ERT och TEM som verktyg för grundvattenundersökningar - Jämförande studie i Skåne

Sammanfattning: Geophysical methods can create 2D- and 3D-models that reflects the geology. These models can be used as a tool for mapping groundwater resources. Four profiles, located in Vombsänkan in southeastern Skåne, have been investigated with Electrical Resistivity Tomography (ERT) and ground-based Transient Electromagnetic Soundings (TEM) as a study of the two methods. The resistivity profiles from the geophysical methods have also been studied. ERT, also called CVES, is carried out to get a better understanding of the coarse sediments in the soil layers. This is complemented with TEM soundings that can provide an understanding of the character of the deeper soil layers as well as the depth and the condition of the bedrock. This study from Lund University is a collaboration with the Geological Survey of Sweden (SGU) and the consulting firm WSP. The results favor the SGU and their further hydrogeological mapping of critical aquifers in Sweden. WSP has been commissioned by Sjöbo municipality to investigate a new main water source to either replace or supplement the existing water supply. Collaboration with WSP involves data from the geophysical methods and drillings (performed by WSP) will serve as a basis for assessing the area with respect to groundwater and its suitability as a water source. Inversion of resistivity data from ERT is relatively user-friendly with pre-settable settings in Res2DINV used for all profiles to be interpreted by the same conditions. Modeling of TEM-soundings requires practical experience from the user, something that has improved during the project. The TEM models was modeled exclusively with 3-layer models for the sake of consistency. Even for an experienced user, the method has its limitations at high resistivity where insufficient data exists because of the low current density. TEM-soundings fit best in determining differences in resistivity below 150 Ohm-m and is therefore well suited for Skåne with underlying sedimentary rocks. Data have been obtained with high resistivity in the top layer through all the profiles. This is common to both the TEM and the ERT. Generally, the resistivity is declining with depth. Profile 2 is the only exception which is located in the hilly landscape where a clayey till/moraine dominates in the soil surface. Indications of aquifers exist with coarse formations above fine-grained formations. These indications can be seen from the drilling reports on profile 3 and 4. TEM and ERT complement each other well. The TEM-method is a quick preliminary investigation method and does not require as much labor or staff as ERT, and may therefore be termed as cost effective. No current injection into the ground via electrodes is required, why no damage on the ground is made. However, an environment without coupling is required. ERT requires a large distance between electrodes to compete with the depth penetration of TEM soundings. With TEM-soundings and its large depth penetration, saline water in bedrock can be detected with a resistivity around 10 Ohm-m at a relatively great depth. No indications of saline groundwater have been observed in the project. There are differences in resolution between ERT and TEM in the uppermost 50-60 meters. ERT is good at detecting differences in resistivity near the soil surface and down to 60 meters, depending on the electrode configuration and spacing. There was no indication of any resistivity-layers the uppermost tens of meters. This is because the coarse-grained formations have naturally high resistivity. Drilling gives a good indication of the complex uppermost ten meters and its results comes in handy when interpreting the geological profiles.