Planning of urban underground infrastructure using a broadband seismic landstreamer — Tomography results and uncertainty quantifications from a case study in southwestern Sweden

Abstract

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 an approximately 3-km-long underground tunnel in the city of Varberg in the southwestern Sweden. Prospective targets were imaging of the shallow (<20 m) bedrock surface and weak zones, such as fracture and shear zones. Over the course of three weeks, 25 profiles were acquired with a total length of approximately 7.5 km using a source and receiver spacing of 2–4 m. A novel approach of the data acquisition was to integrate the landstreamer with wireless sensors in areas in which the accessibility was restricted by roads and also to increase the source-receiver distances (offsets). Although the area was highly noisy, the seismic data, in conjunction with available boreholes, successfully led to delineation of the bedrock surface, its undulations, and areas of poor rock quality. To overcome challenges due to geologic complexities and crooked-line data acquisition, 3D tomographic inversion of first breaks was carried out. Comparisons of the results with the existing boreholes indicated that in most places, the bedrock surface was well resolved by the method, which supported the indication of weak zones in the bedrock, represented by low-velocity structures in the tomographic results. We also evaluated the effect of poor geodetic surveying, particularly regarding elevation data, which adulterated the tomography results toward undulating bedrock surfaces or zones of low velocities.

Delineating fracture zones using surface‐tunnel‐surface seismic data, P‐S, and S‐P mode conversions

“…An attempt to do a 3-D inversion of the first arrivals was made using a grid size of 12 m in the crossline direction, which resulted in numerous gaps in the inverted velocity model, making the interpretation of the results difficult. To ensure that features in the velocity models are not inversion Journal of Geophysical Research: Solid Earth 10.1002/2017JB014304 artifacts, we tried to perturb the picked first arrival travel times by adding a series of randomly generated travel times with a mean value of ±3 ms [Malehmir et al, 2015b]. This did not significantly influence the result, with the anomalous zones still being preserved.…”

“…To address some of the above‐mentioned issues, we conducted a novel surface‐tunnel‐surface seismic survey at the Äspö Hard Rock Laboratory (HRL) in southern Sweden during April 2015. Well‐documented fracture systems, extending from the surface and intersecting the tunnel at different depths [ Kornfält and Wikman , ; Kornfält et al ., ; Rhén et al ., ; Berglund et al ., ], provided a unique opportunity to evaluate their seismic response using a digital three‐component (3C) seismic landstreamer [ Brodic et al ., ; Malehmir et al ., , ] in the tunnel. Both sources and receivers were located on the surface and inside the tunnel and the seismic wavefield simultaneously recorded on all receivers.…”

Interlobate esker architecture and related hydrogeological features derived from a combination of high-resolution reflection seismics and refraction tomography, Virttaankangas, southwest Finland

“…By definition, a landstreamer is an array of seismic sensors that can be towed behind a vehicle without the need for planting the sensors (Kruppenbach and Bedenbender 1975). While most landstreamers use geophones (analogue systems) for data acquisition, a newly developed advanced MEMs-based (micro-electro mechanical) broadband seismic landstreamer (Brodic et al 2015;Malehmir et al 2015aMalehmir et al , 2015bMalehmir et al , 2016aMalehmir et al , 2016b) was employed in this study for delineating subsurface structures of a major interlobate esker system and a MAR aquifer in Virttaankangas, in the southwest of Finland.…”

Geophysical pre‐investigation for a Stockholm tunnel project: joint inversion and interpretation of geoelectric and seismic refraction data in an urban environment

“…To counter the poor signal‐to‐noise ratio, Malehmir et al . () suggests to perform measurements during periods of reduced noise, like weekends or night‐time. However, getting permissions for blasting during the night can be difficult.…”