Microseismic monitoring using a fibre-optic Distributed Acoustic Sensor (DAS) array

We present a case study demonstrating the use of an “L”-shaped downhole fibre-optic array to monitor microseismicity. We use a relatively simple method to detect events from continuous waveform data, and develop a workflow for manual event location. Locations are defined with a cylindrical coordinate system, with the horizontal axis of the DAS cable being the axis of symmetry. Events are located using three manual “picks”, constraining (1) the zero-offset “broadside” channel to the event (2) the P-S wave arrival time difference at the broadside channel, and (3) the angle, ? of the event from the array. Because the one-component DAS array is unable to record P-wave energy on the broadside channel, the P-wave pick is made indirectly by ensuring that the modeled P- and S-wave moveout curves match the observed data. The ? angle requires that signal is observed on the vertical part of the array, in our case this is possible because an engineered fiber, rather than standard telecommunications fiber, provided a significant reduction in the noise level. Because only three picks need to be made, our manual approach is significantly more efficient than equivalent manual processing of downhole geophone data, where picks for P- and S-waves must be made for each receiver. We find that the located events define a tight cluster around the injection interval, indicating that this approach provides relatively precise and accurate event locations. A surface microseismic array was also used at this site, which detected significantly fewer events, the locations of which had significantly greater scatter than the DAS array locations. We conclude by examining some other aspects of the DAS microseismic data, including the presence of multiple events within very short time windows, and the presence of converted phases that appear to represent scattering of energy from the hydraulic fractures themselves.

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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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Seismic reflectivity of hydraulic fractures approximated by thin fluid layers

Geophysics ◽

10.1190/geo2012-0269.1 ◽

2013 ◽

Vol 78 (4) ◽

pp. T79-T87 ◽

Cited By ~ 7

Author(s):

A. Oelke ◽

D. Alexandrov ◽

I. Abakumov ◽

S. Glubokovskikh ◽

R. Shigapov ◽

...

Keyword(s):

Analytical Solutions ◽

Fluid Layer ◽

Synthetic Data ◽

P Wave ◽

Hydraulic Fractures ◽

Reflection Coefficients ◽

S Wave ◽

Reservoir Properties ◽

Thin Fluid ◽

Theoretical Results

We have analyzed the angle-dependent reflectivity of microseismic wavefields at a hydraulic fracture, which we modeled as an ideal thin fluid layer embedded in an elastic, isotropic solid rock. We derived full analytical solutions for the reflections of an incident P-wave, the P-P and P-S reflection coefficients, as well as for an incident S-wave, and the S-S and S-P reflection coefficients. The rather complex analytical solutions were then approximated and we found that these zero-thickness limit approximations are in good agreement with the linear slip model, representing a fracture at slip contact. We compared the analytical solutions for the P-P reflections with synthetic data that were derived using finite-difference modeling and found that the modeling confirmed our theoretical results. For typical parameters of microseismic monitoring by hydraulic fracturing, e.g., a layer thickness of [Formula: see text] and frequencies of [Formula: see text], the reflection coefficients depend on the Poisson’s ratio. Furthermore, the reflection coefficients of an incident S-wave are remarkably high. Theoretical results suggested that it is feasible to image hydraulic fractures using microseismic events as a source and to solve the inverse problem, that is, to interpret reflection coefficients extracted from microseismic data in terms of reservoir properties.

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Elastic-wavefield interferometry using P-wave source VSPs at the Wamsutter field, Wyoming

Geophysics ◽

10.1190/geo2011-0162.1 ◽

2012 ◽

Vol 77 (2) ◽

pp. Q27-Q36 ◽

Cited By ~ 1

Author(s):

James Gaiser ◽

Ivan Vasconcelos ◽

Rosemarie Geetan ◽

John Faragher

Keyword(s):

Stationary Phase ◽

Wave Velocity ◽

P Wave ◽

Phase Point ◽

S Wave ◽

Wave Source ◽

S Waves ◽

Wave Splitting ◽

Zero Offset ◽

Tube Wave

In this study, elastic-wavefield interferometry was used to recover P- and S-waves from the 3D P-wave vibrator VSP data at Wamsutter field in Wyoming. S-wave velocity and birefringence is of particular interest for the geophysical objectives of lithology discrimination and fracture characterization in naturally fractured tight gas sand reservoirs. Because we rely on deconvolution interferometry for retrieving interreceiver P- and S-waves in the subsurface, the output fields are suitable for high-resolution, local reservoir characterization. In 1D media where the borehole is nearly vertical, data at the stationary-phase point is not conducive to conventional interferometry. Strong tube-wave noise generated by physical sources near the borehole interfere with S-wave splitting analyses. Also, converted P- to S-wave (PS-wave) polarity reversals occur at zero offset and cancel their recovery. We developed methods to eliminate tube-wave noise by removing physical sources at the stationary-phase point and perturbing the integration path in the integrand based on P-wave NMO velocity of the direct-arrival. This results in using nonphysical energy outside a Fresnel radius that could not have propagated between receivers. To limit the response near the stationary-phase point, we also applied a weighting condition to suppress energy from large offsets. For PS-waves, a derivative-like operator was applied to the physical sources at zero offset in the form of a polarity reversal. These methods resulted in effectively recovering P-wave dipole and PS-wave quadrupole pseudosource VSPs. The retrieved wavefields kinematically correspond to a vertical incidence representation of reflectivity/transmissivity and can be used for conventional P- and S-wave velocity analyses. Four-component PS-wave VSPs retrieve S-wave splitting in transmitted converted waves that provide calibration for PS-wave and P-wave azimuthal anisotropy measurements from surface-seismic data.

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Some comments on common-asymptotic-conversion-point (CACP) sorting of converted-wave data in isotropic, laterally inhomogeneous media

Geophysics ◽

10.1190/1.1925750 ◽

2005 ◽

Vol 70 (3) ◽

pp. U29-U36 ◽

Cited By ~ 2

Author(s):

Mirko van der Baan

Keyword(s):

Wave Velocity ◽

Velocity Ratio ◽

P Wave ◽

Pure Mode ◽

S Wave ◽

Converted Wave ◽

Wave Data ◽

Conversion Point ◽

Isotropic Media ◽

Zero Offset

Common-midpoint (CMP) sorting of pure-mode data in arbitrarily complex isotropic or anisotropic media leads to moveout curves that are symmetric around zero offset. This greatly simplifies velocity determination of pure-mode data. Common-asymptotic-conversion-point (CACP) sorting of converted-wave data, on the other hand, only centers the apexes of all traveltimes around zero offset in arbitrarily complex but isotropic media with a constant P-wave/S-wave velocity ratio everywhere. A depth-varying CACP sorting may therefore be required to position all traveltimes properly around zero offset in structurally complex areas. Moreover, converted-wave moveout is nearly always asymmetric and nonhyperbolic. Thus, positive and negative offsets need to be processed independently in a 2D line, and 3D data volumes are to be divided in common azimuth gathers. All of these factors tend to complicate converted-wave velocity analysis significantly.

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A simple method of predicting S-wave velocity

Geophysics ◽

10.1190/1.2357833 ◽

2006 ◽

Vol 71 (6) ◽

pp. F161-F164 ◽

Cited By ~ 37

Author(s):

Myung W. Lee

Keyword(s):

Wave Velocity ◽

P Wave ◽

Unconsolidated Sediments ◽

S Wave ◽

Amplitude Analysis ◽

Simple Method ◽

Low Frequencies ◽

Wave Velocities ◽

Aspect Ratios ◽

P Wave Velocity

Prediction of shear-wave velocity plays an important role in seismic modeling, amplitude analysis with offset, and other exploration applications. This paper presents a method for predicting S-wave velocity from the P-wave velocity on the basis of the moduli of dry rock. Elastic velocities of water-saturated sediments at low frequencies can be predicted from the moduli of dry rock by using Gassmann’s equation; hence, if the moduli of dry rock can be estimated from P-wave velocities, then S-wave velocities easily can be predicted from the moduli. Dry rock bulk modulus can be related to the shear modulus through a compaction constant. The numerical results indicate that the predicted S-wave velocities for consolidated and unconsolidated sediments agree well with measured velocities if differential pressure is greater than approximately [Formula: see text]. An advantage of this method is that there are no adjustable parameters to be chosen, such as the pore-aspect ratios required in some other methods. The predicted S-wave velocity depends only on the measured P-wave velocity and porosity.

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A simple method for resolving large converted‐wave (P-SV) statics

Geophysics ◽

10.1190/1.1443426 ◽

1993 ◽

Vol 58 (3) ◽

pp. 429-433 ◽

Cited By ~ 27

Author(s):

Peter W. Cary ◽

David W. S. Eaton

Keyword(s):

P Wave ◽

S Wave ◽

Converted Wave ◽

P Waves ◽

Simple Method ◽

Near Surface ◽

S Waves ◽

Static Solutions ◽

Surface Fluctuations

The processing of converted‐wave (P-SV) seismic data requires certain special considerations, such as commonconversion‐point (CCP) binning techniques (Tessmer and Behle, 1988) and a modified normal moveout formula (Slotboom, 1990), that makes it different for processing conventional P-P data. However, from the processor’s perspective, the most problematic step is often the determination of residual S‐wave statics, which are commonly two to ten times greater than the P‐wave statics for the same location (Tatham and McCormack, 1991). Conventional residualstatics algorithms often produce numerous cycle skips when attempting to resolve very large statics. Unlike P‐waves, the velocity of S‐waves is virtually unaffected by near‐surface fluctuations in the water table (Figure 1). Hence, the P‐wave and S‐wave static solutions are largely unrelated to each other, so it is generally not feasible to approximate the S‐wave statics by simply scaling the known P‐wave static values (Anno, 1986).

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A CROSS-CORRELATION TECHNIQUE FOR RELOCATION OF SEISMICITY IN THE WESTERN CORINTH RIFT

Bulletin of the Geological Society of Greece ◽

10.12681/bgsg.11392 ◽

2017 ◽

Vol 43 (4) ◽

pp. 2015

Author(s):

V. Kapetanidis ◽

P. Papadimitriou ◽

K. Makropoulos

Keyword(s):

Cross Correlation ◽

Time Lag ◽

P Wave ◽

Double Difference ◽

Correlation Technique ◽

S Wave ◽

Waveform Data ◽

Arrival Times ◽

Linkage Algorithm ◽

Seismogenic Structures

Local seismological networks provide data that allow the location of microearthquakes which otherwise would be dismissed due to low magnitudes and low signal-to-noise ratios of their seismic signals. The Corinth Rift Laboratory (CRL) network, installed in the western Corinth rift, has been providing digital waveform data since 2000. In this work, a semi-automatic picking technique has been applied which exploits the similarity between waveforms of events that have occurred in approximately the same area of an active fault. Similarity is measured by the crosscorrelation maxi-mum of full signals. Events with similar waveforms are grouped in multiplet clusters using the nearest-neighbour linkage algorithm. Manually located events act as masters, while automatically located events of each multiplet cluster act as slaves. By cross-correlating the P-wave or S-wave segments of a master event with the corresponding segments of each of its slave events, after appropriately aligning their offsets, the measured time-lag at the cross-correlation maximum can be subtracted from the arrival-time of the slave event. After the correction of the arrival-times, a double-difference technique is applied to the modified catalogue to further improve the locations of clusters and distinguish the active seismogenic structures in the tectonically complex Western Corinth rift.

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SOURCE LOCATION MECHANISM OF MICROSEISMIC MONITORING USING P-S WAVES AND ITS EFFECT ANALYSIS

DYNA INGENIERIA E INDUSTRIA ◽

10.6036/10370 ◽

2022 ◽

Vol 97 (1) ◽

pp. 39-45

Author(s):

Zhigang Wang ◽

Ji Li ◽

Bo Li

Keyword(s):

Monitoring Network ◽

Seismic Source ◽

Microseismic Monitoring ◽

Source Location ◽

P Wave ◽

Monitoring Networks ◽

Distributed Monitoring ◽

S Wave ◽

Location Accuracy ◽

Seismic Source Location

Seismic source location is the most fundamental and most important problem in microseismic monitoring. However, only P wave has been mostly applied in the existing microseismic monitoring networks, with low location accuracy and poor stability of location result for the microseismic events occurring beyond monitoring networks. The seismic source location was implemented using P wave and S wave in this study to expand the effective monitoring area of a microseismic monitoring network and improve its location accuracy for microseismic events nearby the monitoring network. Then, the seismic source location mechanism using P-S wave was revealed through theoretical derivation and analysis. Subsequently, the program development and numerical simulation were combined to analyze and compare systematically the location effects of differently distributed monitoring networks, those consisting of different quantities of sensors, and those with S wave contained in some sensors under two circumstances: combination of P wave and S wave and single use of P wave. Results demonstrate that adding S wave in the plane enhances the accuracy control in the radius direction of the monitoring network. After S wave is included, the location accuracy within a certain area beyond the monitoring network is improved considerably, the effective monitoring area of the whole network is expanded, and the unstable location zones using only P wave are eliminated. The location results of differently distributed monitoring networks and the influence laws of the quantity of sensors constituting the networks on the location results are acquired. This study provides evidence for microseismic monitoring to realize accurate and stable location within a larger range. Keywords: seismic source location, P wave and S wave, mechanism, location effect

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Appication of ZO-CRS Stack on Residual PP Removal of PS Component in Converted-Wave Sesimic Reflection Processing

Scientific Contributions Oil and Gas ◽

10.29017/scog.43.2.520 ◽

2020 ◽

Vol 43 (2) ◽

pp. 53-58

Author(s):

Wahyu Triyoso ◽

Jefri B. Irawan ◽

Natasha C. Viony ◽

Fatkhan Fatkhan

Keyword(s):

Vertical Component ◽

Complex Model ◽

Hydrocarbon Exploration ◽

P Wave ◽

S Wave ◽

Converted Wave ◽

Waveform Modeling ◽

Imaging Quality ◽

Common Reflection Surface ◽

Zero Offset

A high-quality image of the PS component is needed since applying the converted-wave seismic method has increased significantly in hydrocarbon exploration, especially in interpreting the detail and complexity of structure or reservoir zones. The incident P-wave on a surface produces a reflected and converted P-S wave. Converted-wave seismic uses the multicomponent receiver that records both vertical and horizontal components. The vertical component is assumed to correspond to the compressional PP wave, and the horizontal corresponds to the PS converted-wave. To better understand how to image better the PS component, synthetic seismic data with the shallow gas and relatively complex model are constructed by the full-waveform modeling. This study aims to improve the imaging quality in the PS section to remove the residual PP events on the horizontal data refer to our previous study. In this study, to obtain the more reliable PS data, the residual PP reflections have been removed by applying the Zero Offset Common Reflection Surface (ZO CRS) Stack of the PS component. The results of this study, the imaging quality is better than that in the previous study.

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Post-collisional mantle delamination in the Dinarides implied from staircases of Oligo-Miocene uplifted marine terraces

Scientific Reports ◽

10.1038/s41598-021-81561-5 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Philipp Balling ◽

Christoph Grützner ◽

Bruno Tomljenović ◽

Wim Spakman ◽

Kamil Ustaszewski

Keyword(s):

Balkan Peninsula ◽

Receiver Functions ◽

P Wave ◽

S Wave ◽

Slab Geometry ◽

Surface Uplift ◽

River Incision ◽

Marine Terraces ◽

Convergence System ◽

Internal Dinarides

AbstractThe Dinarides fold-thrust belt on the Balkan Peninsula resulted from convergence between the Adriatic and Eurasian plates since Mid-Jurassic times. Under the Dinarides, S-wave receiver functions, P-wave tomographic models, and shear-wave splitting data show anomalously thin lithosphere overlying a short down-flexed slab geometry. This geometry suggests a delamination of Adriatic lithosphere. Here, we link the evolution of this continental convergence system to hitherto unreported sets of extensively uplifted Oligocene–Miocene (28–17 Ma) marine terraces preserved at elevations of up to 600 m along the Dinaric coastal range. River incision on either side of the Mediterranean-Black Sea drainage divide is comparable to the amounts of terrace uplift. The preservation of the uplifted terraces implies that the most External Dinarides did not experience substantial deformation other than surface uplift in the Neogene. These observations and the contemporaneous emplacement of igneous rocks (33–22 Ma) in the internal Dinarides suggest that the Oligo-Miocene orogen-wide uplift was driven by post-break-off delamination of the Adriatic lithospheric mantle, this was followed by isostatic readjustment of the remaining crust. Our study details how lithospheric delamination exerts an important control on crustal deformation and that its crustal signature and geomorphic imprint can be preserved for millions of years.

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