Coda Attenuation Research Articles

Fast Changes in Seismic Attenuation of the Upper Crust due to Fracturing and Fluid Migration: The 2016–2017 Central Italy Seismic Sequence

The Amatrice–Visso–Norcia seismic sequence struck Central Italy across the Apenninic normal fault system in 2016. Fluids likely triggered the sequence and reduced the stability of the fault network following the first earthquake (Amatrice, Mw 6.0), with their migration nucleating the Visso (Mw 5.9) and Norcia (Mw 6.5) mainshocks. However, both spatial extent and mechanisms of fluid migration and diffusion through the network remain unclear. High fluid content, enhanced permeability, and pervasive microcracking increase seismic attenuation, but different processes contribute to different attenuation mechanisms. Here, we measured and mapped peak delay time and coda attenuation, using them as proxies of seismic scattering and absorption before and during the sequence. We observed that the structural discontinuities and lithology control the scattering losses at all frequencies, with the highest scattering delineating carbonate formations within the Gran Sasso massif. The Monti Sibillini thrust marks the strongest contrasts in scattering, indicating a barrier for northward fracture propagation. Absorption does not show any sensitivity to the presence of these main geological structures. Before the sequence, low-frequency high-absorption anomalies distribute around the NW-SE-oriented Apennine Mountain chain. During the sequence, a high-absorption anomaly develops from SSE to NNW across the seismogenic zone but remains bounded north by the Monti Sibillini thrust. We attribute this spatial expansion to the deep migration of CO2-bearing fluids across the strike of the fault network from a deep source of trapped CO2 close to the Amatrice earthquake. Fluids expand SSE-NNW primarily during the Visso sequence and then diffuse across the fault zones during the Norcia sequence.

Temporal variations in coda attenuation associated with the 2008 Wenchuan (MW7.9) Earthquake in SW, China

In this study, waveform data obtained from Western Sichuan Seismic Array (WSSA) in China was utilized to reveal the temporal variations in coda attenuation around the eastern Tibet Plateau and Western Sichuan. Based on the single-scattering model, coda attenuation factor Qc−1 is calculated in narrower overlapping frequency bands at 1.0–24 Hz by measuring the coda decay rates for local earthquakes before and after the Wenchuan mainshock. The temporal variations in coda attenuation are investigated within three periods. The period I lasted from January 2007 to the end of 2007. The period II lasted from January 2008 till Wenchuan earthquake. The period III lasted from the mainshock to the end of 2008. The resulted temporal variations demonstrate an increase in average Qc−1 by approximately 35%–45% in the vicinity of Longmenshan and 30%–35% in Sichuan Basin after the Wenchuan earthquake in lower frequency bands within 1.25–8 Hz. On the contrary, the average Qc−1 is shown to decrease by approximately 10%–18% in southern segment of Longmenshan, 15%–38% in the Chuan-Dian block, and 10–12% in the South China block. These results are confirmed by a statistical t-test at 99.9% confidence level. No statistically significant change in Qc−1 (<10%) is found in the Songpan-Ganzi block after the mainshock. Temporal variations of coda attenuation differ significantly in individual blocks after the mainshock. The coda attenuation is proposed to be a beacon to tectonic static stress changes associated with the Wenchuan earthquake.

Modelling regional-scale attenuation across Italy and the Tyrrhenian Sea

Modelling regional-scale attenuation of seismic waves at ~1 Hz is challenging, especially when these waves propagate across both continental and oceanic crust. Recent developments in seismic imaging and modelling have provided us with the computational tools necessary to reconstruct these mixed settings using deterministic (coherent) and stochastic (coda) information. Here, we present new tomographic maps of coda-attenuation for both the Italian peninsula and the Tyrrhenian Sea. Kernel-based coda attenuation imaging in the diffusive approximation is tested in the oceanic environment, highlighting a non-diffusive behaviour across the Southern Tyrrhenian Sea. Joint deterministic and Radiative Transfer forward modelling of coherent and scattered waves (Radiative3D) is then performed for the portion of this area showing the lowest coda attenuation. The corresponding parametric study shows that coda attenuation is an efficient marker of variations of Moho depths and crustal reverberations, such as those we observe in a transitional area (characterized by continental crust and thicker Moho) between two oceanic basins (Vavilov and Marsili). Diffusive tomographic imaging informed by Radiative3D modelling reconstructs high-attenuation patterns along the Italian peninsula in agreement with the thick sediments cover across the Po Plain and the Adriatic coast and the magmatic systems in the Central Campanian province. A combined approach of coda-attenuation imaging and modelling can constrain Moho depth and spatial variations of seismic attenuation at regional scale and in mixed continental-oceanic settings.

Petro-mineralogical controls on coda attenuation in volcanic rock samples

SUMMARYSeismic attenuation measurements, especially those obtained from coda decay analysis, are becoming a key data source for the characterization of the heterogeneous Earth due to their sensitivity to small-scale heterogeneities. However, the relation between the scattering attenuation measured from coda waves and physical rock properties is still unclear. The goal of this study is to identify the main petrophysical and mineralogical factors controlling coda attenuation in volcanic rocks at the laboratory scale, as a necessary step before modelling seismic waves in real volcanic media. Coda wave attenuation was estimated from ultrasonic S-wave waveforms. To quantify the heterogeneity of the rocks and link them with this attenuation parameter, we performed several categorizations of the pore and grain systems of volcanic samples. Considering that seismic attenuation in rock samples can be modelled using the framework of wave propagation in random media, a statistical analysis of shear wave velocity fluctuations was performed: this analysis gives correlation lengths ranging from 0.09 to 1.20 mm, which represents the length scale of heterogeneity in the samples. The individual evaluation of the pore space and mineral content revealed that the pores of the samples (characterized by large vesicles) have a bigger effect than the grains on the heterogeneity level. We have developed a framework where intrinsic properties of the host rocks drive seismic attenuation by correlating the petro-mineralogical characteristics obtained from image data processing and analysis, with the coda attenuation measured at ultrasonic frequencies. There is conclusive evidence that porosity alone is not the primary controller of coda attenuation: it is also changed by the alteration level (i.e. oxidation, coating of the vesicles, secondary minerals) and the size of grains and pores. Among all the parameters analysed, it appears that the pore space topology is the main contributor to scattering attenuation in the volcanic samples.

CodaQback

Attenuation study of a province is considered as a basic quantity for seismic hazard assessment. It has already been established that the study of two physical processes, namely the seismic sources and propagation of the waves, is essential for seismic-hazard mapping. Additionally, attenuation plays an important role towards scaling seismic hazard. Accordingly, a computational tool entitled CodaQback is presented. Based on back scattering model, this versatile software is equipped with user-friendly graphical user interface. It also allows quick picking of phases for computing coda attenuation parameter. All outputs after each execution step in CodaQback are efficiently exported step-wise into a separate folder in Excel and text formats. To validate the computing tool, it is tested in real data analysis and there is found to be good matching of computed values with already established ones. It is envisioned that this package will enable user to derive quick and reliable estimation of coda attenuation parameter irrespective of geological and geo-morphological units.

Seismic S-wave coda attenuation in the Dominican Republic as a tool for seismic hazard mitigation

High-quality waveforms of the vertical and horizontal components of 1356 seismic events recorded from 2013 to 2016 by the Dominican Republic’s seismic network, were analyzed to study the local spatial distribution of the coda frequency-dependent attenuation Qc(f)−1. Qc(f) was estimated at central frequencies of 1.5 (± 0.5), 3.0 (± 1.0), 6.0 (± 2.0), and 12.0 (± 4.0) Hz and interpreted by considering the single backscattering model. Our results of the attenuation of coda waves may explain why some regions of the Dominican Republic (DR) are more prone to suffer damage due to earthquakes than other areas. The majority of the cities and towns in the DR that were severely damaged by earthquakes in the past are located in zones of low and very low coda wave attenuation. In contrast, cities located in zones of high Qc(f)−1 tend to suffer less damage. Our findings identify regions with low seismic attenuation that reflect zones with soft soils that could be impacted by future large events in the DR. These results can be used as a tool for planning seismic hazard mitigation and emergency response as well as for land use regulations.

Coda and Intrinsic Attenuations From Ultrasonic Measurements in Tight Siltstones

AbstractWe have performed ultrasonic measurements on tight siltstone samples to obtain thePwave coda and intrinsic attenuations using a single isotropic scattering model and the spectral ratio method. The results indicate that attenuation is predominantly caused by scattering. Intrinsic attenuation increases with gas saturation and permeability and decreases with grain size, whereas coda attenuation is stronger under partial saturation conditions and increases with grain size. Porosity and mineral composition do not have significant effects on attenuation. The triple‐porosity and scattering models are used to simulate the intrinsic and scattering attenuations, respectively. The results show that the intrinsic attenuation is predominately caused by wave‐induced fluid flow between microcracks/micropores and intergranular pores. The model incorporating the scattering effects of microcracks underestimates the scattering loss in the coda, which may thus be better explained by heterogeneities in the clustering structure of grains.

New insights into seismic absorption imaging

In recent years, attenuation has been used as a marker for source and dynamic Earth processes due to its higher sensitivity to small variations of lithospheric properties compared to seismic velocity. From seismic hazard analysis to oil and gas exploration and rock physics, many fields need a better reconstruction of energy absorption, a constituent of seismic attenuation generally considered a reliable marker of fluid saturation in space. Here, we propose absorption tomography (AT), a technique grounded on the principles of scattering tomography and Multiple Lapse Time Window Analysis. We benchmark its efficiency to image absorption in space by comparing its results with those obtained using two of the most common mapping strategies, regionalization and kernel-based inversion for a diffusive regime. We applied these methodologies to three datasets, each characterized by a different tectonic setting and quality of the dataset: the Pollino fault area (Southern Italy), Mount St. Helens Volcano (USA) and Vrancea (Romania). AT overcomes the assignment of a single coda quality factor value between each source-receiver pair by modelling and inverting for the spatial distribution of energy as a function of different lapse times. It can then reconstruct node-dependent envelopes using analytic and computational sensitivity kernels and solve for coda attenuation with a grid-search approach. AT allows for a better reconstruction of the localized absorption anomalies than standard methodologies, even if the efficiency of the technique depends on the quality of the dataset, and identifies outliers in the data that could alter the final result and its interpretation. The implementation of analytical diffusive kernels in AT allows for a fast and highly-resolved imaging of well-sampled seismic faults structures. While slightly reducing resolution on faults and being computationally more expensive, implementing multiple-scattering lapse-time-dependent kernels still provides satisfactory results as well as a more physically-correct forward model.

2D kernel-based imaging of coda-Q space variations in the Gargano Promontory (Southern Italy)

Crustal seismic attenuation in the Gargano Promontory (Southern Italy) was inferred by analyzing coda waves of about one hundred small magnitude earthquakes recorded by the OTRIONS local seismic network. Coda attenuation Qc was computed using the Sato (1977) model, i.e. the assumption of single back-scattering, at different lapse times and at different frequencies. Average Qc is comparable with the same quantity measured in the adjacent area of the Umbria–Marche region (central Italy). A recent kernel-based imaging technique for Qc was then applied and the results evidence a well confined zone located South West of the investigated area, showing a sharp Qc anomaly. While Qc cannot be directly related to the physical properties of the propagation medium, its spatially-dependent modeling allows an interpretation in the light of other geophysical and geological information. Our results were then interpreted following the two extreme hypotheses according to which Qc could be representative of the effect of only intrinsic attenuation or of only scattering from the heterogeneities. Both the hypothesis correlate with the observation of anomalies in several geophysical parameters, such as heat flow, magnetic field, gravity, density, seismic velocity in the southwestern Gargano. Moreover, the Qc images support the results obtained in a recent study that shows clear variations in the rheological properties of the Gargano. The results obtained at local scale confirm and detail previous regional scale studies on attenuation and highlight for the first time the presence of a positive Qc anomaly in the south-western part of the Gargano Promontory.

Investigation of coda and body wave attenuation functions in Central Asia

In this study, we evaluate the body and coda wave attenuation characteristics within Kyrgyzstan and Tajikistan as part of Central Asia. The selected database consists of 354 broadband seismograms from 179 local earthquakes recorded by 24 different stations within the period of 2015 through 2018. First, coda Q has been inferred for different coda window lengths of 20, 30, 40, and 50 s using the single-backscattering interpretation. The coda Q values increase by increasing the coda window length. We show that coda attenuation properties in Central Asia are better modeled by multiple-scattering and surface wave regimes for long-distance records without invoking any depth dependence of the attenuation properties in the crust. Furthermore, standard errors and convergence of different components’ QC indicate that we can fit envelope records of coda waves much better using a coda window length of 50 s. Therefore, we evaluate average coda quality factor functions as QC = 261 f0.601 and QC = 219 f0.633 assuming multiple-scattering and surface wave regimes for a coda window length of 50 s in the frequency range of 1 to 20 Hz for distances up to 200 km. We also show that the source to site distance of records has a significant impact on coda Q estimates. For a shorter distance range up to 100 km, attenuation attributes of Central Asia are better captured by a single-scattering model. We reevaluate the average coda quality factor function as QC = 222 f0.692 assuming a single-backscattering model for a coda window length of 50 s in the frequency range of 1 to 20 Hz for distances up to 100 km. Moreover, we determine QP = 158 f0.706 and QS = 152 f0.856 with a geometrical spreading function of R−1 using the multiple-station coda normalization method.

Three-dimensional kernel-based coda attenuation imaging of caldera structures controlling the 1982-84 Campi Flegrei unrest

Coda-wave attenuation imaging has risen as a state-of-the-art technique to depict volcanic structures using their dispersion effects. The 1982–84 seismic and deformation unrest at Campi Flegrei caldera (Italy) is a unique example of non-eruptive volcanic activity in a structured caldera. Here, we propose the first application of 3D coda-attenuation kernels to image caldera structures at multiple frequencies during unrest. Using sensitivity kernels is necessary to assess the effective resolution of coda imaging in highly heterogeneous volcanoes. The technique relies on the solution of Paasschens' equations in the framework of radiative transfer theory. The results map coda attenuation in the 3D space without need of pre-existent velocity models. The resolution and stability of the inversion solutions were examined by changing the damping parameters and outputting the corresponding images, inverting for different node spacings and performing checkerboard tests. These tests show that the resolution of the multiple-scattering model is much lower than that provided by a standard isotropic-scattering and/or single scattering technique. The best resolution in our model is obtained between depths of 1 km and 3 km in the centre of the model, between Pozzuoli town and Solfatara crater. The results are discussed at a frequency of 3 Hz, due to both longer coda durations and broader kernel illumination: in this frequency range, coda-based 3D imaging had so far failed to provide stable results. The interpretation is performed based on the extensive geological and geophysical knowledge of the caldera. High-attenuation anomalies below Solfatara and Monte Nuovo mark areas either saturated with water or enriched in molten rocks, feeding the respective fumarole fields. The flattening and horizontal elongation of these anomalies below 2 km depth is a manifestation of the blocking and spreading around of the rising hot magmatic fluids below a previously-inferred high-velocity, low-attenuation and highly-deforming caprock. This caprock is not uniform, likely due to the remnants of erupted structure. A SW-to-NE-trending low-attenuation and high-velocity anomaly deeps down to 3 km under Pozzuoli. We infer that the high seismicity in the region is a consequence of the stress sustained by the caprock from a 4-km-deep deformation source in 1983–84. On top of this source, high coda attenuation corresponds to high Vp/Vs ratios and high direct-wave attenuation. The region's characteristics are likely due to the accumulation of magmatic fluids above a magmatic sill.

An Improved Method to Determine Coda‐Q, Earthquake Magnitude, and Site Amplification: Theory and Application to Southern California

AbstractSeismic coda waves can be used to constrain attenuation, estimate earthquake magnitudes, and determine site amplification factors. We have developed a new multistation and multievent method to determine these three important seismic parameters simultaneously. We analyze 642 representative local (≤100 km) and shallow (≤20 km) earthquakes with catalog magnitudes between 1.8 and 5.4 in southern California at multiple frequency bands centered at 1.5, 3, 6, and 12 Hz. We find that the length of the moving average time window can affect the measurement of coda attenuation QC, but our tests indicate that the optimal window length is about 15 times the dominant data period. We use linear regression to fit each coda section and use only those portions that agree with the model decay rate with a correlation coefficient larger than 0.9. For a frequency‐dependent coda‐QC model (QC = Q0fn) at 1‐Hz reference frequency, our results yield estimates for Q0 and n of 107–288 and 0.42–1.14, respectively. Our coda magnitude estimates are linearly correlated with catalog magnitudes, and our observed lateral variations in coda‐QC and our site amplification factors are in general agreement with previous results, although there are notable differences at some locations. This approach provides a unified, accurate, and stable method to measure coda‐QC, earthquake magnitude, and site amplification using coda waves of locally recorded earthquakes.

Attenuation of Coda Waves in the Central Region of the Gulf of California, México

Coda waves were analyzed from events recorded by NARS seismic network deployed along both margins of the Gulf of California, Mexico, to estimate coda attenuation Qc. Sato’s (1977) single scattering model was used for a coda window of 20 to 25 s beginning at twice the S-wave travel time. Events recorded from 2003 to 2007 located in the central region of the Gulf of California were analyzed. Source-to-receiver distances are between 40 and 500 km. Assuming a power law of the form QC (f) = QO f a, QC values were averaged and a value of QO = 83±3 and a frequency-dependence α value of 1.06±0.03 in the frequency range from 1 to 7 Hz was obtained. QO value and the high frequency dependency agree with the values of other regions characterized by a high tectonic activity. Based on source-station distribution two subregions (north and south) were defined. QC values were calculated and correlated with tectonics and morphology of each area. The observed higher attenuation in the south region can be attributed to the fact that south region is more fractured since the greater earthquakes occur in central to south Gulf of California and the oceanic crust is reported to be thinner in the southern region.

Stress-associated scattering attenuation and intrinsic attenuation from ultrasonic measurements

SUMMARY Acoustic attenuation has been proved to be an indicator of stress changes in solid structures. Acoustic coda, as a superposition of incoherent scattered waves, reflects small-scale random heterogeneities in solids. Acoustic coda attenuation, as a combination of intrinsic attenuation and scattering attenuation, contains information on stress changes as a result of changes in the physical state of small-scale heterogeneous structures. Based on the ultrasonic measurements of a rock sample with intra-grain pores and fractures under different pore-pressure induced effective stresses, we compute the stress-associated coda attenuation quality factors QPC and QSC as a function of frequencies. Based on the digital heterogeneous cores of the sample, the experimental results are validated and corrected with numerical results by the finite-difference simulation of Biot’s poroelastic equations and the Monte Carlo simulation of multiple scatterings, respectively. The quality factors characterize its scale dependence of scattering attenuation on stress variations in rocks. We compare them with the intrinsic attenuation quality factors QP and QS calculated by the spectral ratio method and BISQ model, respectively, from ultrasonic measurements. Comparisons demonstrate that the scattering attenuation is much stronger, particularly when ultrasonic wavelengths are comparable to the scale of pores and grains. The intrinsic and coda attenuations versus increasing effective stresses present quite different nonlinear features, where QPC and QSC show a greater sensitivity to pore pressure than QP and QS.

Coda Attenuation Analysis in the West Bohemia/Vogtland Earthquake Swarm Area

Seismic coda represents a valuable source of information about the attenuation of the high-frequency waves in the studied region. The quality factor Q derived from coda is an integral parameter of the volume surrounding the hypocenter and seismic station and, according to the applied method, represents the total attenuation or the intrinsic and scattering parts. We analyzed records of 13 selected earthquakes in the magnitude range 1.7–2.9 of the 2011 swarm from West Bohemian/Vogtland area (central Europe), which were recorded at epicentral distances from 7 to 50 km. Two methods were applied: coda method for estimation of the Qc and the Multiple Lapse Time Windows Analysis for separation of the scattering and intrinsic loss by estimation of Qi and Qsc. Careful selection of the analyzed events was necessary due to the frequent contamination of coda decays by the running seismic swarm activity. The resulting coda Qc is relatively high with respect to the geodynamic activity and varies between 100 and 2500 within the analyzed frequency range of 1–18 Hz. The intrinsic loss dominates over scattering attenuation with Qi increasing from 100 and 1850 and Qsc from 300 to 3400 in the same frequency range, which is consistent with the geodynamic activity of the region. We find that the intrinsic attenuation in West-Bohemia/Vogtland is higher that in neighboring Germany, which could be attributed to the heterogeneity of the crust in central Europe.

Attenuation and scattering tomography of the deep plumbing system of Mount St. Helens

AbstractWe present a combined 3‐D P wave attenuation, 2‐D S coda attenuation, and 3‐D S coda scattering tomography model of fluid pathways, feeding systems, and sediments below Mount St. Helens (MSH) volcano between depths of 0 and 18 km. High‐scattering and high‐attenuation shallow anomalies are indicative of magma and fluid‐rich zones within and below the volcanic edifice down to 6 km depth, where a high‐scattering body outlines the top of deeper aseismic velocity anomalies. Both the volcanic edifice and these structures induce a combination of strong scattering and attenuation on any seismic wavefield, particularly those recorded on the northern and eastern flanks of the volcanic cone. North of the cone between depths of 0 and 10 km, a low‐velocity, high‐scattering, and high‐attenuation north‐south trending trough is attributed to thick piles of Tertiary marine sediments within the St. Helens Seismic Zone. A laterally extended 3‐D scattering contrast at depths of 10 to 14 km is related to the boundary between upper and lower crust and caused in our interpretation by the large‐scale interaction of the Siletz terrane with the Cascade arc crust. This contrast presents a low‐scattering, 4–6 km2 “hole” under the northeastern flank of the volcano. We infer that this section represents the main path of magma ascent from depths greater than 6 km at MSH, with a small north‐east shift in the lower plumbing system of the volcano. We conclude that combinations of different nonstandard tomographic methods, leading toward full‐waveform tomography, represent the future of seismic volcano imaging.

Estimation of seismodynamics differences and lateral variations of coda Q in Eastern Anatolia

This study is to compare, analyze and examine the seismotectonic relationship of the change between dynamic properties of the interior of the upper crust such as crust height and crust pressure that belong to different tectonic regimes of the upper crust and the horizontal coda attenuation (Q). The whole study area was divided into four tectonic blocks according to the distributions of the earthquakes and the place of the fault segments. These four horizontal local coda absorption variations have been used to provide a significant and quantitative source of information about the seismotectonics of the fault systems. The coda quality factor, Qc, was estimated using the single scattering model. The estimated Q C values for the four regions ranged from 79f0.91 to 93f0.95, highlighting the regional differences in the tectonic activity of the crust. The mean Q C value calculated was Q T = 97f0.93. The frequency dependency values range between 0.68 and 1.1 in the study area. The highest Q C values were determined in two stations in the north area while the lowest Q C values were observed in two stations in the south of the study area. The highest frequency values were obtained in the southern region, while the lowest frequency values were obtained in the northern area. In this study, two different attenuation areas were distinguished, which agrees with the geotectonic characteristics of Eastern Anatolia from north to south.

Poroelastic finite-difference modeling for ultrasonic waves in digital porous cores

Scattering attenuation in short wavelengths has long been interesting to geophysicists. Ultrasonic coda waves, observed as the tail portion of ultrasonic wavetrains in laboratory ultrasonic measurements, are important for such studies where ultrasonic waves interact with small-scale random heterogeneities on a scale of micrometers, but often ignored as noises because of the contamination of boundary reflections from the side ends of a sample core. Numerical simulations with accurate absorbing boundary can provide insight into the effect of boundary reflections on coda waves in laboratory experiments. The simulation of wave propagation in digital and heterogeneous porous cores really challenges numerical techniques by digital image of poroelastic properties, numerical dispersion at high frequency and strong heterogeneity, and accurate absorbing boundary schemes at grazing incidence. To overcome these difficulties, we present a staggered-grid high-order finite-difference (FD) method of Biot’s poroelastic equations, with an arbitrary even-order (2L) accuracy to simulate ultrasonic wave propagation in digital porous cores with strong heterogeneity. An unsplit convolutional perfectly matched layer (CPML) absorbing boundary, which improves conventional PML methods at grazing incidence with less memory and better computational efficiency, is employed in the simulation to investigate the influence of boundary reflections on ultrasonic coda waves. Numerical experiments with saturated poroelastic media demonstrate that the 2L FD scheme with the CPML for ultrasonic wave propagation significantly improves stability conditions at strong heterogeneity and absorbing performance at grazing incidence. The boundary reflections from the artificial boundary surrounding the digital core decay fast with the increase of CPML thicknesses, almost disappearing at the CPML thickness of 15 grids. Comparisons of the resulting ultrasonic coda Qsc values between the numerical and experimental ultrasonic S waveforms for a cylindrical rock sample demonstrate that the boundary reflection may contribute around one-third of the ultrasonic coda attenuation observed in laboratory experiments.

Monte Carlo Simulation of Stress-Associated Scattering Attenuation from Laboratory Ultrasonic Measurements

Abstract Seismic coda waves scattered by small‐scale heterogeneities contain information on stress changes of the medium, because of changes in the physical state of materials. Based on the ultrasonic measurements under different stresses for a cylindrical sandstone sample, we investigate the influence of stress changes on ultrasonic S ‐coda attenuation and aim to characterize its stress/frequency‐dependent pattern. Considering the complexity of ultrasonic coda waveforms measured from finite‐size rock samples in laboratory experiments, the Monte Carlo simulation is employed to synthesize ultrasonic envelopes, which act by incorporating the effect of multiple scatterings and boundary reflections on coda waves. The optimal simulation parameters, estimated by minimizing the residual between the observed and synthesized envelopes, indicate that the rock sample under study presents moderate heterogeneities. The relationship between attenuation and stress is similar for direct and coda S waves and remains stable in the range of high‐effective stresses around 30–60 MPa, with less stress sensitivity. Enhanced attenuation for both types of waves occurs at lower‐effective stresses, but with coda attenuation much faster and stronger, presenting a quite different nonlinear behavior with respect to stress. Coda attenuation increases drastically at extremely low‐effective stresses below 15 MPa because of the increase in rock compliance, showing much greater sensitivity to high‐pore pressure than intrinsic attenuation. This study improves our understanding of the mechanism of ultrasonic coda attenuation and its scaling dependence on stress and frequency.

Finite difference modeling of ultrasonic propagation (coda waves) in digital porous cores with un-split convolutional PML and rotated staggered grid

Abstract Ultrasonic wave propagation in heterogeneous porous cores under laboratory studies is an extremely complex process involved with strong scattering by microscale heterogeneous structures. The resulting coda waves, as an index to measure scattering attenuation, are recorded as continuous waveforms in the tail portion of wavetrains. Because of the contamination of reflections from the side ends and reverberations between the sample surfaces, it is difficult to extract pure coda waves from ultrasonic measurements for the estimation of the P - and S -coda attenuation quality factors. Comparisons of numerical and experimental ultrasonic wave propagation in heterogeneous porous cores can give important insight into understanding the effect of boundary reflections on the P - and S -codas in the laboratory experiment. It challenges numerical modeling techniques by three major issues: the creation of a digital core model to map heterogeneous rock properties in detail, the perfect simulation with a controllable and accurate absorbing boundary, and overcoming the numerical dispersions resulting from high-frequency propagation and strong heterogeneity in material. A rotated staggered-grid finite-difference method of Biot's poroelastic equations is presented with an unsplit convolutional perfectly matched layer (CPML) absorbing boundary to simulate poroelastic wave propagation in isotropic and fluid-saturated porous media. The contamination of boundary reflections on coda waves is controlled by the CPML absorbing coefficients for the comparison between numerical and experimental ultrasonic waveforms. Numerical examples with a digital porous core demonstrate that the boundary reflections contaminate coda waves seriously, causing much larger coda quality factors and thus underestimating scattering attenuation.