Identifying underground utilities and predicting their depth are fundamental when it comes to civil engineering excavations, for example, to install or repair water, sewer, gas, electric systems and others. The accidental rupture of these systems can lead to unplanned repair costs, delays in completing the service, and risk injury or death of workers. One way to detect underground utilities is using the GPR-Ground Penetrating Radar geophysical method. To estimate depth, the travel time (two-way travel time) information provided by a radargram is used in conjunction with ground wave velocity, which depends on the dielectric constant of materials, where it is usually assumed to be constant for the area under investigation. This procedure provides satisfactory results in most cases. However, wrong depth estimates can result in damage to public utilities, rupturing pipes, cutting lines and so on. These cases occur mainly in areas that have a marked variation of water content and/or soil lithology, thus greater care is required to determine the depth of the targets. The present work demonstrates how the interval velocity of Dix (1955) can be applied in radargram to estimate the depth of underground utilities compared to the conventional technique of constant velocity applied to the same data set. To accomplish this, synthetic and real GPR data were used to verify the applicability of the interval velocity technique and to determine the accuracy of the depth estimates obtained. The studies were carried out at the IAG/USP test site, a controlled environment, where metallic drums are buried in known positions and depths allowing the comparison of real to estimated depths. Numerical studies were also carried out aiming to simulate the real environment with variation of dielectric constant in depth and to validate the results with real data. The results showed that the depths of the targets were estimated more accurately by means of the interval velocity technique in contrast to the constant velocity technique, minimizing the risks of accidents during excavation.
Geophysical applications within the context of urban planning are increasingly recurrent in the literature due to the success of geophysical methods in providing rapid subsurface information, such as identifying buried utilities, predicting their depth and geometry. Knowing this information prior to any activity that aims to drill or excavate underground, such as engineering jobs that aim to the repair and/or install gas, water, sewage, electricity and telephone networks is of great importance. Accidental contact between drilling/excavation tools and buried urban structures can generate unplanned costs on the worksite, require more time for the initial goal to be achieved, or lead potentially to serious accidents, putting people's lives at risk.
One of the geophysical methods that receives great emphasis in this context is GPR (Ground Penetrating Radar), which is able to map underground structures using the principle of transmission and reflection of electromagnetic waves of high frequency (10 MHz - 2.6 GHz) [
The accuracy of depth estimates using GPR depends on precise knowledge of the velocity of electromagnetic wave in the medium. Once the velocity is determined, the double time is converted to depth, and consequently, the depth of the object is determined. This velocity information usually comes from the adjustment of the hyperbola equation in radargram, where the identified diffraction hyperbolas can be associated with buried targets. Depth estimates can also be obtained by means of direct information from previously known targets, together with assumption that the velocity is constant for the entire GPR profile. This is the usual procedure in studies using GPR as evident in the literature, as can be verified in [
The above approach may be appropriate in situations where there are few variations in water content and/or lithology of the host soil of the targets of interest, but may be inappropriate in others, where such variations are more pronounced and may generate erroneous estimates of depth. In this context, the concept of interval velocity developed by Dix [
In addition to improving the depth estimates of buried underground utilities, the interval velocity technique holds great potential for understanding wave velocity in the soil matrix, leading to more precise information about lithology, soil compaction and concentration of water, thus expanding the field of applications beyond the context of urban planning.
The present study employs a comparison between the interval velocity technique and the more conventional constant velocity technique to estimate the depth of metal drums. Numerical modeling studies were performed aiming to validate the studies with real data. The research was developed at the IAG/USP Geophysics Test Site [
The investigated area was the Shallow Geophysics Test Site located in front of the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) of the University of São Paulo (
In this area, different types of objects are distributed along seven lines, buried in depths ranging from 0.5 to 2.5 m. These lines are geographic references that serve to organize the positioning of the targets in the subsoil. In Line 1 are targets of archaeological interest, such as brick walls and pottery vessels, among others. In Line 2 are found PVC pipes, filled with water and air, simulating water supply networks of cities. In Line 3 there are concrete tubes, simulating the sewage networks and galleries of pluvial water pipes. In Line 4 there are metallic drums, representing the context of chemical waste discards. In Line 5 has empty metallic drums filled with water and brine, simulating contamination environments. Finally, in Lines 6 and 7 contain metallic pipes, electrical cables and PVC conductors, representing water, gas, electricity and telephone networks.
In this area, different types of objects are distributed along seven lines, buried in depths ranging from 0.5 to 2.5 m. These lines are geographic references that serve to organize the positioning of the targets in the subsoil. In Line 1 are targets of archaeological interest, such as brick walls and pottery vessels, among others. In Line 2 are found PVC pipes, filled with water and air, simulating water supply networks of cities. In Line 3 there are concrete tubes, simulating the sewage networks and galleries of pluvial water pipes. In Line 4 there are metallic
drums, representing the context of chemical waste discards. In Line 5 has empty metallic drums filled with water and brine, simulating contamination environments. Finally, in Lines 6 and 7 contain metallic pipes, electrical cables and PVC conductors, representing water, gas, electricity and telephone networks.
Precise information is recorded for each line: depth, target position (horizontal or vertical) and the types of fluids (fresh or salt water, bryne or air). This information can be used for testing geophysical methods, thus simulating real problems and conditions. Another important aspect of this test site is that it is located in an urban context, offering the presence of noise from electric power grids, automobile traffic and people, and the presence of electronic devices. This interference is routine for data acquisition, and permit the verification of noise for instrumentation uses at the site. Further details about the IAG/USP test site construction can be obtained in [
The present work was carried out on Line 4, consisting of metallic drums (
The targets buried under Line 4 used previously by Porsani et al. [
We apply the interval velocity technique of Dix [
The geology of this area is formed by a clay sandy embankment of dark red color, approximately 3 m thick. Just below this stratum there is a predominance of sandy clay sediments from the Resende and São Paulo formations, with a thickness that can reach up to 53 m, passing to the granitic gneiss basement. This information was verified through systematic geophysical studies in this area, together with information from three monitoring wells located at the site [
Numerical modeling studies consist of computational simulation of the propagation of electromagnetic waves in a soil model that hosts metallic drums, reproducing the controlled environment where the real data was acquired. The synthetic data are used with the same objective of the real data, specifically, a comparative study between constant and interval velocity techniques to estimate target depths, as well as to validate the studies done with real data.
Comparing the velocity analysis techniques in a computational environment is owes to the fact that hyperbolas present in synthetic radargrams are very well defined, being the characteristic responses of targets present in the subsurface where the depths of each target are known. This study in a controlled environment with real and synthetic data allows greater control of interpreter error by reading double time and adjusting the hyperbola equation in each target identified in the radargrams. In this way, the analysis of accuracy between the techniques can be done in an appropriate way.
The proposed numerical model is a computational simulation of the propagation of electromagnetic waves in a profile 30 m long by 2.5 m deep, containing empty metallic drums arranged horizontally and vertically (
The chosen model is formed by a sandy clay soil characteristic of the study area with the dielectric constant varying in depth, where this gradient of dielectric constant is justified by differences in lithology and soil compaction. Besides, the rainy season in the data acquisition phase, also have influences in the variation of water content in the first meters of soil. The dielectric constant chosen varied from 12 to 18 at 2.5 m depth (
Although these variations also influence the electrical conductivity and the magnetic permeability of the medium, constant values were attributed to these parameters, being σ = 10−3 S/m e μ = 1, because the influence of the dielectric constant is more significant as regards the velocity of the electromagnetic wave in the medium in relation to the electrical conductivity and magnetic permeability (Equation (1)).
Information on the physical properties associated with the local geology was taken from Porsani et al. [
The numerical modeling was performed using ReflexWIN® processing program [
simulated responses were obtained by using the spacing between the traces of 0.02 m (2 cm) and the frequency of the 270 MHz electromagnetic wave. The frequency used in the numerical model is the same used in the acquisition of real data.
To apply the constant velocity technique to estimate the depth of subsurface targets, it is first necessary to obtain the velocity of the medium in question. This estimation can be done based on dielectric constants of soil lithology that are associated with velocity values already known in the literature (Equation (1)), being a more susceptible method to uncertainties, because it is mean values for each material, not necessarily representing the medium where the work is done. Another way is to use employ Equation (2) to calculate the velocity of the medium using the double time information provided by the GPR referring to some known target present in the subsurface that can be identified in radargrams, The third way is based on hyperbolas present in radargrams (
The relation between the velocity of the electromagnetic wave in the medium (v) and the dielectric constant (ε) is given by Equation (1):
where (c) is the speed of light in the free space.
The velocity can also be defined according to Equation (2). Since (h) the depth of the target in the subsurface and (tdouble) is the transit time (two-way travel time) of the electromagnetic wave in the medium, obtained through GPR, given by:
Equation (3) shows that the hyperbola (
Since the velocity of the electromagnetic wave is predetermined, the constant velocity technique assumes that such velocity is constant for the entire GPR profile (
For the GPR, it is considered that the wave velocity is related to the dielectric constant of the medium (Equation (1)), being dependent on the materials in the
subsoil and the water content [
As an alternative, the concept of interval velocity developed by Dix [
Once velocities and path times have been obtained, the layers are delimited, and then the widths of those layers are calculated by Equations (4)-(6). Note that layer width is determined from the shallower targets to the deeper ones, because in this way, the velocity variations are smoothed.
The depths (H) of each target are calculated by Equations (7)-(9):
The percentage error of the depth estimate is calculated by Equation (10). This error is how far the depth obtained is far from the real depth, serving as a parameter of control and comparison between the techniques approached in this work.
Note that to employ the interval velocity technique in radargrams to estimate the depth of the buried targets it is necessary to have the GPR reflection of at least two targets located at different depths. Another consideration is that the higher the number of targets located at different depths, the more layers can be defined, and consequently more velocity information is added.
The GPR data were acquired by means of the reflection profile technique with constant spacing, using the SIR4000 (GSSI) equipment with a shielded antenna of 270 MHz. A profile 30 m long with spacing between the traces of 0.02 m was acquired on Line 4 of the IAG/USP test site, where empty metallic drums were precisely buried, arranged horizontally and vertically, with depths varying between 0.5 and 2.0 m (
After the data acquisition step, the processing was done to increase the signal to noise ratio. Data processing included several steps with the objective of improving the radargram reflections. The first step was zero time correction, where the arrival of the air wave at the initial time of each trait is attributed, necessary so that the depth estimation of the targets is not impaired. Frequency filters (bandpass) were applied to eliminate high and low frequency noise, based on the spectrum of the signal obtained and the center frequency of the antenna used. Temporal gains were applied to highlight the anomalies of interest and compensate the losses by geometric scattering of the electromagnetic wave. Background removal was applied in order to minimize the effects of the background and evidence the hyperbolas of interest.
In
Once velocities have been determined for each target by means of hyperbolic adjustments, the layers are defined considering the tops of the drums located at different depths and laterally proximate. Taking into account the position of the targets identified in the radargram (
The first model was elaborated using targets A, B and C, since these are located at different depths and close to each other (
The depths of some targets are obtained more than once, since some targets were part of more than one model. An arithmetic mean is assigned to the depths obtained more than once, as the final depth of the target in question.
In
Targets | Velocity (m/ns) | Real depth (m) | Estimated depth (m) | Time (ns) | Error (%) |
---|---|---|---|---|---|
A | 0.080 | 1.97 | 2.01 | 50.359 | 2.3 |
B | 0.085 | 0.50 | 0.49 | 11.761 | 2.0 |
C | 0.083 | 0.98 | 1.00 | 24.124 | 2.2 |
D | 0.085 | 0.50 | 0.49 | 12.062 | 2.0 |
E | 0.082 | 0.90 | 0.89 | 21.712 | 1.0 |
F | 0.082 | 0.97 | 0.96 | 23.521 | 1.0 |
G | 0.083 | 1.00 | 0.98 | 23.823 | 1.1 |
H | 0.078 | 1.98 | 1.97 | 50.661 | 0.2 |
ject to velocity variations than the other targets. However, the discrepancy between the estimated and actual depth for these targets is less than or equal to 4 cm, being an indication that the interval velocity technique is adequate to bypass the effect of the vertical variation of dielectric constant with depth.
In an analogous way, in
In
Targets | Velocity (m/ns) | Real depth (m) | Estimated depth (m) | Time (ns) | Error (%) |
---|---|---|---|---|---|
A | 0.083 Target C | 1.97 | 2.09 | 53.511 | 6.1 |
B | 0.50 | 0.49 | 12.833 | 2.4 | |
C | 0.98 | 1.00 | 25.797 | 2.2 | |
D | 0.50 | 0.50 | 13.085 | 0.1 | |
E | 0.90 | 0.90 | 23.345 | 0.1 | |
F | 0.97 | 0.98 | 25.690 | 0.6 | |
G | 1.00 | 0.99 | 26.271 | 1.1 | |
H | 1.98 | 2.10 | 53.972 | 6.2 |
these targets by the constant velocity technique. For the deeper targets A and H this discrepancy reaches 12 cm, which reinforces the thesis that the marked variation of dielectric constant poses difficulties in applying the constant velocity technique to estimate the depth of buried objects.
Similar to that observed in the analysis of the real data, when comparing the percentage results presented in
Observe in
For target H it was possible to visualize the diffraction hyperbola and then make the hyperbolic adjustment to obtain the velocity. The same does not occur for target G, where the diffraction hyperbola is confused with the geological background, making it difficult to visualize the hyperbola, and consequently to make the hyperbolic adjustment. To overcome this problem, it was assumed that the target velocity L is equal to target F, since they are located close together and at similar depths.
The layer models defined for the actual data are the same as the templates defined for the synthetic data. Once the layers have been fixed and the velocities established the widths of the layers and finally the depths of the targets are cal-
Targets | Velocity (m/ns) | Time (ns) | Real depth (m) | Estimated depth (m) | Error (%) |
---|---|---|---|---|---|
A | 0.084 | 45.337 | 1.97 | 1.90 | 3.3 |
B | 0.078 | 13.163 | 0.50 | 0.51 | 2.6 |
C | 0.076 | 26.883 | 0.98 | 1.02 | 4.2 |
D | 0.080 | 11.720 | 0.50 | 0.47 | 6.2 |
E | 0.068 | 27.223 | 0.90 | 0.92 | 2.8 |
F | 0.070 | 27.336 | 0.97 | 0.95 | 1.3 |
G | 0.070 | 27.284 | 1.00 | 0.96 | 4.5 |
H | 0.075 | 57.163 | 1.98 | 2.05 | 3.9 |
culated.
After the layers are established and the target depths determined, Equation (10) allows the comparison between the estimated and actual depths.
For the use of the constant velocity technique, the velocity of target C was set
Targets | Velocity (m/ns) | Real depth (m) | Estimated depth (m) | Error (%) |
---|---|---|---|---|
A | 0.076 Target C | 1.97 | 1.72 | 13.0 |
B | 0.50 | 0.50 | 0.0 | |
C | 0.98 | 1.02 | 4.0 | |
D | 0.50 | 0.45 | 11.0 | |
E | 0.90 | 1.03 | 15.0 | |
F | 0.97 | 1.04 | 7.0 | |
G | 1.00 | 1.04 | 4.0 | |
H | 1.98 | 2.17 | 10.0 |
as the velocity of the medium, following the same procedure as applied to synthetic data.
In
Comparing the percentage results presented in
Estimates employing the depth interval velocity technique appear more suitable and less susceptible to errors, since they are conducted using proximate targets and more velocity information compared to constant velocity technique. Additionally, the depth estimation by the constant velocity technique is more susceptible to errors, as shown in the analysis of real GPR data. These errors can mean potential hazards associated with installing and/or repairing underground utilities.
The studies of the real and synthetic data permit quantifying the vertical variation of depth associated with the variation of the dielectric constant, demonstrating the limiting effects of the constant velocity technique to obtain the depth of buried utility structures and showing that the use of the interval velocity technique can be applied satisfactorily to overcome this problem.
Since the dielectric constant depends on several parameters, such as soil compaction, lithology and water concentration, the results attained in this study indicate that application of the interval velocity technique where there are several buried utilities provides more detailed information about the velocity of the medium. This method has positive implications for both utility depth estimates, knowledge of soil matrices and can be used beyond urban planning contexts, such as for archaeological excavations in urban contexts.
BP is grateful CNPq-Conselho Nacional de Desenvolvimento Científico e Tecnológico for providing a research scholarship (134647/2015-7). JLP also is grateful CNPq (grants: 301692/2013-0 and 406653/2013-5) and FAPESP-Fun- dação de Amparo à Pesquisa no Estado de São Paulo for providing financial support for the construction of the IAG/USP Geophysics Test Site (2002/07509-1), both are Brazilian research agencies. IAG/USP is acknowledged for providing infrastructure support. We thank Ernande, Marcelo and colleagues for helping in geophysical data acquisition.
Poluha, B., Porsani, J.L., Almeida, E.R., dos Santos, V.R.N. and Allen, S.J. (2017) Depth Estimates of Buried Utility Systems Using the GPR Method: Studies at the IAG/USP Geophysics Test Site. International Journal of Geosciences, 8, 726-742. https://doi.org/10.4236/ijg.2017.85040