Rapid Groundwater Exploration

The Swedish Geological Survey (SGU) has been engaged in groundwater exploration projects to investigate Sweden’s many glacial river deposits. The deposits are spread across the country and are the most significant groundwater resources for drinking water supplies in Sweden. SGU wanted a GPR solution to deploy rapidly to collect large amounts of reliable and high-quality data to aid their studies. For efficiency, the goal was to utilise the country’s road networks, and adjacent verges and land to locate areas below that consisted of sand and gravel with great depths to the underlying bedrock.

Given the depth requirements, ImpulseRadar’s application specialist chose CrossOver CO730 as the system of choice.

Since the CrossOver series of GPR antennas utilise a modern real-time sampling (RTS) design, they are suitable for high-speed road-based measurements in addition to the more usual manual pushing/ pulling configurations. For SGU, ImpulseRadar built a custom-made mounting frame to facilitate the vehicle-mounted measurements over roads. This mounting solution allows the antenna to be moved quickly and easily between the vehicle for road-based measurements and manual pulling configurations.

In addition to the road measurements, SGU also used the GPR system to investigate the verges and adjacent terrain. The goal was to obtain more input data to determine the true extent of potential groundwater reservoirs. For the off-road investigations, the CO730 was pulled manually, but also had a skid arrangement for towing behind an off-road vehicle such as a quad bike or a snowmobile during the winter months.

The use of GPR for water-table detection and mapping is well proven. The benefit of the CO730’s dual-channel design meant that the water-table interface could easily be determined using the 300 MHz channel. In contrast, the 70 MHz channel offered greater penetration to define the sand and gravel interfaces to the underlying bedrock. Having visibility of all the geological elements that make up the aquifer, helps to determine the thickness of each layer. Combined with other data, hydrologists can later use the GPR information to build an aquifer model to more precisely predict water volume.

The vehicle-mount solution will fit any car with a standard 50 mm tow ball. For larger vehicles, an easily adaptable tow hitch arrangement is available. The mounting frame design allows the CO730 (or other CrossOver antennas) to float securely over the ground/ roads surface. The height (air-gap) above the measurement surface can adjust from 5 to 15 cm to allow for different conditions. However, it’s always advisable to get the antenna as close to the measuring surface as possible. If there is contact with the measurement surface, a skid plate protects the antenna, and because it is floating, the antenna can move upwards to minimise the force of any impact.

Note – the RTS-based design of the CrossOver antenna means that it can collect data much faster than the speed at which the survey vehicle can drive. Therefore, this configuration is suitable for any road-based survey, even at highway speeds.

The best way to trigger the CrossOver system is from one of the survey vehicle’s rear wheels, using a wheel encoder kit. Wheel triggering also provides chainage and makes it possible to use hyperbola fitting and migration filtering tools during the post-processing of data. The CrossOver antenna’s built-in GPS receiver for georeferencing can be used during the measurement and subsequent interpretation process. However, rather using the elevation data from the internal GPS, SGU used Sweden’s nationwide GSD-elevation data terrain model to get more accurate elevation data.

Note – in countries where such elevation models don’t exist, you must use an external RTK/GPS to get accurate elevation data.

The figures below show data from two sections of an typical vehicle mount survey, and clearly show the value of this approach. The groundwater-table is visible in the radargram due to the high porosity of the sand/ gravel volume. There is also a large contrast at the bedrock interface due to its low porosity. Consider the big difference in the velocity of the GPR signal in dry sand versus saturated sand. A speed of 140 – 150 m/uS is entirely possible in dry sand but can reduce to 60 – 70 m/uS below the groundwater interface depending on the sand formation’s porosity. These examples also show how the two radar channels complement each other. The CO730 comprises of two real GPR antennas with centre frequencies of 70 MHz and 300 MHz respectively. With its higher resolution, the 300 MHz antenna enables a clear view of the shallower layers, including the groundwater-table interface. With its deeper penetration, the 70 MHz antenna enables a clear picture of the deeper layers, including the sand, gravel, and bedrock interfaces.