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

Model‐based optimization of source locations for 3D acoustic seismic full‐waveform inversion

Besides classical imaging techniques, full-waveform inversion is an increasingly popular method to derive elastic subsurface properties from seismic data. High-resolution velocity models can be obtained, and spatial sampling criteria are less strict than for imaging methods, because the entire information content of the seismic waveforms is used. As high operational costs arise from seismic surveys, the acquirable data volume is often limited by economic criteria. By selecting optimal locations for seismic sources, the information content of the data can be maximized, and the number of sources and thus the acquisition costs can be reduced compared with standard acquisition designs. The computation of such optimized designs for large-size 3D inverse problems at affordable computational cost is challenging. By using a sequential receiver-wise optimization strategy, we substantially reduce the computational requirements of the optimization process. We prove the applicability of this method by means of numerical 3D acoustic examples. Optimized source designs for different receiver patterns are computed for a realistic subsurface model, and the value of the designs is evaluated by comparing checkerboard inversion tests with different acquisition designs. Our examples show that inversion results with higher accuracy can be obtained with the optimized designs, regardless of the number of sources, the number of receivers, or the receiver distribution. Larger benefits of the optimized designs are visible when a sparse receiver geometry is used.ISSN:0016-8025ISSN:1365-247

Estimation of the Primary PS Wave Response in OBC Data Using the Receiver Function Approach

P195 Estimation of the Primary PS Wave Response in OBC Data Using the Receiver Function Approach P. Edme* (Schlumberger Cambridge Research (formerly at the Institut de Physique du Globe de Paris-IPGP)) & S.C. Singh (Institut de Physique du Globe de Paris-IPGP) SUMMARY Multi-component recordings provide informations on both the P and S wave structure of the subsurface but most of the methods are developed for the P waves. It is necessary to develop new techniques to isolate the PS wavefield from mixed components. In OBC data it is also necessary to remove the water multiples to improve the sub-seafloor image ..

Estimation of the Primary PS Wave Response in OBC Data Using the Receiver Function Approach

P195 Estimation of the Primary PS Wave Response in OBC Data Using the Receiver Function Approach P. Edme* (Schlumberger Cambridge Research (formerly at the Institut de Physique du Globe de Paris-IPGP)) & S.C. Singh (Institut de Physique du Globe de Paris-IPGP) SUMMARY Multi-component recordings provide informations on both the P and S wave structure of the subsurface but most of the methods are developed for the P waves. It is necessary to develop new techniques to isolate the PS wavefield from mixed components. In OBC data it is also necessary to remove the water multiples to improve the sub-seafloor image ..

Efficient wave type fingerprinting and filtering by six-component polarization analysis

We present a technique to automatically classify the wave type of seismic phases that are recorded on a single six-component recording station (measuring both three components of translational and rotational ground motion) at the Earth's surface. We make use of the fact that each wave type leaves a unique 'fingerprint' in the six-component motion of the sensor (i.e. the motion is unique for each wave type). This fingerprint can be extracted by performing an eigenanalysis of the data covariance matrix, similar to conventional three-component polarization analysis. To assign a wave type to the fingerprint extracted from the data, we compare it to analytically derived six-component polarization models that are valid for pure-state plane wave arrivals. For efficient classification, we make use of the supervised machine learning method of support vector machines that is trained using data-independent, analytically derived six-component polarization models. This enables the rapid classification of seismic phases in a fully automated fashion, even for large data volumes, such as encountered in land-seismic exploration or ambient noise seismology. Once the wave-type is known, additional wave parameters (velocity, directionality and ellipticity) can be directly extracted from the six-component polarization states without the need to resort to expensive optimization algorithms. We illustrate the benefits of our approach on various real and synthetic data examples for applications such as automated phase picking, aliased ground-roll suppression in land-seismic exploration and the rapid close-to real-time extraction of surface wave dispersion curves from single-station recordings of ambient noise. Additionally, we argue that an initial step of wave type classification is necessary in order to successfully apply the common technique of extracting phase velocities from combined measurements of rotational and translational motion.ISSN:0956-540XISSN:1365-246

Wavefield divergence via hydrophone measurement on land

ISSN:1029-7006ISSN:1607-796

Urban Distributed Acoustic Sensing Using In-Situ Fibre Beneath Bern, Switzerland

Anticipating the risks natural hazards pose to an urban environment requires an understanding of the shallow Earth structure of the region. While urban infrastructure often hinders the deployment of a traditional seismic array, Distributed Acoustic Sensing (DAS) technology facilitates the use of existing telecommunication fibre-optic cables for seismic observation, with spatial resolution down to the metre scale. Through collaboration with the SWITCH foundation, we were able to use existing, in-situ fibres beneath Bern, Switzerland for seismic data acquisition over two weeks, covering a distance of 6 km with a spatial resolution of 2 m. This allowed for not only real-time visualisation of anthropogenic noise sources (e.g. road traffic), but also of the propagation of resulting seismic waves. Data is analysed in the time and frequency domain to explore the range of signals captured and to assess the consistency of data quality along the cable. The local velocity structure can be constrained using both noise correlations and deterministic signals excited by traffic. Initial results reveal the ability of DAS to capture signals over a wide range of frequencies and distances, and show promise for utilising urban DAS data to perform urban seismic tomography and hazard analysis

Empirical investigations of the instrument response for distributed acoustic sensing (DAS) across 17 octaves

With the potential of high temporal and spatial sampling and the capability of utilizing existing fiber‐optic infrastructure, distributed acoustic sensing (DAS) is in the process of revolutionizing geophysical ground‐motion measurements, especially in remote and urban areas, where conventional seismic networks may be difficult to deploy. Yet, for DAS to become an established method, we must ensure that accurate amplitude and phase information can be obtained. Furthermore, as DAS is spreading into many different application domains, we need to understand the extent to which the instrument response depends on the local environmental properties. Based on recent DAS response research, we present a general workflow to empirically quantify the quality of DAS measurements based on the transfer function between true ground motion and observed DAS waveforms. With a variety of DAS data and reference measurements, we adapt existing instrument‐response workflows typically in the frequency band from 0.01 to 10 Hz to different experiments, with signal frequencies ranging from 1/3000 to 60 Hz. These experiments include earthquake recordings in an underground rock laboratory, hydraulic injection experiments in granite, active seismics in agricultural soil, and icequake recordings in snow on a glacier. The results show that the average standard deviations of both amplitude and phase responses within the analyzed frequency ranges are in the order of 4 dB and 0.167π radians, respectively, among all experiments. Possible explanations for variations in the instrument responses include the violation of the assumption of constant phase velocities within the workflow due to dispersion and incorrect ground‐motion observations from reference measurements. The results encourage further integration of DAS‐based strain measurements into methods that exploit complete waveforms and not merely travel times, such as full‐waveform inversion. Ultimately, our developments are intended to provide a quantitative assessment of site‐ and frequency‐dependent DAS data that may help establish best practices for upcoming DAS surveys.ISSN:0037-1106ISSN:1943-357