GPR Survey Design

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Workshop Notes July 2001 6-Survey Design

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6 SURVEY DESIGN

Proper design of GPR surveys is critical to success. Setting expectations and optimizing data acquisition to meet

expectations requires planning. This chapter builds on the paper Annan & Cosway (1992) on the subject of surveydesign.

6.1 EVALUATING GPR SUITABILITY

Prediction of whether GPR will "work" for the problem at hand is not clear cut. In general it is easier to rule out situ-ations where radar is totally unsuitable than to state with confidence that radar will be successful. Again, this is not aunique feature of the GPR method but is a fact of life with all geophysical methods. GPR tends to have more mysterybecause people have not normally had as much experience with it as with some other methods.

There are some basic tools which assist the GPR user in the decision making process. The two most important are theradar range equation, and numerical simulation techniques. Some examples are described by Annan and Chua(1988).

The radar range equation (RRE for short) does a basic allocation of available power against all the loss mechanismsto yield a "yes/no" answer on whether a target will return sufficient power to be detectable. The RRE has to simplifythe problem at hand; therefore, the results are good guides, not absolute predictors of success or failure. The basicsteps of the radar range equation are depicted in Figure 6-1. Example results of an automated program to carry outthese calculations are shown in Figure 6-2. Nomograms for specific systems and targets can also be generated suchas shown in Figure 6-2.

Figure: 6-1 Block Diagram of radar range equation.


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6-Survey Design Workshop Notes July 2001

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System Performance Factors

System Q (dB) 133.98 Tx voltage (V) 1000.00

Tx to antenna effcy (dB) -20-00 Rx noise (uV) 200.00Transmitter antenna gain (dB) 3.00

Antenna to receiver effcy (dB) -20.00Receiver antenna gain (dB) -1.45

_____Net system Q (dB) 98.53

Propogation Factors

Spreading losses (dB) -74.02

Attenuation Losses (dB) -4.00

Target Factors POINTS ROUGH SPEC SPEC THIN Target

backscatter fain (dB) -5.37 8.60 19.85 -38.72

Net Performance 15.14 29.11 40.36 -18.21

Target amplitude (uV) 1143 5709 20853 25

Stacks 1 1 1 66

Windows (ns 667RADAR RANGE PAPER COEFFTS

Model Parameters POINT ROUGH SPEC PEC THIN

Centre Freq. (MHz) 100.00 B1-8.54 1.56 0.62 -6.47

Target range (m) 20.00 B20.00 1.00 2.00 2.00Attenuation (dB/m) 0.10 B34.00 -1.00 0.00 0.00

Target diameter (m) 0.50

Target Reflection (dB) -11.14Host K 25.00

*Thin layer reflection (dB) -69.71Target K 8.00

Layer

Thickness (m) 0.00 *Thin layer beta (dB) -58.57

Conductivity Attenuation *Diameter/wavelength (dB) -1.58

5.00 mS/m 1.69 dB/M Wavelength (m) 0.60

* If these values exceed -10 dB then, the assumption and approximations used to derive these formulas are invalid.

Figure: 6-2 Example of a radar range calculation.

Numerical simulation techniques (NST for short) are now becoming well developed for GPR. Simple programs for2D earth structures are commercially available and are instructive to use. More complex 2- and 3-dimensional mod-elling programs are not available for general use but are available at research institutions. The basic concepts forplane (flat) layered earth modelling are shown in Figure 6-3 accompanied by an example synthetic generated by thecommercial Sensors & Software Inc. synthetic radargram program. Simple 2D earth responses using ray tracing andfinite difference approaches are shown in Figure 6-4. (McMechan) (1981)


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Figure: 6-3 Radar range equation nomogram example.]

Figure: 6-4 Illustration of a synthetic radargram to predict a GPR response.


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Figure: 6-5 Modeling such as this 2 dimensional ray trace stimulation helps users utility of GPR .

Answering the question "Will GPR work???" is neither easy nor exact. Addressing the following three questions willcertainly help in anticipating the answer.

Question 1: Is the target within the detection range of the radar irrespective of anyunusual target characteristics?

Figure: 6-6 What is the target depth?


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Discussion of Question 1

The way to answer this question is to calculate or measure the host attenuation coefficient. Using the radar rangeequation and the system performance factor (example inFigure 6-2), compute the maximum range that a reflector of

the anticipated target type can be detected. If the target is at a depth greater than this range, radar will not be effec-tive. A conservative rule-of-thumb is to state that radar will be ineffective if the actual target depth is greater than50% of the maximum range.

Commercial radar systems can typically afford to have a maximum of 60 dB attenuation associated with conductionlosses. A rough guide to penetration depth is

(6-1)

where is attenuation in dB/m and is conductivity in mS/m. These equations are not universal but are applicablewhen attenuation is moderate to high > 0.1 dB/m or > 1 mS/m which is typical of most geologic settings.

Question 2: Will the target generate a response detectable above the backgroundclutter? In other words, does the target have sufficient contrast in electrical prop-erties and is it physically large enough to reflect or scatter a detectable amount of

energy?

Figure: 6-7 What is the target geometry?

Figure: 6-8 What are the target electrical properties?

35=D


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Figure: 6-9 What is the host material?


Power reflectivity is estimated using the expression

(6-2)

Two conservative rules-of-thumb for predicting success are as follows. First, the electrical properties of the targetshould be such that the power reflectivity be at least 0.01. (Note that a metal target is equivalent to KTarget.) Thesize of the target also is a factor in the amount of energy scattered. While specific shape is also important, the size

effect dominates and can be best seen from the scattering cross section of a sphere versus wavelength shown inFigure6-10. For targets small with respect to the wavelength, scattering cross section depends on wavelength. For targetslarger than the wavelength, the cross section stabilizes to a constant.

Figure: 6-10 Variation of the scattering cross section of a spherical target as a function of dimension normalized

against wavelength (diagram from Skolnik(1970)). While specific to a sphere, similar behaviour is displayed by any

object of finite dimension. The response increases until the object is on the same order of size as the wavelength. For

GPR to penetrate through a heterogeneous material, it highly advantageous that the GPR wavelength be large com-

pared to the scale of heterogeneity.

2

TargetHost

TargetHost

+

=

KK

KKPr


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Question 3: Can you confirm that other obvious conditions will not preclude useof GPR?

Figure: 6-11 What is the survey environment like?


One example would be a radio transmitter located at the site. Another example would be a tunnel lined with metalmesh to prevent loose rock from falling. In the first case external signals may saturate the sensitive receiver electron-ics. In the later, all the radar signal would be reflected at the tunnel wall and none would penetrate into the tunnelwall.

If the answers to all the above questions are "yes", there is a good chance GPR will work. The above conditions areposed in a conservative manner and intended to err on the pessimistic side. More detailed analyses can employ RREand NST techniques. In general it is almost impossible to obtain reliable estimates of all of the parameters involvedin RRE and NST; these procedures are most effectively used as part of a sensitivity analysis. The conclusions drawnwill be fuzzy but informed.

As with all predictions nothing beats a real field trial. If practical, a field evaluation stage should be an integral com-ponent in survey design optimization. Unfortunately, financial constraints usually are a real and limiting factor.

6.2 REFLECTION SURVEY DESIGN

The most common mode of GPR surveying is common-offset, single-fold reflection profiling as depicted in Figure 6-12. In such a reflection survey, a system with a fixed antenna geometry is transported along a survey line to mapreflections versus position.

Figure: 6-12 Schematic illustration of common-offset single-fold profiling.


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There are seven parameters to define for a common-offset, single-fold GPR reflection survey. These are the fre-quency, the t ime window, the time sampling interval, the station spacing, the antenna spacing, the line location andspacing, and the antenna orientation.

6.2.1 SELECTING OPERATING FREQUENCY

Selection of the optimal operating frequency for a radar survey is not simple. There is a trade off between spatial res-olution, depth of penetration and system portability. As a rule, it is better to trade off resolution for penetration.There is no use in having great resolution if the target cannot be detected!! The following is a brief summary of moreextensive discussions of this subject given by Annan & Cosway (1994).

There are three main issues which control frequency selection, namely,

i. Spatial resolution desired,

ii. clutter limitations, and

iii. exploration depth.

Each of these issues yields a constraint on frequency. A brief description of each topic is presented and the frequencyconstraint given without detailed derivation.

Resolution of two events requires that the radar pulse envelope time duration be shorter than twice the separationdelay time between two features to be resolved. Assuming a centre frequency to bandwidth ratio of 1, the constrainton the centre frequency, fc, takes the form

(6-3)

where

Z is the spatial separation to be resolved in metres and K is the dielectric constant or relative permittivity. Inother words, spatial resolution places a lower bound on the centre frequency. (and required bandwidth).

Clutter in GPR systems refers to the radar signals returned from material heterogeneity in soils and rock. The radarresponse of small scale features (i.e., fine scale bedding, cracks and joints, laminations), increases rapidly as radarfrequency increases as evident in Figure 6-10. The data example in Figure 6-13 clearly show how clutter increaseswith increasing frequency). If the radar frequency becomes too high, one can often reach the point where one "can'tsee the forest for the trees"!! In order to "see" to depth into the ground, the amount of energy scattered by cluttershould be minimized. To achieve this, the signal wavelength should be much longer (we use a factor of 10) than thetypical heterogeneity or clutter dimension, L, in the host environment. The clutter centre frequency constraint takesthe form

(6-4)

Note that this constraint also implies that the target sought must be considerably larger than the clutter dimension. Ifthis is not true, then identifying the target response becomes the problem of looking for the "needle in the haystack".

z75

>R

cf

z30

>

KL

fC

c


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Figure: 6-13 The above data sets clearly illustrate the clutter frequency concept. These data were acquired over

two tunnels in an area of gneissic bedrock. The rock texture had a spatial scale of 30 cm. At 100 MHz the clutter is

clearly visible. At 50 MHz much of the clutter from the rock texture is suppressed.

The third frequency, referred to as the exploration depth frequency, requires that the target cross section occupy amajor fraction of the radar beam in order that sufficient energy be returned for detection. Furthermore, the targetdimension should be as close in size as possible to a Fresnel zone in order that the returned signal arrive coherently.

Derivation of a simple guide to encompass these trade offs without resorting to a full radar range analysis is not sim-ple but a basic constraint on the centre frequency is that

(6-5)

where is the ratio of radar beam footprint to target size ratio and D is depth in metres. An assumption of = 4 isreasonable for GPR applications and one obtains

(6-6)

as a centre frequency constraint.

As a general rule, the three frequencies should be computed and, if the survey problem has been properly posed, oneshould find that

(6-7)

If the resolution frequency is greater that the depth or the clutter frequency, the desired spatial resolution is incompat-ible with the clutter dimension or depth of exploration.

D

Kf

D

c

1v

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GPR Survey Design