

3.2Basic theory

If two magnetic poles of strength m1 and m2 are separated by a distance r, a force, F,

exists between them. If the poles are of the same polarity, the force will push the poles

apart, and if they are of opposite polarity, the force is attractive and will draw the poles

together. The equation for F is the following:

m1m2

F = -------------------

4 2

where is the magnetic permeability of the medium separating the poles; m1 and m2are pole strengths and r the distance between them.

3.3 Magnetic units

The magnetic flux lines between two poles per unit area, is the flux density B (and is

measured in Weber/m2

= Tesla). B, which is also called the magnetic induction, is a

vector quantity. The unit of Tesla are too large to be practical in geophysical work, so

a sub-unit called a nanotesla (1 nT = 10-9 T)

is used instead, where 1 nT is numerically equivalent to 1 gamma in c.g.s. units (1 nTis equivalent to 10-5 gauss). The magnetic field can also be defined in terms of a force

field which is produced by electric currents. This magnetizing field strength H is

defined, following Biot-Savarts Law, as being the field strength at the centre of a loop

of wire of radius r through which a current I is flowing such that H = I/2r.

Consequently the units of the magnetizing field strength H are amperes per meter

(A/m). The ratio of the flux densityB to the magnetizing field strength H is a constant

called the absolute magnetic permeability ().

3.4 TheEarths magnetic field

The geomagnetic field at or near the surface of the Earth originates largely from within

and around the Earths core. The geomagnetic field can be described in terms of the

declination, D, inclination, I, and the total force vector F (Figure 6). The vertical

component of the magnetic intensity of the Earths magnetic field varies with latitude,


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Local variations, or anomalies, in the Earths magnetic field are the result of

disturbances caused mostly by variations in concentrations of ferromagnetic material

in the vicinity of the magnetometers sensor. Magnetic data can be acquired in two

configurations:

1) A rectangular grid pattern

2) Along a traverse

Grid data consists

of readings taken

at the nodes of a

rectangular grid;

traverse data is

acquired at fixed

intervals along a

line.E

achconfiguration has

its advantages and

disadvantages,

which are

dependent upon

variables such as

the site onditions,

size and rientation

of the target, and

financial esources.An example of a grid along lines is shown in Figure 8. This took place in the hinameca

geothermal area in El Salvador.

In both traverse and grid configurations, the station spacing, or distance between

magnetic readings, is important. Single-point or erroneous anomalies are more

easily recognized on surveys that utilize small station spacing.Ground magnetic

measurements are usually made with portable instruments at regular intervals along

more or less straight and parallel lines that cover the survey area. Often the interval

between measurement locations (stations) along the lines is less than the spacing

between lines. It is important to establish a local base station in an area away from

suspected magnetic targets or magnetic noise and where the local field gradient is

relatively flat. The base-station memory magnetometer, when used, is set up every day

prior to the collection of the magnetic data. Ideally the base station is placed at least

100 m from any large metal objects or travelled roads and at least 500 m from any

power lines when feasible. The base station location must be very well described in

the field book, as others may have to later locate it based on the written description.


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There are certain limitations in the magnetic method. One limitation is the problem of

cultural noise in certain areas. Man-made structures that are constructed using

ferrous material, such as steel, have a detrimental effect on the quality of the data.

Features to be avoided include steel structures, power lines, metal fences, steel

reinforced concrete, surface metal, pipelines and underground utilities. When

these features cannot be avoided, their locations should be noted in a field notebook

and on the site map.

The incorporation of computers and non-volatile memory in magnetometers has

greatly increased their ease of use and data handling capability. The instruments

typically will keep track of position; prompt for inputs, and internally store the data for

an entire day of work. Downloading the information to a personal computer is

straightforward, and plots of the day's work can be prepared each night.

To make accurate anomaly maps, temporal changes in the Earth's field during the

period of the survey must be considered. Normal changes during a day, sometimes

called diurnal drift, are a few tens of nT, but changes of hundreds or thousands of nTmay occur over a few hours during magnetic storms. During severe magnetic storms,

which occur infrequently, magnetic surveys should not be made. The correction for

diurnal drift can be made by repeating measurements of a base station at frequent

intervals. The measurements at field stations are then corrected for temporal variations

by assuming a linear change of the field between repeat base station readings.

Continuously recording magnetometers can also be used at fixed base sites to monitor

the temporal changes. If time is accurately recorded at both the base site and field the

location, the field data can be corrected by subtraction of the variations at the base site.

Some QC/QA procedures require that several field-type stations be occupied at the

start and end of each day's work. This procedure indicates that the instrument isoperating consistently. Where it is important to be able to reproduce the actual

measurements on a site exactly (such as in certain forensic matters), an additional

procedure is required. The value of the magnetic field at the base station must

be asserted (usually a value close to its reading on the first day) and each day's data

corrected for the difference between the asserted value and the base value read at the

beginning of the day. As the base may vary by 10 to 25 nT or more from day to day,

this correction ensures that another person using the same base station and the same

asserted value will get the same readings at a field point to within the accuracy of the

instrument. This procedure is always a good technique but is often neglected by

persons interested in only very large anomalies (> 500 nT, etc.).

Intense fields from man-made electromagnetic sources can be a problem in magnetic

surveys. Most magnetometers are designed to operate in fairly intense 60-Hz and

radio frequency fields. However, extremely low frequency fields caused by equipment

using direct current or the switching of large alternating currents can be a problem.

Pipelines carrying direct current for cathodic protection can be particularly


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troublesome. Although some modern ground magnetometers have a sensitivity of 0.1

nT, sources of cultural and geologic noise usually prevent full use of this sensitivity in

ground measurements.

The magnetometer is operated by a single person. However, grid layout, surveying, or

the buddy system may require the use of another technician. If two magnetometers are

available, production is usually doubled as the ordinary operation of the instrument

itself is straightforward.

3.7 Data reduction and interpretation

The data should be corrected for diurnal variations, if necessary. If the diurnal does not

vary more than approximately 15 to 20 gammas over a one-hour period, correction

may not be necessary. However, this variation must be approximately linear over time

and should not show any extreme fluctuations.

The global magnetic field is calculated through a previous established model (IGRF-

International Geomagnetic Reference), Figure 9, and obtained analytically with the

help of field observations. Due to the fact that the global magnetic field is variable,

these maps are generated every 5 years.

There are filters used for highlighting the contrast of anomalies; these are:

Derivatives of different order or gradients

Upward or downward continuation regarding the anomaly

Band pass or high pass filters

Pole reduction

After all corrections have been made, magnetic survey data are usually displayed as

individual profiles or as contour maps. Identification of anomalies caused by cultural

features, such as railroads, pipelines, and bridges is commonly made using field

observations and maps showing such features.

3.8 Presentation of results

The final results are presented in profile and contour map form. Profiles are usually

presented in a north-south orientation, although this is not mandatory. The orientation

of the traverses must be indicated on the plots. A listing of the magnetic data,

including the diurnal monitor or looping data should be included in the report. The

report must also contain information pertinent to the instrumentation, field operations,

and data reduction and interpretation techniques used in the investigation.


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4.Ground-penetrating radar


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Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image thesubsurface. This non-destructive method uses electromagnetic radiation in the microwave band

(UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurfacestructures. GPRcan be used in a variety of media, including rock, soil, ice, fresh water,

pavements and structures. It can detect objects, changes in material, and voids and cracks.

GPRuses transmitting and receiving antennas or only one containing both functions. Thetransmitting antenna radiates short pulses of the high-frequency (usually polarized) radio waves

into the ground. When the wave hits a buried object or a boundary with different dielectricconstants, the receiving antenna records variations in the reflected return signal. The principles

involved are similar to reflection seismology, except that electromagnetic energy is used insteadof acoustic energy, and reflections appear at boundaries with different dielectric constants

instead of acoustic impedances.The depth range of GPRis limited by the electrical conductivityof the ground, the transmitted center frequency and the radiated power. As conductivity

increases, the penetration depth decreases. This is because the electromagnetic energy is morequickly dissipated into heat, causing a loss in signal strength at depth. Higher frequencies do not

penetrate as far as lower frequencies, but give better resolution. Optimal depth penetration isachieved in ice where the depth of penetration can achieve several hundred meters. Good

penetration is also achieved in dry sandy soils or massive dry materials such as granite,limestone, and concrete where the depth of penetration could be up to 15 m. In moist and/or

clay-laden soils and soils with high electrical conductivity, penetration is sometimes only a fewcentimetres.

Ground-penetrating radar antennas are generally in contact with the ground for the

strongest signal strength; however, GPRair launched antennas can be used above theground.Cross borehole GPRhas developed within the field of hydrogeophysics to be a valuable

means of assessing the presence and amount of soil water.

4.1 Applications

GPRhas many applications in anumber of fields. In the Earth

sciences it is used to studybedrock, soils, groundwater, and

ice. Engineering applicationsinclude nondestructive testing

(NDT) of structures andpavements, locating buried

structures and utility lines, andstudying soils and bedrock. In

environmental remediation, GPRis used to define landfills,

contaminant plumes, and otherremediation sites, while in

archaeology it is used for and

Ground Penetrating Radar


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cemeteries. GPRis used in

law enforcement for locating clandestine graves and buried evidence. Military uses includedetection of mines, unexploded ordnance, and tunnels.

In the early 1970's several different teams of scientists began to develop radars for viewing intothe earth. Radars of this type were first developed for military applications-such as locatingtunnels under the DMZ between North and South Korea. GPRuse in locating and mapping

utility lines has been the subject of much on-going research conducted by both military andcommercial organizations.

GPR's usually operate in the VHF-UHF region of the electromagnetic spectrum. The frequencyused is a compromise. One desires to use the lowest possible frequency because low frequencies

give reasonably high penetration depths into the earth. But a sufficiently high frequency must beselected so that the radar wavelength is short, allowing detection and resolution of small objects

such as pipes. For cart mounted radars, 150 MHz is a typical center frequency, however 300 and

500M

Hz are sometimes used for shallow, high-resolution probing, and frequencies as low as 20MHz are used for locating deep caves or mine tunnels.

GPR's are also known as "impulse radars" because the transmitted pulse is very short and isordinarily generated by the transient voltage pulse generated from an overloaded avalanche

transistor. Resistively-loaded antennas are employed because one can not tolerate antennaringing. The distances in the ground where such radars look is measured from inches to tens of

feet. This corresponds to travel times measured in nanoseconds, that is, billionths of a second.Short transmitted pulses imply wide radar bandwidths, so a GPRoperating at a center frequency

of 150 MHz actually radiates substantial energy from 75 to 300 MHz.

The performance capability of this type of radar is strongly dependent on the soil electricalconductivity at the site. If the soil conductivity is high, attenuation of the radar signal in the soil

can severely restrict the maximum penetration depth of the radar signal. In California where soilsin many areas are often high in clay content the soil absorptive losses can be quite high. Whereas

maximum penetration depth achievable with these radars can be tens of feet in favorableenvironments, these numbers are reduced to a few feet or less at many sites in California.

GPRsurveys should be performed in the dry season if at all possible, especially at Californiasites. Soil moisture, especially in high-clay soils, only increases the radar attenuation rates,

further limiting the radar performance.

Spurious radar echoes (known as "clutter") can also be expected in many test areas because ofburied debris such as old rails, wire scraps, boulders, and small metal objects. Usually a trained

operator can interpret the desired radar signatures in the midst of a moderate amount of suchclutter.

It is not possible to built GPRantennas so that the antenna beamwidth is narrow. The wideantenna beamwidth of cart-mounted ground-penetrating radars makes it difficult to resolve

closely spaced objects, such as two parallel pipes in a common trench. In some cases the fill in


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the trench or the trench walls may be detected on the radar, but the pipes in the trench may not beradar discernible. Interpretation of GPRrecords is an art as well as a science, even with the best

available state of the art radars.

Ground-penetrating radars in principal are capable of locating plastic pipes as easily as metallic

pipes since the radar signal reflection from the pipe depends on contrasting dielectric propertiesof the soil and pipe, not just a high electrical conductivity for the pipe. In actual practice (1) soilattenuation may restrict the use of GPRto shallow depths. (2) The GPRantenna beamwidth is

broad making it difficult for radar to discriminate between closely-spaced pipes. (3) In disturbedground the radar may detect the walls of a trench but not the pipe it contains. Nevertheless GPR

can be very useful when a thorough search of the site is required. GPRnormally has an accuracyof several feet or less when measuring the depth of a buried object.

Cart GPR's rely on motion of the cart to generate a continuous radar record of traverse distance

vs. depth in the earth. GPRdata is ordinarily recorded on video tape with voice comments forarchive purposes, and printed out on thermal printer paper on site for immediate analysis. The

successful interpretation of GPR

records is an art as well as a science requiring considerableoperator experience for good results.

GPR's are also used in boreholes, and for point-to-point exploration in either a monostatic orbistatic mode. Monostatic sounding means the transmitter and receiver are located in the same

borehole, bistatic sounding implies the transmitter and receiver are emplaced in separateboreholes.

4.2 Three-dimensional imaging

Individual lines of GPRdata represent a sectional (profile) view of the subsurface. Multiple lines

of data systematically collected over an area may be used to construct three-dimensional ortomographic images. Data may be presented as three-dimensional blocks, or as horizontal orvertical slices. Horizontal slices (known as "depth slices" or "time slices") are essentially

planview maps isolating specific depths. Time-slicing has become standard practice inarchaeological applications, because horizontal patterning is often the most important indicator

of cultural activities.

4.3 Limitations

The most significant performance limitation of GPRis in high-conductivity materials such as

clay soils and soils that are salt contaminated. Performance is also limited by signal scattering inheterogeneous conditions (e.g. rocky soils).

Other disadvantages of currently available GPRsystems include:

y Interpretation of radargrams is generally non-intuitive to the novice.y Considerable expertise is necessary to effectively design, conduct, and interpret GPR

surveys.


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y Relatively high energy consumption can be problematic for extensive field surveys.Recent advances in GPRhardware and software have done much to ameliorate thesedisadvantages, and further improvement can be expected with ongoing development.

4.4 Survey MethodologyFirstly using Radiodetection, a survey is carried out of the services that we can identify fromthe presence of inspection covers on and around the area of interest. Secondly using

Radiodetection, we would carry out a scan of the survey area searching for passive signalsthat may exist in buried cables. Finally we would carry out a GPRSurvey with 5m cross

sections using a 500mhz antenna in order to locate services up to a depth of 2m. A furtherscan using a 250mhz antenna would be carried out on a 10m grid in order to locate services

up to 3.5m deep. GPRis an echo sounding method where a transducer (transmitter/receiver)is passed over the ground. The transmitter sends out low powered radio energy and

reflections from material boundaries and embedded features such as metal, voids or buried

cables are picked up by the receiver. The results are viewed on-screen and recorded onto diskfor later analysis in the office. Radar is effective on rough surfaces and through multi-layeredmedia, but penetration and resolution is hampered when "looking" through highly conductive

materials such as multi-layered reinforcement, ash or clinker, saline saturated material andwet clay.

4.5 SurveyEffectiveness

The material surrounding, and particularly above buried services can affect whether the

service can be resolved by GPR. Reinforced concrete, buried metallic objects, the presence of

moisture or clay and changes in the material construction can lead to poor data resolution.

Depth results using GPRare typically accurate to within +/- 10% with a horizontal accuracyof +/- 150mm. Radiodetection is able to penetrate far deeper than GPRand has an accuracy

of +/- 10% of depth for positional accuracy.

The effectiveness of GPRis reduced on slopes of greater than 1:3 and in woodland areaswhere the signal is denigrated by tree roots.

It is imperative that the area to be surveyed is reasonably level as it is not possible to scandeep vehicle ruts, etc.

It is not possible to carry out a GPRsurvey in the areas of shrubbery, undergrowth and

flowerbed unless the site is cleared.


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5.Seismic survey

Fig. 1. Schematic of overall field setup for a seismic survey.

The seismic survey is one form of geophysical survey that aims at measuring the earths

(geo-) properties by means of physical (-physics) principles such as magnetic, electric,

gravitational, thermal, and elastic theories. It is based on the theory of elasticity and

therefore tries to deduce elastic properties of materials by measuring their response to elastic

disturbances called seismic (or elastic) waves.

5.1 What Are Seismic Waves?A seismic source-such as sledgehammer-is used to generate seismic waves, sensed

by receivers deployed along a preset geometry (called receiver array), and then recorded


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Fig. 2. Major types of seismic waves based on propagation characteristics.

by a digital device called seismograph (Fig. 1). Based on a typical propagation mechanism used

in a seismic survey, seismic waves are grouped primarily into direct, reflected, refracted, and

surface waves (Fig. 2). There are three major types of seismic surveys: refraction, reflection, and

surface-wave, depending on the specific type of waves being utilized. Each type of seismic

survey utilizes a specific type of wave (for example, reflected waves for reflection survey) and

its specific arrival pattern on a multichannel record (Fig. 3). Seismic waves for the survey

can be generated in two ways: actively or passively. They can be generated actively by using an

impact source like a sledgehammer or passively by natural (for example, tidal motion and under)

and cultural (for example, traffic) activities. Most of the seismic surveys istorically implemented

have been the active type. Seismic waves propagating within the vertical plane holding both

source and receivers are also called inline waves, whereas those coming off the plane are calledoffline waves (Fig. 4).

Refraction Survey Reflection Survey Surface-Wave Survey


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Fig. 3. (Right) A field record and interpretation of different seismic events based on

the arrival pattern

. Fig. 4. Illustration of active versus passive waves and

inline versus offline waves


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5.2 Refraction Survey Method

A refraction survey uses refracted (or head) waves to deduce velocities of the layered-earth

model. So-called first arrival information is used for the analysis (Fig. 3). More

generalized methods based on the turning waves from an arbitrary velocity model have also beenused in recent days. This is called seismic refraction tomography. Historically

the refraction method has been commonly used to map depth and velocity of bedrock.

5.3 Reflection Survey Method

Reflected waves from the interfaces between materials of significant different

elastic properties (density and seismic velocity) are used for this type of survey.

More specifically, a special acquisition and processing method called the CDP(common-depth-point) method is used and the final product from this survey is a

section that depicts a cross-sectional image of the subsurface below the surveyed

line (Fig. 5). This method was invented and has been used traditionally in

exploration for natural resources (oil, coal, etc.). Since the early 1980s, it has been

used mostly for shallow geotechnical engineering projects (Fig. 5). Comparing

these types of reflection surveys together, they are different in dimensions surveyed

and resolution achieved. Field acquisition and data processing procedures are

normally much more costly than with the other types of seismic survey.


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Fig. 5

5.4 Surface Wave Survey Method

When seismic waves are generated, there is a special type of wave propagating along the freesurface called surface waves whose penetration depth is wavelength-

dependent; the longer wavelength influences the deeper portion of the earth (Fig. 1). Because of

this property, surface waves are usually dispersive (Fig. 2), meaning different

frequencies have different propagation velocities, whereas body waves (refraction, reflection,

head, etc., waves) rarely take such property to a noticeable extent. Two types of

surface waves are generally known: Rayleigh and Love waves. The disturbance (vibration)


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direction of the former is mainly perpendicular to the surface, whereas it is parallel

for the latter. Theoretically, the dispersion property of surface waves is determined by several

elastic properties including density (rho), and depth-variation of S- and P-wave

velocities (Vs and Vp). Among these parameters, the depth-variation of Vs is the most

influencing factor. Because of this, surface waves are often used to deduce Vs

properties of near-surface earth materials. In comparison to using conventional body-wave

methods to achieve similar Vs information (for example, S-wave refraction,

reflection, down-hole, cross-hole surveys), the surface-wave method has several advantages:

Field data acquisition is very simple and tolerant because surface waves always take the

strongest energy.The data processing procedure is relatively simple and easy even for the non-experienced. A

large area can be covered within a relatively short time period. Because of all above reasons, itis highly cost effective and time efficient.

Utilization of surface waves for geotechnical engineering purposes has a history dating back to

the early 1950s. Since the early 2000s a multichannel approach called theMASW (multichannel analysis of surface waves) method has been widely used.

Fig. 1.


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Fig. 2. With seismic velocity increasing with depth, longer wavelengths (lower

frequencies) of surface waves penetrating deeper depths travel with faster

velocity than shorter wavelengths (higher frequencies) do. As a result, different

frequencies arrive at different times on a seismic record, making a dispersive

seismic event.

5.5 Applications

Reflection seismology is extensively used in exploration for hydrocarbons (i.e., petroleum,natural gas) and such other resources as coal, ores, minerals, and geothermal energy.

Reflection seismology is also used for basic research into the nature and origin of the rocksmaking up the Earth's crust. Reflection Seismology is also used in shallow application for

engineering, groundwater and environmental surveying. A method similar to reflectionseismology which uses electromagnetic instead of elastic waves is known as Ground-

penetrating radar or GPR. GPRis widely used for mapping shallow subsurface (up to a few

meters deep).

Field Practicals of Geo-Physical Exploration

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