Mapping fresh water and saltwater encroachment

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.