GPR Under Water
GPR typically is used for above ground applications and generally is not believed to work well in wet conditions. This is typically correct as water can increase host material conductivity and in turn reduce GPR resolution, depth penetration and therefore any success. Saline sea water is definitely a material to avoid with GPR due to the high conductivity. However, GPR can be used in specific freshwater applications, most notably freshwater lake bed profiling.
CASE STUDY ONE: FRESHWATER STORAGE TANK - CONCRETE SLAB INTEGRITY
A freshwater storage reservoir serving large towns in Northern Queensland reported that significant washouts of sand and no fines concrete aggregate debris were discovered exiting the drains under the large tank. This was believed to be the result of high winds and cyclonic events.
The reservoir includes two individual reservoirs, one larger tank built around a smaller tank. The small tank is approximately 25m in diameter and the large tank is approximately 37m in diameter. The concrete floor slab in both tanks was approximately 150mm-200mm thick with one layer of steel reinforcing. The depth of water coverage within both tanks was approximately 5m.
Ground Penetrating Radar technology was requested by the client. MALA GPR acted to build a custom GPR to accommodate the survey environment.
The Geophysical investigation aimed to identify potential subsurface voids within and under a section of concrete floor.
Access to the survey area involved entering the tank from the roof. MALA GPR staff were stationed within the reservoir, standing on top of the smaller tank concrete roof. Commercial divers were employed to manoeuvre the Ground Penetrating Radar instrument on the underwater floor slab.
The area was surveyed using the MALA Ground Explorer 750MHz HDR. The 750Mhz GPR antenna was custom fit into a waterproof enclosure that was tested to 5m depth pressure. The enclosure included a custom magnetic distance encoder wheel to record horizontal chainage and trigger GPR data. A single 25m waterproof Ethernet umbilical penetrated the enclosure and was run to the surface to a GPR monitor/controller. The system was pushed by hand by qualified commercial divers
Positioning for the project area was created in the form of a local grid. This was designed around fixed objects (pylons) to allow for easy relocation of GPR results. The two tanks were treated as independent projects.
Local grid plans were provided to the diving team (from MALA GPR Australia) and it was requested that the grid be physically marked on the floor slab with underwater crayon.
After weeks of building and testing and 3 full days in the field, the custom GPR setup was a success. Depth penetration achieved from the GPR antenna appears to be around 550mm.
The GPR survey within the both tanks achieved near complete data collection over the entire proposed grid with 250mm and 500mm profile spacing.
A number of areas within the small and large tanks contained anomalies within the data that could represent potential zones of voiding or reduced structural integrity.
Consistent features were seen that were noted to occur across neighbouring profiles which included anomalies that could relate to slab thickness (~180mm depth), 1 layer of steel reinforcing, potential pipes/drains as well as anomalies relating to surface or very near surface features.
In conclusion, the survey returned no 'significant' evidence of subsurface voiding however inconsistencies within the dataset (displaying potential void like anomalies) warranted interest towards further invasive investigation in specific areas.
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