2014

RAGHU VANSH BHUSHAN SINGH

Ground Penetrating Radar-Technology

Ground Penetrating Radar

Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface.

This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF

frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR can

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.

GPR uses high-frequency (usually polarized) radio waves and transmits into the ground. When the wave

hits a buried object or a boundary with different dielectric constants, the receiving antenna records

variations in the reflected return signal. The principles involved are similar to reflection seismology,

except that electromagnetic energy is used instead of acoustic energy, and reflections appear at

boundaries with different dielectric constants instead of acoustic impedance.

GPR Principle

The operating principle of Ground Penetrating Radar is straightforward. A GPR couples EM waves in

the ground and samples the backscattered echoes. An EM wave will be backscattered on any electrical

parameter contrast in the ground, i.e. the permittivity ε, the permeability μ or the conductivity σ. All

these three macroscopic parameters are in general a function of frequency. In practice it will be

primarily the contrast in permittivity, which leads to a reflection of the radiated EM waves. Earth

materials are mostly nonmagnetic materials, having a relative magnetic permeability μrμ / μ0 of 1, with

μ0=4.10-7 H /mbeing the permeability of free space. This means that no contrast in permeability will be

encountered. The change in conductivity primarily affects absorption of the radar signal by the medium.

The variation in permittivity has the largest impact on the variation of the characteristic impedance of

the medium, so the encountered contrasts in permittivity e between materials in the ground will lead to a

reflection.

The data recorded by a GPR are generally represented as a one, two or three dimensional dataset,

denominated by the acoustic terminology A- B- and C-scans.

A-scan A single waveform b(xi , yj ,t) recorded by a GPR, with the antennas at a given fixed position (xi , yj ) is

referred to as an A-scan. The only variable is the time, which is related to the depth by the propagation

velocity of the EM waves in the medium.

B-scan

When moving the GPR antennas on a line along the x-axis, one can gather a set of A-scan, which form a

two dimensional data set b(x, yj, t), called a B-scan Fig-2. When the amplitude of the received signal is

represented by a color scale (or grey-scale), a 2D image is obtained. The 2D image represents a vertical

slice in the ground. The time axis or the related depth axis is usually pointed downwards.

Reflections on a point scatterer located below the surface appear, due to the beam width of the

transmitting and the receiving antenna, as hyperbolic structures in a B-scan. This can be easily verified,

using the geometry shown in Fig.3 Suppose a homogeneous half-space with a propagation velocity

equals v and the transmitting and receiving antenna close to each other, so that they can be considered as

one antenna (monostatic case). The co-ordinate system is represented on Fig.3. A point scatterer at a

position (0,z0 ) in the half space, will be located by the antenna pair, situated in (x,0) at a distance (x² +

z02)1/2 . So in the recorded data b(x,t) , represented in Fig.3, the reflection on the point scatterer appears

in each A-scan after a time

t1/2 =(x² + z02)1/2/v

Equation 1

Equation (1) represents a hyperbola with a vertical axis and an apex in (0,2z0 /v). The shape of the

hyperbola is function of the antenna configuration (monostatic, bistatic), the depth of the point scatterer 0

z and the propagation velocity profile of the ground. The hyperbolic defocusing of an object can be

corrected for in the data processing, this is called migration or SAR processing.

Fig. 3: (a) Point scatterer at position (0,z0 ), (b) recorded data b(x,t)

C-scan

Finally, when collecting multiple parallel B-scans or in other words, when moving the antenna over a

(regular) grid in the xy-plane, a three dimensional data set b(x, y,t) can be recorded, called a C-scan

(Fig.4). Usually a C-scan is represented as a two dimensional image by plotting the amplitudes of the

recorded data at a given time ti. The image b(x, y, ti) represents then a horizontal slice at a certain depth,

parallel to the recording plane. Nowadays, many user-software packages have integrated functions to

plot directly three-dimensional representations of the recorded C-scans. In this case, an arbitrary cut in

the 3-D volume (Fig.5 (a)) or an iso-surface (surface with the same amplitude) is usually represented

(Fig.5 (b)).

Fig.4 Multiple parallel B-scans forming a C-scan

Fig. 5: (a) Arbitrary cut in the 3-D volume, (b) iso-surface representation

Types of GPR

Impulse Ground Penetrating Radars

Impulse radars acquire data on the reflected energy as a function of time. It consists of an emitter

of high frequency waves in the microwave UHF (ultrahigh frequency) or VHF (very high

frequency) band, and a receiver that detects and amplifies the scattered and reflected waves.

Since impulse radar has low cost parts and deploys simple impulse waveform generating, it is

commercially very powerful. It however has one major disadvantage in that the resolution of its

imaging is restricted by the width of the pulse that is used. Impulse ground penetrating radar is

broadly used in data collection techniques (whether it is in the time or frequency domain).

(Band width) BW=2((fmax-fmin)/(fmax+fmin))100%

Equation 2

Continuous Wave Ground Penetrating Radars

Continuous wave radars acquire and continuously transmit data on the reflected energy as a

function of frequency. Technique involved is to transmit a frequency sweep over a fixed

bandwidth, such as from a beginning to an end frequency. Reflections waves are mapped as a

function of frequency and are a measure of the energy that has been scattered from subsurface

objects.

Stepped Frequency Radar

A further variation of ground penetrating radar is stepped frequency radar that transmits data on

reflected energy in stepped linear increments over fixed bandwidths. These could further be

frequency modulated as well. Further, to eliminate the issue of weaker signals from deeper

targets getting masked by the stronger signals, compensation is to be made for lateral scattering

and taking cognizance of the unambiguous range that can be measured by the stepped frequency

radar.

Other Types of Ground Penetrating Radars

Further variations of ground penetration radars are ultra wide band radars, synthetic aperture

radars, noise source radars and arbitrary waveform radars. Some custom ground penetration

radars may also be designed, such as borehole radars.

Propagation of GPR Signal For a good understanding of the physics behind GPR, it is indispensable to study the EM wave

propagation in a loose dielectric material. Without loss of generality, the study is limited to the

propagation of uniform plane waves. More complicated wave fronts can always be described as a

combination of plane waves. Earth materials, and soils in particular, can be considered as a conductive

dielectric medium. Maxwell’s equations describing the wave propagation in a conductive dielectric are

Equation 3

Equation 4

In harmonic regime, equation (3) and (4) can be written as

Equation 5

Equation 6

ν = c/|εr|1/2

Equation 7

Reflection Coefficient of layer 1 w.r.t. layer 2 is given as-

Where-

At critical angle refraction sin φ1 = v1/v2 = ε11/2

/ε21/2 and Snell's Law- sin φ1/sin φ2 = v1/v2, gives

Reflection Coefficient (R) = (ε11/2

-ε21/2)/(ε1

1/2+ε2

1/2)

Equation 8

Where-

σ σ'jσ" the complex conductivity of the medium.

ε ε'jε" the complex permittivity.

μ = the permeability of the medium.

ν = velocity in medium.

c = velocity of light (EM signal in free space).

Eq. (8) shows that the amplitude of the reflected wave is defined by the ratio of the dielectric constant of

the two material. The reflection coefficient R takes value between _1≤ R ≤1. If the lower material is

metal, the reflection coefficient is R = -1 and it takes the maximum amplitude. Therefore, the reflection

from metallic material is always very obvious. This condition stands even when the metallic material is

a thin sheet, because all the EM energy is reflected by the metallic sheet. In actual GPR measurement,

the radar target is not infinitively large, but it has a finite size. Generally, a larger target reflects stronger

signals, when the size of the target is smaller than the wavelength. The GPR wavelength is normally

more than 1m, and this condition is valid for most of the GPR targets. In this case, the amplitude of the

reflected wave is a good measure of the size of the radar target. This amplitude can normally be related

to the RCS (Radar Cross Section). On the contrary, a linear object such as pipes has a strong dependency

of reflectivity of the EM wave polarization. When the direction of the pipe is collinear to the incident

EM wave, even the diameter of the pipe is very small compared to the wavelength, the reflectivity is

very large. When the incident polarization is perpendicular, a metallic pipe does not reflect EM wave.

Therefore, GPR polarization is very important in applications such as pipe and cable detection.

Table: Attenuation and relative permittivity (dielectric constant) of subsurface material measured

at 100MHz

GPR Data Acquisition (SIR 3000):

Grid pattern is used for GPR data acquisition, we normally use 2m x 2m for geological feature and 10m

x 2m and 20m x 5m for sub-surface utility engineering survey. The GPR Antennas 100MHz, 270MHz,

300MHz, 450MHz and 800MHz are used as per depth of investigation and GPR Antenna runs at the

grid line of survey area. The example of grid pattern is given as-

GPR Data Processing (RADAN SOFTWARE):

General Processing steps of GPR Data

1. Time Zero

2. Static Correction

3. FIR and IIR Filter

4. Background removal

5. Stacking

6. Decovolution

7. Migration

1. Time Zero:

Sometimes it is necessary to vertically adjust the position of the whole profile in the data window (adjust

time-zero). You may want the first positive peak of the direct wave from a ground coupled, bistatic

antenna to be centered at the top edge of the screen so that you can consider ground surface to be at the

top of the window (at Time Zero). A corrected 0-position will give you a more accurate depth

calculation because it sets the top of the scan to a close approximation of the ground surface.

When you click on the Time Zero icon, the left pane displays the Time Zero Process Bar. Simply “grab”

the positive peak and hold your left mouse button down. Drag the positive peak and adjust it until it lines

up with the 0.0 line. Click Apply to Test your position and, if necessary, click Reset and re-adjust.

Once you are satisfied with the Time Zero Position, click OK.

2. Static Correction: Static Corrections compensates for variations in elevation, phase shifts, and high frequency noise served

in the horizontal direction and is generally one of the last processing steps undertaken.

Static Corrections assume that near-horizontal discontinuous due to poor antenna coupling, time zero

tracking problems, or some localized changes in the velocity. Sometimes after a lot of processing, once

horizontal (or near-horizontal) and continuous layer appear discontinuous and slightly shifted in time,

making the reflector difficult to trace from scan to scan. Static Corrections can correct for this.

Static Corrections compensates for noise that is introduced by shifting the reflector within a specified

time window (specified by the number of samples) so that it is realigned. Another function of Static

Corrections is that it allows you to filter horizontally without influencing the vertical frequency of the

data, unlike Horizontal High and Low Pass Filtering. Static Correction proves the horizontal traceability

of layers using cross correlation.

1 A rectangular overlay will appear superimposed upon your data. You can use the mouse to shape the

rectangle to the desired width and move it so it is superimposed upon the reflector of interest.

2 After adjusting the first segment on the reflector, move the cursor with the mouse to the next segment,

and click on it. In this way, create a multi-segmented window that the Static function will use to trace a

reflector.

3 A dialog box will appear in which you will be able to choose the window height (in number of

samples), the filter length, and the type of model (Boxcar or Triangle) you wish to use to filter.

RADAN correlates this model scan of a specified filter length and performs a horizontal boxcar or

triangle filter size of the filter length of the number of samples in the window and compares it to the

highlighted layer. The correlation threshold is the value used to cross-correlate the model data with the

actual data. It is the parameter that tells us how well a layer can be traced from scan to scan. The

correlation threshold is usually from 0.5 to 1.0.

3. Filter:

3a. FIR Filter:

FIR filters have a finite-duration impulse response. FIR filters, when encountering a feature in the data,

are guaranteed to output a finite filtered version of that feature. This property makes it possible to design

filters that are perfectly symmetrical and have linear phase characteristics.

FIR filters will therefore produce symmetrical results so reflections will not be shifted in time or

position. Note, because of the symmetrical nature of FIR filters, FIR filter lengths should always be an

odd number. Generally, FIR filters are normally preferred in digital signal post processing; however,

users more familiar with IIR filters might choose to use them instead.

There are two types of FIR filters available in RADAN, Boxcar and Triangular Filters.

3b. IIR Filter:

IIR filters were introduced before the advent of computers, when simple LRC circuits were used as

analog filters. When an IIR filter encounters a feature in the radar data, it produces an output that decays

exponentially towards zero but never reaches it, hence the name “infinite.” IIR filters are not necessarily

symmetrical and while they achieve excellent amplitude response, their phase response is non-linear and

so they can cause slight phase shifts in the data.

IIR filters having only one pole so that there is not a sharp break at the cutoff frequency, which may

provide limited noise reduction. As a consequence of this, it may be beneficial to run the same filter

more than once. Or, another approach is to modify the Color Transform (under the Display Parameters

Dialog Box) to hide what little noise remains in your data.

3.1 Boxcar Filter: The Boxcar filter is a rectangular window function that performs a simple running

average on the data. A portion of the data, determined by the filter length, is averaged, and the average is

output as a single point at the center of the active portion of the filter window. The filter moves on to the

next sample and the process is repeated. The Boxcar filter assigns equal weight to the data all along the

filter length.

3.2 Triangular Filter: The Triangular filter emphasizes the center of the filter more heavily than the

ends of the filter. This type of filter is a weighted moving average, with the weighting function shaped

like a triangle. A portion of the data, determined by the filter length, is multiplied and summed by this

function. The result is output at the center of the triangle. The filter then advances one sample and the

process repeats.

When the FIR Filter icon is click, the left pane will display the FIR Filter Process Bar.

4. Background Removal:

Background Removal Filter is the best way to remove bands of ringing noise. Low frequency features in

the data will be removed, such as antenna ringing. Note: The length of the filter should always be a

greater number of scans than the length in scans of the longest flat “real” event in the data that you want

to keep. This filter will remove the surface reflection (direct coupling) pulse. In many cases, it is

important to keep that direct coupling pulse as that is a visual check of your surface. If you want to keep

the pulse, change the starting sample number to a value below the surface reflection. This may

necessitate overlap filtering later to remove non-linearities created in the data by “window” filtering

5. Stacking:

Apply a simple running-average to stack the data. Stacking combines the adjacent selected radar scans

and outputs a single scan. When stacking values are used in RADAN, the program will retain the marks

in the file. However, the scans per unit distance and marks per unit distance will be changed in the

header. For example if you had a raw file with 80 scans per meter and 1 meter per mark and you stacked

by a factor of two (2) the output file would have 40 scans per meter written into the header (reduced by a

factor of 2).

Length: Enter the number of scans to filter. High number of Background Removal, low odd number for

Stacking.

6. Deconvolution: Multiples or “ringing” occur when the radar signal bounces back and forth between an object (such as a

piece of metal or layer of wet clay) and the antenna, causing repetitive reflection patterns throughout the

data and obscuring information at lower depths. Multiple reflections may also be observed when

mapping water bottom, bedrock (or till), or voids. Deconvolution is the filtering method used to remove

this type of noise.

RADAN uses a method called Predictive Deconvolution. It is a general technique that includes

“spiking” deconvolution as a special case. Predictive deconvolution is aptly named because the method

tries to approximate the shape of the transmitted pulse as the antenna is coupled to the ground.

Assuming a source wavelet of a specified length, called the operator length, this filter will predict what

the data will look like a certain distance away, called the prediction lag, when the source wavelet is

subtracted (or deconvolved) from it. This results in the compression of the reflected wavelet. Predictable

phenomena, such as antenna ringing and multiples, are moved to distances greater than the prediction

lag and are effectively removed from the data.

Deconvolution Parameter Selection: In order for Deconvolution to work properly, certain parameters,

such as operator length, prediction lag, prewhitening, gain, start sample, and end sample, must be

supplied as input.

Operator Length: The operator length specifies the size of the filter used in terms of the number of

samples making up 1 pulse length. Longer filters give a better approximation of the radar wavelet and

generally give better results, but take longer to process. A good rule to start with is that the operator

length should be about one full cycle of the radar antenna wavelet. A value less than this gives poor

results. To remove reverberation, first measure the width of a reverb packet in number of samples. Then

set the operator length to that value. Increase the operator length slightly for more effect. Tutorial two in

the "Advanced Processing Tutorials" section will detail how to measure the width of a feature and input

an appropriate operator length.

Prediction Lag: The Prediction Lag should be set to the desired length of the output pulses (about one-

half cycle of the antenna wavelet). Smaller values than this will produce more noise.

When using Deconvolution to remove multiple reflections, the lag should be equal to or less than the

spacing between multiples.

A prediction lag between 5 and 1 is used to approximate “spiking deconvolution.” However, this

introduces significantly more noise into the data.

Prewhitening: Prewhitening modifies the autocorrelation function by boosting the white noise (zero

delay) component. Mathematically, prewhitening stabilizes the filter, thereby smoothing the output and

reducing noise. Values between 0.1 and 1 percent are common, 0.8 is a good value to start with.

Additional Gain: Additional Gain may be needed because the deconvolution process attenuates the

signal, especially when the prediction lag is short. Gain values of 3 to 5 are common, but use whatever

achieves an amplitude level equal to the original data.

7. Migration: The radar antenna radiates energy with a wide beam width pattern such that objects several feet away

may be detected. As a consequence of this, objects of finite dimensions may appear as hyperbolic

reflectors on the radar record as the antenna detects the object from far off and is moved over and past it.

Deeper objects may be obscured by numerous shallower objects that appear as constructively interfering

hyperbolic reflectors. Steeply dipping surfaces will also cause diffracted reflections of radar energy.

This diffracted energy can mask other reflections of interest and cause misinterpretation of the size and

geometry of subsurface objects. The apparent geometry of steeply dipping layers are an illusion and

need to be corrected in many cases. Migration is a technique that moves dipping reflectors to their true

subsurface positions and collapses hyperbolic diffractions.

There are two Migration methods available in RADAN: Kirchhoff and Hyperbolic Summation. The

Hyperbolic Summation method is faster but less accurate than the Kirchhoff method.

Hyperbolic Summation: Hyperbolic Summation works by summing along a hyperbola placed on the

data, and placing the resulting average at the apex of the hyperbola. This process is repeated with the

apex on every point of the data.

Kirchhoff (default): The Kirchhoff method is more accurate than the Hyperbolic Summation. An

average value is still derived by summing along a hyperbola placed on the data and placed at the apex.

However, Kirchhoff Migration also applies a correction factor to this averaged value, based upon the

angle of incidence and distance to the feature. It also applies a filter to compensate for the summation

process. This filter improves resolution by emphasizing the higher frequencies and applying a phase

correction. Generally, because of speed considerations, the hyperbolic method is used if it provides

adequate results. If the Hyperbolic Summation method does not provide good results, the Kirchhoff

Migration method should be tried.

Both the Hyperbolic Summation and Kirchhoff Migration require that the hyperbolic width and relative

velocity are specified.

The following file header parameters must also be assigned a value before the data can be migrated:

• Samples/scan

• Range (ns)

• Scans/meter

CMP (Common Mid-Point) Processing:

Subsurface Structure and GPR Profile

In the profile measurement, the transmitting and the receiving antennas are moved together on the

ground surface. We can estimate the rough subsurface structure directly from the continuous travel time

of the GPR signal as shown in Fig. However, we have to notice that this GPR profile is not always the

true subsurface structure. For example, if we have a point reflector such as a metal pipe, the reflected

signal is scattered into wide angle and are measured in wide areas as shown in Fig. When the object has

relatively flat structure, the GPR profile is almost the same as the true subsurface structure, because the

strongest reflection is measured when the antennas are located just above the reflecting point. In many

cases in geological survey, this can be applied. Fig. shows one example of actual GPR profiles. In this

example, we have three steel pipes buried at 75cm below the ground surface. The corresponding three

hyperbolic curves can be observed. A boundary of filled soil and the mixture of concrete and soil is

located at 2.75m. This layer is almost flat, therefore, it is directly imaged. We also observe some

disturbance of reflected signal at 3.75-5m, and we expect there are some buried objects.

Pre-Processing Fig. shows the GPR Profile of the wide angle (CMP) measurement. When the antenna separation gets

larger, the arriving time of the signal delays. The directly coupled signal, which propagates from the

transmitting antenna to the receiving antenna through air, which is often referred as “air wave” has a

straight arriving time delay, but the reflected signal form subsurface objects are on a curved line. The

reflection from an object on the ground surface, which is standing on the line of antenna movement does

not change in time. We can use this arriving time information for discriminating each object, which

causes the reflection.

This GPR profile is acquired by unshielded antennas. The noise rejection can be processed by a band-

pass filter for time domain signal, and two dimensional filter such as f-k filter are effective. Fig. shows

an example of a signal after rejection of the high frequency noise and reflection from an object above a

ground surface by the f-k filtering. In the profile measurement, we sometimes encounter a strong

reflection form metallic objects on the ground surface such as steel fence. Identification of such a

reflection signal is difficult in the profile measurement. The EM wave velocity in subsurface material

and I air is very different, therefore, it is easy to discriminate them by the wide-angle measurement.

Estimation of the Dielectric Constant When we have a subsurface reflection object whose depth is known, the EM velocity can be determined

by the measured travel time. Even when we do not know the exact buried depth, a strong point reflector

such as a metal pipe show clear hyperbolic reflection curve, and we can estimate the depth of the object

and the velocity simultaneously. When the buried object is located at depth of z and the horizontal

distance of x0, and assume the EM wave velocity as v, the travel time of the reflected signal from this

object can be given by:

t1/2=((x-x0)² + z02)1/2/2v

Eq. (1) contains three unknown parameters; z, v and x0. When we have a clear hyperbolic curve as

shown in B-scan, we can graphically fit the theoretical curve and estimate these parameters. In the wide-

angle measurement, when the antenna positions are fixed, theoretical travel time can be calculated and

the EM velocity can be estimated. By the wide-angle measurement, we can use reflecting objects such as

a horizontal boundary, and inclined boundary, even multiple boundaries. Many signal processing

techniques develop for seismic signal processing can be applied for GPR analysis.

CMP Processing This technique has been widely accepted in seismic signal processing. In GPR, EM wave is sometimes

suffered from a strong attenuation and dispersion. In this case, the signals measured at different offsets

lose the coherency and the CMP technique cannot be applied. However, we have found in many cases in

dry soil and crystalline rock, CMP for GPR is an effective processing. The wide-angle measurement and

the profile measurement for the same site is compared in Fig. Both GPR profiles have many similarities,

although SNR and continuities of the reflections are improved in the wide-angle measurement.

However, the wide-angle measurement requires much more data acquisition compared to the profile-

measurement. One of the most significant advantages of the GPR measurement is the fast data

acquisition. CMP cannot use this advantage, so it should not always used. Also, we can find that the

radar resolution in shallow region is better in the profile-measurement. It will be cause by the low spatial

coherency of the measured signals in the wide-angle measurement.

GPR Data Interpretation: 1. Geological Layer Interpretation: When Select Range is activated, a multicolored translucent overlay appears over the data, extending

from the beginning to the end of the file. It operates similarly to the Select Block, except that all

operations performed using the Select Range picking tool are performed within the time interval (slice

width) of the selected area on all of the scans in the file.

2. Utility Interpretation:

a. Utility Marking

b. Utility Plot of All Grid (3D):

Survey Applications:

1. Utility Mapping

To locate and map utilities before any excavation begins is a concern to everyone involved. GPR is a

beneficial tool due to its capability to locate both metallic and non-metallic utilities. When using GPR,

both position and depth can be marked out on site and later also visualized into 3D reports.

Key Advantage:

Locates metallic as well as non-metallic utilities - Fast -Easy user interface - Portable - precise locating -

From low cost GPR solution with the Easy Locator for the utility locate professionals to more advanced

solutions.

2. Utility Locating

To utility companies buried services are assets that need to be protected, whilst to the construction

industry they can represent a major hazard. Precise and reliable information about the presence, location

and depth of these utilities and other buried infrastructure is therefore essential.

Key Advantage:

Locates metallic as well as non-metallic utilities - Easy user interface - Portable - Low cost GPR

solution for the utility locate professionals - Fast - precise locating.

Keywords:

Utility GPR Underground utility locating equipment Subsurface utility engineering Utility Locators

Utility Radar Utility Locating Underground Utility Detection Locating buried utilities Underground

utility locates Locating water lines Utility Locating non-metallic utilities Locating PVC pipes Easy

Locater X3M.

3. Void Detection

Buried voids are a hazard, both to engineers and the general public. They can impede construction

operations, undermine building foundations and be the cause of destructive ground subsidence.

Problems associated with hidden voids come in many forms. Naturally formed cavities and sinkholes in

karstic limestone terrain, unknown basements and culverts, abandoned wells and mineshafts, all of

which present serious hazards.

Key Advantage:

Locates metallic as well as non-metallic utilities - Easy user interface - Portable - Low cost GPR

solution for the utility locate professionals - Fast - precise locating, non-destructive.

4. Underground Storage Tank (UST) Location

GPR can locate Underground Storage Tanks (UST) and any associated underground piping. This non-

destructive inspection and geophysical survey can determine the location and depth of a UST or provide

the location of a former tank vault.

Key Advantage:

Locates metallic as well as non-metallic utilities - Easy user interface - Portable - Low cost GPR

solution for the utility locate professionals - Fast - precise locating, non-destructive.

5. Archaeology

Ground penetrating radar survey is one method used in archaeological geophysics. GPR can be used to

detect and map subsurface archaeological artifacts, features, and patterning.

The concept of radar is familiar to most people. With ground penetrating radar, the radar signal – an

electromagnetic pulse – is directed into the ground. Subsurface objects and stratigraphy (layering) will

cause reflections that are picked up by a receiver. The travel time of the reflected signal indicates the

depth. Data may be plotted as profiles, as plan view maps isolating specific depths, or as three-

dimensional models.

GPR can be a powerful tool in favorable conditions (uniform sandy soils are ideal). Like other

geophysical methods used in archaeology (and unlike excavation) it can locate artifacts and map features

without any risk of damaging them. Among methods used in archaeological geophysics it is unique both

in its ability to detect some small objects at relatively great depths, and in its ability to distinguish the

depth of anomaly sources. The principal disadvantage of GPR is that it is severely limited by less-than-

ideal environmental conditions. Fine-grained sediments (clays and silts) are often problematic because

their high electrical conductivity causes loss of signal strength; rocky or heterogeneous sediments scatter

the GPR signal, weakening the useful signal while increasing extraneous noise.

Subsurface Utility Engineering Survey – Scope (from HFCL)

The description of various quality level surveys is mentioned here below:

2.1 Quality Level-D Survey

This process is to gather information on subsurface utilities from various utility owners, ground staff,

local representatives, residents etc. This is accomplished by

Define each authority/utility owner on ground

Identification of owners

Sending out formal requests for utility data

Extensive follow-ups (since owners have no obligation to share this data)

Opening up informal channels of communication

Identification of key ground level personnel 3 of 9

Recording verbal recollections etc.

Open the manhole and check the depth of utility

2.2 Quality Level-C Survey

This is the process to collect information through physical survey about the presence of subsurface

utilities by above ground observations along with undulations of the path targeted for OFC execution

with the use of Topographic equipment such as Total stations, DGPS, etc.

a. Total Station Survey

Total Station is three-dimensional surveying technology unit. Accuracy of total stations is very high, and

they have been in use over a period of time long enough to justify confidence in their accuracy and

reliability.

The total station is an electronic distance measuring device mounted on a tripod, measures the

undulations of the surface of the ground and the distance from the instrument to its target.

The total station sends an infrared laser beam that is reflected from a prism and sent back to it

giving the distance and difference in earth level of the station created.

b. DGPS Survey

Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System that

provides improved location accuracy to about 10 cm for best implementations.

DGPS uses a network of fixed, ground-based reference stations to broadcast the difference

between the positions indicated by the satellite systems and the known fixed positions.

These stations broadcast the difference between the measured satellite pseudo-ranges and actual

(internally computed) pseudo-ranges, and receiver stations may correct their pseudo-ranges by

the same amount.

The digital correction signal is typically broadcast locally over ground-based transmitters of

shorter range.

Mapping surface appurtenances of the section to be surveyed will be done by using Total Station

equipment and same should be submitted as road profile in each of the surveyed section.

Total Station should pick up all surface projections of utilities such as, Hand Holes, Man Holes,

and Fire Hydrants etc.

The DGPS will be integrated to mark precise coordinates to surveyed along with total Stations

DGPS settings will be provided with nearest available permanent BM and level in the drawings

The reference point for start of the grid will be established

The reference points will be associated with various utilities like manhole valves, fire hydrants,

chambers, and junction boxes etc. for better understanding for field team and HDD operator

2.3 Quality Level-B Survey

This is the process involving geophysical methods with associated technologies such as Ground

Penetration Radar (GPR) and Induction Locators 4 of 9.

Ground Penetrating Radar Following are the important steps to be followed during GPR survey:

a. The utility corridor of 5meter will be based on field survey input and availability of ROW authority

permission. This will be conveyed before start of SUE survey.

b. GPR survey to start from the fixed reference points with respect to the universal co-ordinates obtained

through DGPS or reliable local coordinates from local authorities.

c. The QL-B survey should be done with closed intervals of 2metre at road crossings, junctions and at

intermediate grid locations, identified with new utilities

d. Combination of 100MHz, 300MHz and 500MHz shall be used for best result

e. If the utility corridor specified falls on both foot path and on carriage way, GPR scanning to be done

on both foot path and carriage way.

f. The vertical scanning to be done to cover the 5m corridor on either side of the edges and also on

middle wherever required for utility identification.

g. If there are any obstruction in the survey corridor, the readings to be taken before and after the

obstruction and also whatever readings possible in between.

h. During the survey, if any utility is seen to be starting afresh or on road crossings, the scanning need to

be done at an appropriate 2mtr interval.

Induction Locators Induction surveys are carried out along with GPR survey for optimal results. It can accurately locate

Power cables in passive mode and other utilities in active mode.

a. Using reference points from Total Station (for grid marking and picking up end points of GPR lines)

b. Induction locator for detection of live electric cables

2.4 Quality Level A:

This involves the following methods:

a. Taking readings from various manholes and hand holes where ever possible

b. Taking test pits wherever there is a need

c. Safety precautions (proper hard barricades with safety warning tapes) to be provided during execution

of the job

This will add credibility to the entire SUE process as this actually views the utilities.

Trial pits needed for verification/suggestions made by SUE report necessary to ensure OFC deployment

with bare minimum damages to utilities in the designated path of HDD execution.

Limitations of GPR

The most significant performance limitation of GPR is in high-conductivity materials such as clay soils

and soils that are salt contaminated. Performance is also limited by signal scattering in heterogeneous

conditions (e.g. rocky soils).

Other disadvantages of currently available GPR systems include:

Interpretation of radargrams is generally non-intuitive to the novice.

Considerable expertise is necessary to effectively design, conduct, and interpret GPR surveys.

Relatively high energy consumption can be problematic for extensive field surveys.

Recent advances in GPR hardware and software have done much to ameliorate these disadvantages, and

further improvement can be expected with ongoing development.

References:

1. Ground Penetrating Radar Fundamentals by Jeffrey J. Daniels, Department of Geological

Sciences, The Ohio State University Prepared as an appendix to a report to the U.S.EPA, Region

V.

2. Ground Penetrating Radar by A.P. Annan.

3. GPR and Its Application to Environmental Study by Motoyuki Sato Professor, Center for

Northeast Asian Studies (CNEAS) Tohoku University, Japan.

4. Fundamentals of Ground Penetrating Radar by Carey M. Rappaport CenSSIS Dept. Elect. And

Comp. Engineering Northeastern University.