Seismic noise

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In geophysics, geology, civil engineering, and related disciplines, seismic noise is a generic name for a relatively persistent vibration of the ground, due to a multitude of causes, that is often a non-interpretable or unwanted component of signals recorded by seismometers.

Contents

Physically, seismic noise arises primarily due to surface or near surface sources and thus consists mostly of elastic surface waves. Low frequency waves (below 1 Hz) are commonly called microseisms and high frequency waves (above 1 Hz) are called microtremors. Primary sources of seismic waves include human activities (such as transportation or industrial activities), winds and other atmospheric phenomena, rivers, and ocean waves.

Seismic noise is relevant to any discipline that depends on seismology, including geology, oil exploration, hydrology, and earthquake engineering, and structural health monitoring. It is often called the ambient wavefield or ambient vibrations in those disciplines (however, the latter term may also refer to vibrations transmitted through by air, building, or supporting structures.)

Seismic noise is often a nuisance for activities that are sensitive to extraneous vibrations, including earthquake monitoring and research, precision milling, telescopes, gravitational wave detectors, and crystal growing. However, seismic noise also has practical uses, including determining the low-strain and time-varying dynamic properties of civil-engineering structures, such as bridges, buildings, and dams; seismic studies of subsurface structure at many scales, often using the methods of seismic interferometry; Environmental monitoring, such as in fluvial seismology; and estimating seismic microzonation maps to characterize local and regional ground response during earthquakes.

Causes

Research on the origin of seismic noise [1] indicates that the low frequency part of the spectrum (below 1 Hz) is principally due to natural causes, chiefly ocean waves. In particular the globally observed peak between 0.1 and 0.3 Hz is clearly associated with the interaction of water waves of nearly equal frequencies but probating in opposing directions. [2] [3] [4] [5] At high frequency (above 1 Hz), seismic noise is mainly produced by human activities such as road traffic and industrial work; but there are also natural sources, including rivers. [6] Above 1 Hz, wind and other atmospheric phenomena can also be a major source of ground vibrations. [7] [8]

Anthropogenic noise detected during periods of low seismic activity includes "footquakes" from soccer fans stamping their feet in Cameroon. [9]

Non-anthropogenic activity includes pulses at intervals between 26 and 28 seconds (0.036–0.038 Hz) centered on the Bight of Bonny in the Gulf of Guinea that are thought to be caused by reflected storm waves, focused by the African coast, acting on the relatively shallow sea-floor. [9]

Physical characteristics

The amplitude of seismic noise vibrations is typically in the order of 0.1 to 10 μm/s. High and low background noise models as a function of frequency have been evaluated globally. [10]

Seismic noise includes a small number of body waves (P- and S-waves), but surface waves (Love and Rayleigh waves) predominate since they are preferentially excited by surface source processes. These waves are dispersive, meaning that their phase velocity varies with frequency (generally, it decreases with increasing frequency). Since the dispersion curve (phase velocity or slowness as a function of frequency) is related to the variations of the shear-wave velocity with depth, it can be used as a non-invasive tool to determine subsurface seismic structure and an inverse problem.

History

Under normal conditions, seismic noise has very low amplitude and cannot be felt by humans, and was also too low to be recorded by most early seismometers at the end of 19th century. However, by the early 20th century, Japanese seismologist Fusakichi Omori could already record ambient vibrations in buildings, where the amplitudes are magnified. He determined building resonance frequencies and studied their evolution as a function of damage. [11] Globally visible 30 s–5 s seismic noise was recognized early in the history of seismology as arising from the oceans, and a comprehensive theory of its generation was published by Longuet-Higgins in 1950. [2] Rapid advances beginning around 2005 in seismic interferometry driven by theoretical, methodological, and data advances have resulted in a major renewed interest in the applications of seismic noise.

Civil engineering

After the 1933 Long Beach earthquake in California, a large experiment campaign led by D. S. Carder [12] in 1935 recorded and analyzed ambient vibrations in more than 200 buildings. These data were used in the design codes to estimate resonance frequencies of buildings but the interest of the method went down until the 1950s. Interest on ambient vibrations in structures grew further, especially in California and Japan, thanks to the work of earthquake engineers, including G. Housner, D. Hudson, K. Kanai, T. Tanaka, and others. [13]

In engineering, ambient vibrations were however supplanted - at least for some time - by forced vibration techniques that allow to increase the amplitudes and control the shaking source and their system identification methods. Even though M. Trifunac showed in 1972 that ambient and forced vibrations led to the same results, [14] the interest in ambient vibration techniques only rose in the late 1990s. They have now become quite attractive, due to their relatively low cost and convenience, and to the recent improvements in recording equipment and computation methods. The results of their low-strain dynamic probing were shown to be close enough to the dynamic characteristics measured under strong shaking, at least as long as the buildings are not severely damaged. [15]

Scientific study and applications in geology and geophysics

The recording of global seismic noise expanded widely in the 1950s with the enhancement of seismometers to monitor nuclear tests and the development of seismic arrays. The main contributions at that time for the analysis of these recordings came from the Japanese seismologist K. Aki [16] in 1957. He proposed several methods used today for local seismic evaluation, such as Spatial Autocorrelation (SPAC), Frequency-wavenumber (FK), and correlation. However, the practical implementation of these methods was not possible at that time because of the low precision of clocks in seismic stations.

Improvements in instrumentation and algorithms led to renewed interest on those methods during the 1990s. Y. Nakamura rediscovered in 1989 the horizontal to vertical spectral ratio (H/V) method to derive the resonance frequency of sites. [17] Assuming that shear waves dominate the microtremor, Nakamura observed that the H/V spectral ratio of ambient vibrations was roughly equal to the S-wave transfer function between the ground surface and the bedrock at a site. (However, this assumption has been questioned by the SESAME project.)

In the late 1990s, array methods applied to seismic noise data started to yield ground properties in terms of shear waves velocity profiles. [18] [19] [20] [21] The European Research project SESAME [22] (2004–2006) worked to standardize the use of seismic noise to estimate the amplification of earthquakes by local ground characteristics.

Current uses of seismic noise

Characterization of subsurface properties

The analysis of the ambient vibrations and the random seismic wavefield motivates a variety of processing methods used to characterize the subsurface, including via power spectra, H/V peak analysis, dispersion curves and autocorrelation functions.

Single-station methods:

[25] [26]

Array methods: Using an array of seismic sensors recording simultaneously the ambient vibrations allow for greater understanding of the wavefield and to derive improved images of the subsurface. In some cases, multiple arrays of different sizes may be realized and the results merged. The information of the Vertical components is only linked to the Rayleigh waves, and therefore easier to interpret, but method using the all three ground motion components are also developed, providing information about Rayleigh and Love wavefield. Seismic Interferometry methods, in particular, use correlation-based methods to estimate the seismic impulse (Green's Function) response of the Earth from background noise and have become a major area of application and research with the growth in continuously recorded high quality noise data in a wide variety of settings, ranging from the near surface [29] to the continent scale [30]

Characterization of the vibration properties of civil engineering structures

Like earthquakes, ambient vibrations force into vibrations the civil engineering structures like bridges, buildings or dams. This vibration source is supposed by the greatest part of the used methods to be a white noise, i.e. with a flat noise spectrum so that the recorded system response is actually characteristic of the system itself. The vibrations are perceptible by humans only in rare cases (bridges, high buildings). Ambient vibrations of buildings are also caused by wind and internal sources (machines, pedestrians...) but these sources are generally not used to characterize structures. The branch that studies the modal properties of systems under ambient vibrations is called Operational modal analysis (OMA) or Output-only modal analysis and provides many useful methods for civil engineering. The observed vibration properties of structures integrate all the complexity of these structures including the load-bearing system, heavy and stiff non-structural elements (infill masonry panels...), light non-structural elements (windows...) [31] and the interaction with the soil (the building foundation may not be perfectly fixed on the ground and differential motions may happen). [32] This is emphasized because it is difficult to produce models able to be compared with these measurements.

Single-station methods: The power spectrum computation of ambient vibration recordings in a structure (e.g. at the top floor of a building for larger amplitudes) gives an estimation of its resonance frequencies and eventually its damping ratio.

Transfer function method: Assuming ground ambient vibrations is the excitation source of a structure, for instance a building, the Transfer Function between the bottom and the top allows to remove the effects of a non-white input. This may particularly be useful for low signal-to-noise ratio signals (small building/high level of ground vibrations). However this method generally is not able to remove the effect of soil-structure interaction. [32]

Arrays: They consist in the simultaneous recording in several points of a structure. The objective is to obtain the modal parameters of structures: resonance frequencies, damping ratios and modal shapes for the whole structure. Notice than without knowing the input loading, the participation factors of these modes cannot a priori be retrieved. Using a common reference sensor, results for different arrays can be merged.

Several methods use the power spectral density matrices of simultaneous recordings, i.e. the cross-correlation matrices of these recordings in the Fourier domain. They allow to extract the operational modal parameters (Peak Picking method) that can be the results of modes coupling or the system modal parameters (Frequency Domain Decomposition method).

Numerous system identification methods exist in the literature to extract the system properties and can be applied to ambient vibrations in structures.

Social sciences

The COVID-19 pandemic produced a unique situation in which human transportation, industrial, and other activities were significantly curtailed across the world, particularly in densely populated areas. An analysis of the attendant strong reductions in seismic noise at high frequencies demonstrated that these exceptional actions resulted in the longest and most prominent global anthropogenic seismic noise reduction ever observed. [33] Seismic noise has additionally been investigated as a proxy for economic development. [34]

Inversion/model updating/multi-model approach

Direct measurements of noise properties cannot directly give information on the physical parameters (S-wave velocity, structural stiffness...) of the ground structures or civil engineering structures that are typically of interest. Therefore, models are needed to compute these observations (dispersion curve, modal shapes...) in a suitable forward problem that can then be compared with the experimental data. Given the forward problem, the process of estimating the physical model can then be cast as an Inverse problem.

Material needed

The acquisition chain is mainly made of a seismic sensor and a digitizer. The number of seismic stations depends on the method, from single point (spectrum, HVSR) to arrays (3 sensors and more). Three components (3C) sensors are used except in particular applications. The sensor sensitivity and corner frequency depend also on the application. For ground measurements, velocimeters are necessary since the amplitudes are generally lower than the accelerometers sensitivity, especially at low frequency. Their corner frequency depends on the frequency range of interest but corner frequencies lower than 0.2 Hz are generally used. Geophones (generally 4.5 Hz corner frequency or greater) are generally not suited. For measurements in civil engineering structures, the amplitude is generally higher as well as the frequencies of interest, allowing the use of accelerometers or velocimeters with a higher corner frequency. However, since recording points on the ground may also be of interest in such experiments, sensitive instruments may be needed. Except for single station measurements, a common time stamping is necessary for all the stations. This can be achieved by GPS clock, common start signal using a remote control or the use of a single digitizer allowing the recording of several sensors. The relative location of the recording points is needed more or less precisely for the different techniques, requiring either manual distance measurements or differential GPS location.

Advantages and limitations

The advantages of ambient vibration techniques compared to active techniques commonly used in exploration geophysics or earthquake recordings used in Seismic tomography.

Limitations of these methods are linked to the noise wavefield but especially to common assumptions made in seismic:

Related Research Articles

<span class="mw-page-title-main">Seismology</span> Scientific study of earthquakes and propagation of elastic waves through a planet

Seismology is the scientific study of earthquakes and the generation and propagation of elastic waves through the Earth or other planetary bodies. It also includes studies of earthquake environmental effects such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, glacial, fluvial, oceanic microseism, atmospheric, and artificial processes such as explosions and human activities. A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of Earth motion as a function of time, created by a seismograph is called a seismogram. A seismologist is a scientist working in basic or applied seismology.

<span class="mw-page-title-main">Seismic wave</span> Seismic, volcanic, or explosive energy that travels through Earths layers

A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake, volcanic eruption, magma movement, a large landslide and a large man-made explosion that produces low-frequency acoustic energy. Seismic waves are studied by seismologists, who record the waves using seismometers, hydrophones, or accelerometers. Seismic waves are distinguished from seismic noise, which is persistent low-amplitude vibration arising from a variety of natural and anthropogenic sources.

<span class="mw-page-title-main">Seismometer</span> Instrument that records seismic waves by measuring ground motions

A seismometer is an instrument that responds to ground noises and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The output of such a device—formerly recorded on paper or film, now recorded and processed digitally—is a seismogram. Such data is used to locate and characterize earthquakes, and to study the internal structure of Earth.

<span class="mw-page-title-main">Tuned mass damper</span> Device designed to reduce vibrations in structures

A tuned mass damper (TMD), also known as a harmonic absorber or seismic damper, is a device mounted in structures to reduce mechanical vibrations, consisting of a mass mounted on one or more damped springs. Its oscillation frequency is tuned to be similar to the resonant frequency of the object it is mounted to, and reduces the object's maximum amplitude while weighing much less than it.

Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, wave source distance, and the seismograph array coverage. The data received at seismometers are used to solve an inverse problem, wherein the locations of reflection and refraction of the wave paths are determined. This solution can be used to create 3D images of velocity anomalies which may be interpreted as structural, thermal, or compositional variations. Geoscientists use these images to better understand core, mantle, and plate tectonic processes.

<span class="mw-page-title-main">Earthscope</span> Earth science program exploring the structure of the North American continent

The EarthScope project was an National Science Foundation (NSF) funded earth science program that, from 2003-2018, used geological and geophysical techniques to explore the structure and evolution of the North American continent and to understand the processes controlling earthquakes and volcanoes. The project had three components: USArray, the Plate Boundary Observatory, and the San Andreas Fault Observatory at Depth. Organizations associated with the project included UNAVCO, the Incorporated Research Institutions for Seismology (IRIS), Stanford University, the United States Geological Survey (USGS) and National Aeronautics and Space Administration (NASA). Several international organizations also contributed to the initiative. EarthScope data are publicly accessible.

Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at a given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram. Magnitude scales vary on what aspect of the seismic waves are measured and how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used.

In acoustics, microbaroms, also known as the "voice of the sea", are a class of atmospheric infrasonic waves generated in marine storms by a non-linear interaction of ocean surface waves with the atmosphere. They typically have narrow-band, nearly sinusoidal waveforms with amplitudes up to a few microbars, and wave periods near 5 seconds. Due to low atmospheric absorption at these low frequencies, microbaroms can propagate thousands of kilometers in the atmosphere, and can be readily detected by widely separated instruments on the Earth's surface.

Microtremor is a low amplitude ambient vibration of the ground caused by man-made or atmospheric disturbances. The term Ambient Vibrations is now preferred to talk about this phenomenon. Observation of microtremors can give useful information on dynamic properties of the site such as predominant period and amplitude. Microtremor observations are easy to perform, inexpensive and can be applied to places with low seismicity as well, hence, microtremor measurements can be used conveniently for seismic microzonation. More detailed information on the shear wave velocity profile of the site can be obtained from microtremor array observation.

In seismology, a microseism is defined as a faint earth tremor caused by natural phenomena. Sometimes referred to as a "hum", it should not be confused with the anomalous acoustic phenomenon of the same name. The term is most commonly used to refer to the dominant background seismic and electromagnetic noise signals on Earth, which are caused by water waves in the oceans and lakes. Characteristics of microseism are discussed by Bhatt. Because the ocean wave oscillations are statistically homogenous over several hours, the microseism signal is a long-continuing oscillation of the ground. The most energetic seismic waves that make up the microseismic field are Rayleigh waves, but Love waves can make up a significant fraction of the wave field, and body waves are also easily detected with arrays. Because the conversion from the ocean waves to the seismic waves is very weak, the amplitude of ground motions associated to microseisms does not generally exceed 10 micrometers.

Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive testing for detecting defects. Rayleigh waves are part of the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves.

<span class="mw-page-title-main">Acoustic metamaterial</span> Material designed to manipulate sound waves

An acoustic metamaterial, sonic crystal, or phononic crystal is a material designed to control, direct, and manipulate sound waves or phonons in gases, liquids, and solids. Sound wave control is accomplished through manipulating parameters such as the bulk modulus β, density ρ, and chirality. They can be engineered to either transmit, or trap and amplify sound waves at certain frequencies. In the latter case, the material is an acoustic resonator.

A seismic metamaterial, is a metamaterial that is designed to counteract the adverse effects of seismic waves on artificial structures, which exist on or near the surface of the Earth. Current designs of seismic metamaterials utilize configurations of boreholes, trees or proposed underground resonators to act as a large scale material. Experiments have observed both reflections and bandgap attenuation from artificially induced seismic waves. These are the first experiments to verify that seismic metamaterials can be measured for frequencies below 100 Hz, where damage from Rayleigh waves is the most harmful to artificial structures.

<span class="mw-page-title-main">Surface wave inversion</span>

Seismic inversion involves the set of methods which seismologists use to infer properties through physical measurements. Surface-wave inversion is the method by which elastic properties, density, and thickness of layers in the subsurface are obtained through analysis of surface-wave dispersion. The entire inversion process requires the gathering of seismic data, the creation of dispersion curves, and finally the inference of subsurface properties.

Ground vibrations is a technical term that is being used to describe mostly man-made vibrations of the ground, in contrast to natural vibrations of the Earth studied by seismology. For example, vibrations caused by explosions, construction works, railway and road transport, etc. - all belong to ground vibrations.

Ambient modal identification, also known as operational modal analysis (OMA), aims at identifying the modal properties of a structure based on vibration data collected when the structure is under its operating conditions, i.e., no initial excitation or known artificial excitation. The modal properties of a structure include primarily the natural frequencies, damping ratios and mode shapes. In an ambient vibration test the subject structure can be under a variety of excitation sources which are not measured but are assumed to be 'broadband random'. The latter is a notion that one needs to apply when developing an ambient identification method. The specific assumptions vary from one method to another. Regardless of the method used, however, proper modal identification requires that the spectral characteristics of the measured response reflect the properties of the modes rather than those of the excitation.

Bayesian operational modal analysis (BAYOMA) adopts a Bayesian system identification approach for operational modal analysis (OMA). Operational modal analysis aims at identifying the modal properties (natural frequencies, damping ratios, mode shapes, etc.) of a constructed structure using only its (output) vibration response (e.g., velocity, acceleration) measured under operating conditions. The (input) excitations to the structure are not measured but are assumed to be 'ambient' ('broadband random'). In a Bayesian context, the set of modal parameters are viewed as uncertain parameters or random variables whose probability distribution is updated from the prior distribution (before data) to the posterior distribution (after data). The peak(s) of the posterior distribution represents the most probable value(s) (MPV) suggested by the data, while the spread of the distribution around the MPV reflects the remaining uncertainty of the parameters.

Seismic site effects are related to the amplification of seismic waves in superficial geological layers. The surface ground motion may be strongly amplified if the geological conditions are unfavorable. Therefore, the study of local site effects is an important part of the assessment of strong ground motions, seismic hazard and engineering seismology in general. Damage due to an earthquake may thus be aggravated as in the case of the 1985 Mexico City earthquake. For alluvial basins, we may shake a bowl of jelly to model the phenomenon at a small scale.

Fluvial seismology is the application of seismological methods to understand river processes, such as discharge, erosion, and streambed evolution. Flowing water and the movement of sediments along the streambed generate elastic (seismic) waves that propagate into the surrounding Earth materials. Seismometers can record these signals, which can be analyzed to illuminate different fluvial processes such as turbulent water flow and bedload transport. Seismic methods have been used to observe discharge values that range from single-digits up through tens of thousands of cubic feet per second (cfs).

Subsurface mapping by ambient noise tomography is the mapping underground geological structures under the assistance of seismic signals. Ambient noise, which is not associated with the earthquake, is the background seismic signals. Given that the ambient noises have low frequencies in general, the further classification of ambient noise include secondary microseisms, primary microseisms, and seismic hum, based on different range of frequencies. We can utilize the ambient noise data collected by seismometers to create images for the subsurface under the following processes. Since the ambient noise is considered as diffuse wavefield, we can correlate the filtered ambient noise data from a pair of seismic stations to find the velocities of seismic wavefields. A 2-dimensional or 3-dimensional velocity map, showing the spatial velocity difference of the subsurface, can thus be created for observing the geological structures. Subsurface mapping by ambient noise tomography can be applied in different fields, such as detecting the underground void space, monitoring landslides, and mapping the crustal and upper mantle structure.

References

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  33. ↑ Lecocq, T. (2020). "Global quieting of high-frequency seismic noise due to COVID-19 pandemic lockdown measures". Science. 369 (6509): 1338–1343. Bibcode:2020Sci...369.1338L. doi: 10.1126/science.abd2438 . hdl: 10044/1/81027 . PMID   32703907.
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