Waveform Cross-correlation Research Articles

Bowhead whale localization using time-difference-of-arrival data from asynchronous recorders

This paper estimates bowhead whale locations and uncertainties using nonlinear Bayesian inversion of the time-difference-of-arrival (TDOA) of low-frequency whale calls recorded on onmi-directional asynchronous recorders in the shallow waters of the northeastern Chukchi Sea, Alaska. A Y-shaped cluster of seven autonomous ocean-bottom hydrophones, separated by 0.5-9.2 km, was deployed for several months over which time their clocks drifted out of synchronization. Hundreds of recorded whale calls are manually associated between recorders. The TDOA between hydrophone pairs are calculated from filtered waveform cross correlations and depend on the whale locations, hydrophone locations, relative recorder clock offsets, and effective waveguide sound speed. A nonlinear Bayesian inversion estimates all of these parameters and their uncertainties as well as data error statistics. The problem is highly nonlinear and a linearized inversion did not produce physically realistic results. Whale location uncertainties from nonlinear inversion can be low enough to allow accurate tracking of migrating whales that vocalize repeatedly over several minutes. Estimates of clock drift rates are obtained from inversions of TDOA data over two weeks and agree with corresponding estimates obtained from long-time averaged ambient noise cross correlations. The inversion is suitable for application to large data sets of manually or automatically detected whale calls.

GrowClust: A Hierarchical Clustering Algorithm for Relative Earthquake Relocation, with Application to the Spanish Springs and Sheldon, Nevada, Earthquake Sequences

Accurate earthquake locations are essential for providing reliable hazard assessments, understanding the physical mechanisms driving extended earthquake sequences, and interpreting fault structure. Techniques based on waveform cross correlation can significantly improve the precision of the relative locations of event pairs observed at a set of common stations. Here we describe GrowClust, an open‐source, relative relocation algorithm that can provide robust relocation results for earthquake sequences over a wide range of spatial and temporal scales. The method uses input differential travel times, cross‐correlation values, and reference starting locations, and applies a hybrid, hierarchical clustering algorithm to simultaneously group and relocate events within similar event clusters. The method is computationally efficient and numerically stable in its capacity to process large data sets and naturally applies greater weight to more similar event pairs. Additionally, it outputs location error estimates that can be used to help interpret the reliability and resolution of relocation results. As an example, we apply the GrowClust method to the recent Spanish Springs and Sheldon, Nevada, earthquake swarms. These sequences highlight the future potential for applying the GrowClust relocation method on a much larger scale within the region, where existing relocation results are sparse but vital for understanding the seismotectonics and seismic hazard of Nevada and eastern California.

THE APRIL 2007 SWARM IN TRICHONIS LAKE USING DATA FROM A MICROSEISMIC NETWORK

A series of strong earthquakes took place at the Trichonis lake area, during April 2007. Three events, with M~5.0 occurred within one day, causing damages to almost all nearby villages and especially at the small town of Thermo. The sequence lasted for more than one month with small to moderate size events. It was recorded by regional networks and results related to its time and space evolution have already been published. Just after the major events a microseismic network was deployed in the area, by the University of Patras, Seismology Laboratory. Eight stations were installed, with short period, three component sensors and portable digital recorders. The network was in operation for a period of one month and a lot of events were recorded by enough stations to provide a reliable location. We present here the analysis of these data using modern methodologies like waveform cross-correlation techniques and further relocation of the events using waveform based differential time. The results partially support the findings of the regional networks, as regards the gross characteristics of the aftershock sequence, but provide an enhanced description of it. The space time evolution of the aftershock sequence reveals the activation of more than one fault in the area and suggests the connection of the sequence with an unmapped fault.

Fully probabilistic seismic source inversion – Part 2: Modelling errors and station covariances

Abstract. Seismic source inversion, a central task in seismology, is concerned with the estimation of earthquake source parameters and their uncertainties. Estimating uncertainties is particularly challenging because source inversion is a non-linear problem. In a companion paper, Stähler and Sigloch (2014) developed a method of fully Bayesian inference for source parameters, based on measurements of waveform cross-correlation between broadband, teleseismic body-wave observations and their modelled counterparts. This approach yields not only depth and moment tensor estimates but also source time functions. A prerequisite for Bayesian inference is the proper characterisation of the noise afflicting the measurements, a problem we address here. We show that, for realistic broadband body-wave seismograms, the systematic error due to an incomplete physical model affects waveform misfits more strongly than random, ambient background noise. In this situation, the waveform cross-correlation coefficient CC, or rather its decorrelation D = 1 − CC, performs more robustly as a misfit criterion than ℓp norms, more commonly used as sample-by-sample measures of misfit based on distances between individual time samples. From a set of over 900 user-supervised, deterministic earthquake source solutions treated as a quality-controlled reference, we derive the noise distribution on signal decorrelation D = 1 − CC of the broadband seismogram fits between observed and modelled waveforms. The noise on D is found to approximately follow a log-normal distribution, a fortunate fact that readily accommodates the formulation of an empirical likelihood function for D for our multivariate problem. The first and second moments of this multivariate distribution are shown to depend mostly on the signal-to-noise ratio (SNR) of the CC measurements and on the back-azimuthal distances of seismic stations. By identifying and quantifying this likelihood function, we make D and thus waveform cross-correlation measurements usable for fully probabilistic sampling strategies, in source inversion and related applications such as seismic tomography.

PQC: A novel approach for robust automatic near-surface analysis in low-relief geology

A novel approach, named pQC, was developed to automatically derive near-surface velocities and statics by applying surface-consistent analysis to refraction data, where the data consist of first-break (FB) traveltimes and early-arrival waveforms. This approach is facilitated by the introduction of a new sorting domain in which FB traveltimes and traces are organized in a multidimensional space consisting of a common-midpoint (CMP) location, offset, and azimuth, called the XYOA domain. The FB traveltime data sorted in this domain can be analyzed efficiently with statistical methods to obtain localized average depth domain velocity trends for calculation of long-wavelength statics. Short-wavelength (residual) refraction statics are then calculated based on the cross correlation of early-arrival waveforms in the XYOA gathers over multiple offset classes and inverted for surface-consistent source and receiver corrections. The application of the combined statics solution to the traces enhances the continuity and power of the stacked reflected events without the need to perform tomography. Surface-consistent analysis of refraction data in the XYOA domain is fully automatic, provides robust solutions for noisy data, and enhances subsequent reflection residual statics calculations. This method can also be exploited for quality-control analysis of FB picks and for automating, in an iterative procedure, the picking of large 3D seismic data sets. The method is demonstrated on a 3D land survey over complex geology where the novel refraction surface-consistent solution enhances the imaging results and employs a fraction of the time needed by a typical tomographic workflow.

An Assessment of the Precision and Accuracy of Altimetry Retrievals for a Monterey Bay GNSS-R Experiment

Global navigation satellite systems (GNSS) provide signals of opportunity for bistatic radar remote sensing, called GNSS reflectometry (GNSS-R), that have potential to be used for ocean altimetry. These signals have advantages over traditional mono-static radar that include reduced cost, high density of measurements in time and space, and an inherent reference to a highly accurate time-space frame. Here, we examine GNSS-R data collected from an aircraft flying over Monterey Bay, California. A downward-looking dual-frequency left-hand circularly polarized patch antenna recorded reflected signals. An upward-looking commercial antenna recorded the direct signals. Dual-frequency carrier phase data from this antenna were also used to produce precise coordinates for the aircraft. The L1 P-code GPS data were collected over four days with two flights per day and consist of nine transects covering a 1 deg× 1 deg grid. The performance of three timing retrieval algorithms has been evaluated based on measurement precision. From the observed cross-correlation waveform, the specular reflection timing was derived from the delay of the 70% peak correlation power (HALF method), the waveform leading edge peak first derivative (DER method), or the delay associated with a best fit function approximating the nominal waveform shape (PARA3 method). It was found that the HALF method produced the most precise measurements for a 5 s integration time with a standard deviation of σ = 0.6 m. The measurement accuracy is characterized by comparison with well-established models including neutral atmospheric delay, mean sea surface height, and ocean and solid Earth tides. Biases on the order of 1-4 m are observed with respect to a modeled mean sea surface and between each flight. However, the measurements are shown to track changes in sea surface height along the ground track to within 0.6 m.

Seismicity along St. Lawrence Paleorift Faults Overprinted by a Meteorite Impact Structure in Charlevoix, Québec, Eastern Canada

Abstract The Charlevoix seismic zone (CSZ) in Quebec has the highest number of earthquakes in eastern Canada, with cataloged events diffusely distributed between the St. Lawrence paleorift faults. In this study, we relocate 1242 out of 1639 earthquakes in the CSZ from 1988 to 2010 using double‐difference relocations with phase arrivals from manual picks and waveform cross correlations and requiring a strict set of quality control criteria. Our results show a diffuse distribution of earthquakes beneath the St. Lawrence River within a meteorite impact structure and clear lineations of event hypocenters outside the inner boundary of the impact structure. The diffuse seismicity distribution coupled with a lower cumulative seismic moment within the impact structure, despite a higher number of events, suggests the presence of highly fractured crust and weakened pre‐existing faults. A lack of M N ≥4.0 earthquakes and a wide variation of focal mechanism P‐axis orientations also provide evidence for weakened pre‐existing faults. Relocated earthquakes beneath the north shore of the St. Lawrence River illuminate a broad southeast‐trending linear structure, suggesting the geometry of the Gouffre northwest River fault. A paucity of seismicity between the north shore and the river may correspond to fault‐controlled basins of unconsolidated sediments at the surface and volumes of high‐seismic‐velocity material at midcrustal depths. Online Material: Earthquake catalogs.

Optimizing Event Detection and Location in Low‐Seismicity Zones: Case Study from Western Switzerland

Obtaining robust event catalogs in regions of low seismicity can be time‐consuming, because quality events are less frequent and sensor coverage is generally sparse. Optimizing event detection and location in such regions is all the more crucial because these areas tend to host a higher density of sensitive infrastructures. The methodology proposed consists of reprocessing existing data recorded by a permanent network and boosting the final catalog resolution by temporarily deploying portable sparse mini‐arrays in the target area. Sonogram analysis is applied on both existing and new datasets to detect waveforms barely emerging from the background noise. A visual interactive event analysis module is used to test for phase picking, event association, waveform cross correlation, and location ambiguities. It also estimates back azimuth and slowness when sparse array data are available. The method is applied to a low‐seismicity region in the western Swiss Molasse basin where two sparse mini‐arrays were temporarily deployed. The detection of earthquakes is improved by a factor of 9 when reprocessing four yrs (2009–2013) of available data recorded by two accelerometers and one broadband station in a 2500 km2 target area. Magnitude estimations are empirically calibrated over four magnitude units, down to −1.7 M L, lowering the existing catalog completeness by close to one magnitude unit. After validating picking and location accuracies with a standard residual‐based scheme, 174 newly detected events are relocated, illuminating zones of previously undetected microseismic activity. Online Material: Earthquake catalog FRICAT.

Detailed velocity ratio mapping during the aftershock sequence as a tool to monitor the fluid activity within the fault plane

The rheological properties of Earth materials are expressed by their seismic velocities and VP/VS ratio, which is easily obtained by the Wadati method. Its double-difference version based on cross-correlated waveforms enables focusing on very local structures and allows tracking, monitoring and analysing the fluid activity along faults. We applied the method to three 2014 mainshock–aftershock sequences in the West Bohemia/Vogtland (Czech Republic) earthquake swarm area and found pronounced VP/VS variations in time and space for different clusters of events located on a steeply dipping fault zone at depths ranging from 7 to 11 km. Each cluster reflects the spatial distribution of earthquakes along the fault plane but also the temporal evolution of the activity. Low values of VP/VS ratio down to 1.59±0.02 were identified in the deeper part of the fault zone whereas higher values up to 1.73±0.01 were estimated for clusters located on a shallower segment of the fault. Temporally the low VP/VS values are associated with the early aftershocks, while the higher VP/VS ratios are related only to later aftershocks. We interpret this behaviour as a result of saturation of the focal zone by compressible fluids: in the beginning the mainshock and early aftershocks driven by over-pressured fluids increased the porosity due to opening the fluid pathways. This process was associated with a decrease of the velocity ratio. In later stages the pressure and porosity decreased and the velocity ratio recovered to levels of 1.73, typical for a Poissonian medium and Earth's crust.

The Aftershock Sequence of the 2008 Achaia, Greece, Earthquake: Joint Analysis of Seismicity Relocation and Persistent Scatterers Interferometry

On 8 June 2008 an earthquake of Mw6.4 took place in the northwestern part of Peloponnese, Greece. The main shock was felt in a wide area and caused appreciable damage along the main rupture area and particularly at the antipodal of the main shock epicenter fault edge, implying strongly unilateral rupture and stopping phase effects. Abundant aftershocks were recorded within an area of ~50 km in length in the period 8 June 2008–end of 2014, by a sufficient number of stations that secure location accuracy because the regional network is adequately dense in the area. All the available phases from seismological stations in epicentral distances up to 140 km until the end of 2014 were used for relocation with the double difference technique and waveform cross-correlation. A quite clear 3-D representation is obtained for the aftershock zone geometry and dimensions, revealing the main rupture and the activated adjacent fault segments. SAR data are processed using Stanford Method for Persistent Scatterers (StaMPS) and a surface deformation map constructed based on PS point displacement for the coseismic period. A variable slip model, with maximum slip of ~1.0 m located at the lower part of the rupture plane, is suggested and used for calculating the deformation field which was found in adequate agreement with geodetic measurements. With the same slip model the static stress changes were calculated evidencing possible triggering of the neighboring faults that were brought closer to failure. The data availability allowed monitoring the temporal variation of b values that after a continuous increase in the first 5 days, returned and stabilized to 1.0–1.1 in the following years. The fluctuation duration is considered as the equivalent time for fault healing, which appeared very short but in full accordance with the cessation of onto-fault seismicity.

Towards global seismic monitoring of underground nuclear explosions using waveform cross correlation. Part II: Synthetic master events

Towards global seismic monitoring of underground nuclear explosions using waveform cross correlation. Part II: Synthetic master events

Fully automated near-surface analysis by surface-consistent refraction method

Industry practices for near-surface analysis indicate difficulties in coping with the increased number of channels in seismic acquisition systems, and new approaches are needed to fully exploit the resolution embedded in modern seismic data sets. To achieve this goal, we have developed a novel surface-consistent refraction analysis method for low-relief geology to automatically derive near-surface corrections for seismic data processing. The method uses concepts from surface-consistent analysis applied to refracted arrivals. The key aspects of the method consist of the use of common midpoint (CMP)-offset-azimuth binning, evaluation of mean traveltime and standard deviation for each bin, rejection of anomalous first-break (FB) picks, derivation of CMP-based traveltime-offset functions, conversion to velocity-depth functions, evaluation of long-wavelength statics, and calculation of surface-consistent residual statics through waveform crosscorrelation. Residual time lags are evaluated in multiple CMP-offset-azimuth bins by crosscorrelating a pilot trace with all the other traces in the gather in which the correlation window is centered at the refracted arrival. The residuals are then used to build a system of linear equations that is simultaneously inverted for surface-consistent shot and receiver time shift corrections plus a possible subsurface residual term. All the steps are completely automated and require a fraction of the time needed for conventional near-surface analysis. The developed methodology was successfully performed on a complex 3D land data set from Central Saudi Arabia where it was benchmarked against a conventional tomographic work flow. The results indicate that the new surface-consistent refraction statics method enhances seismic imaging especially in portions of the survey dominated by noise.

A large refined catalog of earthquake relocations and focal mechanisms for the Island of Hawai'i and its seismotectonic implications

AbstractWe present high‐quality focal mechanisms based on a refined earthquake location catalog for the Island of Hawai'i, focusing on Mauna Loa and Kīlauea volcanoes. The relocation catalog is based on first‐arrival times and waveform data of both compressional and shear waves for about 180,000 events on and near the Island of Hawai'i between 1986 and 2009 recorded by the seismic stations at the Hawaiian Volcano Observatory. We relocate all the earthquakes by applying ray tracing through an existing three‐dimensional velocity model, similar event cluster analysis, and a differential‐time relocation method. The resulting location catalog represents an expansion of previous relocation studies, covering a longer time period and consisting of more events with well‐constrained absolute locations. The focal mechanisms are obtained based on the compressional‐wave first‐motion polarities and compressional‐to‐shear wave amplitude ratios by applying the HASH program to the waveform cross correlation relocated earthquakes. Overall, the good‐quality (defined by the HASH parameters) focal solutions are dominated by normal faulting in our study area, especially in the active Ka'ōiki and Hīlea seismic zones. Kīlauea caldera is characterized by a mixture of approximately equal numbers of normal, strike‐slip, and reverse faults, whereas its south flank has slightly fewer strike‐slip events. Our relocation and focal mechanism results will be useful for mapping the seismic stress and strain fields and for understanding the seismic‐volcanic‐tectonic relationships within the magmatic systems.

Calibrating one-dimensional velocity model for downhole microseismic monitoring using station-pair differential arrival times based on the differential evolution method

For microseismic monitoring, in order to obtain accurate microseismic locations, a good estimation of the velocity model for the region covering the monitoring stations and microseismic sources is crucial. Perforation shots are generally used to estimate a velocity model suitable for microseismic location. However, the origin times of the perforation shots are generally not accurate. To mitigate the effect of inaccurate origin times of perforation shots on calibrating the velocity model, we propose to search for a velocity model fitting for station-pair differential arrival times instead of absolute arrival times from perforation shots. Another advantage of using station-pair differential arrival times is that waveform cross-correlation can be used to estimate more accurate differential times because of waveform similarity among stations for perforation shots. Due to high nonlinearity of the objective function for estimating one-dimensional velocity model, the differential evolution (DE) method for solving high-dimensional global optimization problems is utilized in the optimization. Compared to the grid-search method, the DE method is much more efficient. Synthetic tests based on a downhole microseismic monitoring system show the effectiveness of the proposed method to recover the velocity model. We also test our DE-based method by using perforation shots for a real microseismic monitoring project. Compared to the well sonic velocity model, station-pair differential arrival times are better fitted and perforation shots are also more accurately relocated with the calibrated velocity model.

Evidence for non-self-similarity of microearthquakes recorded at a Taiwan borehole seismometer array

We investigate the relationship between seismic moment M_0 and source duration t_w of microearthquakes by using high-quality seismic data recorded with a vertical borehole array installed in central Taiwan. We apply a waveform cross-correlation method to the three-component records and identify several event clusters with high waveform similarity, with event magnitudes ranging from 0.3 to 2.0. Three clusters—Clusters A, B and C—contain 11, 8 and 6 events with similar waveforms, respectively. To determine how M_0 scales with t_w, we remove path effects by using a path-averaged Q. The results indicate a nearly constant t_w for events within each cluster, regardless of M_0, with mean values of t_w being 0.058, 0.056 and 0.034 s for Clusters A, B and C, respectively. Constant t_w, independent of M_0, violates the commonly used scaling relation t_w ∝ M^(1/3)_0. This constant duration may arise either because all events in a cluster are hosted on the same isolated seismogenic patch, or because the events are driven by external factors of constant duration, such as fluid injections into the fault zone. It may also be related to the earthquake nucleation size.

Fault Reactivation Analysis Using Microearthquake Clustering Based on Signal-to-Noise Weighted Waveform Similarity

The cluster formation of about 2000 induced microearthquakes (mostly M L < 2) is studied using a waveform similarity technique based on cross-correlation and a subsequent equivalence class approach. All events were detected within two separated but neighbouring seismic volumes close to the geothermal powerplants near Landau and Insheim in the Upper Rhine Graben, SW Germany between 2006 and 2013. Besides different sensors, sampling rates and individual data gaps, mainly low signal-to-noise ratios (SNR) of the recordings at most station sites provide a complication for the determination of a precise waveform similarity analysis of the microseismic events in this area. To include a large number of events for such an analysis, a newly developed weighting approach was implemented in the waveform similarity analysis which directly considers the individual SNRs across the whole seismic network. The application to both seismic volumes leads to event clusters with high waveform similarities within short (seconds to hours) and long (months to years) time periods covering two magnitude ranges. The estimated relative hypocenter locations are spatially concentrated for each single cluster and mirror the orientations of mapped faults as well as interpreted rupture planes determined from fault plane solutions. Depending on the waveform cross-correlation coefficient threshold, clusters can be resolved in space to as little as one dominant wavelength. The interpretation of these observations implies recurring fault reactivations by fluid injection with very similar faulting mechanisms during different time periods between 2006 and 2013.

Time-difference-of-arrival localization of bowhead whales using asynchronous recorders

This paper estimates bowhead whale locations and uncertainties using linearized Bayesian inversion of the time-difference-of-arrival (TDOA) of whale calls recorded on omni-directional asynchronous recorders in the Chukchi Sea, Alaska. A Y-shaped cluster of seven autonomous ocean-bottom hydrophones, separated by 0.5–9.2 km, was deployed for several months over which time their clocks drifted out of synchronization. Hundreds of recorded whale calls are manually annotated with time-frequency bounds and associated between recorders. The TDOA between all hydrophone pairs are calculated from filtered waveform cross-correlations and depend on the whale locations, hydrophone locations, relative recorder clock drifts, and an effective waveguide sound speed. The inversion estimates all of these parameters and their uncertainties as well as data error statistics using prior information to constrain the otherwise underdetermined problem. Whale location uncertainties are estimated to be approximately 100 m which allow...

Improving the location of induced earthquakes associated with an underground gas storage in the Gulf of Valencia (Spain)

On September 2013, increased seismic activity was recorded near the CASTOR offshore underground gas storage (UGS), in the Gulf of Valencia (Spain). According to the reports by the Spanish Instituto Geográfico Nacional (IGN), more than 550 events occurred during two months, the strongest having a magnitude of Mw=4.2 which took place two weeks after the gas injection stopped. The low magnitude of the events (with only 17 earthquakes having mbLg greater than 3), the lack of nearby stations, and the inhomogeneous station distribution made the location problem a great challenge. Here we present improved locations for a subset of 161 well recorded events from the earthquake sequence using a probabilistic nonlinear earthquake location method. A new 3-D shear-wave velocity model is also estimated in this work from surface-wave ambient noise tomography. To further improve the locations, waveform cross-correlations are computed at each station for every event pair and new locations are obtained from an inverted set of adjusted travel time picks. The resulting hypocentral solutions show a tighter clustering with respect to the initial locations and they are distributed in a NW–SE direction. Most of the earthquakes are located near the injection well at depths of about 6km. Our results indicate that the observed seismicity is closely associated with the injection activities at the CASTOR underground gas storage and may have resulted from the reactivation of pre-existing unmapped faults, located a few kilometers below the reservoir.

Towards global seismic monitoring of underground nuclear explosions using waveform cross correlation. Part I: Grand master events

Waveform cross correlation is an efficient tool for detection and characterization of seismic signals. For the purposes of the Comprehensive Nuclear-Test-Ban Treaty, cross correlation can globally reduce the threshold of detection by 0.3 to 0.4 magnitude units. However, the technique critically depends on the availability of master events. In Part I of this paper, we have demonstrated that in seismically active regions the best master events (grand-masters) replicated over a regular grid allow improving the efficiency of signal detection and event finding. In aseismic areas, there are two approaches to populate the global grid of master events for the International Monitoring System: the replication of grand-masters and calculation of synthetic seismograms for master-events in the global grid nodes. The efficiency of synthetic templates depends on the accuracy of shape and amplitude predictions controlled by focal depth and mechanism, source function, velocity structure and attenuation along the master/station path. Here we test three focal mechanisms (explosion, thrust fault, and actual Harvard CMT solution for one of the April 11, 2012 Sumatra aftershocks) and two velocity structures (ak135 and CRUST 2.0). Sixteen synthetic master events were distributed over a 1° × 1° grid covering the zone of aftershocks. We built five cross correlation standard event lists (XSEL) and compared detections and events with those built using the real and grand master events as well as with the Reviewed Events Bulletin of the International Data Centre. The XSELs were built using an explosion source and ak135 and the reverse fault with isotropic radiation pattern to demonstrate the performance similar to that of the real and grand masters. Here we have proved quantitatively that it is possible to cover all aseismic areas with synthetic masters without significant loss in seismic monitoring capabilities based on cross correlation.

Application of a three-component seismic array to improve the detection efficiency of seismic events by the matched filter method

It is found that the best results, in terms of the efficiency of detecting regional seismic phases from blast sources, are achieved by using the method of waveform cross correlation applied to the data from a small-aperture array of three-component sensors