The challenges include delineation of aquifers, mapping saltwater encroachment/ intrusion, aquifer vulnerability by mapping variations in the clay cover above the aquifer, surface water - groundwater interaction to support managed aquifer recharge, mapping the extent of landfills and pollution plumes from landfills, mapping clays that can initiate a landslide, mapping and monitoring seepage beneath a levee, and many more.
Geophysical methods are great tools to investigate subsurface hydrogeological parameters and processes in a cost-effective and minimally or non-invasive manner, and at relevant spatial scales. Ramboll has utilized geophysics for more than 35 years and gained extensive experience from numerous hydrogeological surveys around the world. These competences include designing geophysical surveys in different climate and geographical conditions, data processing and inversion, and very importantly interpretation of the geophysical data together with borehole information and other types of data to establish three-dimensional (3D) geological models.
An important task to achieve a successful geophysical project is a complete evaluation of the hydrogeological settings and subsequently selection of the most suited geophysical techniques. Ramboll has over the years developed a large and innovative geophysical toolbox that allow selection of the right tool for any specific survey.
Aquifer delineation
Detailed knowledge of the general framework of the aquifers is extremely important when building numerical groundwater models and fundamentally to enhance groundwater management.
Managed Aquifer Recharge (MAR)
Recent climate change has caused more extreme weather events. Regions characterized by moderate and frequent weather changes now experience longer periods of drought and short periods with extreme rain fall. MAR is a way to capture the storm water, infiltrate and store the water locally. A crucial step in MAR is to assess potential sits for infiltration. Geophysics provide very valuable information about where it is most suitable to infiltrate water and where the infiltrated water will flow.
Saltwater Intrusion
In many geographical regions saltwater intrusion is a critical threat to the aquifers for domestic water supply. Therefore, it is very important to obtain information about the extent of saltwater intrusion. Electrical and electromagnetic geophysical investigations are ideally suited to provide this crucial information, since water salinity has a strong impact on the subsurface electrical conductivity.
Download TDEM – Delineation of groundwater salinity (PDF) here.
Aquifer Vulnerability
Clay layers above the aquifers can capture and reduce nitrate within farmland. Therefor it is important to obtain knowledge about the extent and continuity of the clay layers. Using geophysical methods, we can map variations within the clay cover and also point out area where the aquifers are vulnerable, i.e. without a clay cover (windows), and where the aquifer are less or invulnerable.
Techniques
The desired depth of investigation, resolution and the geology determine which geophysical method is most suitable. The advantage of most geophysical methods is that they are non-invasive and leave none or minimal evidence on the ground. At the same time geophysical methods are truly in-situ methods since the ground is not disturbed by the measurement.
Geophysical methods measure different subsurface properties. Often measured properties are electrical resistivity or conductivity and acoustic velocity. The following is an introduction to a selection of the methods that are routinely employed at Ramboll.
Time-Domain Electromagnetics (TDEM)
Single site TDEM method has been used extensively in the last three decades for mapping the subsurface. Ground-based TDEM provides point information on the subsurface electrical conductivity. It is common practice to conduct a TDEM survey by acquiring data at multiple points by which 3D distribution of electrical conductivity can be obtained. The depth of investigation depends on what system setup is used and electrical conductivity of the subsurface materials, and can be as large as 500 m.
For single site TDEM measurements Ramboll uses ProTEM from Geonics and the WalkTEM system from GuidelineGeo/ABEM.
Download TDEM Surveys (PDF) here.
Airborne ElectoMagnetics (AEM)
Airborne ElectroMagnetic (AEM) methods are efficient tools for large-scale investigations or in areas where ground access is limited. Helicopter-borne EM transient EM methods, such as SkyTEM, are developed specifically for accurate measurement of the electrical conductivity of the subsurface, which is vital in groundwater investigations where the variation of electrical conductivity is much less than its variation in mineral explorations.
Ramboll has a great experience working together with SkyTEM Surveys ApS on different AEM projects. With the SkyTEM system large areas can be mapped cost-effectively and in a short time period. Results obtained with the SkyTEM system are comparable with result from the TDEM soundings; however, an airborne system measures a continuous profile, thus much faster coverage. The depth of investigation varies depending on the system setup and the subsurface electrical conductivity, and can exceed 300 m.
Download AEM - Airborne Geophysics (PDF) here.
Towed TEM (tTEM)
Towed Transient ElectroMagnetic (tTEM) is a new innovative TEM technology. With tTEM, large geographic areas can be mapped rapidly and with at a very high lateral and vertical resolution, down to a depth of ~ 60-80 m.
Download tTEM - Towed Transient Electromagnetics (PDF) here.
Floating TEM (FloaTEM)
Surface water- groundwater interaction is often a very important to describe the hydraulics of an aquifer as well as the river inflow. Similar to tTEM, a floating TEM (FloaTEM) system was recently developed for rapid data acquisition on water.
Magnetic Resonance Sounding (MRS)/ Nuclear Magnetic Resonance (NMR)
The NMR/MRS method is a method to directly estimate water content and permeability without a need for drilling. The NMR technology is used on the ground surface and in boreholes. The NMR/MRS method is used to optimize borehole locations among several possible locations, to improve coverage of hydraulic parameters and to determine water content when designing dewatering or groundwater table lowering in construction projects.
Download Magnetic Resonance Sounding (MRS/NMR) (PDF) here.
Electrical Resistivity Tomography (ERT) and Induced Polarization (IP)
Electrical Resistivity Tomography (ERT) is an electrical method to measure the electrical resistivity of the subsurface. Cables with multiple outtakes are laid out and current is injected to the subsurface by spears connected to the outtakes. Potential voltage differences are measured between each pair of electrodes.
Using the entire data set, sections or profiles with detailed variation of the resistivity are obtained with an investigation depth of up to 150 m depending of the system setup. Ramboll uses the Terrameter LS instrument from GuidelineGeo/ABEM.
Download Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) (PDF) here.
Ground Conductivity Meter (GCM)
The Ground Conductivity Meter (GCM) is a frequency-domain EM instrument used to obtain very dense and detailed information of the shallow subsurface, i.e. the top ~ 8 meters. Even in urban environments the GCM method has successfully been applied to identify suitable areas for infiltration from the surface and to compare different sites.
Ramboll uses the DualEM421 system from Dualem Inc.
Download Continuous Surveying - DualEm421S (PDF) here.
Download DualEM for the Optimization of Artificial Infiltration (PDF) here.
