Improving the Calibration of the Swiss Plate Geophone Bedload Monitoring System by Filtering Out Seismic Signals From Extraneous Particle Impacts

Abstract

The spatio‐temporal variability of bedload transport processes poses considerable challenges for bedload monitoring systems. One such system, the Swiss plate geophone (SPG), has been calibrated in several gravel‐bed streams using direct sampling techniques. The linear calibration coefficients linking the signal recorded by the SPG system to the transported bedload can vary between different monitoring stations by about a factor of six, for reasons that remain unclear. Recent controlled flume experiments allowed us to identify the grain‐size distribution of the transported bedload as a further site‐specific factor influencing the signal response of the SPG system, along with the flow velocity and the bed roughness. Additionally, impact tests performed at various field sites suggested that seismic waves generated by impacting particles can propagate over several plates of an SPG array, and thus potentially bias the bedload estimates. To gain an understanding of this phenomenon, we adapted a test flume by installing a partition wall to shield individual sensor plates from impacting particles. We show that the SPG system is sensitive to seismic waves that propagate from particle impacts on neighboring plates or on the concrete bed close to the sensors despite isolating elements. Based on this knowledge, we designed a filter method that uses time‐frequency information to identify and eliminate these “apparent” impacts. Finally, we apply the filter to four field calibration datasets and show that it significantly reduces site‐to‐site differences between calibration coefficients and enables the derivation of a single calibration curve for total bedload at all four sites.

Analysis of the Vibration Modes of Impact Geoplates and Implications for Bedload Flux and Grain Size Measurements

“…Recent findings by Nicollier, Antoniazza, Rieckenmann, et al. (2022) showed that it is possible to filter the spurious vibrations generated by impacts on nearby plates and on the concrete sill surrounding the plates array checking the spectral characteristics of impacts. Particles directly impacting the plate always presented a first part of the impact with high frequencies (larger than 1 kHz), whereas spurious impacts caused the plate to vibrate predominantly with a frequency in the range of the fundamental mode.…”

“…(2016c), Nicollier, Antoniazza, Rieckenmann, et al. (2022), and Nicollier, Antoniazza, Ammann, et al. (2022) developed a procedure to estimate grain size from the spectral centroid frequency.…”

“…However, they pointed out the significant role of vibration propagation across plate arrays, with non‐negligible effects on the number of recorded impulses in the case of multiple plates attached to the same structure. Filtering these spurious impacts strongly enhances the calibration of the geoplate (Nicollier, Antoniazza, Rieckenmann, et al., 2022). Nicollier et al.…”

Anatomy of an Alpine Bedload Transport Event: A Watershed‐Scale Seismic‐Network Perspective

“…Note that the two bedload transport monitoring systems may have different capabilities in terms of minimal grain‐size detection, which may translate into differences in bedload transport estimates. The SPG system has a detection threshold for particles greater than ∼10–20 mm (Nicollier et al., 2021; Nicollier, Antoniazza, Ammann, et al., 2022; Nicollier, Antoniazza, Rickenmann, et al., 2022; Wyss et al., 2016a, 2016b, 2016c), while seismic inversion may detect smaller particles if hydraulic and transport conditions allow it.…”

“…Antoniazza et al. (2020, 2022) and (Nicollier, Antoniazza, Ammann, et al., 2022; Nicollier, Antoniazza, Rickenmann, et al., 2022) describe the system and its calibration in detail. In this contribution, we used data from the monitoring station (a) to set the investigated bedload transport event into comparative context with other events that took place over the period 2016–2020 and (b) as a benchmark method to evaluate the seismic inversion approach for bedload transport sensing.…”