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31496 results in Solid Earth Geophysics

Page 83 of 1575

  • The first occurrence of the earliest species of Acadoparadoxides outside West Gondwana (Cambrian; Holy Cross Mountains, Poland)

  • JAKUB NOWICKI, ANNA ŻYLIŃSKA
  • Journal:
    Geological Magazine / Volume 156 / Issue 6 / June 2019
    Published online by Cambridge University Press:
    28 May 2018, pp. 1027-1051

  • Two hundred and eighty specimens of paradoxidids from two localities in the Holy Cross Mountains (Poland) have been reanalysed using morphometric techniques. Revision of the dataset provided evidence for the presence of two endemic taxa: Acadoparadoxides kozlowskii (Orłowski, 1959) and Acadoparadoxides samsonowiczi (Orłowski, 1959), both belonging to the earliest group of Acadoparadoxides, initially considered to be present only in Gondwanan successions. Thus, this is the first description of the members of this group outside West Gondwana. The taxonomic revision, and the presence of representatives of the earliest acadoparadoxidines coupled with the absence of taxa typical of Scandinavia that were previously described from this locality have resulted in the modification of the biostratigraphic scheme hitherto used in the Holy Cross Mountains. The newly established A. samsonowiczi – A. kozlowskii Assemblage Zone is correlated with the Morocconus notabilis Zone of Morocco, and thus represents the Cambrian Series 2 and 3 boundary interval. Links with West Gondwana challenge the existing palaeogeographic interpretations for the southern part of the Holy Cross Mountains and point to an urgent need to revise the position of the Małopolska Block during the middle Cambrian.

  • Pliocene–Pleistocene sedimentary development of the syntectonic Polis graben, NW Cyprus: evidence from facies analysis, nannofossil biochronology and strontium isotope dating

  • ELIZABETH M. BALMER, ALASTAIR H. F. ROBERTSON, ISABELLA RAFFI, DICK KROON
  • Journal:
    Geological Magazine / Volume 156 / Issue 5 / May 2019
    Published online by Cambridge University Press:
    28 May 2018, pp. 889-917

  • The recently uplifted and exposed Pliocene and Pleistocene sedimentary infill of the neotectonic Polis graben provides an excellent opportunity to understand extensional basin development in a marine setting. Fieldwork, facies analysis and dating using nannofossils and strontium isotopes reveal how the sedimentary conditions evolved during infill of the Polis graben during Pliocene and Pleistocene time, and allow a composite succession for the depocentre to be determined for the first time. Six lithofacies are recognized in the northern Polis graben, allowing evolving palaeoenvironments to be inferred. By the end of Miocene time (Messinian) a major c. N–S-trending graben was established; extensional faulting continued during the Pliocene–Pleistocene until recent time. Post-Messinian salinity crisis deposition began with deposition of hemipelagic muds (c. 5.08–2.76 Ma), equivalent to the Nicosia Formation. This was followed by upwards incoming of repeated normal-graded bioclastic carbonates (couplets) (c. 2.76–1.6 Ma), which are interpreted as age-equivalents of the Athalassa Formation elsewhere in Cyprus. The upwards sudden facies change is explained by tectonically controlled shallowing which enabled neritic carbonate production on the basin margins. The appearance of basement-derived material (e.g. ophiolitic extrusive detritus) in the highest stratigraphic levels of the basin fill in the north (c. 1.7–1.6 Ma) reflects onset of rapid surface uplift focused on the Troodos ophiolitic massif. Overall, the syntectonic basin infill appears to document a two-stage, pulsed uplift related to early-stage collision of the African and Eurasian plates in the easternmost Mediterranean region.

  • Integrated Cambrian biostratigraphy and carbon isotope chemostratigraphy of the Grönhögen-2015 drill core, Öland, Sweden

  • PER AHLBERG, FRANS LUNDBERG, MIKAEL ERLSTRÖM, MIKAEL CALNER, ANDERS LINDSKOG, PETER DAHLQVIST, MICHAEL M. JOACHIMSKI
  • Journal:
    Geological Magazine / Volume 156 / Issue 6 / June 2019
    Published online by Cambridge University Press:
    28 May 2018, pp. 935-949

  • The Grönhögen-2015 core drilling on southern Öland, Sweden, penetrated 50.15 m of Cambrian Series 3, Furongian and Lower–Middle Ordovician strata. The Cambrian succession includes the Äleklinta Member (upper Stage 5) of the Borgholm Formation and the Alum Shale Formation (Guzhangian–Tremadocian). Agnostoids and trilobites allowed subdivision of the succession into eight biozones, in ascending order: the uppermost Cambrian Series 3 (Guzhangian) Agnostus pisiformis Zone and the Furongian Olenus gibbosus, O. truncatus, Parabolina spinulosa, Sphaerophthalmus? flagellifer, Ctenopyge tumida, C. linnarssoni and Parabolina lobata zones. Conspicuous lithologic unconformities and the biostratigraphy show that the succession is incomplete and that there are several substantial gaps of variable magnitudes. Carbon isotope analyses (δ13Corg) through the Alum Shale Formation revealed two globally significant excursions: the Steptoean Positive Carbon Isotope Excursion (SPICE) in the lower–middle Paibian Stage, and the negative Top of Cambrian Excursion (TOCE), previously referred to as the HERB Event, in Stage 10. The δ13Corg chemostratigraphy is tied directly to the biostratigraphy and used for an improved integration of these excursions with the standard agnostoid and trilobite zonation of Scandinavia. Their relations to that of coeval successions in Baltoscandia and elsewhere are discussed. The maximum amplitudes of the SPICE and TOCE in the Grönhögen succession are comparable to those recorded in drill cores retrieved from Scania, southern Sweden. The results of this study will be useful for assessing biostratigraphic relations between shale successions and carbonate facies on a global scale.

  • Errata in June Number

  • Journal:
    Geological Magazine / Volume 6 / Issue 7 / July 1909
    Published online by Cambridge University Press:
    23 May 2018, p. 336
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  • Metamorphic evolution and age constraints of the garnet-bearing mica schist from the Xindaduo area of the Sumdo (U)HP metamorphic belt, Tibet

  • CONG ZHANG, THOMAS BADER, LINGMIN ZHANG, TINGTING SHEN, PENG LI, XUPING LI
  • Journal:
    Geological Magazine / Volume 156 / Issue 7 / July 2019
    Published online by Cambridge University Press:
    15 May 2018, pp. 1175-1189

  • As one of the major components of the Himalayan–Tibetan Orogeny, the Lhasa terrane plays a key role in understanding the origin and evolution of this giant orogenic belt and the opening and closure of the Tethys oceans. The eclogite-bearing Sumdo Complex in the central Lhasa terrane was recognized as the main suture of the Palaeo-Tethys Ocean between the north and south Lhasa sub-terranes. Despite the eclogite having been studied for a long time, no attempts have been applied to studying the country rocks, causing confusion in understanding the relationship between the eclogite and the adjacent schist. Petrological investigations and phase equilibrium calculations on the garnet-bearing mica schist of the Sumdo Complex have been performed to constrain its metamorphic evolution. The P–T conditions for three metamorphic stages are constrained as P1 (480–500°C, 2.6–2.7 GPa), P2 (580–600°C, 1.3–1.4 GPa) and R (530°C, 0.9 GPa), which represent the prograde, temperature peak and retrograde stages. Two possible P–T paths were constructed, which experienced isothermal decompression (PT1) or heating with a decompression process (PT2), corresponding to the growth and extinction of garnet porphyroblasts in the matrix. The LA-MC-ICP-MS U–Pb dating method yielded a metamorphic age of 229.7±3.5 Ma, which was interpreted as the age of amphibolite-facies metamorphism at c. 600°C, 1.2–1.4 GPa during the closure of the Palaeo-Tethys Ocean, resulting in the aggregation of the north and south Lhasa sub-terranes. The close relationship between the eclogite and garnet-bearing mica schist, and their similar P–T–t paths indicate an in situ tectonic evolution rather than tectonic juxtaposition during exhumation.

  • After the ice: Lateglacial and Holocene landforms and landscape evolution in Scotland

  • Colin K. BALLANTYNE
  • Journal:
    Earth and Environmental Science Transactions of The Royal Society of Edinburgh / Volume 110 / Issue 1-2 / March 2019
    Published online by Cambridge University Press:
    15 May 2018, pp. 133-171
    Print publication:
    March 2019

  • During Lateglacial cold periods, permafrost developed throughout Scotland, sediment-mantled slopes were extensively modified by solifluction and other forms of periglacial mass movement, large-scale sorted patterned ground formed on plateaux, and enhanced rockfall resulted in talus accumulation below cliffs. Most rock-slope failures occurred within five millennia following ice-sheet deglaciation, with many probably triggered by uplift-induced earthquakes; numerous debris-free scarps represent sites where Lateglacial rockslide debris was excavated by glaciers during the Younger Dryas Stade (∼12.9–11.7 ka). Sandar and outwash fans deposited by glacial rivers during ice-sheet retreat were incised to form high-level terraces. Under the cool temperate but relatively stable climate of the Holocene, solifluction and patterned ground formation continued to operate on high ground, though accumulation of high-level aeolian deposits on most mountains was terminated by erosion during the Little Ice Age. Drift-mantled slopes and talus slopes have been extensively eroded by translational failures and debris flows, the latter depositing debris cones on upland valley floors. The incidence of Holocene rockslides has been much lower than during the Lateglacial period. Dating of alluvial deposits and low Holocene terraces suggests no consistent pattern of Holocene floodplain evolution: incision has apparently dominated in the Highlands, aggradation in the lowlands, and floodplains in the Southern Uplands have asynchronous histories of incision and aggradation. Studies of floodplain behaviour over the past 200–300 years suggest that though major floods rework the floodplains of braided and piedmont rivers, there is no tendency towards net floodplain aggradation or incision. Most valley-side alluvial fans accumulated episodically in the last 4000 years, many in response to lowering of hillslope stability by woodland clearance. For many postglacial landsystems, disentangling the effects of declining paraglacial sediment supply, climate change and local influences (extreme rainstorm events or anthropogenic impacts) remains challenging.

  • The Last Scottish Ice Sheet

  • Colin K. BALLANTYNE, David SMALL
  • Journal:
    Earth and Environmental Science Transactions of The Royal Society of Edinburgh / Volume 110 / Issue 1-2 / March 2019
    Published online by Cambridge University Press:
    15 May 2018, pp. 93-131
    Print publication:
    March 2019

  • The last Scottish Ice Sheet (SIS) expanded from a pre-existing ice cap after ∼35 ka. Highland ice dominated, with subsequent build-up of a Southern Uplands ice mass. The Outer Hebrides, Skye, Mull, the Cairngorms and Shetland supported persistent independent ice centres. Expansion was accompanied by ice-divide migration and switching flow directions. Ice nourished in Scotland reached the Atlantic Shelf break in some sectors but only mid-shelf in others, was confluent with the Fennoscandian Ice Sheet (FIS) in the North Sea Basin, extended into northern England, and fed the Irish Sea Ice Stream and a lobe that reached East Anglia. The timing of maximum extent was diachronous, from ∼30–27 ka on the Atlantic Shelf to ∼22–21 ka in Yorkshire. The SIS buried all mountains, but experienced periods of thickening alternating with drawdown driven by ice streams such as the Minch, the Hebrides and the Moray Firth Ice Streams. Submarine moraine banks indicate oscillating retreat and progressive decoupling of Highland ice from Orkney–Shetland ice. The pattern and timing of separation of the SIS and FIS in the North Sea Basin remain uncertain. Available evidence suggests that by ∼17 ka, much of the Sea of the Hebrides, the Outer Hebrides, Caithness and the coasts of E Scotland were deglaciated. By ∼16 ka, the Solway lowlands, Orkney and Shetland were deglaciated, the SIS and Irish Ice Sheet had separated, the ice margin lay along the western seaboard, nunataks had emerged in Wester Ross, the ice margin lay N of the Cairngorms and the sea had invaded the Tay and Forth estuaries. By ∼15 ka, most of the Southern Uplands, the Firth of Clyde, the Midland Valley and the upper Spey valley were deglaciated, and in NW Scotland ice was retreating from fjords and valleys. By the onset of rapid warming at ∼14.7 ka, much of the remnant SIS was confined within the limits of Younger Dryas glaciation. The SIS, therefore, lost most of its mass during the Dimlington Stade. It is uncertain whether fragments of the SIS persisted on high ground throughout the Lateglacial Interstade.

  • Role of pressure solution in the formation of bedding-parallel calcite veins in an immature shale (Cretaceous, southern UK)

  • QINGFENG MENG, JOHN HOOKER, JOE CARTWRIGHT
  • Journal:
    Geological Magazine / Volume 156 / Issue 5 / May 2019
    Published online by Cambridge University Press:
    15 May 2018, pp. 918-934

  • Bedding-parallel fibrous calcite veins in black shales (Cretaceous, southern UK) were investigated using a combined field, stable isotopic geochemistry, petrographic and crystallographic method to examine their formation mechanism. Calcite veins occur in all shale beds and are most abundant in the bituminous shales of the Chief Beef Beds. The calcite fibres in these veins exhibit either an antitaxial fibre growth with curvy stylolites as the median zone, or a predominantly syntaxial, upwards growth. The calcite veins range from –0.49 to 1.78‰ of δ13C values, and –6.53 to –0.03‰ of δ18O values, which are both similar to those of their host shales. Our petrographic observations demonstrate that subhorizontal and interconnecting microstylolite networks commonly occur within the calcite veins. Equant calcite grains in the median zones exhibit indenting, truncating and also interpenetrating grain contacts. It is interpreted that the fibrous calcite veins were sourced by neomorphic calcite from their host shales, with evidence from the δ13C signatures, pressure-solution features (stylolites, microstylolites and grain contact styles) and embedded fossil ghosts within the veins. The diagenetic fluids, from which calcite was precipitated, were a mixing of the original seawaters and 18O-depleted meteoric waters. Development of bedding-parallel calcite veins is considered to have been enhanced by pressure solution as a positive feedback mechanism, which was facilitated by the overburden pressure as the maximum principal stress. Calcite fibres, with a predominant subvertical c-axis orientation, exhibit a displacive growth in porous shales and a replacive growth at vein-limestone contacts. This study highlights the critical role of pressure solution in the formation of bedding-parallel calcite veins during burial and diagenesis of immature black shales.

  • Multi-stage growth and fluid evolution of a hydrothermal sulphide chimney in the East Pacific Ridge 1–2° S hydrothermal field: constraints from in situ sulphur isotopes

  • XINGWEI MENG, XIAOHU LI, FENGYOU CHU, BIN FU, JIJIANG LEI, ZHENGGANG LI, HAO WANG, LIN CHEN
  • Journal:
    Geological Magazine / Volume 156 / Issue 6 / June 2019
    Published online by Cambridge University Press:
    11 May 2018, pp. 989-1002

  • Sulphur isotopes can be used as a powerful tool to trace fluid evolution and explore the formation of chimneys. To clarify the in situ S isotopic variations of sulphides at the micro-scale, we analyzed a sulphide chimney collected from the hydrothermal field in the East Pacific Rise 1–2° S using a sensitive high-mass-resolution ion micro-probe for stable isotopes (SHRIMP SI). Three mineral zones can be identified in the chimney: an external outer wall of porous anhydrite and colloform pyrite, an internal middle zone of sub-euhedral pyrite and massive chalcopyrite, and an inner zone of massive pyrite. The δ34SV-CDT values of the sulphides fall within the range 1.83–7.51 ‰ (avg. 4.05 ‰, n = 16), and S isotopic values increase from the core (3.09 ‰, n = 3) to the middle (3.78 ‰, n = 11) to the edge (6.99 ‰, n = 2). These results illustrate mineral crystallization processes and the mixing between seawater-derived S and magmatic–hydrothermal fluids during the growth of the chimney. The zones from the edge to the core are characterized by crystal morphologies of colloform/anhedral pyrite to massive pyrite with decreasing δ34S values, revealing multi-stage mineral deposition and sulphur isotopic fractionation. In contrast to the increase in δ34S values from the core to the edge in one profile (profile A), anomalously low δ34S values in fine-grained pyrite relative to chalcopyrite in another profile (profile B) in the middle zone result from S isotopic exchange between seawater SO42− and fluid H2S due to different fluid–seawater mixing, possibly caused by variations in permeability and porosity across the chimney.

  • The tectonometamorphic evolution of the Uppermost Unit south of the Dikti Mountains, Crete

  • SILVIU O. MARTHA, GERNOLD ZULAUF, WOLFGANG DÖRR, JANNES J. BINCK, PATRICK M. NOWARA, PARASKEVAS XYPOLIAS
  • Journal:
    Geological Magazine / Volume 156 / Issue 6 / June 2019
    Published online by Cambridge University Press:
    10 May 2018, pp. 1003-1026

  • We present a new geological map and new structural, petrographical and geochronological data from the Uppermost Unit of the Cretan nappe pile exposed south of the Dikti Mountains in eastern Crete (Greece). Based on these data, the Uppermost Unit in the study area can be subdivided (from bottom to top) into the Arvi Unit, Theodorii Greenschist and Asterousia Crystalline Complex (ACC)-type rocks. The ACC-type rocks have been affected by polyphase deformation (D1–D3) and metamorphism. Relics of the D1 phase are preserved as internal foliation in garnet porphyroblasts. D2 top-to-the SE shearing under upper amphibolite facies conditions led to the dominant foliation. After post-D2 exhumation, parts of the ACC-type rocks were affected by contact metamorphism of a non-exposed pluton, which intruded at a depth below 10 km during Campanian time (74±2Ma; laser ablation inductively coupled plasma mass spectrometry on zircon). This age, obtained from zircon of chiastolite hornfels, is in line with intrusion ages of ACC-type (meta)granitoids exposed on Crete and on Anafi. The S2-foliation of the ACC-type rocks was reactivated during the late phase of contact metamorphism by D3 top-to-the SE shearing. Latest Cretaceous cross-mica with low silicon content post-dates this shearing event. During middle Paleocene time, the ACC was thrust on top of the Theodorii Greenschist. This thrusting event as well as subsequent brittle thrusting of the greenschists and the ACC-type rocks on top of the prehnite-pumpellyite facies metamorphic Arvi Unit was still accommodated by top-to-the SE kinematics, which is the dominant kinematics of the Uppermost Unit on Crete and on Anafi.

  • Appendix A - Glossary

  • David W. Eaton, University of Calgary
  • Book:
    Passive Seismic Monitoring of Induced Seismicity
    Published online:
    07 June 2018
    Print publication:
    26 April 2018, pp 281-291

  • Summary

    Passive seismic monitoring of induced seismicity is a highly interdisciplinary field, combining aspects of seismology, continuum mechanics, fracture mechanics, geomechanics, structural geology, reservoir engineering and many other disciplines. Consequently, unfamiliar terminology can be a significant barrier to understanding. This glossary provides a concise explanation for some of the terms used in this book.

    Acoustic medium: A medium in which shear waves are not considered or do not propagate, such as liquids or gases (Sheriff, 1991). Imaging methods used in exploration seismology commonly invoke an acoustic approximation and thus consider only P -wave propagation.

    Aftershock: Earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and located within 1–2 rupture lengths distance from the mainshock. Aftershocks can continue over a period of weeks, months or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue (USGS, 2017).

    Anisotropy: Variation of one or more physical properties of a homogeneous material with the direction of measurement (Winterstein, 1990). A common form is transverse isotropy (TI), which is characteristic of shale, thinly bedded media and media with a single set of cracks; TI media have an infinite-fold axis of rotation (symmetry axis) and an infinite set of twofold axes of rotation in the perpendicular plane. The symmetry axis may be vertical (VTI), horizontal (HTI) or tilted (TTI). Another common form is orthorhombic, which has three mutually perpendicular symmetry axes and can arise from two sets of cracks, or a single set of vertical cracks in a layered medium (Figure 1.7). TI media are described by five independent elastic parameters, while orthorhombic media require nine independent parameters.

    Anomalous seismicity: Seismicity that would not normally occur when performing an operation such as hydraulic fracturing (CAPP, 2017).

    Asperity: In a literal sense, asperity means “roughness” (IRIS, 2017). An area on a fault this is stuck or locked.

    b value: The slope of the magnitude-frequency distribution for the Gutenberg–Richter relation, which describes the relative size distribution of earthquakes. Most earthquake fault systems have a b value close to unity, implying that for each unit increase in magnitude there are ten times fewer earthquakes.

  • 6 - Downhole Microseismic Processing

  • from Part II - Applications of Passive Seismic Monitoring
  • David W. Eaton, University of Calgary
  • Book:
    Passive Seismic Monitoring of Induced Seismicity
    Published online:
    07 June 2018
    Print publication:
    26 April 2018, pp 158-189

  • Summary

    We are what we repeatedly do. Excellence, then, is not an act, but a habit.

    Aristotle, paraphrased by Durant (The Story of Philosophy, 1926)

    Downhole microseismic data are distinct from surface and near-surface recordings inasmuch as the sensors are generally installed in relatively close proximity to the treatment zone. This characteristic is both a help and a hindrance. The close proximity to the source region, coupled with the avoidance of highly attenuating near-surface layers, typically results in high-fidelity waveforms from events that might otherwise be undetectable; but, the constraints imposed by limited availability of deep, nearby wellbores means that the acquisition geometry is rarely close to ideal. In many cases, an expedient but sub-optimal configuration is employed with a single vertical observation well; it then becomes a matter of doing more with less.

    Determining hypocentre locations using downhole microseismic data involves phase picking and quality control (QC) of P and S arrivals, estimation of hypocentral distance using the picked arrival times, and estimation of backazimuth based on waveform polarization. If more than one observation well is available, there are additional requirements for correlating observations between different wells (Warpinski et al., 2005a). While the need to pick P and S arrivals is shared by hypocentre location methods used for local or regional seismograph networks, there are important differences. For the most part, seismograph networks are deployed at the surface and sample a substantially greater aperture of the focal sphere. The techniques used for downhole microseismic are thus more akin to the seldom-used method of hypocentre location with a single seismograph station (Roberts et al., 1989; Farahbod et al., 2015b).

    The goal of microseismic data processing is to transform continuous wavefield observations into precise and accurate estimates of event locations, magnitudes and other source characteristics. Figure 6.1 shows a simplified flow chart for the processing of downhole microseismic data after the monitoring program is complete. The basic processing sequence consists of two parallel workflows:

  • • a primary workflow, in which the waveform data are processed to produce an event catalogue;

  • • a secondary workflow that is used for estimation of sensor orientations, as well as construction and validation of a calibrated background model.

  • 9 - Induced Seismicity

  • from Part II - Applications of Passive Seismic Monitoring
  • David W. Eaton, University of Calgary
  • Book:
    Passive Seismic Monitoring of Induced Seismicity
    Published online:
    07 June 2018
    Print publication:
    26 April 2018, pp 241-280

  • Summary

    Logarithmic plots are a device of the devil.

    Charles F. Richter (Earthquake Information Bulletin, 1980)

    The term induced seismicity, as used in this book, refers to earthquakes or other seismic events that have a clear anthropogenic association. Human activities with the potential to produce appreciable seismic activity include: underground mining (Gibowicz and Kijko, 1994) and construction of tunnels (Husen et al., 2013); impoundment of surface water reservoirs (Simpson et al., 1988); development of engineered geothermal systems (Breede et al., 2013); detonation of underground explosions (Massé, 1981); and, subsurface fluid injection or withdrawal (National Research Council, 2013). Among these different causal mechanisms for induced seismicity, this chapter focuses on injection-induced seismicity from energy technologies, including deep-subsurface saltwater disposal (SWD), hydraulic fracturing (HF) of low-permeability unconventional reservoirs and engineered (or enhanced) geothermal systems (EGS). These three causal mechanisms of induced seismicity occur in different settings and arise from distinct industrial applications, yet they share broadly similar underlying physical mechanisms.

    The material in this chapter builds upon previous sections of the book, especially the fundamental theory discussed in the first four chapters. The chapter begins with a review of basic concepts, commencing with definitions for a few key terms. Pioneering work on injection-induced seismicity is then considered, which frames our current understanding of this phenomenon, including criteria for discriminating between induced and natural events. A fluid/stress systems approach is then used to explore necessary conditions for induced seismicity, such as nucleation of dynamic slip on a fault. Seismological tools of the trade that are employed for observations of induced seismicity are then covered, followed by a case histories for various causal mechanisms of deep saltwater disposal, hydraulic fracturing and geothermal applications. Finally, methods used for risk assessment and hazard mitigation are presented, along with a brief discussion of important societal factors.

    Background

    We set the stage for this chapter by considering several important semantic issues. First, some investigators reserve the adjective “induced” for seismicity caused by human activities that lead to shear stress preturbations on a fault that are comparable in magnitude to the shear stress from the ambient (unperturbed) stress field (McGarr et al., 2002).

  • 8 - Microseismic Interpretation

  • from Part II - Applications of Passive Seismic Monitoring
  • David W. Eaton, University of Calgary
  • Book:
    Passive Seismic Monitoring of Induced Seismicity
    Published online:
    07 June 2018
    Print publication:
    26 April 2018, pp 209-240

  • Summary

    Take care of the sense and the sounds will take care of themselves.

    Lewis Carroll (Alice's Adventures in Wonderland, 1865)

    Interpretation is the science of finding meaning in data. The primary purpose of microseismic monitoring is to develop insights into the nature and extent of brittle deformation processes in the subsurface. Passive-seismic monitoring has many applications, including monitoring of fluid stimulation for enhanced geothermal systems and mine-safety protocols. The main focus of this chapter is passive seismic monitoring of MW < 0 induced microseismicity associated with hydraulic fracturing, although the interpretation workflow is more broadly applicable and could be used, for example, in passive seismic monitoring of geothermal stimulation programs. Interpretation of passive-seismic data for induced seismicity, considered here as events with MW ≥ 0, is the subject of the next chapter. In the case of hydraulic-fracture monitoring, microseismic data are generally acquired for the following purposes:

  • • surveillance of the growth of fracture networks;

  • • estimation of the spatial extent of the stimulated region within the reservoir;

  • • characterization of the complexity of induced fracture networks;

  • • diagnosing potential operational issues during well completion.

    • There is considerable research potential for development of more advanced microseismic interpretation methods, such as quantifying the state of stress, imaging discrete fracture networks, time-lapse studies, understanding possible slow-slip processes and reservoir characterization, including facies analysis.

    In this chapter, a systematic approach to microseismic interpretation is introduced. This approach is guided by an underlying philosophy that the interpretation of any experimental data should be repeatable, observation-driven and, to the greatest extent possible, quantitative. As part of this approach, applicable uncertainties in measured quantities should be reported, along with metadata describing how reported uncertainties were obtained. The proposed interpretation procedure is built upon four key concepts:

  • Attributes, following Taner (2001), are physical characteristics derived from observations, either by direct measurements or by logical or experience-based reasoning. There are two fundamentally different types of attributes: measured and inferred (indirect). Examples of measured attributes are the length of a microseismic cluster and the dimensions of the estimated stimulated volume (ESV); examples of inferred attributes are fracture length and SRV.

  • 7 - Surface and Shallow-Array Microseismic Processing

  • from Part II - Applications of Passive Seismic Monitoring
  • David W. Eaton, University of Calgary
  • Book:
    Passive Seismic Monitoring of Induced Seismicity
    Published online:
    07 June 2018
    Print publication:
    26 April 2018, pp 190-208

  • Summary

    Indeed, we often mark our progress in science by improvements in imaging.

    Martin Chalfie (Nobel Lecture, 2008)

    Although the goals of surface-based microseismic monitoring are the same as those for downhole monitoring, the data-processing methods are quite distinct. For acquisition programs that are deployed at or near the surface, the relatively large number of sensors (up to thousands), lower S/N environment and larger aperture lead to a processing approach that is typically more akin to imaging methods used for controlled-source seismic exploration than to methods used for earthquake seismology (Duncan and Eisner, 2010). In order to detect weak events, surface microseismic monitoring exploits the power of stacking and other signal-processing methods for noise reduction and enhancement of signal coherence. Shallow-well arrays constitute a hybrid approach; with this acquisition design, the seismic wavefield is recorded with sparser spatial sampling in a higher S/N environment, but the aperture and overall recording geometry – that is viewing from above – remain almost the same as for surface arrays.

    The theoretical groundwork for source imaging using surface arrays was laid by McMechan (1982), who observed that seismic sources can be imaged by propagation of the observed wavefield backwards in time. This approach, in which the observed seismograms are treated as time-dependent boundary values, provided a foundation for reverse-time imaging (RTI) (Artman et al., 2010). Kao and Shan (2004) proposed an alternative method, which they called a source-scanning algorithm (SSA). This method operates by summing absolute amplitudes observed at all stations at predicted arrival times for a set of trial hypocentre locations and origin times. SSA belongs to a class of source-imaging methods that are based on a form of Kirchhoff migration.

    The emergence, starting in 2004, of surface-based microseismic methods for monitoring hydraulic fracturing (Duncan, 2005; Lakings et al., 2006) was accompanied by vigorous technical debate (Eisner et al., 2011b, a). Given then-established low-magnitude characteristics of microseismicity induced by hydraulic fracturing (Warpinski et al., 1998; Maxwell et al., 2002; Rutledge and Phillips, 2003), the debate centred on the ultimate detection capabilities of sensors deployed in noisy surface environments. Skeptics argued that S/N improvements from stacking saturate at an upper limit, as demonstrated by Cieslik and Artman (2016), and suggested that amplitude loss from geometrical spreading and inelastic attenuation are insurmountable, irrespective of array design.

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