Geophone Array Research Articles

Theoretical Issues with Rayleigh Surface Waves and Geoelectrical Method Used for the Inversion of Near Surface Geophysical Structure

We numerically simulate the field measurements of Rayleigh surface waves and electrical resistivity in which the target depth is set to be less than 50-m. The Rayleigh surface waves are simulated in terms of fundamental mode group and phase velocities. The seismic field data is assumed to be collected through a conventional shot-gather. The group velocities are found from the application of the multiple filter technique in a single-station fashion while for the phase velocities the slant stacking, or linear radon transform are applied in fashion of multichannel analysis of surface waves (MASW). The average seismic structure from the source to the receiver (or geophone) is represented by the group velocity curve while the average seismic structure underneath the geophone array is represented by the phase velocity curve. The single-station group velocity curves are transformed into local group velocity curves by setting a linear system through grid points. The shear-wave velocity cross section underneath the examined area is constructed by inverting these local group velocity curves. The electrical resistivity structure of the underground is similarly studied. The field compilation of the resistivity data is assumed to be completed by the application of the multiple electrode Pole-Pole array. The actual resistivity assemble underneath the analyzed area is inverted by considering the apparent (measured) resistivity values. Unique forms such as ore body, cavity, sinkhole, melt, salt, and fluid within the Earth may be examined by joint interpretation of electrical resistivities and seismic velocities. These formations may be better outlined by following their distinct signs such as high/low resistivities and high/low seismic velocities. Doi: 10.28991/HEF-2021-02-03-01 Full Text: PDF

Near-field strain in distributed acoustic sensing-based microseismic observation

Microseismic monitoring with surface or downhole geophone arrays has commonly been used in tracking subsurface deformation and fracture networks during hydraulic fracturing operations. Recently, the use of fiber-optic distributed acoustic sensing (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 for very close observation of these microsize earthquakes and captures their full wavefield details. We have found that DAS-based microseismic profiles present a seldomly reported near-field strain signal between the P- and S-wave arrivals. This near-field signal indicates a 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 use 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.

Introduction to this special section: Surface-wave applications

The use of surface waves can be found at all levels of seismology, from the near surface to exploration to earthquakes. Once the scourge of reflection processing, the need to remove this coherent noise source led to a variety of in-field techniques to minimize its impact, including the application of geophone arrays, low-cut analog filters, and high-frequency geophones, not to mention the multitude of processing schemes devised to reduce surface-wave amplitudes after data collection. Although commonly labeled as noise, surface waves have proven to be versatile and resilient over the years, lending themselves to engineering applications in the shallow subsurface and developing velocity models to improve complex imaging in the oil and gas sector. This special section includes six papers that are equally representative, running the gamut from the first few meters of the surface to hundreds of meters deep. It includes examples of active and passive acquisition, modeling and field data, as well as geophones and fiber-optic sensors.

Ambient noise and ERT data provide insights into the structure of co-seismic rock avalanche deposits in Sichuan (China)

The post-seismic history of the 2008 Mw7.9 Wenchuan earthquake shows that marginally stable deposits of large co-seismic landslide dams can pose persistent debris flow hazards for the downstream areas. Here, we combine analyses of single-station recordings of ambient noise with electrical resistivity tomography (ERT) surveys to explore the potential of drawing information on structure and geometry of the deposit of a large rock avalanche triggered by the Mw 7.9 2008 Wenchuan earthquake, which dammed the Yangjia stream in the Sichuan Province (China). The substantial thickness and heterogeneity of this kind of deposits limit the application of standard geophysical techniques, like active seismic surveys, which require highly energetic sources and long linear geophone arrays to reach adequate investigation depths. Passive single-station methods, relying on ambient noise recordings to determine site resonance properties, controlled by the contrast between soft surface layers and a stiffer substratum, offer the opportunity of investigating subsoil properties down to larger depths. In particular, we use a recently developed technique, which isolates the contribution of Rayleigh waves to ambient noise and draws information on sub-soil properties from the inversion of Rayleigh wave ellipticity curves plotted as function of frequency. In this framework, the ERT data can support the ellipticity curve inversion, typically affected by highly non-univocal solutions, by providing constraints for defining of the thickness of the uppermost surficial layers. The results allowed inferring the overlap of different layers within the 2008 rock avalanche deposit, as well as estimating lateral variations in their thickness and S-wave (Vs) velocities.

Distributed Acoustic Sensing (DAS) for Natural Microseismicity Studies: A Case Study From Antarctica

AbstractIcequakes, microseismic earthquakes at glaciers, offer insights into the dynamics of ice sheets. For the first time in the Antarctic, we explore the use of fiber optic cables as Distributed Acoustic Sensors (DAS) as a new approach for monitoring basal icequakes. We present the use of DAS for studying icequakes as a case study for the application of DAS to microseismic datasets in other geological settings. Fiber was deployed on the ice surface at Rutford Ice Stream in two different configurations. We compare the performance of DAS with a conventional geophone network for: microseismic detection and location; resolving source and noise spectra; source mechanism inversion; and measuring anisotropic shear‐wave splitting parameters. Both DAS array geometries detect fewer events than the geophone array. However, DAS is superior to geophones for recording the microseism signal, suggesting the applicability of DAS for ambient noise interferometry. We also present the first full‐waveform source mechanism inversions using DAS anywhere, successfully showing the horizontal stick‐slip nature of the icequakes. In addition, we develop an approach to use a 2D DAS array geometry as an effective multi‐component sensor capable of accurately characterizing shear‐wave splitting due to the anisotropic ice fabric. Although our observations originate from a glacial environment, the methodology and implications of this work are relevant for employing DAS in other microseismic environments.

A Frequency-Domain-Based Algorithm for Detecting Microseismicity Using Dense Surface Seismic Arrays

ABSTRACT We propose a new frequency-domain-based algorithm for detecting small-magnitude seismic events using dense surface seismic arrays. Our proposed method takes advantage of the high energy carried by S waves, and approximate known source locations, which are used to rotate the horizontal components to obtain the maximum amplitude. By surrounding the known source area with surface geophones, we achieve a favorable geometry for locating the detected seismic events with the backprojection method. To test our new detection method, we used a dense circular array, consisting of 151 5 Hz three-component geophones, over a 5 km aperture that was in operation at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) in southcentral Utah. We apply the new detection method during a small-scale test injection phase at FORGE, and during an aftershock sequence of an Mw 4.1 earthquake located ∼30 km north of the geophone array, within the Black Rock volcanic field. We are able to detect and locate microseismic events (Mw<0) during injections, despite the high level of anthropogenic activity, and several aftershocks that are missing from the regional catalog. By comparing our method with known algorithms that operate both in the time and frequency domain, we show that our proposed method performs better in the case of the FORGE injection monitoring, and equally well for the off-array aftershock sequence. Our new method has the potential to improve microseismic event detections even in extremely noisy environments, and the proposed location scheme serves as a direct discriminant between true and false detections.

Estimation of bottom parameters in the New England Mud Patch using geophone and hydrophone measurements

Measurements of acoustic pressure and particle velocity were made during the Seabed Characterization Experiment (SCEx) in the New England Mud Patch south of Cape Cod in about 70 m of water. The University of Rhode Island and Woods Hole Oceanographic Institution deployed the “geosled” with a four-element geophone array, a tetrahedral array of four hydrophones, and Several Hydrophone Receive Units (SHRUs) as data acquisition packages. In addition, a recently developed low frequency source, the interface Wave Sediment Profiler (iWaSP), was deployed to excite interface waves (Scholte waves) on the seabed. The geophone array was localized using the known locations of the small explosive sources and noise from the research vessel. Modal arrivals from broadband sources on geophones and hydrophones were used to invert for compressional and shear wave speeds and attenuation in the mud layer and the underlying sand layer. The Scholte wave arrivals from the iWaSP source measured by the geophone array also provided estimates for these parameters. The inversion results are compared to modeling using the viscous grain shearing (VGS) theory. [Work supported by the Office of Naval Research, Code 322 OA.]

Dispersion curves for media with lateral-variation at different angles

In recent years, the studies have been focused on using the surface wave methods in determining the soil specifications. The surface layer of the soil with lateral variation was modeled at different angles in finite element software. The shot was applied on two sides of the geophone array and the seismic wave data were recorded by geophones. Using the windowing methods of different lengths and moving along the array, the various geophone data were placed in different windows. Next, for each windowing, the frequency-wavenumber spectrum was obtained using the double Fourier transform and then, the dispersion curve was plotted. In this regard, the variations in the resolution of frequency-wavenumber spectrum and dispersion curve were investigated for different window lengths. The results show that for the media with lateral variation at different angles, the dispersion curves could be obtained along the array and the location of the start and end of lateral variation along with the corresponding phase velocity range could be achieved with an acceptable accuracy, and the estimated phase velocity could be used as initial velocities in the inversion and specification of soil surface layers.

A distortion matrix framework for high-resolution passive seismic 3-D imaging: application to the San Jacinto fault zone, California

SUMMARYReflection seismic imaging usually suffers from a loss of resolution and contrast because of the fluctuations of the wave velocities in the Earth’s crust. In the literature, phase distortion issues are generally circumvented by means of a background wave velocity model. However, it requires a prior tomography of the wave velocity distribution in the medium, which is often not possible, especially in depth. In this paper, a matrix approach of seismic imaging is developed to retrieve a 3-D image of the subsoil, despite a rough knowledge of the background wave velocity. To do so, passive noise cross-correlations between geophones of a seismic array are investigated under a matrix formalism. They form a reflection matrix that contains all the information available on the medium. A set of matrix operations can then be applied in order to extract the relevant information as a function of the problem considered. On the one hand, the background seismic wave velocity can be estimated and its fluctuations quantified by projecting the reflection matrix in a focused basis. It consists in investigating the response between virtual sources and detectors synthesized at any point in the medium. The minimization of their cross-talk can then be used as a guide star for approaching the actual wave velocity distribution. On the other hand, the detrimental effect of wave velocity fluctuations on imaging is overcome by introducing a novel mathematical object: The distortion matrix. This operator essentially connects any virtual source inside the medium with the distortion that a wavefront, emitted from that point, experiences due to heterogeneities. A time reversal analysis of the distortion matrix enables the estimation of the transmission matrix that links each real geophone at the surface and each virtual geophone in depth. Phase distortions can then be compensated for any point of the underground. Applied to passive seismic data recorded along the Clark branch of the San Jacinto fault zone (SJFZ), the present method is shown to provide an image of the fault until a depth of 4 km over the frequency range 10–20Hz with an horizontal resolution of 80 m. Strikingly, this resolution is almost one eighth below the diffraction limit imposed by the geophone array aperture. The heterogeneities of the subsoil play the role of a scattering lens and of a transverse waveguide which increase drastically the array aperture. The contrast is also optimized since most of the incoherent noise is eliminated by the iterative time reversal process. Beyond the specific case of the SJFZ, the reported approach can be applied to any scales and areas for which a reflection matrix is available at a spatial sampling satisfying the Nyquist criterion.

Using Array‐Derived Rotational Motion to Obtain Local Wave Propagation Properties From Earthquakes Induced by the 2018 Geothermal Stimulation in Finland

AbstractWe estimate vertical rotation rates for 204 earthquakes that were induced by the 2018 stimulation of the Espoo/Helsinki geothermal reservoir from wavefield gradients across geophone arrays. The array‐derived rotation rates from seismograms recorded at 6–9 km hypocentral distances vary between 10−9 and 10−7 rad s−1, indicating a comparable sensitivity to portable rotational instruments. Using co‐located observations of translational and rotational motion, we estimate the local propagation direction and the apparent phase speed of SH waves, and compare these estimates with those obtained by S wave beamforming. Propagation directions generally align with the earthquake back azimuths, but both techniques show deviations indicative of heterogeneous seismic structure. The rotational method facilitates a station‐by‐station approach that resolves site specific variations that are controlled by the local geology. We measure apparent S wave speeds larger than 5 km s−1, consistent with steep incidence angles and high propagation velocities in the Fennoscandian Shield.

Active and passive seismic methods to explore areas of active faulting. The case of Lamezia Terme (Calabria, southern Italy)

This paper presents the results of a geophysical investigation aimed to illustrate the architecture of the shallow stratigraphic horizons across an area believed to be crossed by a NW-SE trending trascurrent fault called the Lamezia Terme-Catanzaro Fault. In spite of the available information on fault segments and tectonic activity associated with faulting, the local scale fault characteristics are currently poorly known and the surface displacements are difficult to detect in a such a densely populated area. A traditional integrated geophysical exploration method was proposed as an effective solution to assist in the location of structures in the subsurface where surface geological and morphological evidence is inconclusive in terms of defining the location and the potential of the local seismogenic sources, responsible for recurrent catastrophic earthquakes. Two active High Resolution (HR) seismic reflection/refraction profiles were acquired using an array of vertical geophones with a Swept Impact Seismic Source (VibSist-500) in order to obtain seismic imaging of the subsurface of the study area. Additionally, passive Microtremor Array Method (MAM), deep wells and commercial seismic profiles were used to better constrain the geological trend of deeper seismostrata. The integrated interpretation of both geophysical and geological data allowed to infer subsurface pictures of the investigated areas identifying the depth of the seismic bedrock and characterizing the buried morphology, enhancing lineaments with possible relevance to faults. Discontinuities in the main horizons in seismic sections indicate that faulting is present in a Pliocene unit and may extend to the shallow subsurface in Quaternary sediments. The results of this study can provide a basis for earthquake prevention and construction work and demonstrate the applicability of the seismic methods to explore buried hidden faults in the perspective of applications on studies of physical stratigraphy of syn-tectonic deposits.

Research and Application of Rayleigh Wave 3-D Phase Velocity Imaging Based on Wavelet Transform

Rayleigh wave detection is a recently developed method for shallow seismic exploration. Current Rayleigh wave data processing and interpretation methods can only provide the transverse average wave velocity of rock-soil bodies under the geophone array range, resulting in a low lateral resolution of wave velocity. To solve this problem, this paper presents a Rayleigh wave data processing method based on wavelet transform. First, the Hankel matrix is constructed from the intercepted Rayleigh wave, and the effective singular value is preserved by singular value decomposition to filter the Rayleigh wave. Then, the appropriate center frequency is selected and the corresponding relationship between the time and frequency of the Rayleigh wave is obtained via wavelet transform. The waveform of each frequency component can be extracted and the complete time difference of each frequency component between two geophones will be obtained and used to calculate the phase velocity-depth profile of the Rayleigh wave in a rock-soil body. This method is applied to examine unfavorable geological bodies that are underground in a yard. By combining the phase velocity-depth profiles of several survey lines, the 3-D image of phase velocity of Rayleigh wave underground can be obtained. This method can provide the phase velocity distribution of the formation below the survey line by only one measurement, which greatly improves upon the work efficiency and lateral resolution of the traditional Rayleigh wave data processing method.

Seismicity during and after stimulation of a 6.1 km deep enhanced geothermal system in Helsinki, Finland

Abstract. In this study, we present a high-resolution dataset of seismicity framing the stimulation campaign of a 6.1 km deep enhanced geothermal system (EGS) in the Helsinki suburban area and discuss the complexity of fracture network development. Within the St1 Deep Heat project, 18 160 m3 of water was injected over 49 d in summer 2018. The seismicity was monitored by a seismic network of near-surface borehole sensors framing the EGS site in combination with a multi-level geophone array located at ≥ 2 km of depth. We expand the original catalog of Kwiatek et al. (2019), including detected seismic events and earthquakes that occurred 2 months after the end of injection, totaling 61 163 events. We relocated events of the catalog with moment magnitudes between Mw −0.5 and Mw 1.9 using the double-difference technique and a new velocity model derived from a post-stimulation vertical seismic profiling (VSP) campaign. The analysis of the fault network development at a reservoir depth of 4.5–7 km is one primary focus of this study. To achieve this, we investigate 191 focal mechanisms of the induced seismicity using a cross-correlation-based technique. Our results indicate that seismicity occurred in three spatially separated clusters centered around the injection well. We observe a spatiotemporal migration of the seismicity during the stimulation starting from the injection well in the northwest–southeast (NW–SE) direction and in the northeast (NE) direction towards greater depth. The spatial evolution of the cumulative seismic moment, the distribution of events with Mw≥1, and the fault plane orientations of focal mechanisms indicate an active network of at least three NW–SE- to NNW–SSE-oriented permeable zones, which is interpreted to be responsible for the migration of seismic activity away from the injection well. Fault plane solutions of the best-constrained focal mechanisms and results for the local stress field orientation indicate a reverse faulting regime and suggest that seismic slip occurred on a sub-parallel network of pre-existing weak fractures favorably oriented with the stress field, striking NNW–SEE with a dip of 45∘ ENE parallel to the injection well.

Unusual Seismic Signals in the Sevier Desert, Utah Possibly Related to the Black Rock Volcanic Field

AbstractOn September 12, 2018 and April 14, 2019 shallow (<2.5 km from the surface, <1 km relative to sea level) earthquakes (Mw 4.0 and 4.1, respectively) struck a relatively aseismic area in south‐central Utah, within the Black Rock Desert volcanic field. Seismic waveforms are dominated by long‐period energy, no clear direct S‐wave arrivals, and signal durations of ∼100 s. Using co‐seismic interferograms, we detect clear and distinguishable surface deformation confirming the shallowness of the two mainshocks, while full moment tensor solutions rule out an isotropic source, such as an explosion or collapse. For the April 2019 sequence, we build an earthquake catalog using a dense temporary geophone array. The enhanced earthquake catalog shows a rapidly decaying aftershock sequence, with 85% of aftershocks occurring during the first three hours. Combining these observations, we argue that these earthquake sequences are related to the Black Rock Desert volcanic field.

Passive seismic recording of cryoseisms in Adventdalen, Svalbard

Abstract. A series of transient seismic events were discovered in passive seismic recordings from 2-D geophone arrays deployed at a frost polygon site in Adventdalen, Svalbard. These events contain a high proportion of surface wave energy and produce high-quality dispersion images using an apparent offset re-sorting and inter-trace delay minimisation technique to locate the seismic source, followed by cross-correlation beamforming dispersion imaging. The dispersion images are highly analogous to surface wave studies of pavements and display a complex multimodal dispersion pattern. Supported by theoretical modelling based on a highly simplified arrangement of horizontal layers, we infer that a ∼3.5–4.5 m thick, stiff, high-velocity layer overlies a ∼30 m thick layer that is significantly softer and slower at our study site. Based on previous studies we link the upper layer with syngenetic ground ice formed in aeolian sediments, while the underlying layer is linked to epigenetic permafrost in marine-deltaic sediments containing unfrozen saline pore water. Comparing events from spring and autumn indicates that temporal variation can be resolved via passive seismic monitoring. The transient seismic events that we record occur during periods of rapidly changing air temperature. This correlation, along with the spatial clustering along the elevated river terrace in a known frost polygon, ice-wedge area and the high proportion of surface wave energy, constitutes the primary evidence for us to interpret these events as frost quakes, a class of cryoseism. In this study we have proved the concept of passive seismic monitoring of permafrost in Adventdalen, Svalbard.

Novel Geophone Array Measures Exact Location of Fluid Movement Behind Casing

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 197560, “Enhanced Well Remedial Decisions From Exact Location of Fluid Movement Behind Casing Identification,” by Marcel Croon, SPE, Perry Huber, SPE, and Jacob Wright, SPE, Weatherford, SPE, prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed. Historically, acoustic wellbore monitoring is among the primary methods of detecting fluid movement behind the casing. However, analysis of the complex acoustic environment in the wellbore can be challenging. A standard hydrophone noise tool is unable to measure flow directions (vertical and horizontal) and cannot detect low flow, low-pressure sporadic events, or multiple sources. The complete paper discusses a geophone array, including four, three-component geophones deployed by wireline, that provides a solution for this problem by creating a 3D map of the acoustic environment. Introduction Well-integrity issues such as a poor cement bond can lead to undesirable fluid movements behind the casing, which could have negative effects with regard to health, safety, and the environment. A detailed understanding of annular fluid movement is critical in identifying potential source locations for wells with surface-casing vent-flow (SCVF) issues. Applicable scenarios include the following: Planned well-intervention activities for wells with multiple-source vent/flow situations Confirming the presence of flow (gas/oil/water) behind the casing Identifying areas with suspected crossflow between zones Integrity confirmation for gas storage caverns or zones Identifying unwanted flow for well-abandonment activities Well abandonment is bound by stringent regulations in some countries where the testing and repair of SCVF and gas-migration (GM) issues before final abandonment are required. An ex-ample is western Canada, where surface casing vents must be tested through a bubble test at minimum. This involves connecting a small container with 2.54 cm of water using fittings to the surface casing vent and monitoring for gas bubbles for 10 minutes. Often the flow rates are very low (less than 35 cu ft/d) with minimal pressure buildup. If bubbles are detected during the monitoring period, the well is deemed to have a positive vent flow. Consequently, the source of the vent flow must be identified by a survey as approved by the energy regulator, which could be conducted by means of acoustic logs, temperature logs, or gas- isotope analysis. Additionally, GM measurements must be conducted before final abandonment in specific areas within western Canada but are recommended for all wells. The complete paper details this process.

Quest CCS Facility: Microseismic System Monitoring and Observations

In August 2015, CO2 injection commenced at the Quest Carbon Capture and Storage (CCS) Facility located near Fort Saskatchewan, Alberta, Canada. Quest is a fully integrated CCS facility with a capture target of just over one million metric tonnes of CO2 per year. CO2 is injected into a deep saline aquifer, the Basal Cambrian Sandstone (BCS), at a depth of about two kilometers below ground. In order to demonstrate containment and conformance of the injected CO2, a Measurement, Monitoring and Verification (MMV) plan has been implemented. Although Quest is in an extremely quiet tectonic location, induced seismicity has been recognized as a potential risk for all large-scale injection sites. As a result, microseismic monitoring is a key component of the Quest MMV Plan to ensure the continued assessment of that risk and to provide early notice of any changes. The aim of this presentation is to detail why we monitor for microseismic activity and how we do this efficiently using proactive technical monitoring (PTM) and exception based surveillance (EBS), to report on six years of microseismic monitoring observations through the pre- and post-start of injection periods and to describe the empirical analysis that has allowed the Quest CCS Facility to de-risk microseismicity as an active threat to containment. Microseismic data is continuously recorded at the Quest CCS storage site using a commercial downhole geophone array installed in a deep monitoring well (DMW). The data are processed to produce trigger files using a common algorithm for event detection and triggering, based on the ratio of short-time-average to long-time-average (STA/LTA) amplitudes. The triggered events are analyzed, classified and reported daily. Categories for various trigger types are defined based on the characteristics of each event. Operationally, the status of the microseismic array is continually assessed by the presence of trigger events. A trigger event is defined to be locatable when it shows clear primary (P) and secondary (S) wave field arrivals. The locatable events are processed and located using an anisotropic velocity model. The p-wave velocities were derived from a vertical seismic profile survey, and the s-wave velocities were obtained from sonic well logs. Pre-CO2 injection recording of microseismic data began in November 2014. Most of the trigger files generated were related to surface, automatic and noise triggers. These events were used to assess the health of the system. No locatable events were detected during the pre-injection monitoring phase. The first locatable event was recorded 10 months after start of CO2 injection in July 2016 with a magnitude of -1.3. Since January 2017, sustained low level, small magnitude microseismic activity have been located within the Quest microseismic area of review (AOR) within the Precambrian basement. The AOR as defined in the 2017 Quest MMV plan, extends 10 km radially outwards from each active injection well (IW). As of December 31st, 2020, 486 locatable events have been detected, with an average magnitude of -0.7, a maximum magnitude of 0.8 and a typical occurrence rate of 1-2 events per week. Using the current methodology, all locatable events have been located within the Precambrian basement, below the injection zone. None of this microseismic activity has represented a risk to containment.

Near-surface evaluation of Bora office area using MASW methods on land-streamer

A limited seismic investigation of the Bora Office Area, was conducted using multiple seismic methods including multi-channel analysis of surface waves (MASW), using land streamer originally designed by the author. The main feature of the land streamer tool is the non-stretch woven belt on which geophone units are mounted to form a multichannel geophone array similar. As a result, it was concluded that it might be possible to avoid higher-mode contaminantion at low frequencies at sites with hight (Vs), it can be seen that the distribution of Vs values varies over the range of 240 m/s to 420 m/s. In general, can be divided into 2 layers based on the value of Vs 100 - 250 m/s as soft rock and > 250 m/s as hard rock. There is a soft rock at a depth range of 0 to 5 m with a velocity value of 140-260 m/s, and hard rock layers at a depth range of below 5m with a velocity value of 280 - 400 m s.

Underwater records of submarine volcanic activity: El Hierro (Canary Islands 2011–2012) eruption

We present the monitoring of a submarine volcanic eruption that took place near the southernmost emerged land of El Hierro Island (Canary Islands, Spain), from October 2011 to February 2012. Right after the onset of the eruption, in mid-October 2011, we deployed an offshore geophone array for the purpose of monitoring the submarine eruptive activity signals. It acquired continuous data from October 2011 to May 2012, sometime after the end of the eruption. The array consisted of 8 high-frequency, 3-component geophones assembled into a cable string, with 6 m of separation. The geophone string was installed in La Restinga Harbor at a distance of less than 2 km from the volcanic edifice. The dataset acquired with the array is a unique continuous acoustic record of the activity associated with this eruption. We analyzed the continuous signal of the eruptive activity, with special interest in those events reflecting the eruption dynamics. Our results show that the geophone array was recording acoustic waves from a back-azimuth source corresponding to the position of the submarine vent, traveling at a speed of 1510 m/s, compatible with the speed of sound in water. Acoustic data shows a good correlation with the seismic data recorded on land for the case of earthquake occurrence. In addition, it provides relevant information towards the understanding of the eruptive surface activity. Therefore, this methodology can be successfully used in cases of submarine eruptions.

Convolutional neural networks for automated microseismic detection in downhole distributed acoustic sensing data and comparison to a surface geophone array

ABSTRACTDistributed acoustic sensing is a growing technology that enables affordable downhole recording of strain wavefields from microseismic events with spatial sampling down to ∼1 m. Exploiting this high spatial information density motivates different detection approaches than typically used for downhole geophones. A new machine learning method using convolutional neural networks is described that operates on the full strain wavefield. The method is tested using data recorded in a horizontal observation well during hydraulic fracturing in the Eagle Ford Shale, Texas, and the results are compared to a surface geophone array that simultaneously recorded microseismic activity. The neural network was trained using synthetic microseismic events injected into real ambient noise, and it was applied to detect events in the remaining data. There were 535 detections found and no false positives. In general, the signal‐to‐noise ratio of events recorded by distributed acoustic sensing was lower than the surface array and 368 of 933 surface array events were found. Despite this, 167 new events were found in distributed acoustic sensing data that had no detected counterpart in the surface array. These differences can be attributed to the different detection threshold that depends on both magnitude and distance to the optical fibre. As distributed acoustic sensing data quality continues to improve, neural networks offer many advantages for automated, real‐time microseismic event detection, including low computational cost, minimal data pre‐processing, low false trigger rates and continuous performance improvement as more training data are acquired.