Using airborne electromagnetic data from the Netherlands, quantitative study reveals the effect of inversion methods on groundwater mapping programmes

Published: 06-04-2018

Using airborne electromagnetic data from the Province of Zeeland, the Netherlands, research has shown that the choice of geophysical inversion method has significant effects on typical hydrological mapping objectives.

Jude King, March 2018

Using airborne electromagnetic data from the Province of Zeeland, the Netherlands, research has shown that the choice of geophysical inversion method has significant effects on typical hydrological mapping objectives.

An inversion is required to translate observed data into a distribution of physical properties. Specifically, during airborne electromagnetic mapping programmes, data collected from an instrument towed beneath a helicopter is inverted into electrical conductivity (EC). As EC relates to groundwater salinity and clay, it is a useful and widely used hydrological mapping tool. Eight of the most commonly used inversion methods were investigated, including a quantitative assessment of each. Results indicated that the most important factor is model smoothness, which was shown to affect the thickness of the groundwater brackish zone – and subsequently the volume estimates of fresh groundwater. Significantly, fresh groundwater estimates were shown to differ by up to 195 million m3 in an area of only 20 x 20km depending on the method used. In general all inversion types resolved fresh, brackish or saline groundwater interfaces with an accuracy of around 3m.

 

Inversion results for a single inversion type, showing 3D models of electrical conductivity (top left), fresh-brackish-saline regions (top right) and fresh-brackish-saline interfaces (bottom left).

 

Fresh-brackish-saline groundwater volume estimates for each of the eight tested inversion methods

The research was presented in New Orleans at the American Geophysical Union (AGU) 2017 fall meeting, and will be presented again in Gdansk, Poland in June 2018 as a part of the biennial Salt Water Intrusion Meeting (SWIM). A full publication of the work is expected this year and involves researchers from Utrecht University, Deltares (Utrecht) and the German Institute for Geosciences and Natural Resources (BGR).

Airborne data were collected in 2014-2015, for a separate project called FRESHEM Zeeland (Fresh Salt groundwater distribution by Helicopter Electromagnetic survey in the Province of Zeeland), and is a research programme involving Deltares, the Geological Survey of the Netherlands (TNO) and BGR.