Setting

Denmark is a world leader in mapping groundwater resources. In the 1990s in order to locate and protect the country’s natural resources the Danish Government embanked on the ambitious plan to map the subsurface of the entire country. The Danish Environmental Protection Agency (EPA) set out to determine the extent of the groundwater resources in the country as well as to gain an understanding of where they could be vulnerable and in need of protection.

Solution

The SkyTEM TDEM method is designed to be sensitive to subtle resistivity variations in soil and rock and is the ideal tool for mapping the interface between the quarternary sediments and the more conductive sticky tertiary clay. For this reason the EPA and the hydrogeological team in Denmark quickly identified the SkyTEM method as the method for outlining the lower boundary of the aquifers. They were able to rapidly collect and assess valuable information describing the lower boundary of the quarternary sediments. These were characterized by a large number of distinct paleochannels/buried valleys ranging in depth from 10 to 20 m and down to more than 300 m.

Findings

Depth to a strong conductor (surface of the tertiary clay) was extracted by modeling the results of the TEM data.

The paleohannels can be traced over long distances. Typically the channels can be divided into primary and secondary channels (secondary channels are those drained by the primary channels). It can be seen that few of the boreholdes drilled into the aquifer are deeper than a few tens of metres. It is obvious that a leap of faith and some creativity would be required to arrive at an interpretation similar to that provided by the SkyTEM system.

An area that was going to be developed as a new industrial area was located directly above one of the primary groundwater aquifers. To protect the water, the country re-evaluated these plans and it was decided to leave the land as farmland and forest to avoid the risk of contaminating the aquifers. The farmers subsequently made agreements with the local water works to reduce the usage of fertilizer and pesticides/herbicides.

Conclusion

This new knowledge made it possible to determine the size of groundwater resources as well as to understand where to protect them. These results have had a significant influence on urban planning. This case is evidence that the SkyTEM method is a valuable investment, a major step towards securing clean drinking water for future generations and an essential input for sustainable groundwater management planning.

Setting:

The Galapagos Island of Santa Cruz. Santa Cruz supports 13,000 inhabitants and 120,000 annual visitors and there is very little evidence of groundwater on the island. Drilling restrictions and challenging terrain prevent traditional hydrogeological investigation.

Solution:

The Darwing Foundation and several universities employed the SkyTEM³⁰⁴ to collect data over the island. Helicopter flying speed was 45 km/h sensor altitude was 35-45 m and South-North flight lines had a spacing of 200 m. A few lines were flown cross island to obtain a full picture of the salt-fresh water interface. SkyTEM filters avoid smoothing of early time data and the near-surface resistivity variations were enhanced. Later times data were more severe filtered to obtain as much depth penetration as possible. The data were gathered into soundings with a spatial distance of about 25 m. The SkyTEM method was sensitive to low-resistivity layers of hydrogeological interest [50-200 ohm-m] to a depth of approximately 300 m.

Findings:

The resistivity data below shows the electrical properties of the Santa Cruz rock. High resistivity values (reds) indicate competent rock, or basalts, while low resistivities (reds) represent gravels and soils. Unit IV, shown in blue-green to yellow is a perched aquifer that lies beneath the weathered, vegetated and permeable layer (yellow). Downward movement of water is constrained by impermeable Unit I (orange) basalts that make up the bulk of the volcano’s composition. Unit II is electrically conductive and is a seawater intrusion wedge underlying the island. Unit III, outcrop, has the same geophysical signature of Unit IV but is interpreted differently based on existing geological data. Large-scale faults that facilitate internal drainage were identified and helped explain the absence of surface seepage.

Conclusions:

An aquifer can be difficult to identify due to similarities in resistivity of the aquifer and the layers masking it. SkyTEM however was able to delineate the subtle contrasts required for accurate interpretation. The unique spatial resolution and 3-D view of the subsurfface resistivity structures allowed identification of the previously unknown perched aquifer and the salt-water wedge in the basal aquifer, located with an accuracy of a few meters. Beyond the vital implications for water resource and ecosystem management the high resolution SkyTEM data has forced scientists to reconsider their established conceptual flow models of volcanic islands.

Postscript:

The identification of weathered layers in a volcanic edifice may help to explain the cuase of potentially catastrophic landslides. In 2011 the US Geological Survey contracted SkyTEM to map the slopes and bases of Mt St Helens and Mt Iliamna, two volcanoes in the USA, to study slope stability and the presence of water.