Seismic Amplitude Measurements Research Articles

Examining 22 Years of Ambient Seismic Wavefield at Mount St. Helens

Abstract An increase in seismic activity precedes most volcanic eruptions. Whereas event-based forecasting approaches have been successful, some eruptions remain unanticipated, resulting in casualties and damage. Our study leverages the recent advancements in ambient field seismology. We explore features extracted from continuous ambient fields using traditional methods, for example, peak ground velocity, peak ground acceleration, root mean square, root median square, real-time seismic amplitude measurement, and novel methods (displacement seismic amplitude ratio and spectral width). In addition, we explore unsupervised learning of higher order wavelet features using scattering networks. We find that combining all the methods was necessary to disentangle the effects of seismic sources from structural changes at Mount St. Helens. Although the ambient wavefield-based approach does not yield additional or more significant precursory signals than event-based methods at Mount St. Helens, our study demonstrates that the ambient wavefield provides supplementary information, mainly about structural changes and complements traditional methods. The ambient seismic wavefield offers additional insights into long-lasting processes. We find enhanced wave attenuation correlating with geochemical measurements. We interpret this as ongoing structural changes, such as dome growth or the evolution of the volcanic conduit system. On annual and decadal timescales, we interpret seasonal seismic attenuation in the shallow subsurface as groundwater fluctuations, corroborated by observations at the nearby Spirit Lake level. This multimethod approach at Mount St. Helens sheds light on a volcanic system’s underlying dynamics and structure.

Enhanced Reservoir Description via Areal Data Integration and Reservoir Fluid Geodynamics: A Case Study From Deepwater Gulf of Mexico

Accurate reservoir characterization is vital for effective decisions made throughout the life cycle of an oilfield reservoir, including management and development. Of all the components of reservoir description, hydraulic connectivity carries the highest amount of uncertainty, where inaccurate connectivity evaluation often results in production underperformance. Shortcomings are faced when applying conventional approaches of connectivity assessment. Seismic surveys are not always sufficient to evaluate lateral connectivity as detected faults can be transmissive or partially transmissive, while some faults are below the detection limits of seismic amplitude measurements. Vertical connectivity represents another uncertainty, where pressure measurements and well logs are often either unable to detect the baffles along oil columns or cannot assess whether detected baffles are relevant seals or flow diverters. Although conventional downhole fluid analysis (DFA) workflows have proven effective in delineating reservoir connectivity, enough DFA data are not always available, and with added complexity, uncertainties arise. Additionally, while equilibrated asphaltene gradients, measured through DFA probes, imply connectivity, ongoing reservoir fluid geodynamics (RFG) processes, such as current hydrocarbon charging, can preclude equilibration in a connected reservoir. Thus, a comprehensive assessment approach, that utilizes all available data streams, is needed to overcome the significant spatial complexity associated with moderately and heavily faulted reservoirs. In this paper, we employed our recently introduced interpretation workflow to evaluate the connectivity of a heavily faulted reservoir in the deepwater Gulf of Mexico. The field was divided into five investigation areas penetrated by 12 wells. Areal downhole fluid analysis (ADFA) was applied to assess local connectivity leading to reservoir-scale connectivity. Through integrating fluid/dynamic and rock/static data, each data type provided insights that were pieced together to enhance consistency and reduce uncertainty. Analyzed data included pressure-volume-temperature (PVT) reports, pressure surveys, well logs, and geochemistry. The study resulted in a verifiable connectivity description where faults, previously regarded as sealing, were classified into sealing or partially transmissive faults; unresolved faults were detected. Fault-block migration was detected, and fault throw was estimated; asphaltenes behavior was used to deduce original field structures prior to faulting. We also examined RFG processes to investigate oil biodegradation, where an asphaltene clustering trend was observed, causing high oil viscosities toward the bottom of one sandstone. A correlation was then derived and successfully implemented to estimate oil viscosity.

Effects of the 2021 La Palma volcanic eruption on groundwater resources (part I): Hydraulic impacts

The 2021 volcanic eruption in the Cumbre Vieja mountain range on La Palma Island (Canary Islands, Spain) raised concerns regarding the potential impact on groundwater resources. This study is the first part of a series of papers investigating those impacts, and focuses on the hydraulic impacts of the eruption, while subsequent papers will explore the geochemical consequences. Three boreholes equipped with sensors to measure hydraulic head, temperature, and electrical conductivity of groundwater were installed near the volcano. Monitoring started during the eruption and continued a year after it. Statistical analysis were performed to assess the relationship between the measured variables and real-time seismic-amplitude measurements (RSAM). In addition, the possibility of groundwater vaporization due to magma emergence was assessed with a groundwater flow numerical modelling of the island. Correlation coefficients were computed to assess the linear relationship between groundwater parameters and seismic signals, observing a statistically significant association, and suggesting near-instantaneous variations in parameters such as groundwater levels and EC. Different response patterns of groundwater levels were observed in recharging areas in highlands compared to discharge areas, showing an opposite correlation direction. Deduction of natural trends from the linear regression models of head and RSAM two months after the eruption revealed a more predictable impact on the groundwater system, as the hydrogeological system adjusts to the volcanic activity and its effects on the aquifer. The hydrogeological simulation of the "magma pumping effect" suggested that groundwater extraction was possible, but the absence of an appropriate groundwater monitoring network made it impossible to determine the amount of water extracted from the aquifer. The uncertainty analysis showed values up to 2000 m3·day−1. These findings have important implications for understanding the negative impacts of volcanic eruptions on groundwater resources, highlighting the need for regular monitoring and assessment by hydrogeologists and water management professionals.

Seismic signature of a river flooding in La Réunion Island during the tropical cyclone Dumazile (March 2018)

Monitoring of sediment transport during extreme flood events is difficult and often impossible. Fluvial seismology can provide constraints on the mechanisms of this transport and on the seismic sources but few cases of application in context of extreme events such as tropical cyclones were realized. Recordings from three seismic stations temporarily installed along a river (the Rivière du Mât) located in La Réunion Island (Indian Ocean) are analysed to characterize high-frequency (> 1 Hz) seismic noise induced by the extreme flood generated by the tropical cyclone Dumazile (March 2018). We evidence a good correlation (r2 = 0.94) between the amplitude of the seismic signal and the water level at lower frequencies (2–7 Hz), particularly during the rising limb of the river flood. The relationship between seismic amplitude and water level measurements tested at multiple frequency ranges are consistent with a signal dominated by water flow at lower frequencies and by sediment transport at higher frequencies (15–45 Hz). We show that the use of seismic measurement, particularly at stations located very close to the riverbed can provide comprehensive information on the mechanisms involved during sediment transport associated with extreme flood events when direct measurements are not possible.

Analysis of magma flux and eruption intensity during the 2021 explosive activity at La Soufrière, St Vincent, West Indies

Abstract Real-time Seismic Amplitude Measurement signals and eruption cloud height measurements were used to estimate peak intensities of 40 explosive events during the 8–22 April 2021 activity of La Soufrière volcano. We estimated magma supply rates and erupted volumes in each explosion, characterized uncertainty by stochastic modelling and identified four eruptive stages. Stage 1 included an intense period of 9.5 hours with 11 explosive events with peak eruption intensity between 2000 and 4000 m 3 s −1 and magma supply rate reaching 828 m 3 s −1 . Twelve high-intensity explosions ( c. 4000 m 3 s −1 ) occurred in Stage 2 with average magma supply rate of 251 m 3 s −1 . Stage 3 involved both declining intensity and magma supply rate and lengthening repose periods between explosions. Stage 4 involved three much weaker explosions. The total erupted volume of magma is estimated at 38.5 × 10 6 m 3 (90% credible interval: [22.0 .. 61.9] × 10 6 m 3 ) consistent with independent estimates from analysis of tephra deposits and volcano subsidence sourced at c. 6 km depth. The 150-fold increase in magma supply rate, from the preceding effusive phase to Stage 1 of the explosive phase, is attributed to replacement of very high-viscosity degassed magma occupying the shallow conduit system with new, lower-viscosity, volatile-rich magma from the magma chamber.

Forecasting and Seismic Detection of Proglacial Debris Flows at Mount Rainier National Park, Washington, USA

ABSTRACT The glaciated Mount Rainier volcano in southwestern Washington State (United States) has a rich history of outburst floods and debris flows that have adversely impacted infrastructure at Mount Rainier National Park in the 20th and 21st centuries. Retreating glaciers leave behind vast amounts of unconsolidated till that is easily mobilized during high-precipitation-intensity storms in the fall months, and during outburst floods during warm summer months. Over 60 debris flows and outburst floods have been documented between 1926 and 2019 at Mount Rainier. Debris-flow activity has led to the closure of campgrounds and visitor destinations, which has limited visitor access to large swaths of the park. This paper documents efforts to characterize and seismically monitor debris flows, map hazards, and develop forecasting approaches for wet and dry weather debris flows. Using the day-of and historic antecedent weather conditions on past debris-flow days, we developed a debris-flow hazard model to help predict those days with a higher relative hazard for debris-flow activity park-wide based on prevailing and forecasted weather conditions. Debris flows are detected in near-real-time using the U.S. Geological Survey Real-time Seismic Amplitude Measurement (RSAM) tool. If an event is detected, we can then provide evacuation alerts to employees and visitors working and recreating in the areas downstream. Our goal is to accurately forecast the debris-flow hazards up to 7 days ahead of time and then use RSAM to detect debris flows within minutes of their genesis.

Capturing, Preserving, and Digitizing Legacy Seismic Data from the Montserrat Volcano Observatory Analog Seismic Network, July 1995–December 2004

Abstract An eruption of the Soufrière Hills Volcano (SHV) on the eastern Caribbean island of Montserrat began on 18 July 1995 and continued until February 2010. Within nine days of the eruption onset, an existing four-station analog seismic network (ASN) was expanded to 10 sites. Telemetered data from this network were recorded, processed, and archived locally using a system developed by scientists from the U.S. Geological Survey (USGS) Volcano Disaster Assistance Program (VDAP). In October 1996, a digital seismic network (DSN) was deployed with the ability to capture larger amplitude signals across a broader frequency range. These two networks operated in parallel until December 2004, with separate telemetry and acquisition systems (analysis systems were merged in March 2001). Although the DSN provided better quality data for research, the ASN featured superior real-time monitoring tools and captured valuable data including the only seismic data from the first 15 months of the eruption. These successes of the ASN have been rather overlooked. This article documents the evolution of the ASN, the VDAP system, the original data captured, and the recovery and conversion of more than 230,000 seismic events from legacy SUDS, Hypo71, and Seislog formats into Seisan database with waveform data in miniSEED format. No digital catalog existed for these events, but students at the University of South Florida have classified two-thirds of the 40,000 events that were captured between July 1995 and October 1996. Locations and magnitudes were recovered for ∼10,000 of these events. Real-time seismic amplitude measurement, seismic spectral amplitude measurement, and tiltmeter data were also captured. The result is that the ASN seismic dataset is now more discoverable, accessible, and reusable, in accordance with FAIR data principles. These efforts could catalyze new research on the 1995–2010 SHV eruption. Furthermore, many observatories have data in these same legacy data formats and might benefit from procedures and codes documented here.

Design Of Real-Time Seismic Amplitude Measurement (RSAM) System Using Geophone as the Detection of Seismic Vibration

Indonesia merupakan salah satu negara dengan gunungapi terbanyak di dunia. Rangkaian gunungapi membentang dari Pulau Sumatera, Jawa, Bali dan Nusa Tenggara. Dari kepulauan di Laut Bdana sampai bagian utara Pulau Sulawesi merupakan daerah gunungapi terpanjang di dunia. Indonesia memiliki 129 gunungapi aktif, 70 buah diantaranya berancaman dan 500 buah tidak aktif. Sejak tahun 1800, paling tidak telag tercatat 600 kali letusan oleh 70 gunungapi di Indonesia. Karena itu, Indonesia dapat dikatakan sebagai negara yang rawan bencana, terutama ancaman bencana gunung berapi. Pengamatan, pemantauan, pencatatan, penyebaran informasi dan peringatan tanda bahaya terhadap aktivitas gunungapi menjadi salah satu fokus dalam penanggulangan bencana gunungapi. Untuk mendukung hal tersebut diperlukan instrumentasi deteksi aktivitas vulkanik gunungapi. Salah satu instrumentasi yang digunakan untuk memantau aktivitas gunung berapi Gunung Merapi adalah Real-time Seismic Amplitude Measurement (RSAM). RSAM adalah sistem yang menyediakan pengukuran kontinu dari amplitudo seismik rata-rata absolut dari jumlah stasiun seismik. RSAM menempati peran strategis dalam memantau aktivitas seismik gunung berapi terutama pada saat krisis sebelum letusan. Dalam penelitian ini, perancangan sistem RSAM menggunakan sensor geophone untuk mendeteksi getaran seismik. Frekuensi tes yang diberikan dalam penelitian ini termasuk 10 Hz, 30 Hz, dan 50 Hz. Sistem yang telah dirancang bangun dapat mendeteksi frekuensi dengan baik sebagaimana dibuktikan bahwa semakin besar frekuensi uji maka semakin banyak output grafik yang dihasilkan. Selain itu, besaran yang dihasilkan juga semakin besar. Pada frekuensi 10 Hz, magnitudo yang dihasilkan adalah 0,997, pada 30 Hz magnitudo yang dihasilkan 1,559 sedangkan pada 50 Hz magnitudo yang dihasilkan adalah 1,962. Sistem RSAM yang dirancang bangun menghasilkan amplitudo yang memiliki hubungan linier dengan besarnya getaran sumber disediakan..

Short-Term Forecasting and Detection of Explosions During the 2016–2017 Eruption of Bogoslof Volcano, Alaska

We describe a multidisciplinary approach to forecast, rapidly detect, and characterize explosive events during the 2016–2017 eruption of Bogoslof volcano, a back-arc shallow submarine volcano in Alaska’s Aleutian arc. The eruptive sequence began in December 2016 and included over 60 discrete explosive events. Because the volcano has no local monitoring stations, we used distant stations on the nearest volcanoes, Okmok (54 km) and Makushin (72 km), combined with regional infrasound sensors and lightning detection from the Worldwide Lightning Location Network (WWLLN). Monitoring of activity used a combination of scheduled checks combined with automated alarms. Alarms triggered on real-time data included real-time seismic amplitude measurement (RSAM); infrasound from several arrays, the closest being on Okmok; and lightning strokes detected from WWLLN within a 20-km radius of the volcano. During periods of unrest, a multidisciplinary response team of four people fulfilled specific roles to evaluate geophysical and remote-sensing data, event-specific ash-cloud dispersion modeling, interagency coordination, and development and distribution of formalized warning products. Using this approach, for events that produced ash clouds ≥7.5 km above sea level, AVO called emergency response partners 15 minutes, and issued written notices 30 minutes, after event onset (mean times). Factors that affect timeliness of written warnings include event size and number of data streams available; bigger events and more data both decrease uncertainty and allow for faster warnings. In remote areas where airborne ash is the primary hazard, the approach used at Bogoslof is an effective strategy for hazard mitigation.

Evaluating Local Distance Seismic Amplitude Measurements as Predictors of Event Magnitude in a Tectonically Complex Setting

Evaluating Local Distance Seismic Amplitude Measurements as Predictors of Event Magnitude in a Tectonically Complex Setting

Using a process-based model of pre-eruptive seismic patterns to forecast evolving eruptive styles at Sinabung Volcano, Indonesia

Most volcanoes worldwide are not monitored in real-time; for those that are, patterns of pre-eruptive earthquakes coupled with conceptual models of magma ascent enable short-term forecasting of eruption onset. Basic event locations, characterization of background seismicity, and recognition of changes in earthquake types and energy release are most important to successful eruption forecasting. During renewed activity at Sinabung volcano, Indonesia, this approach was used by the Center for Volcanology and Geological Hazards Mitigation (CVGHM) and the USGS Volcano Disaster Assistance Program to forecast eruption onset, identify changes in eruptive styles and raise or lower alert levels and extend or contract evacuation zones. After >400years of quiescence, Sinabung began erupting in August 2010. The volcano was unmonitored at the onset of these eruptions, which were phreatic, but soon after a monitoring network was installed by CVGHM. Increasing swarms of high-frequency volcano tectonic (VT) earthquakes were used to forecast continuing phreatic eruptions. Volcanic activity decreased in mid-September 2010, while additional intrusions at depth (inferred from continued distal VT swarms) continued through September 2013, when explosive phreatic eruptions recurred. Explosive eruptions were forecast based on increases in real-time seismic amplitude measurement (RSAM) and VT seismicity. Seismicity changed markedly in late November and early December 2013 with the occurrence of deep earthquakes and an overall transition from low-frequency (LF) dominated and irregular (in time and magnitude) earthquakes to more regular LF and hybrid seismicity – a transition that accompanied the continued rise, eventual emergence and growth of a lava dome in the summit crater. This lava dome was first observed on 18 December. In late December 2013 to early January 2014, the eruptive style changed again as additional ascending magma deformed the summit and the dome grew beyond the capacity of the summit crater, resulting in the en masse collapse of the lava dome (2Mm3) on 11 January and the largest pyroclastic flow to date. The collapse was forecast on the basis of a several order of magnitude increase in RSAM, continued strong distal VT seismicity, an increase in proximal seismicity, and large-scale observed deformation of the summit area. Similarly, a later collapse of a second summit lava dome on 1 February 2014 was forecast on the basis of increased distal seismicity. Here, we demonstrate how a process-based volcano seismicity model was used in combination with real-time data to forecast the time and magnitude of eruptions, as well as changes in eruption style.

Linking SO2 emission rates and seismicity by continuous wavelet transform: implications for volcanic surveillance at San Cristóbal volcano, Nicaragua

San Cristobal volcano is the highest and one of the most active volcanoes in Nicaragua. Its persistently high activity during the past decade is characterized by strong degassing and almost annual VEI 1–2 explosions, which present a threat to the local communities. Following an eruption on 8 September 2012, the intervals between eruptions decreased significantly, which we interpret as the start of a new eruptive phase. We present here the results of semi-continuous SO2 flux measurements covering a period of 18 months, obtained by two scanning UV-DOAS instruments installed as a part of the network for observation of volcanic and atmospheric change project, and the results of real-time seismic amplitude measurements (RSAM) data. Our data comprise a series of small to moderately explosive events in December 2012, June 2013 and February 2014, which were accompanied by increased gas emissions and seismicity. In order to approach an early warning strategy, we present a statistical method for the joint analysis of gas flux and seismic data, by using continuous wavelet transform and cross-wavelet transform (XWT) methods. This analysis shows that the XWT coefficients of SO2 flux and RSAM are in good agreement with the occurrence of eruptive events and thus may be used to indicate magma ascent into the volcano edifice. Such multi-parameter surveillance efforts can be useful for the interpretation and surveillance of possible eruptive events and could thus be used by local institutions for the prediction of upcoming volcanic unrest.

Changes in seismic velocity during the first 14 months of the 2004–2008 eruption of Mount St. Helens, Washington

AbstractMount St. Helens began erupting in late 2004 following an 18 year quiescence. Swarms of repeating earthquakes accompanied the extrusion of a mostly solid dacite dome over the next 4 years. In some cases the waveforms from these earthquakes evolved slowly, likely reflecting changes in the properties of the volcano that affect seismic wave propagation. We use coda‐wave interferometry to quantify small changes in seismic velocity structure (usually <1%) between two similar earthquakes and employed waveforms from several hundred families of repeating earthquakes together to create a continuous function of velocity change observed at permanent stations operated within 20 km of the volcano. The high rate of earthquakes allowed tracking of velocity changes on an hourly time scale. Changes in velocity were largest near the newly extruding dome and likely related to shallow deformation as magma first worked its way to the surface. We found strong correlation between velocity changes and the inverse of real‐time seismic amplitude measurements during the first 3 weeks of activity, suggesting that fluctuations of pressure in the shallow subsurface may have driven both seismicity and velocity changes. Velocity changes during the remainder of the eruption likely result from a complex interplay of multiple effects and are not well explained by any single factor alone, highlighting the need for complementary geophysical data when interpreting velocity changes.

Using volcanic tremor for eruption forecasting at White Island volcano (Whakaari), New Zealand

Eruption forecasting is a challenging task because of the inherent complexity of volcanic systems. Despite remarkable efforts to develop complex models in order to explain volcanic processes prior to eruptions, the material Failure Forecast Method (FFM) is one of the very few techniques that can provide a forecast time for an eruption. However, the method requires testing and automation before being used as a real-time eruption forecasting tool at a volcano. We developed an automatic algorithm to issue forecasts from volcanic tremor increase episodes recorded by Real-time Seismic Amplitude Measurement (RSAM) at one station and optimised this algorithm for the period August 2011–January 2014 which comprises the recent unrest period at White Island volcano (Whakaari), New Zealand. A detailed residual analysis was paramount to select the most appropriate model explaining the RSAM time evolutions. In a hindsight simulation, four out of the five small eruptions reported during this period occurred within a failure window forecast by our optimised algorithm and the probability of an eruption on a day within a failure window was 0.21, which is 37 times higher than the probability of having an eruption on any day during the same period (0.0057). Moreover, the forecasts were issued prior to the eruptions by a few hours which is important from an emergency management point of view. Whereas the RSAM time evolutions preceding these four eruptions have a similar goodness-of-fit with the FFM, their spectral characteristics are different. The duration–amplitude distributions of the precursory tremor episodes support the hypothesis that several processes were likely occurring prior to these eruptions. We propose that slow rock failure and fluid flow processes are plausible candidates for the tremor source of these episodes. This hindsight exercise can be useful for future real-time implementation of the FFM at White Island. A similar methodology could also be tested at other volcanoes even if only a limited network is available.

Introduction to this special section: Well geosteering

Drilling operations typically account for the largest percent of the expenses incurred in the exploration and development of hydrocarbon reservoirs. Modern well placement and real-time navigation methods take into account uncertainties in a predefined “earth model” to reach and produce hydrocarbon-bearing rocks in an efficient manner. Because the construction of the initial earth model is commonly based on low-spatial-resolution data such as seismic-amplitude measurements or is from extrapolation of conditions encountered in nearby wells, the well trajectory needs to be adjusted continuously as the drill bit encounters rocks formations, geometric conditions, and rock properties unaccounted for by the initial earth model. Most modern drilling operations are accompanied not only by measurements while drilling (e.g., rate of penetration, weight on bit, torque, and so forth) but also by logging-while-drilling (LWD) measurements such as electrical resistivity, density, neutron-porosity, acoustic velocities, and magnetic resonance, among others. The latter measurements are placed in close proximity to the drill bit and hence provide useful information to modify the well trajectory in real time to maximize drilling efficiency and to steer the well toward the best storage and production zones.

Introduction to this special section: Borehole geophysics and sonic logging

Sonic logs are most frequently used to tie surface and borehole seismic amplitude measurements to P- and S-wave velocities encountered along wellbore trajectories. Other common uses of sonic logs include the estimation of dynamic elastic properties for geomechanical analysis, such as wellbore stability studies and the design and planning of hydrofracturing operations. Modern sonic logs are acquired with multiple transmitters and an array of closely spaced receivers (anywhere between eight and 13 receivers) in the form of time waveforms. Sonic transmitters can come in the form of monopole, dipole, and quadrupole actuators. By design, sonic transmitters are immersed in a fluid — the borehole mud — whereby formation shear waves can be detected and quantified only by the elastic coupling that exists between wave motion taking place in the borehole fluid and in the surrounding rock formations. Such an elastic coupling gives rise to markedly frequency-dispersive behavior of the detected waves that presents some technical challenges when one interprets their speeds.

Varying seismic‐acoustic properties of the fluctuating lava lake at Villarrica volcano, Chile

AbstractVillarrica volcano outgasses through an open lava lake, with bubbles ranging in size from submillimeter to several meters, the largest of which produce strombolian bursting events that are visible from the crater rim. Thousands of shallow strombolian events identified through seismic waveform cross correlation were found to produce discrete and repetitive long‐period seismic and infrasonic signals. We identified variations of up to 0.7 s in seismic‐acoustic arrival delay times between April and July 2010 at a station ~750 m from the vent, which we interpret as due to fluctuations in the level of lava lake. During time periods interpreted as having high lava lake levels, based on reduced time delays, interevent times were also reduced, and average seismic amplitude measurements, seismic and acoustic event energies, and volcano acoustic‐seismic ratios were all high as compared to times when the lava lake was lower. The crater is also a source of nearly continuous, monotonic infrasonic tremor. We found that the peak frequency of this infrasonic tremor, typically around 0.5–1.0 Hz, was inversely correlated with seismic‐acoustic delay times and therefore an indicator of lava lake level. We use this correlation to propose a new model for infrasonic tremor generation, namely, using crater geometry to approximate a Bessel horn. We interpret the two clearest cycles of elevated seismicity and lava lake level as due to an increase in exsolved gas, resulting from an injection of volatile‐rich magma or an overturn in a deeper magma reservoir.

The ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009): Introduction

In this introductory article we give a general description of the ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009). We also provide the background for four further articles that describe the effort in collecting and digitizing parametric earthquake bulletin data as well as the methodologies developed to compute homogeneous earthquake parameters.The result of the two and a half year project is a catalogue of approximately 20,000 large earthquakes covering 110years with hypocentres and uncertainties computed using the same technique and velocity model. We show that the overall homogeneity of the main earthquake parameters in the catalogue was achieved despite changes in instrumentation and routine measurement practices concurrent with developments in instrumental seismology from almost a ground level to its current state.For each earthquake, MW magnitude values and uncertainties were computed either based on available estimates of seismic moment or using new empirical relationships between MW, MS and mb. Further important results of this project include the electronic availability of a considerable volume of seismic wave arrival time and amplitude measurements from early instrumental printed station bulletins. These newly recovered amplitude measurements provided a basis for computation of many previously unavailable MS magnitudes with uncertainties.In this article we describe why such a catalogue is required for a comprehensive assessment of global and regional seismic hazard. We also describe other potential uses of the catalogue in many other fields of Earth Sciences. We discuss the catalogue availability and lay out the plans of further development.

Introduction to this special section: Unconventional resources technology

The detection, appraisal, and development of unconventional resources invoke some of the greatest technical challenges faced by the hydrocarbon-producing industry today. There exist vast gas and oil reserves in organic mudrocks, but unlocking them in a financially and ecologically viable manner demands the combined and effective implementation of geologic, geophysical, geomechanical, drilling, and production concepts in ways that we have not witnessed before. The very genesis and diagenesis of kerogen give rise to large and complex spatial variations of the storage/flow properties of organic mudrocks, anywhere from nano-size flow confinements to macroscopic sedimentary sequencing (see the accompanying photograph of an outcrop of the Eagle Ford Formation in west Texas). Quantifying the production potential of organic mudrocks requires the integration of geochemical, petrophysical, mechanical, and elastic measurements in nontraditional ways. New effective-medium theories are needed to describe the mechanistic relationships among a multitude of core, borehole, and seismic amplitude measurements. It is necessary not only to detect and quantify presence of kerogen but also to infer geomechanical properties for reliable prediction of hydrofracture efficiency, proppant placement, and fluid stimulation. Experience also shows that the “release” of hydrocarbon present in mudrocks through hydrofractures often involves fast depletion and poor recovery factors. Much work and experience are still needed to assess hydrofracture efficiency and forecast hydrocarbon depletion via in situ or remote measurements (e.g., microseismic amplitude data).

Multiphysics borehole geophysical measurements, formation evaluation, petrophysics, and rock physics — Introduction

This special section showcases some emerging trends in the acquisition, processing, and interpretation of borehole geophysical measurements, with emphasis on multiphysics and multiscale applications that integrate geologic information, petrophysical properties, rock physics, and core measurements. Borehole geophysical measurements are routinely acquired to appraise, produce, and monitor subsurface resources, including fossil fuels, minerals, and water. They also are used in shallow reconnaissance projects, for civil-engineering studies, and for soil characterization and remediation, to cite a few salient applications. Wells usually contain the most dense and diverse types of information concerning geology, fluid flow, and rock physics, when assessing subsurface resources. During the last decade, borehole geophysical measurements have experienced significant developments in their acquisition, processing, and interpretation; they no longer can be viewed in isolation but rather as representing an important component of the quantitative description of petrophysical and rock physical properties under a geologic framework. Relatively recent entries to this endeavor are the petrophysical and geophysical appraisal/monitoring of subsurface CO 2 sequestration and the petrophysical/rock-physics characterization of hydrocarbon-bearing shale. It is expected that future interpretation challenges in borehole geophysics will consistently require the solid quantitative integration of measurements across disciplines and physical scales in formation-evaluation and rock-physics studies. The papers included in this issue consider topics as diverse as new multiphysics methods to combine electromagnetic and seismic amplitude measurements to estimate petrophysical properties between wells and the basis of Archie’s laws. Several of these contributions formally and rigorously integrate the physics of fluid flow in porous media with borehole geophysical measurements to estimate static and dynamic petrophysical properties. New rock-physics methods also are introduced to estimate elastic properties …