Active Seismic Tomography Research Articles

Reviews

Active Seismic Tomography: Theory and Applications, by Kalachand Sain and Damodara Nara, 2023 Distributed Acoustic Sensing for Seismic Measurements — What Geophysicists and Engineers Need to Know, by Mark E. Willis, 2022

Theoretical and Experimental Study on Parameter Optimization of Active Sources in Seismic Tomography

Abstract Given the high cost of active seismic tomography and the large influence on mining production, as well as the uncontrollable inversion period and accuracy of the target area during the passive seismic tomography, an optimization algorithm based on adding a certain number of artificial microseism (active sources) was proposed through the distribution characteristic of microseismic events (passive sources). The active source parameter optimization method realizes the combination of active source and passive source and improves the accuracy of inversion at the same time. The method uses a multiobjective optimization genetic algorithm to fuse the accuracy index and stability index that preevaluates the inversion effect and solves the best combination of active sources to improve the inversion effect. At the same time, experimental studies were carried out. After the active source was added, the inversion accuracy of high-stress areas and the ability to resist data errors were greatly improved. The inversion results were more in line with the law of stress transfer and stress distribution, which enhanced the connectivity of the low-velocity area of the fault zone or weakened zone.

Improvement of the accuracy of active seismic tomography by eliminating blasting delay in coal mines

Improvement of the accuracy of active seismic tomography by eliminating blasting delay in coal mines

Stress Redistribution in a Longwall Yield Pillar — a Comparison between Active Seismic Tomography and Theory

Stress Redistribution in a Longwall Yield Pillar — a Comparison between Active Seismic Tomography and Theory

Geophysical and analytical determination of overstressed zones in exploited coal seam: a case study

A variety of geophysical methods and analytical modeling are applied to determine the rockburst hazard in Polish coal mines. In particularly unfavorable local conditions, seismic profiling, active/passive seismic tomography, as well as analytical state of stress calculating methods are recommended. They are helpful in verifying the reliability of rockburst hazard forecasts. In the article, the combined analysis of the state of stress determined by active seismic tomography and analytical modeling was conducted taking into account the relationship between the location of stress concentration zones and the level of rockburst hazard. A longwall panel in the coal seam 501 at a depth of ca.700 m in one of the hard coal mines operating in the Upper Silesian Coal Basin was a subject of the analysis. The seismic tomography was applied for the reconstruction of P-wave velocity fields. The analytical modeling was used to calculate the vertical stress states basing on classical solutions offered by rock mechanics. The variability of the P-wave velocity field and location of seismic anomaly in the coal seam in relation to the calculated vertical stress field arising in the mined coal seam served to assess of rockburst hazard. The applied methods partially proved their adequacy in practical applications, providing valuable information on the design and performance of mining operations.

Fault sealing and caprock integrity for CO<sub>2</sub> storage: an in situ injection experiment

Abstract. The success of geological carbon storage depends on the assurance of permanent containment for injected carbon dioxide (CO2) in the storage formation at depth. One of the critical elements of the safekeeping of CO2 is the sealing capacity of the caprock overlying the storage formation despite faults and/or fractures, which may occur in it. In this work, we present an ongoing injection experiment performed in a fault hosted in clay at the Mont Terri underground rock laboratory (NW Switzerland). The experiment aims to improve our understanding of the main physical and chemical mechanisms controlling (i) the migration of CO2 through a fault damage zone, (ii) the interaction of the CO2 with the neighboring intact rock, and (iii) the impact of the injection on the transmissivity in the fault. To this end, we inject CO2-saturated saline water in the top of a 3 m thick fault in the Opalinus Clay, a clay formation that is a good analog of common caprock for CO2 storage at depth. The mobility of the CO2 within the fault is studied at the decameter scale by using a comprehensive monitoring system. Our experiment aims to close the knowledge gap between laboratory and reservoir scales. Therefore, an important aspect of the experiment is the decameter scale and the prolonged duration of observations over many months. We collect observations and data from a wide range of monitoring systems, such as a seismic network, pressure temperature and electrical conductivity sensors, fiber optics, extensometers, and an in situ mass spectrometer for dissolved gas monitoring. The observations are complemented by laboratory data on collected fluids and rock samples. Here we show the details of the experimental concept and installed instrumentation, as well as the first results of the preliminary characterization. An analysis of borehole logging allows for identifying potential hydraulic transmissive structures within the fault zone. A preliminary analysis of the injection tests helped estimate the transmissivity of such structures within the fault zone and the pressure required to mechanically open such features. The preliminary tests did not record any induced microseismic events. Active seismic tomography enabled sharp imaging the fault zone.

APASVO: A free software tool for automatic P-phase picking and event detection in seismic traces

The accurate estimation of the arrival time of seismic waves or picking is a problem of major interest in seismic research given its relevance in many seismological applications, such as earthquake source location and active seismic tomography. In the last decades, several automatic picking methods have been proposed with the ultimate goal of implementing picking algorithms whose results are comparable to those obtained by manual picking. In order to facilitate the use of these automated methods in the analysis of seismic traces, this paper presents a new free, open source, software graphical tool, named APASVO, which allows picking tasks in an easy and user-friendly way. The tool also provides event detection functionality, where a relatively imprecise estimation of the onset time is sufficient. The application implements the STA-LTA detection algorithm and the AMPA picking algorithm. An autoregressive AIC-based picking method can also be applied. Besides, this graphical tool is complemented with two additional command line tools, an event picking tool and a synthetic earthquake generator. APASVO is a multiplatform tool that works on Windows, Linux and OS X. The application can process data in a large variety of file formats. It is implemented in Python and relies on well-known scientific computing packages such as ObsPy, NumPy, SciPy and Matplotlib.

Electric effects induced by artificial seismic sources at Somma-Vesuvius volcano

<p>In this paper, we present a series of self-potential measurements at Somma-Vesuvius volcanic area acquired in conjunction with an active seismic tomography survey. The aim of our study is both to provide further confirmation to the occurrence of seismo-electric coupling and to identify sites suitable for self-potential signal monitoring at Somma-Vesuvius district. The data, which were collected along two perpendicular dipoles, show significant changes on the natural electric field pattern. These variations, attributable to electrokinetic processes triggered by the artificial seismic waves, were observed after explosions occurred at a distance less than 5 km from the SP dipole arrays. In particular, we found that the NW-SE component of the natural electric field was more sensible to the shots than the NE-SW one, and the major effects did not correspond to the nearest shots. Such evidences were interpreted considering the underground electrical properties as deduced by previous detailed resistivity and self-potential surveys performed in the study area.</p>

Crustal structure of northern Gran Canaria, Canary Islands, deduced from active seismic tomography

Seismic P-wave travel times collected during METEOR cruise M24 are inverted to derive a three-dimensional model of the P-wave velocity structure of the northern part of Gran Canaria, Canary Islands. The data consist of 6689 P-wave travel times from 1487 offshore air-gun shots which were recorded by both land-based seismometers and ocean bottom hydrophones. The crustal structure is well imaged by the data set as demonstrated by analysis of the resolution and tests with synthetic data. The volcanic island is characterized by generally high P-wave velocities (>5.5 km/s) and a heterogeneous structure with large lateral velocity variations. High P-wave velocities are found around the centers of the Miocene shield volcanoes in the vicinity of Agüimes, San Nicolas, and Agaete as well as the center of the Pliocene Roque Nublo volcano. The velocity structure suggests a high percentage of dense intrusive rocks. Some of the intrusive rocks were emplaced during the eruption of >1000 km 3 of Miocene felsic magmas following the basaltic shield phase. The velocity structure beneath La Isleta peninsula and its submarine continuation is interpreted as a volcanic rift zone with abundant dikes. The velocities decrease to <5 km/s north of the coastline. A high velocity zone thinning away from the central edifice is interpreted as the massive island flank extending up to 50 km off the coast which is underlain by prevolcanic Neogene–Jurassic sediments. The igneous part of the oceanic crust exhibits an anomalous structure with a relatively small thickness (∼3 km) layer 3 and a 2–4-km-thick layer 2, probably reflecting a modification of the crust due to long-lasting magmatic intrusive activity during the evolution of the Canary Islands. The Moho north of Gran Canaria is found at a depth of ∼15 km. The structure of Gran Canaria and the adjacent ocean basin is thought to be the result of a diffuse mantle upwelling under a slowly moving plate.

Space and Time Behavior of Seismic Activity at Mt. Vesuvius Volcano, Southern Italy

We analyze the space and time behavior of seismicity at Mt. Vesuvius during the last 20 yr to characterize the seismic regime of the volcano during the present quiescent period. The new results on the volcano structure inferred from active seismic tomography experiments, the newly implemented 10-yr arrival time catalog, and a high-quality digital waveform data set have been analyzed. The background seismicity is concentrated near and beneath the Mt. Vesuvius crater, with depths lying above and below the discontinuity, which marks the transition from the shallow alluvium/volcanic sediments and the Mesozoic carbonate basement. The focal mechanisms of microearthquakes show variable stress-axis orientations as a function of depth, although there is evidence for a clustering around roughly the north–south to vertical directions for the tension axes and east-southeast–west-northwest to vertical directions for the pressure axes. The statistical analysis of the seismic catalog confirmed the tendency of background seismicity to cluster in time, according to a trigger model (as denoted by Vere-Jones and Davies, 1966), that is, the generalized Poisson process. A significant increase of the average seismic energy release with time is observed, which is related to the occurrence of several M D > 3 events in the past 10 yr, accompanied by intense swarm activity. This is consistent with the decrease of the b -value from about 2 to 1 during the same period. The ( M D > 3) events are located in the same area and depth range of the whole seismicity, and their fault-plane solutions also show variable stress-axis and nodal-plane orientations. In particular, the moment tensor inversion of P and S waveforms from the largest earthquake in the catalog ( M D 3.6, on 9 October 1999) shows no significant departure from a pure shear, double-couple mechanism, thus suggesting a dominant tectonic-like fracture mechanism. The decrease of parameter b with time is interpreted as a dominant effect of fluid pressure variations on the present seismic regime at Mt. Vesuvius, which could be driven by the progressive cooling of the volcanic system.

Three‐dimensional structure of the Torngat Orogen (NE Canada) from active seismic tomography

The crustal velocity structure and the Moho depth of the Proterozoic Torngat Orogen, NE Canada, is determined by active seismic tomography using travel times of crustal turning rays and Moho reflections. The orogen developed during oblique convergence of the Archean Superior and Nain Provinces, which trapped an interior belt of Archean crust (Core Zone) between them, with the Torngat Orogen evolving between the Core Zone and the Nain Province. Beneath the central orogen a crustal root is found with a preserved depth of &gt;52 km and a width of ∼100 km. To the north, the root shallows to &lt;44 km and narrows to a width of ∼45 km. The root correlates with a set of major, late orogenic shear zones that accommodated oblique convergence of the Superior and Nain Provinces. This correlation suggests that the transpressional shearing focused strain in the region of the root and contributed to the crustal thickening. Absence of postorogenic magmatic activity prevented reworking or thermal relaxation of the root. The lack of late magmatism is probably related to the depleted and refractory nature of the Archean lithosphere underlying the orogen. Upper crustal velocities are lowest in the Core Zone (∼5.7 km/s at the surface) and are compatible with laboratory measurements carried out on gneissic rock samples from that area. Higher velocities in the Nain Province (∼5.9 km/s) correlate with felsic gneiss and anorthosite rock samples. A high‐velocity region immediately to the north of the crustal root is associated with a Moho uplift (∼34 km). This is explained by extension along the Ungava transform fault, and possibly in Hudson Strait, at ∼55 Ma when rifting in the Labrador Sea was transferred into Baffin Bay.

Seismic attenuation structure of the East Pacific Rise near 9°30′N

We describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9°32′N on the East Pacific Rise. Over 3500 estimates of t* are obtained from 0.4‐ to 0.7‐s‐long windows aligned with crustal P phases, including diffractions above and below a midcrustal magma chamber and Moho reflections which cross the rise axis at a range of lower crustal depths. We apply a smoothest model inversion algorithm to the t* measurements to derive images of apparent crustal Q−1 both 20 km off axis and within a 16×16 km area centered on the rise crest. The models resolve regions of high attenuation in the uppermost crust and in a low‐Q zone which extends from midcrustal to lower‐crustal depths beneath the rise axis. Off axis, Q values in the upper 1 km average 35–50, while at depths greater than 2–3 km Q is at least 500–1000. The high levels of attenuation in the uppermost crust probably result from the combined effect of factional, fluid flow, and scattering mechanisms. Within 1–3 km of the rise axis, Q increases markedly in the uppermost 1 km to about 65. If the increase in attenuation off axis is entirely due to the ∼300‐m increase in the thickness of layer 2A extrusives required by seismic velocity measurements, then Q in layer 2A must be about 10–20. No measurements of Q are obtained in the immediate vicinity of the 1.6‐km‐deep axial magma lens because no wave paths cross the rise axis through this region. The diffractions beneath the magma chamber and the Moho reflections require a low‐Q region, with minimum Q values of 20–50, which extends from no more than 2.5 km depth to the base of the crust. These values are similar to laboratory measurements of Q obtained at solidus temperatures and constrain the low‐Q region to contain no more than a few percent melt. The axial magma chamber, which comprises a melt lens and an underlying crystal mush zone, must be confined to a narrow, 1‐km‐thick region through which no rise‐crossing paths pass. Inversions for along‐axis structure in the low‐Q anomaly show a 20–25% increase in attenuation at 2–3 km depth north of 9°34′N but resolve no such trend at 4–6 km depth. The along‐axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along‐axis variations in magma supply.