APPLICATION OF THE GPR METHOD TO GROUND WATER

EXPLORATION AND ENVIRONMENTAL STUDIES.

GROUP 8BISUGA OLUBUKOLA - 110813011ODUBOTE OLUWASEUN - 100813033AJOSE ADEMIDE - 110813005

OUTLINE

• Introduction

• Literature Review

• Methodology

• Case Studies

• Conclusion

INTRODUCTIONEssential need for the sustenance of life and mankind as a whole.

Pollution escalades from the activities of mankind.

Fig 1. Groundwater

Fig 2. Pollution as a result of human activitiesFig 3. Arsenic Poisoning in Bangladesh

INTRODUCTION

Fig 4. The GPR method.

LITERATURE REVIEW• The GPR is a geophysical method of obtaining information about the subsurface with

extremely high resolution. GPR waves are sensitive to contrasts in dielectric properties; such changes can be detected, imaged, and characterized. GPR’s high frequency results in high resolution which makes it sensitive to geometry, texture, and structure, which are very useful in understanding and describing heterogeneity of the subsurface (Olhoeft, 1991a, 1991b).

• GPR is used to map geologic conditions that include depth to bedrock, depth to the water table (Knoll et. al., 1997), depth and thickness of soil and sediment strata on land and under fresh water bodies (Beres and Haeni, 1991; Smith and Jol, 1997), and the location of subsurface cavities and fractures in bedrock. Other applications mapping contaminants (Daniels et. al., 1995; Guy et. al., 2000)

• The interpretation of GPR reflection data, however, is currently highly subjective and based primarily on an interpreter’s ability to recognize patterns in a radar image. Attributes commonly calculated for seismic data can be extracted from GPR data. Visualization of certain GPR data attributes (such as energy, instantaneous amplitude, and steepness event) can enhance the accuracy and confidence of subsurface structural interpretations (Böniger and Tronicke, 2010, and Udphuay et al., 2010).

METHODOLOGY

Fig 5. The GPR survey illustration. (After John Milsom 2011)Table 1. Typical Dielectric Constant, Conductivity and Velocity Value of Common Materials and Medium.

METHODOLOGY

Fig 6. A Block Diagram of a GPR System

Fig 7. Radar Tomography

Fig 8. Various Borehole Measurements (a) Single-Hole Measurement and (b) Cross-Hole Measurements

METHODOLOGY

Fig 9. Bi-static Mode Antenna (Radar reflection profiling)Fig 10. Mono-static Mode Antenna(Radar reflection profiling)

Fig 11. Wide-Angle Reflection and Refraction (WARR) Sounding (Reynolds 1997)

METHODOLOGY

Fig 12. Common Midpoint (CMP) Sounding. (Reynolds 1997)

Table 2 - Basic Processes in the Processing and Interpretation of a GPR Survey.

• GPR’s processing stages bearing striking resemblance with that of seismic reflection.

• Velocity plays a crucial role in GPR processing and interpretation.

METHODOLOGYPROCESSING OF GPR DATA

After stacking, GPR data are passed through low-cut filters, to removenoise due to inductive effects and the limitations in instrument frequencyresponse, and high-cut filters, to eliminate noise spikes.

• The decrease in signal amplitude with time is then reversed by time-variant amplification.Automatic gain control (AGC) is used to do this in the field, producingrecords for quality control, but data are normally stored in unmodified form.

• Compensation, in the processing centre, for propagation effects, using SEC(spherical and exponential compensation) filters based on physical modelsof the subsurface, comes after any frequency-based filtering, because time-variantgain functions distort wavelets and must be applied with care ifamplitude integrity is to be preserved.

•Migration algorithms operate on the entire data set rather than individual traces (Figure 13),and are usually the last to be applied.

METHODOLOGYPROCESSING OF GPR DATA

Fig 13. Example of processing used to image steel reinforcing bars in a concrete slab. (a) The raw data, with hyperbolic responses from the bars. (b) Migration has been used to collapse the hyperbolic tails and highlight the reflection sources. (c) The energy envelope produced by a Hilbert transform. (After John Milsom 2011).

METHODOLOGYINTERPRETATION OF GPR DATA

Figure 14. Three-dimensional (3D) display of a high-resolution ground penetrating radar (GPR) data set (left) collected on a 0.1-m orthogonal grid to locate a safe area to drill a hole through a reinforced concrete slab. On the left, unprocessed data are presented as a 3D cube showing X and Y image sections. In the centre, a time slice (in the Z plane) has been selected to highlight a conduit cross-cutting the slab. The location of therecommended drill hole is shown (right) superimposed on the data cube. No data processing other than 3D presentation and selection of interpolation parameters was required, and nobody was hurt in the making of the hole.

METHODOLOGYINTERPRETATION OF GPR DATA

There are situations when the water table is detectable on a radargram, and others where it is not the reason for this is that the ratio of the thickness of the capillary zone to the wavelength of the incident radiowaves need to be small (i.e d<wavelenght) in order to provide sufficient contrast in relative dielectic constant between the unsaturated and saturated material to reflect a significant proportion of energy. If the capillary zone is thick with respect to the wavelength,then the rate of charge if relative dielectric constant with depth through this zone is small.

The effect of this is that, for each incremental increase in the relative dielectric constant, a proportion of the incident energy is reflected so that the total reflected energy is smeared from the capillary zone and hence the resulting reflection amplitude is too low to be detected with any clarity.

IMPORTANT TO NOTE

GPR EXPLORATION FOR GROUND WATER AND ENVIRONMENTAL STUDIES IN CHANGDE CITY, HUNAN

PROVINCE, CHINA. (Ziaqiang Zhu et al., 2009)

Changde

Fig 15. Location of study area.

Fig 16. A conceptual model of the water saturation profile in the subsurface.

• This project was carried out to differentiate between the capillary fringe and the saturated zone.•Special attention was paid to the central frequency used for this purpose.

Fig 17. GPR sections at profile show changes in the electromagnetic signal during the exploration. Antenna frequency is centered in 400 MHz.

GPR EXPLORATION FOR GROUND WATER AND ENVIRONMENTAL STUDIES IN CHANGDE CITY, HUNAN

PROVINCE, CHINA. (Ziaqiang Zhu et al., 2009)

Fig 18. GPR sections at profile show changes in the electromagnetic signal during the exploration. Antenna frequency is centered in 400 MHz.

GPR EXPLORATION FOR GROUND WATER AND ENVIRONMENTAL STUDIES IN CHANGDE CITY, HUNAN

PROVINCE, CHINA. (Ziaqiang Zhu et al., 2009)

GPR exploration for groundwater in a crystalline rock terrain: A case study of the district of Petro´polis (Brazil). (Jandyr de Menezes Travassos et al.,

2004)

Fig 19. Location of study area.

Fig 20. CMP profile at the studied spring. The airwave is represented by the first straight line in time. The groundwave is represented by the second straight line, note that beyond 10 m, thiswave is highly attenuated. Subsurface reflectors are represented by hyperbolas.

• Most GPR acquisition is done in sedimentary terrain. Work done with the GPR in Crystalline terrain is very rare.

• Water exploration will not be associated with extensive/continuous reflections.

Fig 21. Velocity analysis for the CMP profile. Velocity is incremented in 0.001 m/ns steps from 0.01 to 0.15 m/ns.

Fig 22.(a) Raw GPR time section on crystalline rock, note the several diffraction patterns (bowties) below 100 ns, specially between traces 40 and 50 in the profile. (b) Same GPR section migrated and converted to depth, a uniform velocity of Vr = 0.07 m/ns is assumed throughout. Compare with (a), migration untied the bowties on the section and turn them into dipping reflectors. A producing well, with static hydrostatic level at 4.9 m depth is located at trace 32 m.

GPR exploration for groundwater in a crystalline rock terrain: A case study of the district of Petro´polis (Brazil). (Jandyr de Menezes Travassos et al.,

2004)

Fig 23. Interpretation of the radar section shown in Fig. 22. The F’s are interpreted as fractures and B is interpreted as a boudin structure.

GPR exploration for groundwater in a crystalline rock terrain: A case study of the district of Petro´polis (Brazil). (Jandyr de Menezes Travassos et al.,

2004)

Merits/Demerits of GPR

MERITS• Very good resolution at shallow depth of penetration.

• Relatively easy to carry out.

DEMERITS• In very loose snow pack or areas where trees create void spaces in the snow,

the energy may encounter air pockets and reflect these in the data. This can obscure and complicate the reflection profile, so extra care is needed during interpretation.

• Expensive

• Shallow depth of penetration.

CONCLUSION• GPR has two major advantages over most other non-invasive geophysical

techniques:– 1. GPR provides a 3-D pseudo-image that can easily be converted to

depths that are accurate down to a few centimeters; and– 2. GPR responds to both metallic and non-metallic objects.– 3. The GPR method is highly useful in both sedimentary and crystalline

environments for the exploration of groundwater. The method has proven successful in those works cited in this paper (report).

• The GPR method has also proven useful in the determination of contamination in water which is directly linked to the study of the environment.

• GPR is not an invasive method and with suitable constraints, has given an 70-80% success rate.

Thanks.