Over the past few years, the demand for urban infrastructures has continuously increased worldwide and in particular, in Sweden. However, there is a lack of knowledge about subsurface geology and structures in the urban environment. Occasionally, information about former or hidden outcrops exists or is available from, for example, municipalities, consultants, and construction companies. Accurate knowledge about the bedrock depth and condition is important for planning a trench or a tunnel because it may imply what kind of excavation and rock reinforcement methods should be used.
The urban environment is, however, challenging for most geophysical methods due to the multiple sources of noise (e.g., ground vibrations caused by vehicles and electromagnetic noise from power lines) and spatial and temporal restrictions imposed on geophysical surveys by infrastructure. The geophysical survey equipment used needs to be flexible and versatile, and highly insensitive to electromagnetic noise. In this project, we have developed a multicomponent broadband seismic landstreamer system based on digital sensors and particularly suitable for noisy environments and areas in which high-resolution images of the subsurface are desired.
We have evaluated results, interpretations, and approaches using the streamer in the planning of several underground infrastructure projects in Sweden, Norway and Finland. We have also developed a new data acquisition system and technique to measure the radio magnetotelluric (RMT) signals from distant radio transmitters with the objective of mapping and modeling electric resistivity structures below a river or lake. A boat tows the acquisition system; therefore, we refer to it as boat-towed RMT.
The data acquisition is fast with a production rate of approximately 1 km∕hr using a nominal sampling spacing of 10–15 m. Owing to the ample number of radio transmitters available in most parts of the world, the method can be used for near-surface studies of various targets. We have developed boat-towed RMT measurements on Lake Mälaren near the city of Stockholm in Sweden and at the Äspö Hard Rock Laboratory to determine the feasibility of the method. The boat-towed RMT technique is well suited for water bodies with moderate electric resistivity such as in brackish and freshwater environments.