Vibroseis Source Research Articles

Time‐frequency domain correlation method based on the synchrosqueezed wavelet transform for tunnel vibroseis and its application

AbstractTo meet the demand for long‐distance and highly accurate detection in tunnel construction, a vibroseis source has been introduced into tunnel forward prospecting. However, serious noise in tunnels reduces the resolution of vibroseis signals, and small‐scale structure recognition in tunnels requires higher signal resolution. Thus, we develop a time‐frequency domain correlation method based on the synchrosqueezed wavelet transform for tunnel vibroseis data. Time‐frequency domain correlation may capture more information than a single time or frequency domain correlation method, and its effectiveness depends on the time‐frequency transformation method. The synchrosqueezed wavelet transform allows us to obtain high‐resolution time‐frequency spectra and thus helps to extract high‐resolution effective signals. The advantages and disadvantages of different correlation methods are highlighted using numerical examples, in which the high resolution and strong robustness of the time‐frequency domain correlation method based on the synchrosqueezed wavelet transform are demonstrated. A detailed field case study in an iron mine tunnel further demonstrates the reliability of the time‐frequency domain correlation method based on the synchrosqueezed wavelet transform for practical data.

Seismic communication data processing based on compressed sensing algorithm

AbstractGeophysical prospecting signals encompass subsurface structural information and incorporate textual messages generated in accordance with a specific pattern. These signals can be employed in places without radio access to ensure public and worker safety. Therefore, the use of seismic signals to transmit information through the earth has attracted the attention of researchers in the last decade. Presently, achievements in seismic communication are mainly in the coding, generation, and propagation of seismic signals. Little work has been done on methods to convert seismic signals generated by vibroseis sources into text and broadcast them vocally. Therefore, we built a seismic communication system with 6‐bit code using the American Standard Code for Information Interchange. To better recommend the seismic communication system scheme, the origin, state and principles of seismic communication system are illustrated in detail. Then, a seismic transmitting system is devised with a 500 N vibroseis source, which compiles seismic signals through amplitude modulation. After seismic signals propagate through the earth, they are received by geophones and recorded in seismographs. Through data acquisition based on a compression algorithm, seismic signals are converted into text and voice signals, which significantly reduces the storage and transmission of seismic data.

Characteristic and processing method of SH-wave data generated by vibroseis source

Enhancing the energy of horizontally-polarized shear (SH) waves while suppressing compressional (P) waves is a crucial prerequisite for processing and interpreting SH-wave data obtained from urban engineering surveys. In the case of processing SH-wave data generated by a hammer-impact source, a widely used and effective technique is to calculate half the difference between the two datasets collected from opposite force directions at the same shot point. However, this method shows the opposite effect when processing SH-wave data generated by a shear wave vibroseis source. To address this issue, we begin with investigating the characteristic of related reflections in SH-wave data generated by the vibroseis source, and then propose an effective processing method that computes half the sum of the two vibroseis-generated datasets collected in the same manner. Synthetic test shows that the proposed method can effectively enhance the energy of SH-waves while suppressing that of P-waves. The field data application suggests that the proposed method can successfully and effectively detect the location of underground limestone caves compared to the traditional method. The proposed method provides a simple but effective alternative for improving the signal-to-noise ratio (SNR) of SH-wave data generated by vibroseis source.

Evidence of Nonlinear Seismic Effects in the Earth from Downhole Distributed Acoustic Sensors

Seismic velocities and elastic moduli of rocks are known to vary significantly with applied stress, which indicates that these materials exhibit nonlinear elasticity. Monochromatic waves in nonlinear elastic media are known to generate higher harmonics and combinational frequencies. Such effects have the potential to be used for broadening the frequency band of seismic sources, characterization of the subsurface, and safety monitoring of civil engineering infrastructure. However, knowledge on nonlinear seismic effects is still scarce, which impedes the development of their practical applications. To explore the potential of nonlinear seismology, we performed three experiments: two in the field and one in the laboratory. The first field experiment used two vibroseis sources generating signals with two different monochromatic frequencies. The second field experiment used a surface orbital vibrator with two eccentric motors working at different frequencies. In both experiments, the generated wavefield was recorded in a borehole using a fiber-optic distributed acoustic sensing cable. Both experiments showed combinational frequencies, harmonics, and other intermodulation products of the fundamental frequencies both on the surface and at depth. Laboratory experiments replicated the setup of the field test with vibroseis sources and showed similar nonlinear combinations of fundamental frequencies. Amplitudes of the nonlinear signals observed in the laboratory showed variation with the saturating fluid. These results confirm that nonlinear components of the wavefield propagate as body waves, are likely to generate within rock formations, and can be potentially used for reservoir fluid characterization.

Development and Application of a Portable Vibroseis Source for Acquisition and Analysis of Seismic Surface Wave Data

Development and Application of a Portable Vibroseis Source for Acquisition and Analysis of Seismic Surface Wave Data

Performance of high-resolution impact and vibration sources for underground seismic exploration of clay formations at the scale of a rock laboratory

SUMMARY Low permeability, high retention capacity and self-sealing ability are advantageous characteristics that are attributed to argillaceous rocks. In contrast, other properties of clay, such as internal heterogeneities, strong attenuation and anisotropic behaviour, pose major challenges for underground exploration techniques. Although with regard to the underground storage of nuclear waste, the seismic exploration in the underground itself is of great importance to fill the gap between surface and borehole investigations. Furthermore, to prevent destruction of the host rock during exploration this demands low to non-invasive techniques. To approach these issues, a seismic survey was carried out in the Mont Terri Underground Rock Laboratory (Switzerland) using a gallery-based acquisition with an operating range up to several decametres. The seismic campaign included three-component borehole sensors and two different seismic source types (pneumatic impact and magnetostrictive vibroseis source). An executed source comparison analysed the characteristics of the different source types, for example frequency or amplitude behaviour of the generated wavefield, to assess their performance under similar conditions at the meso scale and to reveal their strengths and weaknesses in clay. Based on these findings, we performed traveltime and reflection analyses that demonstrate their potential to characterize clay formations and to map internal structures. The highest seismic velocities are found in the carbonate-rich sandy facies (vPmax = 4000 m s−1, vSmax = 2050 m s−1), slower velocities are found in the sandy facies (vPmax = 3720 m s−1, vSmax = 1840 m s−1) and the slowest velocities are found in the shaly facies (vPmax = 3220 m s−1, vSmax = 1480 m s−1). The seismic velocity anisotropy is larger within the shaly facies (AvP = 23 per cent, AvS = 32 per cent) compared to the sandy facies (AvP = 9 per cent, AvS = 12 per cent) and it is more pronounced for S-waves than P-waves. Thus, non-invasive meso-scale seismic techniques are suited to characterize the Opalinus Clay in great detail.

Seismic monitoring of CO2 geosequestration using multi-well 4D DAS VSP: Stage 3 of the CO2CRC Otway project

An important part of any CO2 geosequestration project is to ensure CO2 containment and conformance in the subsurface. This is generally done by implementing a comprehensive, risk-based Measurement, Monitoring and Verification plan, a key element of which is active time-lapse seismic monitoring. However, high cost and environmental impact of the standard surface seismic monitoring dictate the need for a cost-effective and environmentally friendly alternative. An opportunity to develop such method emerges with advances in distributed acoustic sensing (DAS) technology, which turns an optical fibre into a seismic sensor with dense spatial sampling. DAS can be permanently deployed in multiple wells across the geosequestration site providing a robust and non-intrusive network of seismic receivers. This approach was developed and tested in the CO2CRC Otway project, where injection of 15 kt of CO2 at 1.5 km depth was monitored with a 4D vertical seismic profiling (VSP) using five borehole DAS arrays and mobile vibroseis sources. The 4D DAS VSP in each of the five wells provides broadly consistent images of the CO2 plume with some differences due to different illumination of the target horizon, lateral variation of velocities, and seismic anisotropy. When the newly injected CO2 reaches a CO2 plume created as a result of an earlier injection into the same formation ∼600 m updip, 4D DAS VSP shows a change in reflectivity in that area and beyond. This shows a potential of 4D DAS VSP for monitoring gas injection into gas-saturated reservoirs.

The Diminishing Returns of Low-Frequency Vibroseis and How Only Physics Will Get Us out of the Rut

All land seismic sources are inefficient in the conversion of their energy/force, generated at the surface, into useful seismic signals. It is not commonly known that less than 5% of the source’s energy is converted into energy of the propagating wave. Until now, one of the objectives of the designers of land seismic sources has been to increase the maximum force (or energy) output at the surface to increase the amplitude of the seismic signal. Today’s vibroseis sources, at about 40 tonnes, are at their maximum size and weight limits for many road infrastructures limiting their areas of use, as well as being beyond the budget of most seismic operators. As force output is related to vehicle weight (hold down force), the vibroseis source is effectively at its maximum output (and therefore maximum seismic signal) under its current configuration. For this reason, to achieve further increases in the amplitude and frequency content of the propagating seismic signals, the emphasis in future research and development must be in understanding the many factors that are involved in the conversion of the force/energy generated by the surface source into useful seismic signals. This has been missing in the past; it is where only physicists can play a role. In this article, we discuss the physics of impulsive and vibroseis sources and their interactions with the ground to show that improving the amplitude/frequency content and efficiency of the vibrator, especially at low frequencies, cannot be fixed by ‘mere engineering’.

Advances in Vibroseis Technology

The development of vibroseis technology in the 1950s ushered in a new era in seismic data acquisition. The vibroseis gave control of the source wavelet and frequency range to the user. In addition, the vibroseis is a surface-mobile source, which does not require substantial site preparation. Combined, the technical advantages of the vibroseis and its operation benefits make the vibroseis source the primary method for acquiring seismic data on land today. Vibroseis technology has advanced dramatically since the early days. The most recent innovations in vibroseis span areas ranging from mechanical engineering to software and controller development. This article will discuss the latest innovations in vibroseis technology and explain the benefits expected to be derived by contractors and explorationists.

Main Aspects of the Implementation of Seismic Projects

The paper presents the planning and workflow of field seismic explorations. Seismic data acquisition has developed significantly over the last 15 20 years; the paper covers the most important changes and developments of this period and goes step-by-step through the process of implementing a land 3-D seismic project. In order to increase efficiency, the different cost reduction technologies appeared on the vibroseis source side in 3-D seismic, and wireless field technology is becoming more and more accepted among the forms of seismic data acquisition. The advances in seismic acquisition, including simultaneous source acquisition, wireless channel systems, and exponential growth in seismic channel count, are revolutionizing surveys. Improved receiver sampling along with high productivity are the drivers for seismic technology today, which means smaller, lighter and less power-hungry equipment, while also increasing channel counts, minimizing system related remediation and HSE risk. A new era in land 3-D seismic surveys has arrived in Europe and in Hungary, too. The author used her experience in the implementation of many domestic and foreign 2-D and 3-D land seismic projects as an observer, shift leader, field manager, and then as a project leader in Europe. She combines these practical experiences with specific aspects of field project design and implementation. The aspects covered in this paper contribute to the professionally efficient and financially profitable implementation of field projects.

Seismic monitoring of a small CO2 injection using a multi-well DAS array: Operations and initial results of Stage 3 of the CO2CRC Otway project

Active time-lapse seismic is widely employed for monitoring CO2 geosequestration due to its ability to track the distribution of fluids in space and time. However, standard 4D seismic monitoring suffers from several challenges, including high cost, disruption to other land uses, and, consequently, relatively large intervals between monitor surveys. Some of these challenges can be mitigated using permanently installed sources and receivers. Such an approach was tested at the CO2CRC Otway site by continuous offset VSP monitoring of 15,000 t of supercritical CO2 injected into an aquifer 1,500 m deep with nine permanent seismic sources (surface orbital vibrators or SOVs) and five downhole fibre-optic receivers. This continuous monitoring is complemented by multi-well 4D VSP using a mobile vibroseis source and the same DAS receivers, which included one baseline and two monitor surveys after injection of 4,000 and 12,000 t of CO2. The continuous DAS-SOV monitoring detected an abrupt increase of travel times below the injection interval on the second day of injection (after injection of 300 t of CO2) and tracked the growth of the areal CO2 plume by mapping changes of reflection amplitudes. The plume is also detected by time-lapse changes of reflection amplitudes in multi-well 4D VSPs. The plume images obtained from continuous offset VSP and 4D VSP are broadly consistent with each other but with some differences due to differences in illumination, lateral variations of velocities and seismic anisotropy. These differences also serve as a measure of uncertainty of 4D VSP images.

Multi-drive level Vibroseis test to evaluate the non-linear response of soft soils

In this paper we present the results of a seismic experiment using data produced by an active Vibroseis source and varying the drive force level, during a standard seismic reflection survey. We observed a clear non-linear response that can be linked to the presence of shallow soft sediment layers. The variation of the applied vibration force induces a change in both propagation velocity and attenuation of the Rayleigh waves, and this implies that such parameters are non-linear functions of the wave energy. We analyze the soil non-linear behavior with respect to the estimated induced strain, in order to compare the collected data against the observed reduction of shear modulus and damping values as usually measured in laboratory experiments and reported in the literature. The use of multi-component receivers allowed us also to identify the Rayleigh wave ellipticity and hence the resonance frequency of the studied site. Using active source data, rather than passive micro-tremors of unknown origin, for these applications is a novel approach. Thus, we propose an alternative and efficient method to characterize the non-linear key parameters of the soil seismic response, based on the use of controlled sources of widespread use in exploration seismics. This approach may pave the way to the development of utterly new site characterization techniques.

Elastic-waveform inversion and imaging of 2010 walkaway VSP data from the Raft River geothermal field

A vertical seismic profiling (VSP) survey conducted in 2010 in the original RRG-9 well at the Raft River geothermal field is to understand the subsurface geologic structures for geothermal reservoir characterization. This survey consists of 40 three-component (3C) geophones deployed in the upper 600 m of the well and 255 vibroseis sources positioned along eight walkaway lines at the surface. We perform elastic-waveform inversion and reserve-time migration of the VSP data to obtain high-resolution subsurface images. We first process the raw walkaway 3C VSP data and separate the upgoing compressional-to-compressional (PP) and compressional-to-shear (PS) wavefields. We then build an initial velocity model based on sonic logging and zero-offset VSP data, update the initial model with first-arrival traveltime tomography, and refine the model using elastic-waveform inversion to reveal high-resolution velocity variations. We finally apply reverse-time migration to the upgoing wavefields to obtain high-resolution PP and PS migration images of the lithologies encountered in the well. The refined velocity models obtained using elastic-waveform inversion improve the subsurface migration imaging results. Our elastic-waveform inversion and reverse-time migration confirm the lithology interfaces revealed from well logs and provide images of subsurface geologic layers away from well RRG-9 at the Raft River geothermal field.

Application of the Segmented Correlation Technology in Seismic Communication with Morse Code

Seismic communication might promise to revolutionize the theory of seismic waves. However, one of the greatest challenges to its widespread adoption is the difficulty of signal extraction because the seismic waves in the vibration environments, such as seas, streets, city centers and subways, are very complex. Here, we employ segmented correlation technology with Morse code (SCTMC), which extracts the target signal by cutting the collected data into a series of segments and makes these segments cross-correlate with the decoded signal to process the collected data. To test the effectiveness of the technology, a seismic communication system composed of vibroseis sources and geophones was built in an environment full of other vibration signals. Most notably, it improves the signal-to-noise ratio (SNR), extending the relay distance and suppressing other vibration signals by using technology to deal with seismic data generated by the system.

Testing of a permanent orbital surface source and distributed acoustic sensing for monitoring of unconventional reservoirs: Preliminary results from the Eagle Ford Shale

The effective monitoring of hydraulic fracturing in unconventional oil and gas production requires tools to quantify elastic property variations even in the absence of microseismic activity. To track the subtle time-lapse variations in reservoir properties during such activities, monitoring techniques with high repeatability and high resolution, spatially and temporally, are required. Distributed acoustic sensing (DAS) is a rapidly maturing fiber-optic technology for low-cost, permanent, high density, in-well monitoring. Surface orbital vibrators (SOVs) are inexpensive fixed rotary seismic sources that offer the opportunity to frequently interrogate the subsurface with energies comparable to vibroseis sources. We have evaluated a field vertical seismic profile test, conducted in the Eagle Ford play, pairing an SOV source recorded by DAS behind casing in a deviated well to better evaluate the potential of the technology set for unconventional reservoir monitoring. We determine the data processing workflow for reservoir monitoring using the SOV-DAS system. We analyze the data characteristics of the SOV-DAS system, including the signal-to-noise ratio characteristics and source repeatability. High-quality P- and S-wave reflections, as well as mode conversions, are visible in the vertical section. In addition, clear P-P reflections are also observable along the horizontal well sections. Time shifts with a mean value of 10 μs between different data sets demonstrate the high repeatability for the semipermanent SOV source, which is crucial for time-lapse analysis. We also apply reflection imaging on P and S to reveal reflection depths. In a first-of-its-kind deployment, we implemented a rotating SOV with a slewing bearing and discuss the possibility to optimize S-wave construction along the horizontal well with specific SOV orientation directions. Our preliminary results indicate that the combination of repeatable surface sources such as SOVs with DAS has significant potential for providing a low-cost approach for high-resolution seismic monitoring of unconventional reservoirs.

WSGF — Time for change part 2: Forces and energies acting on the seismic source and earth systems

In land geophysical exploration an impulsive or vibratory source, operating at the earth’s surface, is used to generate the seismic signal during the field acquisition. Comparisons of the manufacturer’s output specifications will often be a factor in selecting a seismic source for ‘good S/N’ (signal to noise ratio) in a particular survey area. For impulsive sources, such as dynamite, weight drop, and accelerated weight drop (AWD), the source specifications are given in energy units (Joules, foot-pounds) whereas vibroseis sources are rated in force units (Newtons, pounds-force). For either type of source, the manufacturer’s specifications are the output ‘power’ of the source mechanism and do not take into account any inefficiencies/losses in energy in the source mechanism or the interacting earth volume when generating the seismic signal. In this paper, I examine some of the factors that determine the transfer of energy from the source mechanism to the seismic signal and show, for AWD sources, the energy transfer during the impulse can be greatly improved by optimization of the source parameters. For vibrators, the energy transfer to the ground may be a better indicator of the propagating wave than the current WSGF estimates.

A Modeling Comparison of the Potential Effects on Marine Mammals from Sounds Produced by Marine Vibroseis and Air Gun Seismic Sources

Concerns about the potential environmental impacts of geophysical surveys using air gun sources, coupled with advances in geophysical surveying technology and data processing, are driving research and development of commercially viable alternative technologies such as marine vibroseis (MV). MV systems produce controllable acoustic signals through volume displacement of water using a vibrating plate or shell. MV sources generally produce lower acoustic pressure and reduced bandwidth (spectral content) compared to air gun sources, but to be effective sources for geophysical surveys they typically produce longer duration signals with short inter-signal periods. Few studies have evaluated the potential effects of MV system use on marine fauna. In this desktop study, potential acoustic exposure of marine mammals was estimated for MV and air gun arrays by modeling the source signal, sound propagation, and animal movement in representative survey scenarios. In the scenarios, few marine mammals could be expected to be exposed to potentially injurious sound levels for either source type, but fewer were predicted for MV arrays than air gun arrays. The estimated number of marine mammals exposed to sound levels associated with behavioral disturbance depended on the selection of evaluation criteria. More behavioral disturbance was predicted for MV arrays compared to air gun arrays using a single threshold sound pressure level (SPL), while the opposite result was found when using frequency-weighted sound fields and a multiple-step, probabilistic, threshold function.

Three-dimensional seismic characterization and imaging of the Soda Lake geothermal field

Accurate characterization of subsurface geophysical properties and detection of the fault system are essential for geothermal energy exploration and production. The Soda Lake geothermal field is in western Nevada with a complex fault system. Previous seismic characterization only produced a low-resolution, smooth velocity model along with a simple, conceptual fault model. Using optimized correlation-based full-waveform inversion, wavefield-separation-based reverse-time migration, and automatic fault detection techniques, we present 3D seismic characterization for the Soda Lake geothermal field using 3D surface seismic data acquired with Vibroseis sources. We obtain 3D high-resolution velocity, density, and acoustic impedance models, 3D seismic images with different grid spacings, and a high-resolution fault system. Consistency check between the constructed faults and currently active injection and production geothermal wells verifies that our seismic inversion and imaging results and detected faults are reliable. These results can provide valuable information for optimizing well placement and geothermal energy production at the Soda Lake geothermal field.

Modeling the seismic response of individual hydraulic fracturing stages observed in a time-lapse distributed acoustic sensing vertical seismic profiling survey

In 2017, distributed acoustic sensing (DAS) technology was deployed in a horizontal well to conduct a vertical seismic profiling survey before and after each of 78 hydraulic fracturing stages. From two vibroseis source locations at the surface, time shifts of P- and S-waves were observed but decayed over days. Some stages also showed waves scattered off the stimulated rock volume. We have used 2D finite difference elastic wavefield modeling to understand these observations and connect them to underlying properties of the stimulated rock. We have developed an effective medium model of vertical fractures that close exponentially with time as fluid leaks off into the formation can match the distribution of P- and S-wave time shifts along the well. This has enabled estimates of the height, normal and tangential fracture compliance values, and decay time of the stimulated rock volume. Additionally, the kinematics of scattered waves observed in the data have been found to be consistent with PS conversion across the stimulated rock volume from an individual stage. With higher quality DAS data, stage-by-stage inversion for height, fracture compliance, and decay time attributes may be possible for characterizing variations in the effectiveness of hydraulic fracturing.

OPTIMIZATION-BASED RECURSIVE FILTERING FOR VIBROSEIS HARMONIC NOISE ELIMINATION

The vibroseis sources always produce harmonics, which are considered here as noise to be eliminated. An algorithm for removing such noise has been developed based on the previously obtained mathematical model of a vibroseis wavelet distorted by the harmonics. It is shown that the ideal inversion operator is recursive, i.e. is a filter with infinite impulse response. However, due to the peculiarity of the problem, this operator is successfully approximated by a filter with a short impulse response, allowing linearization of the optimization scheme. An objective being the energy of the noise attenuation result is formed. It allows application of the geometrical divergence correction despite the fact that it introduces distortions in the signal shape and the harmonics.