Passive Seismic Surveys Research Articles

Ambient noise as the new source for urban engineering seismology and earthquake engineering: a case study from Beijing metropolitan area

In highly populated urban centers, traditional seismic survey sources can no longer be properly applied due to restrictions in modern civilian life styles. The ambient vibration noise, including both microseisms and microtremor, though are generally weak but available anywhere and anytime, can be an ideal supplementary source for conducting seismic surveys for engineering seismology and earthquake engineering. This is fundamentally supported by advanced digital signal processing techniques for effectively extracting the useful information out from the noise. Thus, it can be essentially regarded as a passive seismic method. In this paper we first make a brief survey of the ambient vibration noise, followed by a quick summary of digital signal processing for passive seismic surveys. Then the applications of ambient noise in engineering seismology and earthquake engineering for urban settings are illustrated with examples from Beijing metropolitan area. For engineering seismology the example is the assessment of site effect in a large area via microtremor observations. For earthquake engineering the example is for structural characterization of a typical reinforced concrete high-rise building using background vibration noise. By showing these examples we argue that the ambient noise can be treated as a new source that is economical, practical, and particularly valuable to engineering seismology and earthquake engineering projects for seismic hazard mitigation in urban areas.

Aquifer Characterization and Monitoring by Active and Passive Seismic Surveys

A 3D active and passive seismic survey was carried over an aquifer in Italy. We used 1-component vertical receiv ers in a fine areal g rid of 100x100 m. Furthermore, two orthogonal linear profiles were acquired with 3-component receivers, recording the signal of a directional vibrator in the x, y and z direction, so getting a 9-component wave field. The data allow studying the elastic propagation effects o f seismic waves in the aquifer, getting independent measurements of direct P, SH and SV arrivals. The elastic parameters they provide allow exploiting the Rayleigh wave velocities obtained by passive seismic for aquifer monitoring. © 2014 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Organizing Committee of WDSA 2014.

Passive Seismic Ambient Noise Correlation: An Example from the Ketzin Experimental CO2 Storage Site, Germany

Passive seismic ambient noise correlation is potentially a new tool to monitor carbon dioxide storage sites. Unlike conventional monitoring tools, such as time-lapse active seismic surveys, the passive seismic ambient noise correlation method is a relatively low cost method and can be performed together with microseismic and reservoir monitoring. In this study, we test the method at the Ketzin CO2 storage site, Germany. A new passive seismic survey was performed in August 2013 together with an active survey in conjunction with a source test. The survey consisted of two profiles, Line 1 and Line 2. Line 1 contained 42 10Hz geophones spaced at 24 m intervals and Line 2 contained 23 DSU3 (3-component MEMS) sensors spaced at 5 m intervals. In total, 6 nights and 3 daytime series of ambient noise data were recorded. First, we applied autocorrelation to the passive data records to reconstruct the reflection response. The results show good structural correlation with a stacked section from the active data. Then we used passive seismic interferometry to reconstruct common shot gathers for each receiver location. Surface waves were observed in some retrieved gathers from Line 1. However, for Line 2 we also observed refracted and reflected waves in some retrieved shot gathers. Here we review ambient noise correlation methods and compare our results with active seismic data.

Strategy for automated analysis of passive microseismic data based on S-transform, Otsu’s thresholding, and higher order statistics

Small-magnitude seismic events, either natural or induced microearthquakes, have increasingly been used in exploration seismology with applications ranging from hydrocarbon and geothermal reservoir exploration to high-resolution passive seismic tomography surveys. We developed an automated methodology for processing and analyzing continuously recorded, single-channel seismic data. This method comprised a chi-squared-based statistical test for microseismic event detection and denoising filtering in the S-transform domain based on the Otsu thresholding method. An automatic P-phase picker based on higher order statistics criteria was used. The method was used with data from a surface seismic station. The performance of the method was tested and evaluated on synthetic and real data from a microseismic network used in a high-resolution PST survey and revealed a high level of consistency.

Borehole receiver orientation using a 3D velocity model

ABSTRACTThe orientation of three‐component borehole geophones used for recording during a microseismic monitoring experiment is estimated. The standard technology for deploying multi‐component geophones in a deep borehole is wireline‐based, in which the azimuthal rotation of the geophone string cannot be controlled. Each receiver can have a different rotation angle that is compensated by the particle motion analysis of the direct P‐wave arrivals, picked from a walk‐around VSP carried out in the proximity of the well. Knowing the orientation of borehole receivers is critical, as inaccuracies lead to systematic errors in determining the hypocentral coordinates of microseismic events. Additional errors arise from over‐simplifications of P‐ and S‐wave velocity Earth models.In this paper, we propose a tomographic approach for improving the orientation estimates of borehole receivers based on hodogram analysis. The initial velocity model built from well logs and upholes is refined by 3D tomographic inversion of walk‐around VSP data and some string shots fired in a nearby borehole. Taking into account ray bending, the estimated errors due to local velocity anomalies can be reduced. This makes our estimates of fracture orientation and microseismic hypocentres more reliable when borehole receivers are used in passive seismic surveys.

Probabilistic surface reconstruction from multiple data sets: An example for the Australian Moho

Interpolation of spatial data is a widely used technique across the Earth sciences. For example, the thickness of the crust can be estimated by different active and passive seismic source surveys, and seismologists reconstruct the topography of the Moho by interpolating these different estimates. Although much research has been done on improving the quantity and quality of observations, the interpolation algorithms utilized often remain standard linear regression schemes, with three main weaknesses: (1) the level of structure in the surface, or smoothness, has to be predefined by the user; (2) different classes of measurements with varying and often poorly constrained uncertainties are used together, and hence it is difficult to give appropriate weight to different data types with standard algorithms; (3) there is typically no simple way to propagate uncertainties in the data to uncertainty in the estimated surface. Hence the situation can be expressed by Mackenzie (2004): “We use fantastic telescopes, the best physical models, and the best computers. The weak link in this chain is interpreting our data using 100 year old mathematics”. Here we use recent developments made in Bayesian statistics and apply them to the problem of surface reconstruction. We show how the reversible jump Markov chain Monte Carlo (rj‐McMC) algorithm can be used to let the degree of structure in the surface be directly determined by the data. The solution is described in probabilistic terms, allowing uncertainties to be fully accounted for. The method is illustrated with an application to Moho depth reconstruction in Australia.

Extracting subsurface information from ambient seismic noise — A case study from Germany

We analyzed ambient seismic noise from a broadband passive seismic survey acquired in an urban area in Germany. Despite a high level of anthropogenic noise, we observe lateral variations in the quasi-stationary spectra that are of natural origin and indicative of the subsurface in the survey area. The best diagnostic is the ellipticity spectrum which is the spectral ratio of the vertical over the horizontal components. Deviations of the observed spectra from a pure Rayleigh-wave ellipticity allow an approximate separation of surface-wave from body-wave components in the analyzed frequency range, distinguishing shallow (upper tens of meters) from deeper (upper three kilometers) subsurface effects. We observe an increase of vertically polarized body waves between 1 and 4 Hz that is correlated to a subsurface structure that contains an oil reservoir at about 2-km depth. We located the source of the observed body wave microtremor in depth by applying an elastic wavefield back projection and imaging technique. The method includes normalization by the impulse response of the velocity model, effects of the receiver geometry, and lateral variation of incoherent noise. The source region of the low-frequency body wave microtremor is centered above the location of the oil reservoir. Two possible explanations for the deep microtremor are elastic body-wave scattering due to the impedance contrast of the structural trap, and viscoelastic scattering due to poroelastic effects in the partially saturated reservoir.

A new hydrostratigraphic model of Venice area (Italy)

Two environmental problems affect the Venice area: subsidence, which has been increasing due to the intensive water abstraction after the Second World War, and contamination of the soil and shallow aquifers. In order to address these problems, which are decisive factors in the entire Venetian ecosystem, the aquifer structure must be known in detail. The lithologic data are abundant and of good quality up to a depth of 50 m, whereas boreholes beyond this depth are much rarer and more dispersed, making their associated lithological data unreliable. This work, which uses the available data together with fast and low cost passive seismic measurements, provides an improved understanding of the deeper hydrogeologic domain. For this purpose, a MATLAB package (Modalstrata) has been developed to improve the correlation of the stratigraphic succession for each selected homogeneous sub-area and applied to obtain a new, upgraded hydrostratigraphic model. The horizontal to vertical spectral ratio passive seismic surveys have confirmed the lateral correlations among the sample areas at least for the two main aquifer horizons. Analysis and comparisons of several previous studies performed on the data related to the only drilling continuous coring 951 m deep in the Tronchetto Island (Venice), have allowed a satisfactory validation of the proposed hydrogeological model.

Indexes and physical parameters for the litho-stratigraphic model of Venice

We performed passive and controlled source seismic surveys in the historical center of Venice to characterize subsoil layers, seismic indexes and physical parameters of subsoil. The adopted techniques allowed us to identify two main different seismic responses for the Venetian subsoil, in terms of both elastic earth parameters and seismic characteristics. In particular, the horizontal to vertical spectral ratio technique was applied, highlighting a substantial relevant heterogeneity. This is due to the presence of relatively strong acoustic impedance contrast in the shallow subsoil (8–10 m deep), detected only in some sites of historical city center. With the aid of a geological reconstruction, seismic surveys interpretation was able to highlight the presence of a particular alluvial complex, identified as a ‘multistorey sandbody’. The measurements campaign furnished, moreover, indexes and physical parameters quantification for the litho-stratigraphic model of the Venetian subsoil.

Analysis of the surface waves present on the passive seismic data set recorded in the Mizil area (Romania)

A passive seismic survey in April 2009 had two main goals: the use of seismic interferometry to retrieve the seismic signal (reflected waves) from the background noise and to demonstrate that passive surveys can cover spatial gaps between active profiles. Seismic interferometry works well if the passive records are not affected by surface waves with high amplitudes; therefore, such records are neglected in the application of seismic interferometry.

A passive seismic survey over a gas field: Analysis of low-frequency anomalies

Passive seismic low-frequency (from approximately [Formula: see text]) data have been acquired at several locations around the world. Spectra calculated from these data, acquired over fields with known hydrocarbon accumulations, show common spectral anomalies. Verification of whether these anomalies are common to only a few, many, or all hydrocarbon reservoirs can be provided only if more and detailed results are reported. An extensive survey was carried out above a tight gas reservoir and an adjacent exploration area in Mexico. Data from several hundred stations with three-component broadband seismometers distributed over approximately [Formula: see text] were used for the analysis. Severalhydrocarbon reservoir-related microtremor attributes were calculated, and mapped attributes were compared with known gas intervals, with good agreement. Wells drilled after the survey confirm a predicted high hydrocarbon potential in the exploration area. A preliminary model was developed to explain the source mechanism of those microtremors. Poroelastic effects caused by wave-induced fluid flow and oscillations of different fluid phases are significant processes in the low-frequency range that can modify the omnipresent seismic background spectrum. These processes only occur in partially saturated rocks. We assume that hydrocarbon reservoirs are partially saturated, whereas the surrounding rocks are fully saturated. Our real data observations are consistent with this conceptual model.

Georadar and passive seismic survey in the Roman Amphitheatre of Catania (Sicily)

Abstract A geophysical survey was performed at the Roman Amphitheatre of Catania with the aim to study the lithological features of the foundation subsoil of the Amphitheatre and to identify possible underground voids, buried crypts and other heterogeneities. The survey consisted of a number of georadar prospections and of three mechanical drillings. In addition it was experimented a non-invasive technique (passive seismic stratigraphy), which allows to retrieve information on subsoil stratigraphy and on possible seismic site effects. The survey allowed to establish 1) the presence of clays below superficial lava layers in the foundation soil of the Amphitheatre, 2) the presence of 2-D structures below the external ambulatory and 3) the natural resonance frequencies of the structure and of the subsoil in the area of investigation.

Combined use of SPAC, FK and HVSR microtremor survey methods for site hazard study over the 2D Tamar Valley, Launceston, Tasmania.

SUMMARY The presence of the ancient Tamar valley in Launceston, filled with soft Tertiary and Quaternary sediments, is thought to induce a 2D seismic resonance pattern, amplifying the surface motion over the valley. We combine the use of spatially averaged coherency (SPAC), frequency-wavenumber (FK) and horizontal to vertical spectrum ratio (HVSR) microtremor (passive seismic) survey methods to identify and characterize potential 2D effects over the Tamar valley. Array observations are presented at two selected sites; GUNN Company (site GUN) located over an assumed 1D geology, and King's Park (site KPK) inside the valley. We obtain, by interpretation of SPAC data, a precise estimation of the shear wave velocity (or slowness) profile with depth. We calculate the dispersion curves at both sites with SPAC and FK methods for comparison purposes. While both dispersion curves present similar features, SPAC method enables interpretation over a wider range of frequency. HVSR curves are compared with modelled Rayleigh wave ellipticity computed using the slowness profiles obtained by SPAC. At GUN, the peaks on HVSR and ellipticity coincide closely. However, we observe a shift to higher frequency on HVSR peak when compared to ellipticity peak at KPK. We postulate possible 2D effects might explain that shift.

Combining Seismic Data from Passive and Active Sources for understanding the terrane structure of the Eastern Goldfields, Western Australia

A passive seismic survey to investigate variations in crustal structure across the Yilgarn craton has shown significant contrasts in seismic velocity models between neighbouring terranes/superterranes. The Eastern Goldfields showed a unique variability in crustal structure in agreement with a recent reinterpretation of terrane boundaries within the Yilgarn craton. We further investigated the Eastern Goldfields region using a 3-way approach which combined conventional passive seismic analysis with innovative seismic noise-correlation methods and constraints from active source data. Conventional passive seismic analysis enables the Receiver Function S-velocity structure, and hence composition, of the lower crust to be constrained. Noise -correlation analysis allows seismic velocity models in the 5-15 km depth range to be determined and provides medium resolution coverage across regions not previously explored using active seismic methods. Where active source data have been acquired, shallow structure and deeper seismic velocity determinations are added, providing an unprecedented combination of seismic constraints on the structure of this complex and economically important region. We find that, although some individual terrane boundaries within the new Eastern Goldfields reinterpretation are open to question, the concept of the multi-terrane amalgamation is substantially justified by the exceptional variability of the lower crustal structure. Upper crustal structure is often characterised by seismic discontinuities which may represent detachment surfaces or layered structure that varies between terranes over a sub-100 km length scale. The accretionary history of the superterrane and associated regional tectonic setting of numerous formations of economic significance would now appear to be beyond question.

Borehole seismic: what does it bring to the understanding of the reservoir?

When thinking about what we need for a more comprehensive approach to reservoir description and management, the range of information is large and various. The objective is always to improve our ‘vision’ of the geological setting, including a precise rock-type characterization and a good understanding of hydrocarbon flow paths. Each of these elements of the reservoir is determined with a varying level of uncertainty. Today, there is general agreement that integration of competences in business units is the optimal way to take the full benefit of technology breakthroughs in each individual discipline. Borehole seismic is identified as being critical to increasing the quality and the reliability of processing and interpretation of surface seismic data. If until recently, borehole seismic applications were focused on seismic calibration using vertical seismic profiles (VSP) and sometimes on imaging when surface seismic defaulted, today the link to well data is growing stronger. Since not all disciplines involved in reservoir management can work at the same scale, this raises the problem of scale and model compatibility between the respective disciplines. Here again borehole seismic can play a key role. This is becoming possible because advances have been made in a variety of techniques and methods leading to applications such as multi-walkaway and 3D-VSP, 4C and 4D-VSP, single-well and crosswell acquisitions, amplitude variation with offset (AVO) calibration, and scale change. Moreover, improvements in technology make the acquisition and processing cost-effective. Multilevel, three- or four-component receiver strings and permanent seismic sensors are examples of a continued development of operationally efficient hardware. New borehole seismic techniques such as 4D-VSP or the improvement in more mature ones such as crosswell seismic, offer new images of the reservoir. Although these images may be spatially limited, they are usually of a higher resolution than surface seismic and can offer significant benefit for reservoir management and monitoring. This paper illustrates the benefits of borehole seismic with three case studies, illustrating (1) how we can include uncertainties in the VSP imaging process and its impact on structural interpretation of a North Sea reservoir, (2) why AVO calibration using a walkaway technique is mandatory for a better prediction and interpretation of seismic attributes in terms of lithology and fluid content and (3) 4D walkaway and passive seismic surveys using fibre-optic seismic permanent sensors for monitoring an onshore gas storage.

Resolving a velocity inversion at the geotechnical scale using the microtremor (passive seismic) survey method

Resolving a velocity inversion at the geotechnical scale using the microtremor (passive seismic) survey method

Resolving a velocity inversion at the geotechnical scale using the microtremor (passive seismic) survey method

High levels of ambient noise and safety factors often limit the use of "active-source" seismic methods for geotechnical investigations in urban environments. As an alternative, shear-wave velocity-depth profiles can be obtained by treating the background microtremor wave field as a stochastic process, rather than adopting the traditional approach of calculating velocity based on ray path geometry from a known source. A recent field test in Melbourne demonstrates the ability of the microtremor method, using only Rayleigh waves, to resolve a velocity inversion resulting from the presence of a hard, 12 m thick basalt flow overlying 25 m of softer alluvial sediments and weathered mudstone. Normally the presence of the weaker underlying sediments would lead to an ambiguous or incorrect interpretation with conventional seismic refraction methods. However, this layer of sediments is resolved by the microtremor method, and its inclusion is required in one-dimensional layered-earth modelling in order to reproduce the Rayleigh-wave coherency spectra computed from observed seismic noise records.Nearby borehole data provided both a guide for interpretation and a confirmation of the usefulness of the passive Rayleigh-wave microtremor method. Sensitivity analyses of resolvable modelling parameters demonstrate that estimates of shear velocities and layer thicknesses are accurate to within approximately 10% to 20% using the spatial autocorrelation (SPAC) technique. Improved accuracy can be obtained by constraining shear velocities and/or layer thicknesses using independent site knowledge. Although there exists potential for ambiguity due to velocity-thickness equivalence, the microtremor method has significant potential as a site investigation tool in situations where the use of traditional seismic methods is limited.

Case study in NW Greece of passive seismic tomography: a new tool for hydrocarbon exploration

We have learned more about the structure of the Earth and its crust from earthquake seismology in all its facets than from any other single geophysical or geological method. The use of the vast amount of information provided by the natural seismicity of the earth has been largely restricted to the investigation of classical seismological problems or to large scale investigations of the earth’s interior, with relatively little attention being paid towards small scale hydrocarbon exploration. Listening to the earth passively, and using the collected seismological information wisely, can be successfully applied to hydrocarbon exploration, as is shown in the present investigation. Controlled source seismology uses conventional surface sources such as vibroseis, explosives or airguns to generate seismic waves whose travel times and amplitude distribution through the earth are used to determine structural images and bulk physical properties of the subsurface. In contrast, passive seismic tomography uses micro-earthquakes as an energy source to probe earth structure. It is a fairly simple concept, based on the fundamental principle that all small movements and ‘roars’ in the earth are actually seismic sources. Both compressional and shear waves are emitted from an earthquake source and can be used for independent estimates of compressional (Vp) and shear (Vs) velocities of the various geological formations. The definition of velocity structure in a complex tectonic environment is a challenging task for conventional reflection velocity analysis based on NMO methods. In recent years, there has been an increase in exploration activity in geologically complex areas, such as fold and thrust belts, and even seeking good seismic images beneath high impedance layers such as basalt. Exploration in these areas is challenging, as well as expensive, and is driving the oil exploration industry towards the application of state of the art techniques. A key aspect of tackling the ‘complex geology’ problem has been the design and implementation of new types of seismic acquisition and processing strategies. The passive seismic tomography method falls into this category. Conventional land seismic is a labour-intensive business with expensive recording crews and set-ups of hundreds of miles of cable out on the ground, while geophones have to be deployed and retrieved manually. Surface access is needed for vibrators and shot-hole rigs, while permitting and other environmental issues mean high costs. The rationale for the application of passive tomography as a complementary imaging tool is multifold. It is a cost effective manner to image a large area where the terrain is difficult (mountainous or even shallow water) and, as a consequence, conventional seismic is expensive and may be of poor quality. Since the seismic energy from microearthquakes comes from below the target of interest it can be easily used to map complex tectonic regions (i.e. overthrust belts, subbasalts, shallow carbonates etc.) characterised by seismic energy penetration problems. Another advantage of passive seismic tomography is the capability to measure accurately an intrinsic Vp/Vs ratio. This is a direct consequence of the production of high amplitude shear waves by small earthquakes that are reliably recorded by (3-component) surface receivers. In contrast, 3D reflection seismic methods employ man-made sources (e.g. explosions) that do not produce large shear waves, so that detecting and identifying the weak shear waves reflected deep in the medium is not generally reliable, and often not even possible. Consequently, the active seismic reflection methods currently employed by the petroleum industry do not adequately provide material parameter information related to the shear velocities in the medium. Typically, a passive seismic 3D survey will cost less than a conventional 2D survey by several orders of magnitude. Drilling and explosives, for example, generally account for almost half of conventional 3D seismic cost. These costs are eliminated with passive seismic. Furthermore, the recording station density required in this methodology is significantly less, meaning significantly smaller equipment inventory and crew size. A passive seismic crew will be normally five to 10 people, compared with a conventional crew that will number upwards of 50. Another important aspect of the technique is that it has the advantage of being environmentally friendly. The absence of explosives and heavy vehicle support for conventional sources permits activity in terrains that might otherwise be inaccessible, and almost eliminates any environmental issues. In the following sections we present a successful application of passive seismic tomography for hydrocarbon exploration in the area of Epirus, Greece.

High-resolution teleseismic tomography of upper-mantle structure using ana priorithree-dimensional crustal model

SUMMARY The effect of an a priori known 3-D crustal model in teleseismic tomography of upper-mantle structure is investigated. We developed a 3-D crustal P-wave velocity model for the greater Alpine region, encompassing the central and western Alps and the northern Apennines, to estimate the crustal contribution to teleseismic traveltimes. The model is constructed by comparative use of published information from active and passive seismic surveys. The model components are chosen to represent the present large-scale Alpine crustal structure and for their significant effect on the propagation of seismic wavefields. They are first-order structures such as the crust‐mantle boundary, sedimentary basins and the high-velocity Ivrea body. Teleseismic traveltime residuals are calculated for a realistic distribution of azimuths and distances by coupling a finite-difference technique to the IASP91 traveltime tables. Residuals are produced for a synthetic upper-mantle model featuring two slab structures and the 3-D crustal model on top of it. The crustal model produces traveltime residuals in the range between −0.7 and 1.5 s that vary strongly as a function of backazimuth and epicentral distance. We find that the non-linear inversion of the synthetic residuals without correcting for the 3-D crustal structure erroneously maps the crustal anomalies into the upper mantle. Correction of the residuals for crustal structure before inversion properly recovers the synthetic slab structures placed in the upper mantle. We conclude that with the increasing amount of high-quality seismic traveltime data, correction for near-surface structure is essential for increasing resolution in tomographic images of upper-mantle structure.

GIS as a Tool for Seismological Data Processing

A computerized application of an integrated seismological GIS model is presented. An object oriented approach of the GIS topology is introduced and the special functions and features of this system are described. A network topology was selected to simulate the network characteristics of seismological data management and analysis. Each seismological entity is considered as a graphical data object, which is associated to other objects by predefined relationships. The graphical user interface introduced by GIS enables to handle seismological software routines and data in a more intuitive way. Examples of interactive processing of seismic waveforms for detecting, locating and characterizing seismic events using GIS visualization capabilities are presented. The benefits of this system during a passive seismic survey in the framework of the CTBT are highlighted.