Groundwater exploration WATEX applications with Ground Penetrating Radars. Dr.Saud Amer USGS Dr.Alain Gachet Radar Technologies France

Transcription

1 Groundwater exploration WATEX applications with Ground Penetrating Radars Dr.Saud Amer USGS Dr.Alain Gachet Radar Technologies France

2 GPR is a technology that allows rapid and non destructive collection of data. -The result is a 3-D database of the underground geology and hydrology (from surface to 40 meters deep) according to the kind of antenna in use. -It can easily be compared to seismic lines using acoustic waves, with one major difference : -GPR provides an echography using electromagnetic waves (microwaves).

3 The 400 Mhz antenna has a penetration depth from surface down to 5 meters. -It can be used for soil surveys and fractures mapping -The profiles can be acquired out of tracks. -All data recorded in 16 bits -Range 88 ns -10 impulsions per meter, whatever the acquisition speed, imply a spatial resolution of 10 cm Antenna must be kept in contact with soil

4 The 400 Mhz antenna trailed by a chain behind a car and the profiles are recorded continuously with 10 impulses per meter

5 The 400 Mhz GPR program of 10 lines averaging 4 km each has been achieved on pre-recorded targets over WATEX images

6 The 400 Mhz GPR used in tough terrane conditions

7 GPR echographies confirm soil detection by remote sensing. They detect thickness and structure over the first 5 meters along hundreds of kilometers.

8 Ground truthing with an excavator : Discovery in Oman of hectares of new soils in 8 days only

9 The 40 Mhz antenna has a penetration depth from surface down to 40 meters. -It can be used for Geologic surveys and fractures mapping -The profiles can be acquired out of tracks. -All data recorded in 16 bits -10 to 60 impulsions per seconds, No contact needed between antenna and soil, can be operated by helicopter or by car.

10 40 Mhz GPR trailed behind car is Sudan

11 Fractures detected by 40 Mhz GPR with 50 cm accuracy Surface F F F Metamorphic dry basement F F 25 m

12 Why is this hand pump dry?

13 GPR radargram showing dry alluvial deposits overlying granitic basement in Kordofan-Sudan 30 m 0 m Dry alluvial deposits Dry granitic basement

14 Why is this Mahogany forest dying?

15 Mahogany trunks infected by termites?

16 Immediate answer after trailing the 40 Mhz GPR antenna through the mahogany plantation

17 With the private owner-kordofan-sudan

18 30 m Inclined granitic basement overlaid by dry sediments : drought is harming the trees (vegetative stress after severe years of drought) 0 m Dry alluvial deposits Dry granitic basement

19 Perched aquifers over granitic basement : Mahogany trees above this area are not affected by termites 30 m 0 m Alluvial deposits Perched aquifers Dry granitic basement

20 The Ground Penetrating Radar or GPR is a light accurate and quick geophysic instrumentation which can be used anywhere in dry conditions 40 Mhz antenna penetrating down to 40 m. Light equipment of 5 kg can be carried anywhere

21 GPR standard processing sequences 1-Vertical shift to zero 2-Velocity analysis 3-Filtering to erase multiples 4-Gain adaptation and enhancement.

22 1-Vertical shift to zero During acquisition, all radargrams are recorded with a shift in order to better delineate the first echo. The shift is then computed by analysing the maximum signal amplitude as shown below.

23 1-Vertical shift to zero During acquisition, all radargrams are recorded with a shift in order to better delineate the first echo. The shift is then computed by analysing the maximum signal amplitude as shown below. Radargram before shift to 0 Radargram after shift to 0

24 2-Velocity analysis Velocity analysis is achieved by using some hyperboles appearing on the radargrams.using the Kirchhoff migration process, we adapt directly on the screen the size of the hyperbole (whitish cone) to the real one on the radargram.. This gives immediately, on the left screen, the velocity in Meters per nanosecond. Obviously, m/ns is not matching the morphology of the radargram hyperbole. First essay : Radar processing window with velocity of m per nano-second

25 Second essay with velocity of m per nano-second. This velocity of m/ns is better matching the hyperbole shape 2-Velocity analysis The radar waves velocity in the soil, V is dependant of the coefficient ε called dielectric coefficient of the soil (or soil dielectric impedance) and C, the light velocity, according to the formula : V = C/ ε. When applying this formula, = 0.3/ ε we extract the value of ε value = 7 which is generally matching the value of superficial layers all over the survey area in the desert of Oman. This value has been used to convert all data from time to depth on all radargrams.

26 3-Filtering to erase multiples Radar filter window applied to line 29 which shows pseudo-horizontal layers before filter application.

27 3-Filtering to erase multiples 4-Gain adaptation and enhancement Alluvium-detritic progradations Alluvium-detritic progradations Limestones Limestones After the enhanced linear adaptative gain and removing the multiples, we have improved the detailed vision of line 29 showing alluvium progradations or infillings of the southern flanks of Wadi Tharawt along the dipping limestone layer.

28 We can test deep seated major fractures without ambiguity from the first 3 meters with deeper consequences for groundwater circulation Major fracture within Wadi Tharawt on line 3bis with 1,20 meter thrust

29 Sustainable development will result from the discovery of groundwater and new soils. with new radar technologies operated continuously from Space to the field. Such a work can be undertaken anywhere.