Andreas Ahl

Spatial mapping of submerged cave systems by means of airborne electromagnetics: an emerging technology to support protection of endangered karst aquifers

Karst aquifers represent important but very vulnerable sources for water supply to a significant part of the Earth’s population. For sustainable use of these resources, development of integrated management tools based on numerical groundwater models is required. In principle karst aquifers are characterized by the presence of two distinct flow domains: the limestone matrix fractures and the conduits. A flow model of karst aquifers requires detailed, spatially distributed information on the hydrologic characteristics of the aquifer and flow paths. Geophysical methods determining the distribution of the electrical resistivities within the subsurface could provide such information. An international scientific research project was initiated to explore the potential of airborne electromagnetic mapping for providing such innovative information for improving groundwater modelling of karst aquifers. The project study area is located in the Sian Ka’an Biosphere Reserve located in Yucatan, Mexico, a coastal wetland of international importance. As a first step ground geoelectric and ground electromagnetic measurements were performed in March 2006 to determine the electrical properties of the Sian Ka’an Biosphere Reserve subsurface environment. These results were used for 3D forward modelling to calculate the expected airborne electromagnetic response. Based on these promising results, an airborne pilot survey was performed in 2007 to evaluate the applicability of airborne electromagnetic methodology. This survey covers an area of 40 square kilometres above the well-mapped Ox Bel Ha cave system. The results showed that the signature of the cave system could be clearly detected. The pilot survey offered as well the chance to define the limits of current state-of-the-art airborne data acquisition and inversion. The study helped to define the needs for further developments and improvements to establish the frequency domain electromagnetic method as a practical karst exploration method.

Precipitation correction of airborne gamma-ray spectrometry data using monitoring profiles: methodology and case study

Variations of soil moisture content caused by precipitation often complicate the interpretation of airborne gamma-ray spectrometry data. This is particularly the case in repeated surveys designed to monitor the change of near surface abundances of radioactive elements or in large and time-consuming surveys. To counter this precipitation effect we propose a correction method based on repeated survey flights over a monitoring profile. Assuming that the weather and the soil conditions at the monitoring profile are representative for the survey area, the weather dependent effect of soil moisture can be observed and sufficiently corrected.

Airborne and Ground Geophysics for Modelling a Karstic Conduit System: New Results from the 2007-2011 Campaigns in Tulum

Karst aquifers represent important sources for water supply to a significant part of the earth’s population. For sustainable use of these resources, development of management tools based on numerical groundwater models is required. A flow model of karst aquifers requires spatially distributed information on its characteristic flow domains. Methods determining the distribution of the electrical resistivity within the subsurface could provide such information. To explore the potential of airborne electromagnetic (AEM) mapping for providing such information to groundwater modelling of karst aquifers, the international project XPLORE was initiated. The project is carried out around the Sian Ka’an Biosphere Reserve, located near Tulum, Mexico. Airborne surveys were performed in 2007 and 2008 to prove the basic applicability of the AEM method. The results show that the signature of the cave system can be clearly detected by AEM mapping. Additionally, for better coverage of ground truth and calibration of the hydrological model, three extended ground geophysical campaigns have been conducted in 2009-2011 comprising geoelectrics, GPS-water level measurements, GPR, and borehole geophysics. The airborne data as well as mapped caves were used to generate a numerical ground water model of the karst system.

Karst Mapping Using Airborne Electromagnetics – Multi – Layer Inversion to Derive Spatially Distributed Parameter

Karst aquifers represent important but very vulnerable sources for water supply to a significant part of the earth’s population. For sustainable use of these resources, development of integrated management tools based on numerical groundwater models is required. However a flow model for karst aquifers requires detailed, spatially distributed information on the flow- relevant characteristics of the subsurface. Methods determining the distribution of the electrical resistivities within the subsurface could provide such information. To explore the potential of airborne electromagnetic mapping for providing such innovative input information, the international scientific research initiative XPLORE was initiated. Within this paper, successful approaches to derive the subsurface cave structure and to map the depth of the halocline using multi-layer 1D inversion of frequency domain electromagnetic data are presented.

Chapter 14 Detection of AEM anomalies corresponding to dike structures

Publisher Summary This chapter discuses the detection of airborne electromagnetic (AEM) anomalies corresponding to dike structures. The major geoscientific applications of AEM measurements are in the areas, such as investigation of groundwater resources, geotechnical applications (e.g. landslides, mass movements), exploration of raw materials (mass raw materials like clay and gravel, ore resources), and assisting terrain mapping (geology). Because of the enormous quantity of data that will be obtained during such measurements, most of the users of such systems use only simple mathematical-physical models—for example, the homogeneous half-space and the Schwerpunktstiefe. Chiefly these methods are used to keep the calculation time low.