Strain microseismics: Radiation patterns, synthetics, and moment tensor resolvability with distributed acoustic sensing in isotropic media

We have derived analytical formulations for the strain field produced by a moment tensor source in homogeneous isotropic media. Such formulations are important for microseismic projects that increasingly are monitored with fiber-optic distributed acoustic sensing (DAS) systems. We find that the spatial derivative of displacement produces new terms in strain proportional to [Formula: see text] with [Formula: see text]. In viscoelastic media, the derivative also produces an additional far-field term that is scaled by a frequency-dependent factor. When comparing with full wavefield synthetic data, we observe that the new terms proportional to [Formula: see text] can be considered part of a near-field in strain, similar to the practice with the displacement formulation. Analyses of moment tensor resolvability show that full moment tensors are resolvable with P-wave information from two or more noncoplanar vertical DAS cable geometries if intermediate- and far-field terms are considered and that S-wave information alone cannot constrain full moment tensors using only vertical wells. These results mirror previous observations made with displacement measurements. Furthermore, the addition of the new terms proportional to [Formula: see text] in strain improves the moment tensor resolvability but only in the case of a single vertical array. In the case of a single deviated/horizontal well, we can, in theory, resolve a full moment tensor but a case-by-case analysis is necessary to identify regions of full constraint around the well and the necessary noise conditions to guarantee reliable solutions. Real DAS measurements also are affected by the gauge length and interrogator details. In the case of the gauge length, we observe that this operator does not change the resolvability of the problem but it does affect inversion stability. The results derived here represent theoretical limits or in some cases specific examples. Practical implementations require analyses of conditioning, noise, coupling, and the effect of gauge length on a case-by-case basis.

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Distributed Acoustic Sensing Using a Large-Volume Airgun Source and Internet Fiber in an Urban Area

Seismological Research Letters ◽

10.1785/0220200274 ◽

2021 ◽

Author(s):

Zhenghong Song ◽

Xiangfang Zeng ◽

Baoshan Wang ◽

Jun Yang ◽

Xiaobin Li ◽

...

Keyword(s):

Urban Area ◽

Urban Areas ◽

Signal To Noise Ratio ◽

Gauge Length ◽

P Wave ◽

S Wave ◽

Near Surface ◽

Acoustic Sensing ◽

Wave Signal ◽

Distributed Acoustic Sensing

Abstract Seismological methods have been widely used to construct subsurface images in urban areas, for both seismological and engineering purposes. However, it remains a challenge to continuously operate a dense array in cities for high-resolution 4D imaging. In this study, we utilized distributed acoustic sensing (DAS) and a 5.2 km long, L-shaped, telecom, fiber-optic cable to record the wavefield from a highly repeatable airgun source located 7–10 km away. No P-wave signal was observed, but the S-wave signal emerged clearly on the shot-stacked traces, and the arrivals were consistent with collocated geophone traces. Because the signal quality is significantly affected by cable coupling and local noise, three methods can be employed to improve signal-to-noise ratio: (1) stacking contiguous, colinear channels to increase effective gauge length, (2) connecting multiple fibers within a single conduit and stacking collocated channels, and (3) using engineered fiber. In conclusion, the combination of DAS, using internet fiber and an airgun source with proven efficient signal enhancement methods, can provide frequent snapshots of the near surface across an urban area.

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Hydraulic Fracturing Diagnostics Utilizing Near and Far-Field Distributed Acoustic Sensing DAS Data Correspondences

10.2118/204205-ms ◽

2021 ◽

Author(s):

Yinghui Wu ◽

Robert Hull ◽

Andrew Tucker ◽

Craig Rice ◽

Peter Richter ◽

...

Keyword(s):

Hydraulic Fracture ◽

Near Field ◽

Fracture Propagation ◽

Cost Effective ◽

Hydraulic Fractures ◽

Far Field ◽

Distributed Temperature Sensing ◽

Acoustic Sensing ◽

Distributed Acoustic Sensing ◽

Multiple Fibers

Abstract Distributed fiber-optic sensing (DFOS) has been utilized in unconventional reservoirs for hydraulic fracture efficiency diagnostics for many years. Downhole fiber cables can be permanently installed external to the casing to monitor and measure the uniformity and efficiency of individual clusters and stages during the completion in the near-field wellbore environment. Ideally, a second fiber or multiple fibers can be deployed in offset well(s) to monitor and characterize fracture geometries recorded by fracture-driven interactions or frac-hits in the far-field. Fracture opening and closing, stress shadow creation and relaxation, along with stage isolation can be clearly identified. Most importantly, fracture propagation from the near to far-field can be better understood and correlated. With our current technology, we can deploy cost effective retrievable fibers to record these far-field data. Our objective here is to highlight key data that can be gathered with multiple fibers in a carefully planned well-spacing study and to evaluate and understand the correspondence between far-field and near-field Distributed Acoustic Sensing (DAS) data. In this paper, we present a case study of three adjacent horizontal wells equipped with fiber in the Permian basin. We can correlate the near-field fluid allocation across a stage down to the cluster level to far-field fracture driven interactions (FDIs) with their frac-hit strain intensity. With multiple fibers we can evaluate fracture geometry, the propagation of the hydraulic fractures, changes in the deformation related to completion designs, fracture complexity characterization and then integrate the results with other data to better understand the geomechanical processes between wells. Novel frac-hit corridor (FHC) is introduced to evaluate stage isolation, azimuth, and frac-hit intensity (FHI), which is measured in far-field. Frac design can be evaluated with the correlation from near-field allocation to far-field FHC and FHI. By analyzing multiple treatment and monitor wells, the correspondence can be further calibrated and examined. We observe the far-field FHC and FHI are directly related to the activities of near-field clusters and stages. A leaking plug may directly result in FHC overlapping, gaps and variations in FHI, which also can be correlated to cluster uniformity. A near-far field correspondence can be established to evaluate FHC and FHI behaviors. By utilizing various completion designs and related measurements (e.g. Distributed Temperature Sensing (DTS), gauges, microseismic etc.), optimization can be performed to change the frac design based on far-field and near-field DFOS data based on the Decision Tree Method (DTM). In summary, hydraulic fracture propagation can be better characterized, measured, and understood by deploying multiple fibers across a lease. The correspondence between the far-field measured FHC and FHI can be utilized for completion evaluation and diagnostics. As the observed strain is directly measured, completion engineering and geoscience teams can confidently optimize their understanding of the fracture designs in real-time.

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Accuracy of the moment-tensor inversion of far-field P waves

Geophysical Journal International ◽

10.1093/gji/ggz446 ◽

2019 ◽

Vol 220 (1) ◽

pp. 248-256 ◽

Cited By ~ 1

Author(s):

Yue Kong ◽

Min Li ◽

Weimin Chen ◽

Boqi Kang

Keyword(s):

Moment Tensor ◽

Near Field ◽

Radial Component ◽

Moment Tensor Inversion ◽

Far Field ◽

Field Range ◽

P Waves ◽

Moment Tensors ◽

Nodal Lines ◽

The Moment

SUMMARY The far-field assumption is widely used and suitable for the moment-tensor inversion in which the source–receiver distance is quite long. However, the description of far field is uncertain and an explicit far-field range is missing. In this study, the explicit far-field range is determined and the errors of moment-tensor solutions produced by the far-field approximation are analysed. The distance, for which the far-field assumption is satisfied and the effect of the near-field term can be ignored, is directionally dependent. For the shear dislocation, in the directions near the nodal lines of the far-field P waves, the far-field distance is heavily dependent on the displacement component used to invert moment tensors. The radial component of displacement, which is parallel to the wave propagation direction, is recommended for the inversion and the corresponding far-field distance is quite short. In the directions far from the nodal lines, the selection of displacement components has little influence on the far-field distance. The maximum far-field distance appears in the directions of the pressure and tensile axes of the source and the value is about 30 wavelengths of radiated waves. Using more receivers (>6) in the moment-tensor inversion can shorten the far-field distance. The effect of the near-field term on the moment-tensor inversion for tensile dislocations and isotropic sources (explosion or implosion) can be ignored. The conclusions obtained in this study are helpful for determining the positions of receivers and evaluating the accuracy of moment-tensor solutions, with far-field assumption being applied in the inversion.

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Near-field strain in DAS-based microseismic observation

Geophysics ◽

10.1190/geo2021-0031.1 ◽

2021 ◽

pp. 1-49

Author(s):

Ge Jin ◽

Frantisek Stanek ◽

Bin Luo

Keyword(s):

Hydraulic Fracture ◽

Fiber Optic ◽

Moment Tensor ◽

Near Field ◽

Source Parameters ◽

Field Strain ◽

Far Field ◽

Normal Strain ◽

S Wave ◽

Sign Patterns

Microseismic monitoring with surface or downhole geophone arrays has been commonly used in tracking subsurface deformation and fracture networks during hydraulic fracturing operations. Recently, the use of fiber-optic DAS technology has improved microseismic acquisition to a new level with unprecedentedly high spatial resolution and low cost. Deploying fiber-optic cables in horizontal boreholes allows very close observation of these micro-sized earthquakes and captures their full wavefield details. We show that DAS-based microseismic profiles present a seldomly reported near-field strain signal between the P- and S-wave arrivals. This near-field signal shows monotonically increasing (or decreasing) temporal variation, which resembles the previously reported near-field observations of large earthquakes. To understand the near-field strain behavior, we provide a mathematical expression of the analytic normal strain solution that reveals the near-field, intermediate-near-field, intermediate-far-field, and far-field components. Synthetic DAS strain records of hydraulic-fracture-induced microseismic events can be generated using this analytic solution with the Brune source model. The polarity sign patterns of the near-field and far-field terms in these synthetics are linked to the corresponding source mechanism’s radiation patterns. These polarity sign patterns are demonstrated to be sensitive to the source orientations by rotating the moment tensor in different directions. A field data example is compared to the synthetic result and a qualitative match is shown. The microseismic near-field signals detected by DAS have potential value in hydraulic fracture monitoring by providing a means to better constrain microseismic source parameters that characterize the source magnitude, source orientation, and temporal source evolution, and therefore better reflect the geomechanical response of the hydraulically fractured environment in the unconventional reservoirs.

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Detecting anisotropy using Distributed Acoustic Sensing and fibre-optic seismology in a fast-flowing glacier in Greenland

10.5194/egusphere-egu2020-7709 ◽

2020 ◽

Author(s):

Adam Booth ◽

Poul Christoffersen ◽

Charlotte Schoonman ◽

Andy Clarke ◽

Bryn Hubbard ◽

...

Keyword(s):

Wave Energy ◽

Internal Flow ◽

P Wave ◽

Sediment Layer ◽

Fibre Optic ◽

S Wave ◽

Acoustic Sensing ◽

Seismic Properties ◽

Distributed Acoustic Sensing ◽

Zero Offset

Material anisotropy within a glacier both influences and is influenced by its internal flow regime. Anisotropy can be measured from surface seismic recordings, using either active sources or natural seismic emissions. In the past decade, Distributed Acoustic Sensing (DAS) has emerged as a new, and potentially transformative, seismic acquisition technology, involving determining seismic responses from the deformation of optical fibres. Although DAS has shown great potential within engineering and resources sectors, it has not yet been widely deployed in studies of glaciers and ice masses.Here, we present results from a glaciological deployment of a DAS system. In July 2019, a Solifos BRUsens fibre optic cable was installed in a 1050 m borehole drilled on Store Glacier in West Greenland. Vertical seismic profiles (VSPs) were recorded using a Silixa iDAS interrogation unit, with seismic energy generated with a 7 kg sledgehammer striking a polyethene (UHMWPE) impact plate. A three-day sequence of zero-offset VSPs (with the source located ~1 m from the borehole top) were recorded to monitor the freezing of the cable, combined with offset-VSPs in along- and cross-flow directions, and radially at 300 m offset.P-wave energy (frequency ~200 Hz) is detectable through the whole ice thickness, sampled at 1 m depth increments. The zero-offset reflectivity of the glacier bed is low, but reflections are detected from the apparent base of a subglacial sediment layer. S-wave energy is also detectable in the offset VSP records. The zero-offset VSPs show a mean vertical P-wave velocity of 3800 &#177; 140 m/s for the upper 800 m of the glacier, rising to 4080 &#177; 140 m/s between 900-950 m. In the deepest 50 m, velocity reduces to 3890 &#177; 80 m/s. This variation in vertical velocity is consistent with the development of an anisotropic ice fabric in the lowermost 10% of the glacier. The full dataset also contains natural seismic emissions, highlighting the potential of DAS as both an active and passive seismic monitoring tool.DAS offers transformative potential for understanding the seismic properties of glaciers and ice sheets. The simplicity of the typical VSP geometry makes the interpretation of seismic travel-times less vulnerable to approximations, and thus the derivation of seismic properties more robust, than in conventional surface seismic surveys. As an addition, DAS facilitates VSP recording with unprecedented vertical and temporal resolution. Furthermore, the sensitivity of the optical-fibre to both P- and S-wave particle motion means that a comprehensive suite of acoustic and elastic properties can be inferred.

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To: “Strain microseismics: Radiation patterns, synthetics, and moment tensor resolvability with distributed acoustic sensing in isotropic media,” Ismael Vera Rodriguez and Andreas Wuestefeld, Geophysics, 85, no. 3, KS101–KS114, doi: 10.1190/geo2019-0373.1.

Geophysics ◽

10.1190/geo2020-0528-erratum.1 ◽

2020 ◽

Vol 85 (4) ◽

pp. Y5-Y5

Author(s):

Ismael Vera Rodriguez ◽

Andreas Wuestefeld

Keyword(s):

Moment Tensor ◽

Radiation Patterns ◽

Acoustic Sensing ◽

Isotropic Media ◽

Distributed Acoustic Sensing

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A teleseismic body-wave analysis of the May 1980 Mammoth Lakes, California, earthquakes

Bulletin of the Seismological Society of America ◽

10.1785/bssa0730020419 ◽

1983 ◽

Vol 73 (2) ◽

pp. 419-434

Author(s):

Jeffery S. Barker ◽

Charles A. Langston

Keyword(s):

Body Wave ◽

Moment Tensor ◽

Normal Fault ◽

Strike Slip ◽

Body Waves ◽

P Wave ◽

Moment Tensors ◽

Double Couple ◽

The Moment ◽

Mammoth Lakes

abstract Teleseismic P-wave first motions for the M ≧ 6 earthquakes near Mammoth Lakes, California, are inconsistent with the vertical strike-slip mechanisms determined from local and regional P-wave first motions. Combining these data sets allows three possible mechanisms: a north-striking, east-dipping strike-slip fault; a NE-striking oblique fault; and a NNW-striking normal fault. Inversion of long-period teleseismic P and SH waves for the events of 25 May 1980 (1633 UTC) and 27 May 1980 (1450 UTC) yields moment tensors with large non-double-couple components. The moment tensor for the first event may be decomposed into a major double couple with strike = 18°, dip = 61°, and rake = −15°, and a minor double couple with strike = 303°, dip = 43°, and rake = 224°. A similar decomposition for the last event yields strike = 25°, dip = 65°, rake = −6°, and strike = 312°, dip = 37°, and rake = 232°. Although the inversions were performed on only a few teleseismic body waves, the radiation patterns of the moment tensors are consistent with most of the P-wave first motion polarities at local, regional, and teleseismic distances. The stress axes inferred from the moment tensors are consistent with N65°E extension determined by geodetic measurements by Savage et al. (1981). Seismic moments computed from the moment tensors are 1.87 × 1025 dyne-cm for the 25 May 1980 (1633 UTC) event and 1.03 × 1025 dyne-cm for the 27 May 1980 (1450 UTC) event. The non-double-couple aspect of the moment tensors and the inability to obtain a convergent solution for the 25 May 1980 (1944 UTC) event may indicate that the assumptions of a point source and plane-layered structure implicit in the moment tensor inversion are not entirely valid for the Mammoth Lakes earthquakes.

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Distributed acoustic sensing as a tool for exploration and monitoring: a proof-of-concept

10.5194/se-2021-37 ◽

2021 ◽

Author(s):

Nicola Piana Agostinetti ◽

Alberto Villa ◽

Gilberto Saccorotti

Keyword(s):

High Resolution ◽

Plane Wave ◽

Geothermal Field ◽

P Wave ◽

Apparent Velocity ◽

Near Surface ◽

Acoustic Sensing ◽

Geothermal Activity ◽

Distributed Acoustic Sensing ◽

Microseismic Event

Abstract. We use PoroTOMO experimental data to compare the performance of Distributed Acoustic Sensing (DAS) and geophone data in executing standard exploration and monitoring activities. The PoroTOMO experiment consists of two "seismic systems": (a) a 8.6 km long optical fibre cable deployed across the Brady geothermal field and covering an area of 1.5 x 0.5 km with 100 m long segments, and (b) an array of 238 co-located geophones with an average spacing of 60 m. The PoroTOMO experiment recorded continuous seismic data between March 10th and March 25th 2016. During such period, a ML 4.3 regional event occurred in the southwest, about 150 km away from the geothermal field, together with several microseismic local events related to the geothermal activity. The seismic waves generated from such seismic events have been used as input data in this study. For the exploration tasks, we compare the propagation of the ML 4.3 event across the geothermal field in both seismic systems in term of relative time-delay, for a number of configurations and segments. Defined the propagation, we analyse and compare the amplitude and the signal-to-noise ratio (SNR) of the P-wave in the two systems at high resolution. For testing the potential in monitoring local seismicity, we first perform an analysis of the geophone data for locating a microseismic event, based on expert opinion. Then, we a adopt different workflow for the automatic location of the same microseismic event using DAS data. For testing the potential in monitoring distant event, data from the regional earthquake are used for retrieving both the propagation direction and apparent velocity of the wavefield, using a standard plane-wave-fitting approach. Our results indicate that: (1) at a local scale, the seismic P-waves propagation and their characteristics (i.e. SNR and amplitude) along a single cable segment are robustly consistent with recordings from co-located geophones (delay-times δt ∼ 0.3 over 400 m for both seismic systems) ; (2) the interpretation of seismic wave propagation across multiple separated segments is less clear, due to the heavy contamination of scattering sources and local velocity heterogeneities; nonetheless, results from the plane-wave fitting still indicate the possibility for a consistent detection and location of the event; (3) at high-resolution (10 m), large amplitude variations along the fibre cable seem to robustly correlate with near surface geology; (4) automatic monitoring of microseismicity can be performed with DAS recordings with results comparable to manual analysis of geophone recordings (i.e. maximum horizontal error on event location around 70 m for both geophones and DAS data) ; and (5) DAS data pre-conditioning (e.g., temporal sub-sampling and channel-stacking) and dedicated processing techniques are strictly necessary for making any real-time monitoring procedure feasible and trustable.

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Relative moment tensors and deep Yakutat seismicity

Geophysical Journal International ◽

10.1093/gji/ggz375 ◽

2019 ◽

Vol 219 (2) ◽

pp. 1447-1462 ◽

Cited By ~ 1

Author(s):

Alexandre P Plourde ◽

Michael G Bostock

Keyword(s):

Principal Components ◽

Moment Tensor ◽

Synthetic Data ◽

Principal Component ◽

P Wave ◽

Inversion Method ◽

Low Velocity ◽

Stress Regime ◽

Moment Tensors ◽

S Waves

SUMMARY We introduce a new relative moment tensor (MT) inversion method for clusters of nearby earthquakes. The method extends previous work by introducing constraints from S-waves that do not require modal decomposition and by employing principal component analysis to produce robust estimates of excitation. At each receiver, P and S waves from each event are independently aligned and decomposed into principal components. P-wave constraints on MTs are obtained from a ratio of coefficients corresponding to the first principal component, equivalent to a relative amplitude. For S waves we produce constraints on MTs involving three events, where one event is described as a linear combination of the other two, and coefficients are derived from the first two principal components. Nonlinear optimization is applied to efficiently find best-fitting tensile-earthquake and double-couple solutions for relative MT systems. Using synthetic data, we demonstrate the effectiveness of the P and S constraints both individually and in combination. We then apply the relative MT inversion to a set of 16 earthquakes from southern Alaska, at ∼125 km depth within the subducted Yakutat terrane. Most events are compatible with a stress tensor dominated by downdip tension, however, we observe several pairs of earthquakes with nearly antiparallel slip implying that the stress regime is heterogeneous and/or faults are extremely weak. The location of these events near the abrupt downdip termination of seismicity and the low-velocity zone suggest that they are caused by weakening via grain-size and volume reduction associated with eclogitization of the lower crustal gabbro layer.

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Near-field spectral analysis of data-integrative dynamic rupture earthquake simulations of the 1992 Landers earthquake

10.5194/egusphere-egu2020-22673 ◽

2020 ◽

Author(s):

Nico Schliwa ◽

Alice-Agnes Gabriel

Keyword(s):

Spectral Analysis ◽

Ground Motion ◽

Near Field ◽

Slip Distribution ◽

Corner Frequency ◽

Dynamic Rupture ◽

Geophysical Research ◽

Acoustic Sensing ◽

Distributed Acoustic Sensing ◽

Large Earthquakes

The rise of observations from Distributed Acoustic Sensing (e.g., Zhan 2020) and high-rate GNSS networks (e.g., Madariaga et al., 2019) highlight the potential of dense ground motion observations in the near-field of large earthquakes. Here, spectral analysis of >100,000 synthetic near-field strong motion waveforms (up to 2 Hz) is presented in terms of directivity, corner frequency, fall-off rate, moment estimates and static displacements.The waveforms are generated in 3&#8208;D large-scale dynamic rupture simulations which incorporate the interplay of complex fault geometry, topography, 3&#8208;D rheology and viscoelastic attenuation (Wollherr et al., 2019). A preferred scenario accounts for off-fault deformation and reproduces a broad range of observations, including final slip distribution, shallow slip deficits, and spontaneous rupture termination and transfers between fault segments. We examine the effects of variations in modeling parameterization within a suite of scenarios including purely elastic setups and models neglecting viscoelastic attenuation.&#160;First, near-field corner frequency mapping implementing a novel spectral seismological misfit criterion reveals rays of elevated corner frequencies radiating from each slipping fault at 45 degree to rupture forward direction. The azimuthal spectral variations are specifically dominant in the vertical components indicating we map rays of direct P-waves prevailing (Hanks, 1980). The spatial variation in corner frequencies carries information on co-seismic fault segmentation, slip distribution, focal mechanisms and stress drop. Second, spectral fall-off rates are variably inferred during picking the associated corner frequencies to identify the crossover from near-field to far-field spectral behaviour in dependence on distance and azimuth. Third, we determine static displacements with the help of near-field seismic spectra.Our findings highlight the future potential of spectral analysis of spatially dense (low frequency) ground motion observations for inferring earthquake kinematics and understanding earthquake physics directly from near-field data; while synthetic studies are crucial to identify "what to look for" in the vast amount of data generated.References:Hanks, T.C., 1980. The corner frequency shift, earthquake source models and Q.Madariaga, R., Ruiz, S., Rivera, E., Leyton, F. and Baez, J.C., 2019. Near-field spectra of large earthquakes. Pure and Applied Geophysics, 176(3), pp.983-1001.Wollherr, S., Gabriel, A.-A. and Mai, P.M., 2019.&#160; Landers 1992 &#8220;reloaded&#8221;: Integrative dynamic earthquake rupture modeling. Journal of Geophysical Research: Solid Earth, 124(7), pp.6666-6702.Zhan, Z., 2020. Distributed Acoustic Sensing Turns Fiber&#8208;Optic Cables into Sensitive Seismic Antennas. Seismological Research Letters, 91(1), pp.1-15.

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