Seismic Signature Research Articles

Micro‐Seismic Monitoring of a Shear Fault Within a Floating Ice Plate

AbstractThe deformation of a circular fault in a thin floating ice plate imposed by a slow rotational displacement is investigated. Temporal changes in shear strength, as a proxy for the resistance of the fault as a whole, are monitored by the torque required to impose a constant displacement rate. Micro‐seismic monitoring is used to study the relationship between fault average resistance (torque) and micro‐ruptures. The size distribution of ruptures follows a power‐law scaling characterized by an unusually high exponent (b≃3), characteristic of a deformation driven by small ruptures. In strong contrast to the typical brittle dynamics of crustal faults, an 'apparently aseismic' deformation regime is observed in which small undetected seismic ruptures, below the detection level of the monitoring system, control the slip budget. Most (≃71%) of the detected ruptures are organized in bursts with highly similar waveforms, suggesting that these ruptures are only a passive by‐product of apparently aseismic slip events. The seismic signature of this deformation regime has strong similarities with crustal faulting in settings characterized by high temperature and with non‐volcanic tremors.

A regional Alpine graphite décollement level beneath the NW Pannonian Basin

The so-called Transdanubian Conductivity Anomaly (TCA) of the Hungarian part of the NW Pannonian Basin has been well known for more than five decades. The exceptionally low resistivity (i.e. 1–2 Ωm) zone has a very large areal extent (on the order a few thousand km2) and it is an entirely subsurface anomaly occurring at depth between circa 3–15 km, with no corresponding outcrops. Various geological explanations of this enigmatic crustal-scale geophysical anomaly range from invoking sub-horizontal Alpine nappe contacts to sub-vertical dikes with graphite and/or saline fluid content. Only one possible analogue outcrop area was considered for the high conductivity anomaly so far, namely the Drauzug/Gailtal area of the Eastern Alps in Austria, some 300 km to the West from the TCA area. Previous attempts to find correspondence between the TCA and prominent seismic reflectors seen on 2D seismic reflection profiles were based on data acquired by research institutions. This study systematically correlates, for the first time, the TCA with 2D industry seismic reflection data in the same area. Our new results show a very strong correlation between the subsurface extent and location of the TCA with various sub-horizontally oriented Cretaceous Alpine nappe surfaces. In addition, we draw on the latest structural correlation of the Alpine nappe stack of the Transdanubian Range with its proper tectonic counterpart in the Eastern Alps.At the southern edge of the Upper Austroalpine units in northern Styria, in the Veitsch Nappe of the Greywacke Zone, numerous graphite localities are known historically. These laterally extensive graphite units in NW Styria formed as the result of greenschist-grade metamorphism of a Carboniferous coal sequence during the Cretaceous. For the first time, we describe here one well penetration of possibly age-equivalent graphitic units in NW Hungary. Correlation of the magnetotelluric anomaly with the distinct reflection seismic signature suggests that the same Palaeozoic graphitebearing Upper Austroalpine units should be present at 3–15 km depth in our study area.Therefore we propose that the best explanation for the observed extent and geometry of the TCA is the presence of graphite in subhorizontal, tectonically thinned detachment surfaces at the base of the Upper Austroalpine nappe edifice of NW Hungary

Seismicity and outgassing dynamics of Nyiragongo volcano

Active lava lakes at volcanoes can be regarded as open windows to their magmatic systems. The dynamics of such lakes may therefore provide decisive insights into deeper magmatic processes, potentially leading to fundamental theoretical implications and volcano monitoring improvements. Among the rare volcanoes on Earth hosting a persistent lava lake, Nyiragongo in D.R. Congo directly threatens a massive population of roughly 1 million inhabitants. Here we analyze close-range (i.e., summit) and distant (around 17 km) seismic measurements acquired at this African volcano between 2011 and 2018 in order to better understand the seismic signature of the lava lake activity and how it relates to the deeper volcanic processes. Both summit and distant seismic records contain a similar continuous tremor pattern attributable to the lava lake activity. Combining this information with time-lapse camera images and lava lake level measurements confirms the mechanism of gas pistoning at Nyiragongo, which is characterized by short-duration (a few minutes long) and meter-scale level variations during the period of observation. We also characterize the dominant periodicity of this shallow tremor signature of about a few tens of minutes. Because this marked periodic pattern varies during a significant one-month fluctuation of SO2 emissions (estimated from space), we suggest that this particular seismic periodicity corresponds to the convective lake movement driven by the persistent degassing typical of active open-vent volcanoes. Finally, new seismic evidence reveals the effect of deep magmatic intrusion and consequent major pressure changes in the plumbing system, resulting in sudden and large drops of the lava lake level associated with strong degassing. Such transient episodes have similar characteristics to total lava lake drainage associated with flank eruptions already observed at this volcano in 1977 and 2002, or at Kīlauea volcano in 2018.

Validation of a fast semi-analytic method for surface-wave propagation in layered media

SUMMARYGreen’s functions provide an efficient way to model surface-wave propagation and estimate physical quantities for near-surface processes. Several surface-wave Green’s function approximations (far-field, no mode conversions and no higher mode surface waves) have been employed for numerous applications such as estimating sediment flux in rivers, determining the properties of landslides, identifying the seismic signature of debris flows or to study seismic noise through cross-correlations. Based on those approximations, simple empirical scalings exist to derive phase velocities and amplitudes for pure power-law velocity structures providing an exact relationship between the velocity model and the Green’s functions. However, no quantitative estimates of the accuracy of these simple scalings have been reported for impulsive sources in complex velocity structures. In this paper, we address this gap by comparing the theoretical predictions to high-order numerical solutions for the vertical component of the wavefield. The Green’s functions computation shows that attenuation-induced dispersion of phase and group velocity plays an important role and should be carefully taken into account to correctly describe how surface-wave amplitudes decay with distance. The comparisons confirm the general reliability of the semi-analytic model for power-law and realistic shear velocity structures to describe fundamental-mode Rayleigh waves in terms of characteristic frequencies, amplitudes and envelopes. At short distances from the source, and for large near-surface velocity gradients or high Q values, the low-frequency energy can be dominated by higher mode surface waves that can be captured by introducing additional higher mode Rayleigh-wave power-law scalings. We also find that the energy spectral density for realistic shear-velocity models close to piecewise power-law models can be accurately modelled using the same non-dimensional scalings. The frequency range of validity of each power-law scaling can be derived from the corresponding phase velocities. Finally, highly discontinuous near-surface velocity profiles can also be approximated by a combination of power-law scalings. Analytical Green’s functions derived from the non-dimensionalization provide a good estimate of the amplitude and variations of the energy distribution, although the predictions are quite poor around the frequency bounds of each power-law scaling.

Seismic markers of the Messinian salinity crisis in the deep Ionian Basin

AbstractWe conduct the seismic signal analysis on vintage and recently collected multichannel seismic reflection profiles from the Ionian Basin to characterize the deep basin Messinian evaporites. These evaporites were deposited in deep and marginal Mediterranean sedimentary basins as a consequence of the “salinity crisis” between 5.97 and 5.33 Ma, a basin‐wide oceanographic and ecological crisis whose origin remains poorly understood. The seismic markers of the Messinian evaporites in the deep Mediterranean basins can be divided in two end‐members, one of which is the typical “trilogy” of gypsum and clastics (Lower Unit – LU), halite (Mobile Unit – MU) and upper anhydrite and marl layers (Upper Unit – UU) traced in the Western Mediterranean Basins. The other end‐member is a single MU unit subdivided in seven sub‐units by clastic interlayers located in the Levant Basin. The causes of these different seismic expressions of the Messinian salinity crisis (MSC) appear to be related to a morphological separation between the two basins by the structural regional sill of the Sicily Channel. With the aid of velocity analyses and seismic imaging via prestack migration in time and depth domains, we define for the first time the seismic signature of the Messinian evaporites in the deep Ionian Basin, which differs from the known end‐members. In addition, we identify different evaporitic depositional settings suggesting a laterally discontinuous deposition. With the information gathered we quantify the volume of evaporitic deposits in the deep Ionian Basin as 500,000 km3 ± 10%. This figure allows us to speculate that the total volume of salts in the Mediterranean basin is larger than commonly assumed. Different depositional units in the Ionian Basin suggest that during the MSC it was separated from the Western Mediterranean by physical thresholds, from the Po Plain/Northern Adriatic Basin, and the Levant Basin, likely reflecting different hydrological and climatic conditions. Finally, the evidence of erosional surfaces and V‐shaped valleys at the top of the MSC unit, together with sharp evaporites pinch out on evaporite‐free pre‐Messinian structural highs, suggest an extreme Messinian Stage 3 base level draw down in the Ionian Basin. Such evidence should be carefully evaluated in the light of Messinian and post‐Messinian vertical crustal movements in the area. The results of this study demonstrates the importance of extracting from seismic data the Messinian paleotopography, the paleomorphology and the detailed stratal architecture in the in order to advance in the understanding of the deep basins Messinian depositional environments.

Wind gust quantification using seismic measurements

Wind gusts are a major cause of damage to property and the natural environment and a source of noise in seismic networks such as the USArray Transportable Array. Wind gusts cause ground motion through shear stresses, pressure fluctuations and vegetation flexing. Herein, we demonstrate the presence of a seismic response signature to wind gusts at sites across the contiguous USA and explore important geophysical factors that determine the precise nature of wind gust–seismic response relationships. There is a consistent seismic response to wind gusts that is typically manifest at relatively low frequency (0.05–0.1 Hz). However, there is also a marked seasonality in the seismic frequency of peak response, possibly due to seasonal differences in atmospheric conditions and/or vegetation and soil mediation of the atmosphere–ground interaction. The gust–seismic response functions also exhibit a clear dependence on (1) distance from the coast, (2) land cover, (3) topographic complexity and (4) lithology. We propose a generalized methodology to extract wind gust magnitude distributions from seismic networks. Although initial results from this model overestimate the spatial variability in wind gusts as measured by meteorological networks, the analyses described here highlight the potential for new methods to remove wind gust noise from seismic time series and potentially to derive quantitative wind gust estimates from seismic observations.

Moving Ground Target Detection With Seismic Signal Using Smooth Pseudo Wigner–Ville Distribution

The main challenging task in moving ground target detection with seismic signals is the extraction of robust feature vectors for accurate localization of a seismic event. Toward this goal, a new feature extraction technique is proposed for seismic event detection based on time-frequency analysis implemented with the Smooth pseudo Wigner–Ville distribution (SPWVD). The time-frequency coefficients are utilized to constitute a Renyi entropy vector exploiting a probability distribution function. This entropy vector gives information on the localized measurement of a possible seismic event resulting from moving ground vehicles. The likelihood of an actual seismic event is then ensured by using a constant false alarm rate (CFAR) detector which dynamically adjusts the threshold to minimize false alarms. The proposed algorithm is tested on the benchmarked data set SITEX02 consisting of seismic signals generated by moving tracked and wheeled vehicles. Additionally, the algorithm is also applied on a data set generated by collecting the seismic signature of a moving wheeled vehicle, i.e., bus within the campus. The results are also compared with pseudo Wigner–Ville distribution (PWVD) and short-time Fourier transform (STFT)-based seismic event detection technique. Significantly improved detection results are observed for SPWVD technique. An F-score improvement of ~24% as well as lead time enhancement of twofold is observed in comparison with the PWVD for relatively weak seismic signals generated by wheeled Dragon Wagon (DW) vehicles. Similarly, the F-score improvement is of ~13% and the lead enhancement is approximately 10 s in comparison with the STFT method.

Seismic Signatures of Stellar Magnetic Activity—What Can We Expect From TESS?

Asteroseismic methods offer a means to investigate stellar activity and activity cycles as well as to identify those properties of stars which are crucial for the operation of stellar dynamos. With data from CoRoT and \textit{Kepler}, signatures of magnetic activity have been found in the seismic properties of a few dozen stars. Now, NASA's Transiting Exoplanet Survey Satellite (TESS) mission offers the possibility to expand this, so far, rather exclusive group of stars. This promises to deliver new insight into the parameters that govern stellar magnetic activity as a function of stellar mass, age, and rotation rate. We derive a new scaling relation for the amplitude of the activity-related acoustic (p-mode) frequency shifts that can be expected over a full stellar cycle. Building on a catalogue of synthetic TESS time series, we use the shifts obtained from this relation and simulate the yield of detectable frequency shifts in an extended TESS mission. We find that, according to our scaling relation, we can expect to find significant p-mode frequency shifts for a couple hundred main-sequence and early subgiant stars and for a few thousand late subgiant and low-luminosity red giant stars.

Shallow Nonvolcanic Tremor Activity and Potential Repeating Earthquakes in the Chile Triple Junction: Seismic Evidence of the Subduction of the Active Nazca–Antarctic Spreading Center

ABSTRACT In southern Chile, at ∼46.2°S and ∼75.2°W, the active spreading center between the Nazca and Antarctic plates is colliding with the South American plate, forming the Chile triple junction (CTJ). For 1 yr, from March 2009 to February 2010, five ocean‐bottom seismometers (OBSs) were deployed over the CTJ. We used a portion of the OBS data to study the seismic signatures of the subduction of the active Nazca–Antarctic spreading center. Using the envelope technique, we detected long episodes of shallow nonvolcanic tremor (NVT) activity. To improve the identified location of the NVT activity, we cross‐correlated the vertical and horizontal components of all located NVTs. In different months, we measured the local maximum of the lag‐time correlation near 2 s, which is associated with the lag between the S and P waves (S−Ptime). Furthermore, we observed that in the days with intense tremor activity, the maxima corresponding to S−Ptime emerged in windows without observable NVTs. We suggest that days with intense tremor activity correspond to an almost continuous slow slip, which may accelerate and decelerates nearly randomly, with spatial and temporal heterogeneity. In addition, we detected some potential repeating earthquakes with an S−Ptime near 2 s, as well as NVTs. The detected NVT activity and potential repeating earthquakes suggest the existence of a shallow region close to the CTJ that is able to generate brittle (earthquakes) and brittle–ductile (potential repeating earthquakes and NVTs) ruptures.

Convolutional Neural Network-Based Moving Ground Target Classification Using Raw Seismic Waveforms as Input

Seismic vibration signatures are strong criteria to recognize moving ground targets in unattended ground sensor (UGS) systems. However, it is a challenging task because of the complexity of seismic waves and their high dependency on the underlying geology. In order to approach this problem, this paper proposes a novel method called “VibCNN” based on convolutional neural networks (CNNs). Instead of preprocessing signals to extract features, the proposed model takes raw waveforms as input. Another characteristic of the model is that it can handle very short input, which only contains 1024 sample points. The experimental results show that the model yields performance much better than benchmarks and generalizes quite well across different geological types. To further improve the performance of VibCNN, we introduce two auxiliary input channels based on seismic signals and add each auxiliary channel to the input layer of VibCNN separately. Furthermore, we explore different fusion rules of the auxiliary channels at three levels: sample level, feature level, and decision level. The best result achieves relative improvement of 2.05%. In addition, data augmentation for seismic data has not been deeply investigated yet. Thus, we conduce a data augmentation experiment to explore the influence of different augmentation techniques on the performance of the model. The appropriate augmentation improves the accuracy of the model from 93.44% to 95.20%.

Geomorphology and clast assemblages of intertidal beachrock: Implications for submerged shoreline preservation

Beachrock may be considered a reliable indicator of sea level, in addition to a reliable and recognizable source for onshore movement of beachrock clasts by overwash processes associated with extreme events. Key to this is its geomorphology and preservation potential, both of which are poorly understood. Here we examine the processes and deposits associated with beachrock breakdown from Camurupim, NE Brazil and compare its preservation to beachrocks found on continental shelves in South Africa. We study a prominent and continuous 50–60 m-wide beachrock complex that crops out along a 10 km stretch of coastline, and which mostly comprises 1.5 to 2 m-thick trough-crossbedded sandstones. These have generated large joint-bounded blocks that have been transported landwards under extreme energy conditions (based on numerical modelling and comparisons to the contemporary wave climate). Tabular clasts have accumulated as isolated, very coarse boulders on the seaward margin, small clasts in various topographic traps, boulder ridges with large landward- and seaward-dipping clasts on the uppermost part of the outcrop, and subaqueous and subaerial washover fans. Isolated fine meso-boulders emplaced during extreme waves (Hs = ca. 12 m) exhibit fitted textures and pressure-solution contacts, pointing to subsequent rocking and shaking. Smaller clasts in boulder ridges and overwash settings are initially liberated and transported by waves within the annual-decadal frequency spectrum, deposited on the high points of the outcrop. Subsequent movement during less extreme wave action progressively transports the clasts across the surface, where they are often trapped in the fitted fabric on top of the platform. Our breakdown model and overall beachrock preservation scheme is consistent with reported seismic signatures of submerged Holocene beachrock shorelines. Beachrock preservation during transgression is strongly influenced by the original sedimentary facies; thick, trough-crossbedded joint-bound clasts preserve the former shoreline. Clasts from the planar units accumulate as washover fans to landwards of the ridge.

New evidence of perfect overlapping of Haploops and pockmarks field: Is it a coincidence?

In the shallow waters of the Bay of Concarneau (South Brittany, France), previous studies reported a dense settlement of the species Haploops (tube-dwelling bioengineer amphipod) that perfectly overlaps a field of pockmarks. A possible mechanistic link may therefore exist between the Haploops colonies and the shallow pockmarks. To test this hypothesis, interferometry sonar and sub-bottom profiler chirp data were acquired in two new areas located in the marine estuary of the Loire River, “La Lambarde” and “Le Croisic”. The new sites share the same sedimentary and geological characteristics as those found in the Bay of Concarneau. The three sites are similarly located in shallow and muddy environments above a faulted Middle Eocene formation that is incised by paleo-valleys. Despite different hydrodynamic conditions and estuarine influence, in the two new investigated areas, the Haploops also settle within the pockmarks field limits. To better constrain the association pockmarks/Haploops, we compared the sedimentary features, seismic signatures and total organic carbon (TOC) content of the sediment. Active pockmark chimneys displaying highly reworked profiles and slightly higher TOC contents suggest a local deeper source of organic matter, which might be an important source of nutrient for the Haploops. In Southern Brittany the pockmarks/Haploops spatial overlap is not a simple coincidence and may therefore present a general pattern for identification of settlement preferences of the specie.

Seismic and Microseismic Signatures of Fluids in Rocks: Bridging the Scale Gap

AbstractThe aim of this collection is to record a snapshot of our current state of understanding of the impact of fluids on the evolution and monitorability of subsurface rock formations during anthropogenic fluid injection/withdrawal operations, accounting for scale and frequency effects. In this introduction we provide a summary of the key challenges and findings reported in these 23 articles. This suite of articles addresses a variety of issues related to the impact of fluids on subsurface rocks, which can be grouped in three themes: (i) impact of fluids on wave velocity and attenuation/dispersion (five articles); (ii) fluids, fractures, and seismicity (nine articles); and (iii) dynamic fluid injection and substitution (nine articles).

Geological and geophysical signatures of the East Pacific Rise 8°–10°N

The East Pacific Rise (EPR) 8°–10°N is an archetype for fast-spreading mid-ocean ridge (MOR), and has been one of the most comprehensively studied MOR sections for over four decades. It covers all orders of ridge discontinuities: first-order transform faults, second-order overlapping spreading centers, third-order small offset tectonic discontinuities, and fourth-order discontinuities of eruptive fissure zones. This section has been selected as a multi-disciplinary integrated study site because of its abundance of hydrothermal vents where scientists are investigating the linkages between mantle flow, crustal accretion, volcanic, hydrothermal, and biological processes that result in chemical and physical exchanges of energy and matter between the Earth's mantle and oceans. In particular, the segment near 9°50′N is one of the only two sites where repeated seafloor volcanic eruptions were onsite observed along the MOR. In this paper, we investigate the geological and geophysical signatures of the EPR 8°–10°N on behalf of the fast-spreading MOR, present the seismic signatures of the hydrothermal pathways through full waveform inversion, and put forward some thoughts on the remaining questions to be addressed. The skewness of mantle upwelling, on- and off-axis magmatism, are playing a critical role in shaping the crustal architecture. The spatial distribution and physical property of the crustal melt lens are mutually related to the hydrothermal system. The mechanism of hydrosphere-lithosphere interaction beneath the MOR is worth further studying. This compilation should be useful to investigators new to the MOR topic but we also target advanced researchers in presentation and synthesis of findings to date.

Detection of tunnels and boulders using shallow SH-SH reflected seismic waves

We present three case studies on detecting buried glacial boulders, a sewage tunnel, and abandoned coal mine tunnels using shear-wave reflection methods. The seismic signature of such subsurface features is in the form of an isolated diffraction, distinctly recognized on seismic sections obtained from shallow seismic surveys using a transverse horizontal (H2) source and a multichannel landstreamer that consists of H2 geophones. We used H2 impulsive and vibrator sources with varying bandwidth. Based on field experiments with multicomponent recordings, we determined that the H2-H2 source-receiver configuration is the most optimal to generate downgoing horizontally polarized shear (SH) waves and upcoming SH reflected and diffracted waves. A shallow SH-SH image using a microvibe high-frequency sweep exhibits a wavelength between 1 and 2 m, which is comparable to that of a ground-penetrating radar image with the additional advantage of deeper penetration.

Modeling violent rock failures in tunneling and shaft boring based on energy balance calculations

Excavation in highly stressed grounds may lead to violent compressive failures of rock or violent slip along discontinuities in the vicinity of the underground space. Violent failures, usually with anomalous seismic signatures, are characterized with radiating excess energy during the failure, endangering the safety and productivity of the operation. This paper adopts a numerical methodology, developed in the Universal Distinct Element Code (UDEC) for quantitatively studying the occurrence and intensity of violent failures, and advances it for respective analyses in tunneling and shaft-boring operations. The methodology enables estimating the radiated energy as a measure for evaluating the occurrence and relative intensity of violent failures. We first validate the radiated energy calculation in the models against analytical solutions for the formation of a circular opening, analogous to the geometry of tunnels and shafts. After reviewing the concept of violent failures by ideal models, we explore the effects of different factors on the occurrence and intensity of violent failures caused by an excavation into brittle rock or next to a preexisting fault. Limited cases studied in this paper demonstrate that different combinations of the rock elastic modulus and post-failure response of rock and discontinuity can result in a violent failure with various intensities. Larger excavations under higher deviatoric stresses can intensify the potential violent failure mostly through expanding the rock failure zone or the slip area on the activated fault. However, as opposed to the radiated energy, using the failure zone volume and fault slip area alone may lead to underestimating or overestimating the intensity of possible violent failures in a modeling practice. The energy-based analyses outlined in this paper provide a rational yet simple means for quantitatively studying violent failures in potentially complex geological conditions and excavation layouts.

Introduction to Special Issue on “Rock Physics ‐ from microstructure to seismic signatures”

Geophysical ProspectingVolume 67, Issue 4 p. 711-714 Introduction Introduction to Special Issue on “Rock Physics - from microstructure to seismic signatures” Per Avseth, Per Avseth Guest EditorSearch for more papers by this author Per Avseth, Per Avseth Guest EditorSearch for more papers by this author First published: 13 April 2019 https://doi.org/10.1111/1365-2478.12790Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume67, Issue4Special Issue: Rock Physics - from microstructure to seismic signaturesMay 2019Pages 711-714 RelatedInformation

The effect of pore structure complexity and saturation history on the variations of acoustic velocity as function of brine and oil saturation in carbonates

Time lapse seismic is a major technique that has been successfully implemented to monitor reservoir dynamics (such as changes in fluid saturation) in sandstones but its applicability in carbonate rocks is not well understood yet. This study aims to improve our understanding of the time lapse seismic signature in carbonates oil reservoirs by investigating how acoustic velocity changes as a result of variations in the brine and oil saturation. To this scope, we utilize a core flooding system to perform brine-oil displacement experiment in three reservoir carbonate samples, while monitoring the acoustic velocity as function of brine saturation (Sw) changes. The fluid displacement experiment included injection of oil into the fully brine saturated samples until reaching irreducible brine saturation, followed by injection of brine until reaching the residual oil saturation. We also characterize the pore structure complexity of the samples using nuclear magnetic resonance (NMR) and micro-CT scans to investigate its effect on the velocity-saturation relationship. We finally compare the measured acoustic velocity as function of saturation with the predictions of Gassmann theory while considering both uniform and patchy fluid distribution styles.Our results show that both P- and S-wave velocity (Vp and Vs respectively) decrease as oil displaces brine and then they both increase as brine displaces oil. Nevertheless, the velocity-saturation relationship exhibits a hysteresis: velocities measured at a given saturation during brine injection tend to be higher than those measured during oil injection. Our analysis shows that the Vp-Sw trend during oil injection into brine saturated samples follows the lower bound predicted by Gassmann theory (i.e., uniform fluid distribution), while the brine flooding data follows the upper bound of Gassmann theory (i.e., patchy fluid distribution). The transition from the uniform to patchy fluids distribution seems to occur around the irreducible brine saturation. Since oil production or enhanced oil recovery can be represented by the process of brine displacing oil, our results suggest that patchy fluid distribution should be considered in Gassmann theory when performing feasibility studies to predict changes in Vp as function of saturation changes. This study also highlights the importance of pore structure in controlling the changes of saturation and the corresponding changes in acoustic velocity. Larger changes and hysteresis in the measured velocity as function of saturation seem to correlate with lower pore stiffness (i.e., more micro cracks and less content of intraparticle rounded pores).

Simultaneous seismic inversion to identify prospective areas in carbonate rocks of the southeastern part of the West Siberian Platform

This paper focuses on the description of a technology and results of identifying and mapping areas in Devonian carbonate rocks prospective for hydrocarbons and associated with increased fracture-cavern porosity in the southeastern part of the West Siberian Platform. The lack of wide-azimuthal seismic data and recording of only the vertical component in the study area did not allow the use of the direct indicators of fracture systems based either on azimuthal anisotropy of the amplitudes of pressure waves or on splitting converted waves into fast and slow. Instead, abnormally low values of the ratio of pressure-wave and shear-wave velocities derived by deterministic simultaneous pre-stack seismic inversion were used as an indirect indicator of increased fracturing. The choice of this seismic signature of fractures is substantiated by a brief review of publications on its successful use in the identification and delineation of highly fractured and cavernous zones in carbonate reservoirs. The behaviour of the indicator showed good agreement with well productivity in the study area and, therefore, made it possible to predict a number of perspective pay zones, presumably associated with in-creased fracturing. At the same time, however, well log estimates of the velocity ratio related to fractured reservoirs exhibited an opposite trend, which mismatched the facts observed in the study area and reported in the literature. This apparent discrepancy is explained by the impact of near-vertical natural macro-fracturing, which, due to different measurement scales, might substantially reduce seismic estimates while producing no impact on well log estimates. In order to describe this phenomenon quantitatively, the weak-anisotropy approximation of P-wave reflection coefficients at a horizontal boundary derived by Rüger for transversely isotropic media with a horizontal axis of symmetry was used. This equation was rewritten in terms of impedances and density and then was analyzed analytically. To express the impact of fractures in terms of fracture density, the simplest model of thin, isolated, penny-shaped fractures was used.

The seismic signature of heavy oil on carbonate reservoir through laboratory experiments and AVA modelling

Abstract The influence of unconventional oil on the petrophysical properties of carbonate rocks is of primary interest in petroleum science. In this work, we first provided a thorough petrophysical characterization in laboratory by measuring density, porosity, VP and VS of a bitumen carbonate reservoir rock outcropping in the Majella Mountain (Central Italy). The presence of bitumen stiffens the rock and causes an increases of the acoustic impedance (AI) and a strong decrease of the VP/VS ratio respect to brine saturation. On the contrary, lighter oil causes a decrease of the AI and a slight decrease of the VP/VS ratio in comparison with a saturation in brine. Then, we performed a Synthetic Amplitude vs Angles (AVA) modelling showing that through the AVA analysis, it is possible to recognize the type of fluid that saturates the analyzed carbonate reservoir rocks. We also observed that the increasing porosity plays a key role in the AVA response, by decreasing the bulk density and seismic waves velocities and increasing the amount of fluids that saturate the rocks. By applying our laboratory and modelling results to a public stacked seismic line we show that our results may help in recognize heavy oil directly from seismic data.