Distributed acoustic sensing for seismic activity monitoring

Continuous, real-time monitoring of surface seismic activity around the globe is of great interest for acquiring new insight into global tomography analyses and for recognition of seismic patterns leading to potentially hazardous situations. The already-existing telecommunication fiber optic network arises as an ideal solution for this application, owing to its ubiquity and the capacity of optical fibers to perform distributed, highly sensitive monitoring of vibrations at relatively low cost (ultra-high density of point sensors available with minimal deployment of new equipment). This perspective article discusses early approaches on the application of fiber-optic distributed acoustic sensors (DASs) for seismic activity monitoring. The benefits and potential impact of DAS technology in these kinds of applications are here illustrated with new experimental results on teleseism monitoring based on a specific approach: the so-called chirped-pulse DAS. This technology offers promising prospects for the field of seismic tomography due to its appealing properties in terms of simplicity, consistent sensitivity across sensing channels, and robustness. Furthermore, we also report on several signal processing techniques readily applicable to chirped-pulse DAS recordings for extracting relevant seismic information from ambient acoustic noise. The outcome presented here may serve as a foundation for a novel conception for ubiquitous seismic monitoring with minimal investment

Distributed acoustic sensing for seismic activity monitoring

Continuous, real-time monitoring of surface seismic activity around the globe is of great interest for acquiring new insight into global tomography analyses and for recognition of seismic patterns leading to potentially hazardous situations. The already-existing telecommunication fiber optic network arises as an ideal solution for this application, owing to its ubiquity and the capacity of optical fibers to perform distributed, highly sensitive monitoring of vibrations at relatively low cost (ultra-high density of point sensors available with minimal deployment of new equipment). This perspective article discusses early approaches on the application of fiber-optic distributed acoustic sensors (DASs) for seismic activity monitoring. The benefits and potential impact of DAS technology in these kinds of applications are here illustrated with new experimental results on teleseism monitoring based on a specific approach: the so-called chirped-pulse DAS. This technology offers promising prospects for the field of seismic tomography due to its appealing properties in terms of simplicity, consistent sensitivity across sensing channels, and robustness. Furthermore, we also report on several signal processing techniques readily applicable to chirped-pulse DAS recordings for extracting relevant seismic information from ambient acoustic noise. The outcome presented here may serve as a foundation for a novel conception for ubiquitous seismic monitoring with minimal investment

Rotational Motions in Seismology

The seismic waves that spread out from the earthquake source to the entire Earth are usually measured at the ground surface by a seismometer which consists of three orthogonal components (Z (vertical), N (north-south), and E (east-west) or R (radial), T (transversal), and Z (vertical)). However, a complete representation of the ground motion induced by earthquakes consists not only of those three components of translational motion, but also three components of rotational motion plus six components of strain. Altough theoretical seismologists have pointed out the potential benefits of measurements of rotational ground motion, they were not made until quite recently. This was mainly because precise instruments to measure ground rotational motion were not available. The measurement of rotational motion induced by earthquakes is relatively new in the field of seismology. To the best of our knowledge, the first experiment to measure ground rotational motion using rotational sensor was done by Nigbor (1994}. He successfully measured translational and rotational ground motion during an underground chemical explosion experiment at the Nevada Test Site using a triaxial translational accelerometer and a solid-state rotational velocity sensor. The same type of sensor was also used by Takeo (1998} for recording an earthquake swarm on Izu peninsula, Japan. However, because of the limitation of the instrument sensitivity, this kind of sensor was only able to sensing the rotational ground motion near the earthquake sources of other artificial sources. Another type of rotational sensor was assembled using two oppositely oriented seismometers. This is possible since in principle the rotational component of the ground motions is equal to half the curl of the ground velocity. This kind of sensor was intensively researched and developed by the seismology group in Institute of geophysics, Polish Academy of Sciences. However, they report several problems especially due to the small differences in the seismometer's response function. Like the solid state rotational sensors, this sensor was only able to measure rotational motion near the seismic sources. The application of the Sagnac effect for sensing the inertial rotation using optical devices were intensively investigated, since the advent of lasers in the sixties. However, the first application of a ring laser gyroscope as a rotational sensor applied in the field of seismology was reported by Stedman et al. (1995}. Fully consistent rotational motions were recorded by a ring laser gyro installed at the fundamental station Wettzell, Germany (Igel et al., 2005). They showed that the rotational motions were compatible with collocated recordings of transverse acceleration by a standard seismometer, both in amplitude and phase. They mentioned that "standard" rotational sensors with sufficient resolution may be possible in the near future. Among the other type of rotational sensor, ring lasers seem more reliable in seismic applications since it has been provenable to sensing the ground rotational motion from near source as well as teleseismic earthquake events with a broad magnitude range (Igel et al., 2007}. In earthquake engineering, observations of rotational components of seismic strong motions may be of interest as this type of motion may contribute to the response of structures to earthquake-induced ground shaking. Most of rotational/torsional studies of ground motion in earthquake engineering are so far still carried out by indirect measurements. It can be done since the rotational component of motion is a linear combination of the space derivatives of the horizontal component of the motion. However, to the best of our knowledge, there are no comparison of array-derived rotation rate and direct measurement from rotational sensors mentioned in the literature. The first objective of my thesis is to study the effect of noise and various uncertainties to the derivation of rotation rate and to compare directly the result with the ring laser data. Here we present for the first time a comparison of rotational ground motions derived from seismic array with those observed directly with ring laser. Our study suggest that - given accurate measurements of translational motions in an array of appropriate size and number of stations - the array-derived rotation rate may be very close to the "true" rotational signal that would be measured at the center of the array (or the specific reference station). However, it is important to note that it may be dangerous to use only the minimally required three stations as even relatively small noise levels may deteriorate the rotation estimates. Furthermore, it is clear that the logistic effort to determine rotations from array is considerably larger than direct measurements. In the light of this, the necessity to develop field-deployable rotational sensors with the appropriate resolution for use in local and regional seismology remains an outstanding issue. More recently, Igel et al. (2005) introduced a method to estimate the horizontal phase velocity by using collocated measurements from a ring laser and seismometer. A simple relationship between transverse acceleration and rotation rate (around a vertical axis) shows that both signals should be in phase and their ratio proportional to horizontal phase velocity. Comparison with synthetic traces (rotations and translations) and phase velocities determined in the same way showed good agreement with the observations. The second objective of my thesis is to study the accuracy of phase velocity determination using collocated measurement of rotational and translational motion and derive the Love wave dispersion curve using spectral ratio for both synthetic and real observed data. Whether the accuracy of the dispersion curves derived with the approach presented in this thesis is enough for tomographic purposes remains to be evaluated. Nevertheless, the results shown here indicate that through additional measurements of accurate rotational signals, wavefield information is accessible that otherwise requires seismic array data. However, to make this methodology practically useful for seismology will require the development of an appropriate high-resolution six-component broadband sensor. Efforts are underway to coordinate such developments on an international scale (Evans et al., 2006). The ground tilt is generally small but not negligible in seismology, especially in the strong-motion earthquake. It is well known that the tilt signal is most noticeable in the horizontal components of the seismometer. Ignoring the tilt effects leads to unreliable results, especially in calculation of permanent displacements and long-period calculations. The third objective of my thesis is to study the array-derived tilt, a further application of measuring tilt. An interesting result concerning tilt study based on a synthetic study is the possibility to derive the Rayleigh wave phase velocity as well as Rayleigh wave dispersion curve from collocated measurement of tilt rate and translational motions. The synthetic study shows that there is a frequency dependent phase velocity from collocated radial acceleration and transverse tilt

Rotational Motions in Seismology: Theory, Observation, Modeling

Theoretically, to fully describe the change in the medium around a point one needs three components of translation, six components of strain, and three components of rotation. It is expected that collocated measurements of translations and rotations may help (1) correcting translation signals recorded by classical seismometers for contamination by ground rotations, (2) extracting additional information on earthquake source properties, soil-structure interactions, and properties of the subsurface, and (3) providing additional ground motion information to earthquake engineers for seismic design. Thus, in addition to translations and strains, the rotational part of ground motions should also be recorded. However, the lack of instrumental sensitivity did not allow seismologists to observe rotational motions for decades. Recently, ring laser technology has provided the means to develop instruments that allow in principle the observation of rotational motions in a wide frequency band and epicentral distance range. Here we present the observations of rotational ground motions around a vertical axis in the P coda (the section between the onsets of direct P- and S- waves) of tele-seismic signals on a ring laser sensor at the Fundamental Observatory Wettzell, southeast Germany. The studies focus on finding the explanation for the observed P coda rotations as well as the way to extract additional information from the use of co-seismic rotational motions. First, the effects of co-seismic tilts on ring laser measurements are quantified based on magnitude-amplitude relations and translation derived tilts. Then the phenomenon of scattering assuming three dimensional random media and topography that may generate the observed P coda rotations is investigated through analysis of observations and forward modelling. The partitioning of P and S energy indicated by the stabilization of the ratio of energies of the two is used to constrain scattering properties. Finally, an analytical approach focusing on the solution of plane waves in linear elastic anisotropic media is used to quantify the anisotropic behavior through the variations of rotational wavefield. The focus is on quasi-P waves and transverse isotropic media. Kelvin-Christoffel equation and the Thomson parameters, descriptive of the degree of anisotropy, are used. The obtained results show that 1) P-SH scattering in the random crust is the main cause of the P-coda observations; and 2) rotational motions contain additional information (at least) about scattering properties and anisotropic coefficients and that joint measurements of translational and rotational motions at only one point allow the extraction of additional information. The results demonstrate the potential benefit not only of measurements of co-seismic rotational ground motions but also of the use of the amplitude content of seismic signals

Long-range fiber-optic earthquake sensing by active phase noise cancellation

We present a long-range fiber-optic environmental deformation sensor based on active phase noise cancellation (PNC) in metrological frequency dissemination. PNC sensing exploits recordings of a compensation frequency that is commonly discarded. Without the need for dedicated measurement devices, it operates synchronously with metrological services, suggesting that existing phase-stabilized metrological networks can be co-used effortlessly as environmental sensors. The compatibility of PNC sensing with inline amplification enables the interrogation of cables with lengths beyond 1000 km, making it a potential contributor to earthquake detection and early warning in the oceans. Using spectral-element wavefield simulations that accurately account for complex cable geometry, we compare observed and computed recordings of the compensation frequency for a magnitude 3.9 earthquake in south-eastern France and a 123 km fiber link between Bern and Basel, Switzerland. The match in both phase and amplitude indicates that PNC sensing can be used quantitatively, for example, in earthquake detection and characterization.Comment: 7 pages, 4 figure

Seismic Array Processing of High-Rate GPS Data

Seismic Array Processing of High-Rate GPS DataCombinations of high-rate GPS (HRGPS) and broadband seismograms show great potential for augmenting the spatial coverage of existing seismic networks. GPS time series\u27 use as seismograms is limited by high noise levels. The primary noise source in 1 Hz GPS seismograms is GPS multipath. GPS multipath is highly dependent on local station conditions and incoherent in an array setting, while seismic waves travelling across an array are coherent and have predictable phase spectra. We propose that the effects of GPS multipath in HRGPS seismograms can be reduced by seismic array processing techniques that enchance the coherent and predictable seismic signals. We use f-k beamforming to demonstrate GPS multipath reduction within an array, and phase-match filtering to reduce multipath in 1 Hz GPS seismograms.We apply f-k beamforming to 1 Hz GPS seismograms for the December 26, 2004, Sumatra earthquake for GPS stations in central North America. Love wave phase velocity for central North America is recovered. Results agree well with current models for continental Love phase velocity dispersion between 20 and 300 seconds. GPS multipath is observed to be incoherent within the array, but its presence still introduces error into the calculations. Methods for reducing multipath in the seismograms prior to additional processing are needed.We use phase-match filtering to reduce GPS multipath in 1 Hz GPS seismograms from the Sumatra, Maule, and Tohoku-Oki earthquakes. We demonstrate significant reduction of multipath for both teleseismic and nearfield data. Tests with multipath-only GPS seismograms suggest that phase-match filtering is useful for 1 Hz GPS seismograms with signal-to-noise ratios of 2 or greater.Using networks of HRGPS stations as seismic arrays with array processing, such as beamforming and phase-match filtering, shows potential for the incorporation of HRGPS seismograms into seismic datasets. Other methods more robust than f-k beamforming and phase-match filtering exist and may further improve future results

Characterization of transient noise in Advanced LIGO relevant to gravitational wave signal GW150914

On 14 September 2015, a gravitational wave signal from a coalescing black hole binary system was observed by the Advanced LIGO detectors. This paper describes the transient noise backgrounds used to determine the significance of the event (designated GW150914) and presents the results of investigations into potential correlated or uncorrelated sources of transient noise in the detectors around the time of the event. The detectors were operating nominally at the time of GW150914. We have ruled out environmental influences and non-Gaussian instrument noise at either LIGO detector as the cause of the observed gravitational wave signal

Characterization of Transient Noise in Advanced LIGO Relevant to Gravitational Wave Signal GW150914

On September 14, 2015, a gravitational wave signal from a coalescing black hole binary system was observed by the Advanced LIGO detectors. This paper describes the transient noise backgrounds used to determine the significance of the event (designated GW150914) and presents the results of investigations into potential correlated or uncorrelated sources of transient noise in the detectors around the time of the event. The detectors were operating nominally at the time of GW150914. We have ruled out environmental influences and non-Gaussian instrument noise at either LIGO detector as the cause of the observed gravitational wave signal

Characterization of transient noise in Advanced LIGO relevant to gravitational wave signal GW150914

On 14 September 2015, a gravitational wave signal from a coalescing black hole binary system was observed by the Advanced LIGO detectors. This paper describes the transient noise backgrounds used to determine the significance of the event (designated GW150914) and presents the results of investigations into potential correlated or uncorrelated sources of transient noise in the detectors around the time of the event. The detectors were operating nominally at the time of GW150914. We have ruled out environmental influences and non-Gaussian instrument noise at either LIGO detector as the cause of the observed gravitational wave signal