Hydrogeophysics as a multidisciplinary tool on aquifer appraisalFocus on AMT capabilities

Resumen

The shallow subsurface of the earth is an extremely important geological zone that yields much of our water resources, supports our agriculture and ecosystems. Safe and effective management of our natural resources is a major challenge, that is, not to overexploit and pollute the aquifer systems. The hydrogeophysics discipline has emerged in recent years to investigate the potential that geophysical methods hold for providing quantitative information about subsurface hydrogeological parameters and processes. Water is essential for life. It has been and will continue to be a hot topic in both the political and scientific fields for years to come.Chapter two, Hydrogeophysics, aims to review the actual state of the hydrogeophysical discipline showing the paramount perspectives of the geophysical methods applied to the hydrogeology discipline. Hydrogeophysical importance relies on its multidisciplinarity. That is, the joint interpretation of geophysical and hydrogeological data to get better approach and more constrained/reliable hydrogeologic models. The principal geophysical methods that have actually been applied to hydrogeology problems are reviewed, as well as their challenges, limitations and concerns. Electric and electromagnetic methods that provide electrical resistivity models have been examined in detail. The electrical resistivity is considered, given that this property is highly controlled by the hydrogeological properties of the media (lithology, porous structure, water content and quality). However, the complexity of the property contribution of the geophysical response made the transfer step from geophysical model to hydrological properties a complex issue. Chapter three, Audiomagnetotelluric method (AMT), presents further insights of the AMT frequency domain method. Firstly, theoretical basics of the AMT method, investigation depth and magnetotelluric family methods (frequency recording capability) are reviewed. Further on, the determinant mode modelling process is discussed in order to obtain a reasonable approach of three-dimensional media along two-dimensional profiles. This chapter demonstrates in detail Stratagem EH4 equipment, in particular, source effects, optimal transmitter receiver distance, and signal improvement of controlled electromagnetic source.Part II is composed of three different hydrogeophysical case studies at different scales where AMT plays a key role on the approach to each aquifer system.Chapter four, Tordera deltaic aquifer, Spain, is a hydrogeophysical application carried out on a porous aquifer media extensively affected by seawater intrusion. This chapter presents a multidisciplinary approach to a fluviodeltaic aquifer system. Hydrogeological information, AMT, seismic reflection and velocity tomography models have been evaluated together to provide spatially continuous information about aquifer properties processes and boundaries. Two and three-dimensional inverse models have been obtained and validated with hydrogeological data. In addition, an AMT monitoring experiment has been performed every four months to monitor seasonal seawater changes along the main seawater intrusion path. Chapter five, La Soutte, Vosges, France is based on the AMT study on "La Soutte", natural hydrogeophysical laboratory. In this case, 3D AMT forward model look into a small scale catchment area, aimed to provide a mesoscale framework for other geophysical applications carried out on the site. Combined use of AMT and DC surface resistivity has been also used to image different resolution and work scale of each method. Chapter six, Spring Valley, Nevada, USA presents the AMT study out on Spring Valley, a 110 km long valley on the Nevada Desert aimed to provide a structural framework for water resources exploration and hydrological modelling. Five 2D AMT profiles along the valley, together with gravity and magnetometry data have been analysed to map the aquifer bounding structure and sediment infill in a typical Basin and Range setting. Faults, stratigraphy, and estimates of depth to basement are valuable information for assessing groundwater potential.