Transdimensional surface wave tomography of the near-surface: Application to DAS data

Distributed Acoustic Sensing (DAS) is a novel technology that allows sampling of the seismic wavefield densely over a broad frequency band. This makes it an ideal tool for surface wave studies. In this study, we evaluate the potential of DAS to image the near-surface using synthetic data and active-source field DAS data recorded with straight fibers in Groningen, the Netherlands. First, we recover the laterally varying surface wave phase velocities (i.e., local dispersion curves) from the fundamental-mode surface waves. We utilize the Multi Offset Phase Analysis (MOPA) for the recovery of the laterally varying phase velocities. In this way, we take into account the lateral variability of the subsurface structures. Then, instead of inverting each local dispersion curve independently, we propose to use a novel 2D transdimensional surface wave tomography algorithm to image the subsurface. In this approach, we parameterize the model space using 2D Voronoi cells and invert all the local dispersion curves simultaneously to consider the lateral spatial correlation of the inversion result. Additionally, this approach reduces the solution nonuniqueness of the inversion problem. The proposed methodology successfully recovered the shear-wave velocity of the synthetic data. Application to the field data also confirms the reliability of the proposed algorithm. The recovered 2D shear-wave velocity profile is compared to shear-wave velocity logs obtained at the location of two boreholes, which shows a good agreement

Seismic noise in an urban environment

The results of an experiment aimed at identifying the nature of major noise sources within an urban area are described. We found the strongest noise source to be an irrigation pump located adjacent to the geophones. The noise from the pump had a narrow bandwidth centered at 75 Hz with a duration of 5 minutes every 17 and 34 minutes during the day and night, respectively. Traffic noise was mainly restricted to between 10 and 25 Hz, with its strength decreasing between 9 p.m. and 6 a.m. Passing aircraft resulted in noise between 30 and 200 Hz lasting about 1 minute. Electrical noise was observed at the supply frequency of 50 Hz, although additional noise at 45 Hz also was observed. Given these results we recommended that acquisition within the area should be restricted to late evening or early morning, receiver locations should be selected to avoid strong localized sources of electrical and/or mechanical noise, and any cables associated with the recording system should be as short as possible (although nodal systems are preferable). If nodal systems are deployed for logistical reasons, real-time noise monitoring should be deployed to identify and avoid bursts of high-amplitude, short-duration noise. </jats:p