Toward a general calibration of the Swiss plate geophone system for fractional bedload transport

Substantial uncertainties in bedload transport predictions in steep streams have encouraged intensive efforts towards the development of surrogate monitoring technologies. One such system, the Swiss plate geophone (SPG), has been deployed and calibrated in numerous steep channels, mainly in the Alps. Calibration relationships linking the signal recorded by the SPG system to the intensity and characteristics of transported bedload can vary substantially between different monitoring stations, likely due to site-specific factors such as flow velocity and bed roughness. Furthermore, recent flume experiments on the SPG system have shown that site-specific calibration relationships can be biased by elastic waves resulting from impacts occurring outside the plate boundaries. Motivated by these findings, we present a hybrid calibration procedure derived from flume experiments and an extensive dataset of 308 direct field measurements at four different SPG monitoring stations. Our main goal is to investigate the feasibility of a general, site-independent calibration procedure for inferring fractional bedload transport from the SPG signal. First, we use flume experiments to show that sediment size classes can be distinguished more accurately using a combination of vibrational frequency and amplitude information than by using amplitude information alone. Second, we apply this amplitude-frequency method to field measurements to derive general calibration coefficients for 10 different grain-size fractions. The amplitude-frequency method results in more homogeneous signal responses across all sites and significantly improves the accuracy of fractional sediment flux and grain-size estimates. We attribute the remaining site-to-site discrepancies to large differences in flow velocity and discuss further factors that may influence the accuracy of these bedload estimates.ISSN:2196-632XISSN:2196-631

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

The way Alpine rivers mobilize, convey and store coarse material during high-magnitude events is poorly understood, notably because it is difficult to obtain measurements of bedload transport at the watershed scale. Seismic sensor data, evaluated with appropriate seismic physical models, can provide that missing link by yielding time-varying estimates of bedload transport albeit with non-negligible uncertainty. Low cost and ease of installation allow for networks of sensors to be deployed, providing continuous, watershed-scale insights into bedload transport dynamics. Here, we deploy a network of 24 seismic sensors to estimate coarse material fluxes in a 13.4 km2 Alpine watershed during a high-magnitude transport event. First, we benchmark the seismic inversion routine with an independent time-series of bedload transport obtained with a calibrated acoustic system. Then, we apply the procedure to the other seismic sensors across the watershed. Propagation velocities derived from cross-correlation analysis between spatially consecutive bedload transport time-series were too high with respect to typical bedload transport velocity suggesting that a faster-moving water wave (re-)mobilizes local coarse material. Spatially distributed estimates of bedload transport reveal a relative inefficiency of Alpine watersheds in evacuating coarse material, even during a relatively infrequent high-magnitude bedload transport event. Significant inputs estimated for some tributaries were rapidly attenuated as the main river crossed less hydraulically efficient reaches. Only a small proportion of the total amount of material mobilized in the watershed was exported at the outlet. Multiple periods of competent flows are likely necessary to evacuate coarse material mobilized throughout the watershed during individual bedload transport events

In-situ regolith seismic velocity measurement at the InSight landing site on Mars

InSight's seismometer package SEIS was placed on the surface of Mars at about 1.2 m distance from the thermal properties instrument HP3 that includes a self-hammering probe. Recording the hammering noise with SEIS provided a unique opportunity to estimate the seismic wave velocities of the shallow regolith at the landing site. However, the value of studying the seismic signals of the hammering was only realised after critical hardware decisions were already taken. Furthermore, the design and nominal operation of both SEIS and HP3 are non-ideal for such high-resolution seismic measurements. Therefore, a series of adaptations had to be implemented to operate the self-hammering probe as a controlled seismic source and SEIS as a high-frequency seismic receiver including the design of a high-precision timing and an innovative high-frequency sampling workflow. By interpreting the first-arriving seismic waves as a P-wave and identifying first-arriving S-waves by polarisation analysis, we determined effective P- and S-wave velocities of vP = 114+43-20 m/s and vS = 60+11-7 m/s, respectively, from around 2,000 hammer stroke recordings. These velocities likely represent bulk estimates for the uppermost several 10's of cm of regolith. An analysis of the P-wave incidence angles provided an independent vP/vS ratio estimate of 1.84+0.89-0.35 that compares well with the traveltime based estimate of 1.92+0.52-0.28. The low seismic velocities are consistent with those observed for low-density unconsolidated sands and are in agreement with estimates obtained by other methods

Bedload Transport Measurements in Mountain Streams with the Swiss Plate Geophone System: Towards a General Calibration Procedure for Fractional Transport

Flood events across Europe in the summer of 2021 have illustrated the threat of flood-related hazards like bedload transport to human life and infrastructure, especially in small and steep mountainous catchments. Predicting bedload transport in higher gradient streams still represents a considerable challenge because of its large spatio-temporal variability. To cope with these uncertainties, intensive efforts were made to develop surrogate bedload monitoring technologies. The present thesis focuses on one such system, the Swiss plate geophone (SPG), which has been deployed and calibrated in numerous steep water courses, mainly in the Alps. In a first stage, we conducted field calibration measurements at three Swiss field sites recently equipped with a SPG system which are located at the Albula, the Navisence, and the Avançon de Nant streams. There, direct bedload samples were collected with a net sampler to calibrate the signal recorded by the geophones. Earlier studies have shown that the calibration relationships obtained from such campaigns can vary substantially between different monitoring stations, likely due to site-specific factors such as the flow velocity and the bed roughness. In a second stage, we attempted to improve our understanding of these disparities by performing full-scale controlled flume experiments at an outdoor flume facility. The mentioned field sites were replicated one after another in a 24 m-long flume on the basis of their morphological and hydraulic characteristics, and a part of the bedload samples collected in the field served as test material. Even though these flume experiments could not accurately reproduce field-based calibration relationships, they enabled us to relate variations in the SPG signal response to changes in the grain-size distribution of bedload mixtures. Furthermore, we showed that a geophone plate detects vibrations from impacts occurring either on a neighboring plate or on the surrounding concrete sill, which introduces a significant site-specific bias to field calibration relationships. Recent studies have reported combinations of amplitude and frequency information to infer particle sizes and improve the detectability of bedload particles using various surrogate monitoring techniques. Motivated by these findings, we developed in a last stage two amplitude-frequency-based methods aiming to identify these extraneous impacts and reduce their effect on site-specific calibration relationships. Using a field calibration dataset extended with measurements from the Erlenbach site, we showed that including frequency information in the calibration of the SPG signal results in more homogeneous signal responses across all sites and significantly improves the accuracy of fractional sediment flux and grain-size estimates. Thus, it was possible to develop a single general calibration relationship capable of providing fairly accurate bedload transport estimates at all the four investigated field sites. Optimizing the calibration procedure for each field site separately, resulted in a further increase of the accuracy of the predicted bedload transport rates

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

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.ISSN:2333-508

Comparison of calibration characteristics of different acoustic impact systems for measuring bedload transport in mountain streams

The Swiss plate geophone (SPG) system has been installed and tested in more than 20 steep gravel-bed streams and rivers, and related studies generally resulted in rather robust calibration relations between signal impulse counts and transported bedload mass. Here, we compare this system with three alternative surrogate measuring systems. A variant of the SPG system uses the same frame (housing) set-up but with an accelerometer instead of a geophone sensor to measure the vibrations of the plate (GP-Acc, for geophone plate accelerometer). The miniplate accelerometer (MPA) system has a smaller dimension of the impact plate and is embedded in more elastomer material than the SPG system. The Japanese pipe microphone (JPM) is a 1 m long version of the system that has been installed in many streams in Japan. To compare the performance of the four systems, we used calibration measurements with direct bedload samples from three field sites and an outdoor flume facility with controlled sediment feed. At our field sites, the systems with an accelerometer and a microphone showed partly large temporal variations in the background noise level, which may have impaired the calibration measurements obtained during certain time periods. Excluding these periods, the SPG, GP-Acc, and JPM all resulted in robust calibration relations, whereas the calibration of the MPA system showed a poorer performance at all sites.</p

Seismic High-Resolution Acquisition Electronics for the NASA InSight Mission on Mars

The Seismic Experiment for Interior Structures (SEIS) was deployed on Mars in November 2018 and began science operations in March 2019. SEIS is the primary instrument of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission, which was launched by the National Aeronautics and Space Administration (NASA). The acquisition and control (AC) electronics is a key element of SEIS. The AC acquires the seismic signals of the two sets of seismic sensors with high resolution, stores the data in its local nonvolatile memory for later transmission by the lander, and controls the numerous functions of SEIS. In this article, we present an overview of the AC with its connections to the sensors and to the lander, as well as its functionality. We describe the elements of the acquisition chains and filters, and discuss the performance of the seismic and temperature channels. Furthermore, we outline the safety functions and health monitoring, which are of paramount importance for reliable operation on Mars. In addition, we analyze an artefact affecting the seismic data referred to as the "tick-noise" and provide a method to remove this artefact by post-processing the data.ISSN:0037-1106ISSN:1943-357

In Situ Regolith Seismic Velocity Measurement at the InSight Landing Site on Mars

Interior exploration using Seismic Investigations, Geodesy and Heat Transport's (InSight) seismometer package Seismic Experiment for Interior Structure (SEIS) was placed on the surface of Mars at about 1.2 m distance from the thermal properties instrument Heat flow and Physical Properties Package (HP3) that includes a self-hammering probe. Recording the hammering noise with SEIS provided a unique opportunity to estimate the seismic wave velocities of the shallow regolith at the landing site. However, the value of studying the seismic signals of the hammering was only realized after critical hardware decisions were already taken. Furthermore, the design and nominal operation of both SEIS and HP3 are nonideal for such high-resolution seismic measurements. Therefore, a series of adaptations had to be implemented to operate the self-hammering probe as a controlled seismic source and SEIS as a high-frequency seismic receiver including the design of a high-precision timing and an innovative high-frequency sampling workflow. By interpreting the first-arriving seismic waves as a P-wave and identifying first-arriving S-waves by polarization analysis, we determined effective P- and S-wave velocities of vP = 119(+45)(-21) m/s and vS = 63(+11)(-7) m/s, respectively, from around 2,000 hammer stroke recordings. These velocities likely represent bulk estimates for the uppermost several 10s of cm of regolith. An analysis of the P-wave incidence angles provided an independent vP/vS ratio estimate of 1.84(+0.89)(-0.35) that compares well with the traveltime based estimate of 1.86(+0.42)(-0.25). The low seismic velocities are consistent with those observed for low-density unconsolidated sands and are in agreement with estimates obtained by other methods.ISSN:0148-0227ISSN:2169-909