Katherine Hostel Site: Ground Penetrating Radar Survey

POWERImIliN AND

WATER 4"iIr AUTHORITY

,

POWER AND WATER AUTHORITY WATER RESOURCES DIVISION

Transport and Works Report 03/96

KATHERINE HOSTEL SITE GROUND PENETRATING RADAR

SURVEY

REPORT 07/1996D H.T. Calvert Water Resources Division Power and Water Authority Darwin, N.T. Australia January 1996

SUMMARY

A ground penetrating radar (GPR) survey was conducted during December, 1995 on the site for the proposed hostel at Katherine East. The purpose of the GPR survey was to locate areas which may have a risk of sinkhole development The depth of investigation for the GPR was estimated to be about 7m at 100MHz. The survey did not detect any underground voids. This would indicate that shallow voids of significant size (>.25m diameter) are not present and sinkhole development is not likely to occur. However there are two small sinkholes immediately to the east of the site. It is recommended that drainage of surface water into these sinkholes should be avoided in order to protect the aquifer from contamination.

CONTENTS

SUMMARY

CONTENTS

LIST OF FIGURES

INTRODUCTION

THE GROUND PENETRATING RADAR TECHNIQUE

GPR EQUIPMENT

TEST OF GPR OVER KNOWN SINKHOLES

RESULTS OF GPR SURVEY AT KATHERINE HOSTEL SUE,

CONCLUSIONS AND RECOMMENDATIONS

REFERENCES

APPENDIX A: GPR SURVEY OF STUART HIGHWAY SINKHOLES

LIST OF FIGURES

Geology of the Katherine area. Location of sinkholes and ground penetrating radar survey at Katherine East. Field operation of ground penetrating radar and resulting radar section. The PulseEKKO IV Ground Penetrating Radar. GPR survey grid for Katherine Hostel site. Typical GPR responses commonly encountered during the survey. Radar section for line 20SB. Detailed radar section for anomaly on line 20SB. Radar section for line 30SD.

Detailed radar section for anomaly on line 30SD. Location of Stuart Highway sinkholes GPR survey. Map of GPR lines for Stuart Highway sinkholes survey. Radar section for line SH-1. Radar section for lines SH-2, SH-3, and SH-4. Radar section for line SH-5. Detailed radar section for line SH-1.

1. INTRODUCTION

The Katherine area is underlain by the Tindal Limestone, which outcrops along the Stuart Highway as shown in Figure 1. Within the Tindal Limestone, extensive karst systems, consisting - of fractures, caves and sinkholes form the major aquifer for the region. Lauritzen and Karp (1993) have made an assessment of the karst aquifer. They conclude that most of the karst developed under much different hydrological conditions than that of the present day. Many of the underground cavities are now filled with sediments and not active. The construction of roads impedes the surface run-off, which can result in ponding of water above these cavities. As this water percolates downward to connect with an active underground stream, it transports some of the sediment with it, eventually resulting in the formation of a sinkhole. Such sinkholes have recently developed along the side of the Stuart Highway south-east of Katherine.

In December, 1995 the Construction Agency, NT Department of Transport and Works requested the Water Resources Division of the Power and Water Authority to conduct a Ground Penetrating . Radar (GPR) survey at the site of the proposed Katherine Hostel, in the Katherine East subdivision in order to locate areas of potential sinkhole formation. Several sinkholes are located throughout the developed area of Katherine East as shown in Figure 2, with two sinkholes located immediately east of the building site. Where possible, construction of buildings has been avoided near these sinkholes. However much of the surface drainage has been channelled into these sinkholes and contaminants in the run-off, such as fertilisers used on residential lawns, may be entering the town aquifer at these points.

During December, 1995 a ground penetrating radar (GPR) survey was conducted over the site proposed for the Katherine Hostel on Callistemon Drive, Katherine East subdivision (Figure 2), under the supervision of Tom Calvert, Geophysicist, using vehicles and equipment of the Water Resources Division. The purpose of the survey was to locate any underground voids (sinkholes) within the underlying Tindal Limestone which may pose a hazard to building on the site. A similar GPR survey at Katherine East in June, 1995 and at a construction site near Tindal in 1994 successfully located sinkholes.

2. THE GROUND PENETRATING RADAR TECHNIQUE

Ground penetrating radar (GPR) is a relatively new geophysical technique. A pulse of high frequency (10 to 1000MHz) electromagnetic energy is transmitted into the ground. The signal is then reflected from within the ground towards a receiver, as shown in Figure 3. The system is then progressively moved along the surface to provide a profile of the subsurface stratigraphy.

The GPR method shares many similarities with the seismic reflection method and sonar. The output is a wiggle trace plot (radar section or radargram), similar to a reflection seismic section in which depth is plotted versus surface position. Real time processing allows the radar section to be viewed in the field during data acquisition.

The propagation of the radar signal is affected by the dielectric constant of the material. Air has a dielectric constant of 1.0 while water has a dielectric constant of about 80. The presence of water in soils and rocks results in a wide range of dielectric constants. Reflections of the signal will occur where there is a dielectric contrast. A strong reflection would therefore be expected from an underground void such as a sinkhole.

GPR is not effective in areas of high electrical conductivity due to absorption of the radar signal in the ground. In addition, powerlines, underground services (cables and pipelines), vehicles and tree canopies cause interference which may mask the subsurface response.

For a more comprehensive discussion of the ground penetrating radar technique refer to Calvert, (1995).

3. GPR EQUIPMENT

A 400V PulseEKKO IV GPR (manufactured by Sensors and Software) was hired from GeoInstruments, Ramsgate, NSW by the Construction Agency. The system consists of a transmitter and receiver connected to a console by fibre-optic cables as shown in Figure 4. The console is operated by a notebook computer. For this survey the 100MHz antennae were used. The GPR was mounted on a PVC sled that was towed 15m behind a quad bike. The system is provided with software for processing and plotting the data.

4. TEST OF GPR OVER KNOWN SINKHOLES

In order to ensure the GPR system was functioning properly and could detect sinkholes, a survey was conducted over known sinkholes adjacent to the Stuart Highway, 25km south-east of Katherine. Three lines were acquired along, the road pavement, and one along each of the roadside drains. The results, shown in Appendix A, prove the system was operating effectively as it located the known sinkholes, as well as locating voids which have not yet opened to the surface.

5. RESULTS OF GPR SURVEY AT KAI HERINE HOSTEL SITE

A grid was established for the GPR survey, using the previous grid as a baseline. The arid consisted of 30 lines, 100 to 175m in length, spaced 2.5m apart. The area immediately south of the road could not be surveyed due to the interference from the power line. Closely spaced trees, small outcrops and boulders greatly reduced the speed at which the area could be surveyed. The location of these lines are shown in Figure 5. A total of approximately 4.5 km of line was surveyed.

An EM 31 electromagnetic survey was conducted on line 100E in order to measure the electrical conductivity of the subsurface to 6m depth. From previous work it was found that GPR would only be effective in areas in which the electrical conductivity was less than 10 mS/m. The EM 31 readings taken beneath the power line were unreliable, however away from the power line conductivities were less than 4mS/m, allowing sufficient penetration of the GPR signal.

The most common responses encountered during the GPR survey included interference from overhead wires, interference from surface metal such as fencing wire and reflections from tree canopies. Typical responses are shown in Figure 6. Due to the large amount of data, only the radar sections exhibiting anomalies are presented in this report. The complete set of radar sections acquired from the survey was supplied to the client as continuous form plots.

Two anomalies were deemed appropriate for further investigation. These anomalies were then surveyed in detail using a station spacing of 0.25m. The results of the detailed survey are presented below.

Line 20SB. The radar section for line 20SB is shown in Figure 7. A narrow hyperbolic reflection at 87 east was considered to be possibly due to an underground void. A detailed survey over this anomaly at a station spacing of 0.25m (Figure 8) indicated that the anomaly was caused by the constructive interference effects from two trees.

Line 30SD. The radar section for line 30SD is shown in Figure 9. A high amplitude hyperbolic reflection at 80 east was investigated further in order to determine if an underground void was present. The detailed survey. shown in Figure 10, does not show the broad wavelength and multiple reflections which would be expected from an underground void. The anomaly is most likely caused by the interference effects of trees.

6. CONCLUSIONS AND RECOMMENDATIONS

No substantial underground voids were located by the GPR survey. Therefore the Katherine Hostel site should not be prone to sinkhole formation. There is a risk of further development of the sinkholes adjacent to site. Future development should avoid construction in these areas. In order to protect the aquifer, surface run-off should not be channelled into these sinkholes.

REFERENCES

Calvert, H.T., 1995. Katherine East Ground Penetrating Radar Survey. Report 49/1995D, Water Resources Division, Power and Water Authority, Darwin, N.T., 27p.

Calvert, H.T., 1995. Use of Ground Penetrating Radar for Groundwater Investigations in the Northern Territory. Report 16/1995D, Water Resources Division, Power and Water Authority, Darwin, N.T., 60p.

Davis J.L., and Annan, A.P., 1989. Ground-penetrating radar for high resolution mapping of soil and rock stratigraphy. Geophysical Prospecting, v37, 531-551.

Lauritzen S., and Karp D., 1993. Speleological Assessment of Karst Aquifers Developed Within the Tindal Limestone, Katherine, N.T.. Report 63/1993, Water Resources Division, Power and Water Authority, Darwin, N.T., 70p.

Sensors and Software, 1993. PulseEKKO W user guide version 4.0. Technical Manual 20, Sensors & Software Inc., Mississauga, Ontario. 61p.

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APPENDLX A

GPR SURVEY OF STUART HIGHWAY SINKHOLES

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PROJECT BRIEF December 6, 1995

Katherine East Hostel Survey: Construction Agency, Transport and Works

Introduction

The presence of sinkholes within the Tindal Limestone poses some risk to future development in the Katherine area. The Construction Agency, Department of Transport and Works is supervising the construction of a hostel at the east end of Callistemon Rd (see attached map). There are sinkholes located adjacent to this site. A ground penetrating radar (GPR) survey is required over this site to locate areas in which sinkholes might develop. The field work must be completed before the 18th of December 1995.

The Water Resources Division carried out successful GPR surveys to locate sinkholes at Tindal in 1994, and at Katherine East in June, 1995.

Work Required

The area to be covered is approximately 100m by 160m, which will require 3 days field work for a geophysicist. The Construction Agency will provide a technician. Line spacing will be 2.5m. The field work should commence on December 12 and be completed on December 14. In addition, one day preparation time by a technician and four days for report writing by a geophysicist are required.

Cost

The cost of the project is detailed in the attached table. The shipping and hire of the radar equipment from GeoInstruments in Sydney will be paid for directly by the Construction Agency, Department of Transport and Works. Travel allowance, wages and other costs will be reimbursed by the client.

Equipment Preparation: 1 technician for 1 day @ 368/day $368 Field Survey: 1 professional for 3 days @ 440/day $1320 Travel Allowance: 1 person for 3 days @ 127.40/day $382 Miscellaneous: Fuel, survey pegs, etc. $500 Report Preparation: 1 professional for 4 days @ 440/day $1760

TOTAL $4330

Tom Calvert, Geophysicist

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POWER WATER AUTHORITY

WATER RESOURCES — SASCO House, 8 Cavenagh Street • GPO Box 1096, Darwin NT 0801

Mr. Graham Clark, Geotechnical Engineer Telephone (089) 82 7111 Construction Agency Department of Transport and Works Facsimile (089) 41 0703 4th Floor, Highway House Palmerston, N.T.

December 6, 1995

Dear Graham,

In response to your inquiry regarding a ground penetrating radar (GPR) survey over the hostel site at Katherine East, I have completed an estimate for the work required.

The area will be covered with approximately 6.4km of line spaced 2.5m apart. The GPR survey will be operated at a frequency of 100MHz which should give a depth of penetration of up to 10m. An EM-31 survey will also be conducted along the lines.

A surveyed grid will be established on which the locations of interpreted sinkholes will be marked. If there are fallen trees or other obstacles in the area these should be removed before the survey starts.

The GPR equipment will be hired for one week directly by your department from GeoInstruments in Sydney. The cost will be approximately $4000 including shipping and handling. I will make arrangements with them for delivery on December 11. The GPR must arrive back in Sydney on December 18.

The field work will take place from December 12 to December 14 including travel time. In addition, there will be one-day for equipment preparation and 4 days for writing the final report. Below is the estimate of the cost for the survey, excluding GPR equipment hire.

Equipment Preparation: 1 technician for 1 day @ 368/day $368 Field Survey: 1 professional for 3 days @ 440/day $1320 Travel Allowance: 1 person for 3 days @ 127.40/day $382 Miscellaneous: Fuel, survey pegs, etc. $500 Report Preparation: 1 professional for 4 days @ 440/day $1760

TOTAL $4330

Payment will be required before the survey commence. I will contact you regarding payment once I have obtained a project number for the survey. If you have any questions about this estimate, I can be contacted on 827484.

Regards,

Tom Calvert Geophysicist