Geophone Array Research Articles

Detecting microseismicity using distributed vibration

Distributed acoustic sensing (DAS) and, more generally, distributed vibration sensing (DVS) offer new approaches to the detection of seismic waves, based on interrogation of a fibre-optic cable. We investigated the detection of microseismic events using a heterodyne distributed vibration sensor (hDVS), comparing to a pair of borehole geophone arrays co-located in the monitoring well. The aim was to establish the current performance level of hDVS for microseismic monitoring and to understand any differences between geophone and hDVS responses. We compared the microseismic events detected simultaneously on both systems. Under the assumption that hDVS measures strain in the fibre, the comparison to geophone particle velocity was made in the strain-rate domain. We supported these observations with modelling work. We found the hDVS measurements directly comparable to the geophones, indicating a potentially true amplitude response from hDVS. An anomaly in the hDVS frequency response was a function of apparent phase velocity. Modelling verified the observed effect as consistent with the filtering effect of the light pulse used in fibre interrogation. We find that hDVS detects microseismicity but is not yet a replacement for the borehole array. However, improved microseismic response might be achieved with shorter pulse-length systems in the next generation of fibre interrogators. The heterodyne distributed vibration sensor (hDVS), together with other forms of distributed acoustic sensing (DAS), have been reported as observation techniques for microseismic monitoring (Webster et al., 2013; Li et al., 2015; Molteni et al., 2016). We present further results from the study described by Molteni et al. (2016) and extend the discussion on the differences between observation with a borehole array of geophones and observation on a laser-interrogated fibre-optic cable that is cemented behind casing.

Surface Soil Assessment in the Ubhur Area, North of Jeddah, Western Saudi Arabia, Using a Multichannel Analysis of Surface Waves Method

ABSTRACT Ten boreholes drilled in Ubhur area up to the depth of bedrock indicted the shallow depth of bedrock where the average depth ranges between 10 and 15 m. The standard penetration test N-values of these boreholes were measured and averaged. Based on N-values to the depth of bedrock, Ubhur area can be classified as site class C and D. Multichannel analysis of surface waves technique has been applied along seventy six profiles using 24-channel geophone array and 4.5Hz vertical geophones with 1m geophone spacing and sledgehammer and/or weight drop as seismic energy sources. Values of shear wave velocity to 30 m are calculated and then averaged (Vs30) where it ranges between 310.08 m/s and 1139.8 m/s. Therefore, Ubhur area can be classified into site class B, C and D based on site classification of the national earthquake hazards reduction program (NEHRP) recommendations. Accordingly, the greatest part of the study area falls in site class C while class B and D covered limited areas in the western and the eastern parts respectively. Depending on the shallow depth of bedrock in the study area, the Vs30 parameter is not applicable in the study area so the average values of Vs for the soil thickness, excluding the bedrock, have been calculated and mapped for site class C and D only. So Vs30 approach is not applicable for areas with shallow depth of bedrock which gives higher classification.

Passive acoustic monitoring of bed load discharge in a large gravel bed river

AbstractSurrogate technologies to monitor bed load discharge have been developed to supplement and ultimately take over traditional direct methods. Our research deals with passive acoustic monitoring of bed load flux using a hydrophone continuously deployed near a river bed. This passive acoustic technology senses any acoustic waves propagated in the river environment and particularly the sound due to interparticle collisions emitted during bed load movement. A data set has been acquired in the large Alpine gravel‐bedded Drau River. Analysis of the short‐term frequency response of acoustic signals allows us to determine the origin of recorded noises and to consider their frequency variations. Results are compared with ancillary field data of water depth and bed load transport inferred from the signals of a geophone array. Hydrophone and geophone signals are well correlated. Thanks to the large network of deployed geophones, analysis of the spatial resolution of hydrophone measurements shows that the sensor is sensitive to bed load motion not only locally but over distances of 5–10 m (10–20% of river width). Our results are promising in terms of the potential use of hydrophones for monitoring bed load transport in large gravel bed rivers: acoustic signals represent a large river bed area, rather than being local; hydrophones can be installed in large floods; they can be deployed at a low cost and provide continuous monitoring at high temporal resolution.

Opportunities and pitfalls in surface-wave interpretation

Many explorationists think of surface waves as the most damaging noise in land seismic data. Thus, much effort is spent in designing geophone arrays and filtering methods that attenuate these noisy events. It is now becoming apparent that surface waves can be a valuable ally in characterizing the near-surface geology. This review aims to find out how the interpreter can exploit some of the many opportunities available in surface waves recorded in land seismic data. For example, the dispersion curves associated with surface waves can be inverted to give the S-wave velocity tomogram, the common-offset gathers can reveal the presence of near-surface faults or velocity anomalies, and back-scattered surface waves can be migrated to detect the location of near-surface faults. However, the main limitation of surface waves is that they are typically sensitive to S-wave velocity variations no deeper than approximately half to one-third the dominant wavelength. For many exploration surveys, this limits the depth of investigation to be no deeper than approximately 0.5–1.0 km.

Understanding MEMS-based digital seismic sensors

Over the last few decades almost all the electronic devices we use in our daily lives have switched from analog to digital owing to the numerous benefits they offer, such as miniaturization, enhanced functionalities or reduced power consumption, etc. This revolution also reached the seismic industry, primarily when fully digital recorders and telemetries became available in the late 1970s. However, measurement of the Earth’s displacement by sensors remained analog. The last step towards full digital recording was made in the early 2000s with the launch of digital seismic sensors based on MEMS (Micro Electro Mechanical Systems) accelerometers that had the potential to replace analog geophones that had been used since the early days of seismic dating back to the 1930s. However, although digital sensors have established a foothold and indeed even become a reference for a number of different applications, the anticipated revolution has however not lived up to the expectations of the seismic industry, the current situation being perhaps analogous to using audio cassette tapes alongside MP3 music files. More than a decade after the release of the first digital seismic sensors, several explanations for such a situation have become clear: • the operating principles of digital sensors are somehow perceived to be more complicated than those of geophones, and trickier to understand; • the quality of data acquired with digital sensors is sometimes judged to be no better than equivalent to data acquired with geophones (more often than not, this is owing to a comparison of single digital sensors with strings of geophones at the same trace interval); • the cost of digital sensors is often seen to be higher (at least for one-component – 1C – acquisition) than the cost of a conventional Field Digitizing Unit (FDU) connected to a string of geophones. This article aims firstly to review the main operating characteristics of MEMS-based Digital Sensor Units (DSU), especially when compared to geophones, and then discuss the quality and cost issues in the light of experience gained after more than a decade of field operations. We conclude that their use with an adapted high-density geometry makes it possible to achieve much better imaging than with geophone arrays, for an equivalent cost.

Application of the refraction microtremor (ReMi) process for predicting change in rock characterisation and possible correlations with the rock mass rating (RMR)

In an attempt to provide a more efficient means of determining changes in rock characterisation, a study was conducted using the refraction microtremor (ReMi) process in an underground mine. ReMi involves acquiring noise data along a linear array of geophones. The data for our study was analysed using the SeisOpt® ReMi™. Various geophone orientations were investigated in order to determine which produced the most discernible dispersion curve. In order to determine whether the changes seen actually exist, shear wave velocities were measured in location of known rock characterisation. These shear wave velocities were then compared with the rock mass rating (RMR) and previously published correlations between RMR and shear wave velocity. In addition, the use of ReMi as a tool to determine the extent of voids underground was investigated.

Application of the refraction microtremor (ReMi) process for predicting change in rock characterisation and possible correlations with the rock mass rating (RMR)

In an attempt to provide a more efficient means of determining changes in rock characterisation, a study was conducted using the refraction microtremor (ReMi) process in an underground mine. ReMi involves acquiring noise data along a linear array of geophones. The data for our study was analysed using the SeisOpt® ReMi™. Various geophone orientations were investigated in order to determine which produced the most discernible dispersion curve. In order to determine whether the changes seen actually exist, shear wave velocities were measured in location of known rock characterisation. These shear wave velocities were then compared with the rock mass rating (RMR) and previously published correlations between RMR and shear wave velocity. In addition, the use of ReMi as a tool to determine the extent of voids underground was investigated.

Shear Wave Velocity Profiles of Roadway Substructures from Multichannel Analysis of Surface Waves and Waveform Tomography

Assessment of roadway subsidence caused by embedded low-velocity anomalies is critical to the health and safety of the traveling public. Surface-based seismic techniques are often used to assess roadways because of data acquisition convenience and large depths of characterization. To mitigate the negative impact of closing a traffic lane under traditional seismic testing, a new test system that uses a land streamer is presented. The main advantages of the system are the elimination of the need to couple the geophones to the roadway, the use of only one source at the end of the geophone array, and the movement of the whole test system along the roadway quickly. For demonstration, experimental data were collected on asphalt pavement overlying a backfilled sinkhole that was experiencing further subsidence. For the study, a 24-channel land streamer and a propelled energy generator to generate seismic energy were used. The test system was pulled by a pickup truck along the roadway and the data were collected with 81 shots at every 3 m for a road segment of 277.5 m, with a total data acquisition time of about 1 h. The measured seismic data set was analyzed by the standard multichannel analysis of surface waves (MASW) and advanced two-dimensional (2-D) waveform tomography methods. Eighty-one one-dimensional shear wave velocity (VS) profiles from the MASW were combined to obtain a single 2-D profile. The waveform tomography method was able to characterize subsurface structures at a high resolution (1.5- × 1.5-m cells) along the test length to a depth of 22.5 m. Very low S-wave velocity was obtained at the repaired sinkhole location. The 2-D VS profiles from the MASW and waveform tomography methods are consistent. Both methods were able to delineate high- and low-velocity soil layers and variable bedrock.

Improved seismic profiling by minimally invasive multimodal surface wave method with standard penetration test source (MMSW-SPT)

Abstract Surface waves propagating in layered media inherently possess multimodal dispersion characteristics. However, traditional surface wave testing methods employing measurements at the free surface usually capture only a single apparent dispersion curve, especially when using short geophone arrays common to near surface and geotechnical-scale investigations. Such single-mode or fragmented multimode apparent dispersion curves contain only a fraction of the possible dispersion information, thus limiting the accuracy of inverted profiles. To enable more robust measurement of higher Rayleigh-wave modes, a recently developed hybrid minimally invasive multimodal surface wave method is combined herein with the widely used geotechnical standard penetration test (SPT), which is employed as a practical and ubiquitous downhole source. Upon superimposing surface wave dispersion data for a range of SPT impact depths within the soil, higher modes can be measured more consistently and reliably relative to traditional non-invasive testing methods. As a result, misidentification of multiple dispersion modes can be practically eliminated, significantly improving the accuracy and certainty of inversion results.

Ground control monitoring of retreat room–and–pillar mine in Central Appalachia

In order to study pillar and overburden response to retreat mining, a ground control program was conducted at a Central Appalachian Mine. The program consisted of several monitoring methods including a seismic monitoring system, borehole pressure cells in the pillars, and time-lapse photogrammetry of the pillar ribs. Two parallel geophone arrays were installed, one on each side of the panel with the sensors mounted 3m into the roof. A total of fourteen geophones recorded more than 5000 events during the panel retreat. A MIDAS datalogger was used to record pressure from borehole pressure cells (BPCs) located in two adjacent pillars that were not mined during retreat. A series of photographs were taken of the pillars that had the BPCs as the face approached so that deformation of the entire rib could be monitored using photogrammetry. Results showed that pillar stability and cave development were as expected. The BPCs showed an increase in loading when the face was 115m inby and a clear onset of the forward abutment at 30m. The photogrammetry results displayed pillar deformation corresponding to the increased loading. The microseismic monitoring results showed the overburden caving inby the face, again as expected. The significance of these results lies in two points, (1) we can quantify the safe manner in which this mine is conducting retreating operations, and (2) we can use volumetric technologies (photogrammetry and microseismic) to monitor entire volumes of the mine in addition to the traditional point-location geotechnical measurements (BPCs).

Acquisition and deconvolution of seismic signals by different methods to perform direct ground-force measurements

We present the results of a novel borehole-seismic experiment in which we used different types of onshore—transient-impulsive and non-impulsive—surface sources together with direct ground-force recordings. The ground-force signals were obtained by baseplate load cells located beneath the sources, and by buried soil-stress sensors installed in the very shallow-subsurface together with accelerometers. The aim was to characterize the source's emission by its complex impedance, function of the near-field vibrations and soil stress components, and above all to obtain appropriate deconvolution operators to remove the signature of the sources in the far-field seismic signals. The data analysis shows the differences in the reference measurements utilized to deconvolve the source signature. As downgoing waves, we process the signals of vertical seismic profiles (VSP) recorded in the far-field approximation by an array of permanent geophones cemented at shallow-medium depth outside the casing of an instrumented well.We obtain a significant improvement in the waveform of the radiated seismic-vibrator signals deconvolved by ground force, similar to that of the seismograms generated by the impulsive sources, and demonstrates that the results obtained by different sources present low values in their repeatability norm. The comparison evidences the potentiality of the direct ground-force measurement approach to effectively remove the far-field source signature in VSP onshore data, and to increase the performance of permanent acquisition installations for time-lapse application purposes.

Using graph clustering to locate sources within a dense sensor array

We develop a model-free technique to identify weak sources within dense sensor arrays using graph clustering. No knowledge about the propagation medium is needed except that signal strengths decay to insignificant levels within a scale that is shorter than the aperture. We then reinterpret the spatial coherence matrix of a wave field as a matrix whose support is a connectivity matrix of a graph with sensors as vertices. In a dense network, well-separated sources induce clusters in this graph. The geographic spread of these clusters can serve to localize the sources. The support of the covariance matrix is estimated from limited-time data using a hypothesis test with a robust phase-only coherence test statistic combined with a physical distance criterion. The latter criterion ensures graph sparsity and thus prevents clusters from forming by chance. We verify the approach and quantify its reliability on a simulated dataset. The method is then applied to data from a dense 5200 element geophone array that blanketed 7km×10km of the city of Long Beach (CA). The analysis exposes a helicopter traversing the array and oil production facilities.

A source-synchronous filter for uncorrelated receiver traces from a swept-frequency seismic source

We have developed a novel algorithm to reduce noise in signals obtained from swept-frequency sources by removing out-of-band external noise sources and distortion caused from unwanted harmonics. The algorithm is designed to condition nonstationary signals for which traditional frequency-domain methods for removing noise have been less effective. The source synchronous filter (SSF) is a time-varying narrow band filter, which is synchronized with the frequency of the source signal at all times. Because the bandwidth of the filter needs to account for the source-to-receiver propagation delay and the sweep rate, SSF works best with slow sweep rates and moveout-adjusted waveforms to compensate for source-receiver delays. The SSF algorithm was applied to data collected during a field test at the University of California Santa Barbara’s Garner Valley downhole array site in Southern California. At the site, a 45 kN shaker was mounted on top of a one-story structure and swept from 0 to 10 Hz and back over 60 s (producing useful seismic waves greater than 1.6 Hz). The seismic data were captured with small accelerometer and geophone arrays and with a distributed acoustic sensing array, which is a fiber-optic-based technique for the monitoring of elastic waves. The result of the application of SSF on the field data is a set of undistorted and uncorrelated traces that can be used in different applications, such as measuring phase velocities of surface waves or applying convolution operations with the encoder source function to obtain traveltimes. The results from the SSF were used with a visual phase alignment tool to facilitate developing dispersion curves and as a prefilter to improve the interpretation of the data.

S-wave velocity measurement and the effect of basin geometry on site response, east San Francisco Bay area, California, USA

We measured S-wave velocity profiles at eleven sites in the east San Francisco Bay area using surface wave methods. Data acquisition included multichannel analysis of surface waves using an active source (MASW), a passive surface-wave method using a linear array of geophones (Linear-MAM), and a two-station spatial autocorrelation method (2ST-SPAC) using long-period accelerometers. Maximum distance between stations ranged from several hundred meters to several kilometers, depending on the site. Minimum frequency ranged from 0.2 to 2 Hz, depending on the site, corresponding to maximum wavelengths of 10 to 1 km. Phase velocities obtained from three methods were combined into a single dispersion curve for each site. A nonlinear inversion was used to estimate S-wave velocity profiles to a depth of 200–2000 m, depending on the site. Resultant S-wave velocity profiles show significant differences among the sites. On the west side of the Hayward fault and the east side of the Calaveras fault, there is a low velocity layer at the surface, with S-wave velocity less than 700 m/s, to a depth of approximately 100 m. A thick intermediate velocity layer with S-wave velocity ranging from 700 to 1500 m/s lies beneath the low velocity layer. Bedrock with S-wave velocity greater than 1500 m/s was measured at depths greater than approximately 1700 m. Between the Hayward Fault and the Calaveras Fault, thicknesses of the low velocity layer and the intermediate velocity layer are less than 50 m and 200 m respectively, and depth to bedrock is less than 250 m. To evaluate the effect of a lateral change in bedrock depth on surface ground motion due to an earthquake, a representative S-wave velocity cross section perpendicular to the Hayward fault was constructed and theoretical amplification was calculated using a viscoelastic finite-difference method. Calculation results show that the low frequency (0.5–5 Hz) component of ground motion is locally amplified on the west side of the Hayward fault because of the effect of two-dimensional structure.

Inversion-based directional deconvolution to remove the effect of a geophone array on seismic signal

The geophone array, as a directivity filter, has been widely used in seismic data acquisition for suppressing noises and alias-free sampling, and its negative effects have long been recognized by analyzing its response to harmonic waves. We extended the analysis by considering its effects on seismic signals, especially on amplitude variation with offset (AVO). Taking the ratio of the array length to the dominant wavelength as a reference, we analyzed the attenuations of the peak amplitude and the peak frequency with the incident angle for the Ricker wavelet. When an array with a ratio of 1.0 is used, the peak amplitude and the peak frequency decay to around 50% and 80% respectively at an angle of 40°. The effect of an array is directivity-dependent, and it acts on the seismic record as a nonstationary filter. We present an inversion-based solution to remove the directional effects. On the approximation of the horizontal stratified medium, the horizontal velocity of the seismic reflection can be concisely expressed in terms of its reflection time, offset, and rms velocity, which facilitates the implementation of directional deconvolution. The method was tested using model-based and logging-based synthetic data.

Calibration of a local magnitude relationship for microseismic events using earthquake data

Calibration of local seismic arrays is crucial to quantify the magnitude of local microseismic events. This is traditionally done by performing a site survey consisting of calibration shots at known locations with known strength. However, when a calibration survey cannot be conducted, alternative calibration methods are required. We have demonstrated the feasibility of magnitude estimation of microseismic events recorded at the [Formula: see text] storage site at Ketzin (Germany), by analyzing earthquake signals that have been recorded on the noncalibrated geophone array and on standardized seismometers of a regional seismological network. We estimated a linear calibration relation by fitting three different signal attributes for 10 distant earthquakes. Through extrapolation of the linear fit toward lower magnitudes, we estimated the local magnitude of weak local seismic events. Reported magnitudes of the distant earthquakes show significant variation because there are different methods and constants used to calculate a magnitude estimate from recordings on multiple seismometers. As a consequence, there was a considerable spread in our magnitude estimates for the Ketzin events. We found magnitude values in the range between [Formula: see text] and [Formula: see text] for two local events recorded at the Ketzin site. We expect that by incorporating additional distant earthquake data, the uncertainty in this estimate can be further reduced.

Illinois Basin – Decatur Project pre-injection microseismic analysis

The Illinois Basin – Decatur Project (IBDP) is a large-scale carbon dioxide (CO2) injection and storage demonstration project. Over a three-year period one million metric tons of CO2 were injected deep into the Mt. Simon Sandstone, a deep saline reservoir in the Illinois Basin. This study examines the microseismic data gathered for the period from May 2010 to November 2011, which preceded the CO2 injection.Microseismic events are detected through permanent geophone arrays installed in two wells. A CO2 injection well drilled to the depth of 7236 feet (2205m) is outfitted with a 4-component geophone array. Situated approximately 185ft away (56m), the 3502 feet (1067m) deep geophysical monitoring well contains a 3-component geophone array.During the pre-injection period, a total of 7894 microseismic events were detected and 99% of these correlate with drilling and other well-related operations. Eight local microseismic events were identified that appear unrelated to well activity and appear representative of the background level of microseismicity.Regional seismicity was also examined by changing triggering parameters and producing a second dataset. Under this configuration, the system detected twelve regional seismic events that correlate to the United States Geological Survey (USGS) earthquake record, and approximately 1100 distant events are believed associated with quarry-related blasting operations.

Magnetotelluric monitoring of coal-seam gas and shale-gas resource development in Australia

Extraction of unconventional energy has become a major global industry in the last decade and is driven by changes in technology and increasing demand. One of the key factors for the success of gas extraction is establishing sufficient permeability in otherwise low-porosity and low-permeability formations. Permeability can be established through hydraulic stimulation of deep formations, either through existing fracture networks or by creating new pathways for fluids to flow, and through depressurization of coalbeds by extracting existing subsurface fluids. Geophysical monitoring of hydraulic stimulation and depressurization can be used to determine lateral and vertical constraints on fluid movements in the target lithologies. Such constraints help to optimize production and well placement. In addition, independent verification is critical for social and environmental regulation, to ensure that hydraulic stimulations and depressurization do not interact with overlying aquifers. To date, the primary and most successful geophysical technique has been microseismic, which measures small seismic events associated with rock fractures from arrays of surface and downhole geophones. The microseismic approach has been used widely for many types of unconventional energy-resource development. The magnetotelluric (MT) method is an alternative approach to monitoring hydraulic stimulations and depressurization. In contrast to microseismic, which delineates the locations of rock fractures, MT is sensitive directly to the presence of fluid as measured by the earth's bulk electrical resistivity, which is dependent on permeability. MT is sensitive to the direction of fluid connection, so it might yield important information on how fluids migrate with time. Because subsurface fluids conduct electrical current dependent on the porosity, connectivity, and ionic saturation of the fluid, it follows that the introduction or removal of fluids will change the electrical resistivity of the formation. The physics of the approach is outlined, and the feasibility of the MT method for monitoring unconventional energy-resource development is demonstrated. Two case studies are conducted, one for a shallow (CSG) depressurization and the second for a deep hydraulic stimulation of a shale-gas reservoir.

Trends in onshore seismic data acquisition: a case study on cable-free nodal systems

The desire to conduct onshore seismic surveys without cables has been an elusive dream since the dawn of seismic exploration. Since the late 1970s, seismic surveys were conducted with cabled multi-channels acquisition systems. As the number of channels steadily grew, a fundamental restriction appeared with hundreds of kilometres of line cables dragged on the ground. Seismic surveys within rugged terrain—across rivers, steep cliffs, urban areas, and culturally and environmentally sensitive zones—were both challenging and expansive exercises. Modern technology has made different cable-free solutions practical. High-resolution analogue to digital converters are now affordable, as are GPS radios for timing and location. Microprocessors and memory are readily available for autonomous recording systems, along with a battery the size and weight of a field nodal now promising to power an acquisition unit for as long as required for normal seismic crew operations. Many successful 2D and 3D seismic data acquisition using cable-free autonomous nodal systems were attempted in the past few years; however, there remain a number of concerns with these systems. The first concern queries whether the units are working according to manufacturer specifications during the data acquisition window. The second is the limited or no real-time data quality control that inspires sceptics to use the term blind acquisition to nodal operations. The third is the traditional question of geophone array versus point receiver acquisition. Although a string of the geophone can be connected to autonomous nodes, the preference is to deploy a single or internal geophone with the nodes to maintain the proposed flexibility of cable-free recording systems. This case study elaborates on the benefits of the cable-free seismic surveys, with specific examples of 2D and 3D exploration programs conducted in Australia in the past few years. Optimisation of field crew size, field crew resources, cost implications, and footprint to the environment, wildlife and domestic livestock will be discussed. In addition, the study focuses on the data quality/data assurance and the processes implanted during data acquisition to maintain equivalent industry standards to cable recording. Emphases will also include data analysis and test results of the geophone array versus the cable-free point receiver recording.

Velocity model estimation by full waveform inversion of time-lapse 4D passive seismic array data

SUMMARY In passive-source monitoring, an accurate velocity model is important to precisely estimate microseismic source locations, and to understand changes in reservoir properties. In this study, we employ frequency-domain full waveform inversion in order to obtain a high- resolution velocity model by exploiting full wavefields. We demonstrate the feasibility of the method for a surface geophone array by inverting for time-lapse 4D velocity changes in a realistic subsurface model. Our method successfully estimates the small velocity changes of a few percent within layers of 10s of meters, even for a single passive seismic source event. The analysis of wavepaths and gradients suggests that keys for the successful inversion are the use of full wavefields (both first and scattered arrivals), the vicinity of velocity changes to the source, and the wide-aperture surface array.