Use Of Geophones Research Articles

Rigidity effect of the mine geophone mounting on its frequency response

Purpose. To determine the rigidity effect of the changes in mine geophones mounting on the frequency response of the recorded seismoacoustic signals. Methodology. Operational calculus and frequency analysis are used. Findings. The dependences of the frequency characteristics of geophones under coupling conditions with the rock are studied. A possibility is shown of qualitative change in the recorded signal when changing the rigidity of installation of the case of a geophone; moreover, increase in rigidity can result in both increase and decrease in the amplitude of frequency components in a fixed frequency range. The calculations are performed for the parameters characteristic of the current use of geophones and the most common rocks. Electrodynamic velocimeter and piezoelectric accelerometer were considered separately. Originality. Frequency response changes in the seismoacoustic signal recorded by the geophone can be caused by the changes in the rigidity at the coupling between the geophone and the rock. The ratio of the installation frequency to the natural frequency of the geophone can serve as an indicator of the possible rigidity effect. Numerical values of this indicator are proposed. Practical value. Critical analysis of the obtained seismoacoustic data, taking into account the possible changes in the geophone installation rigidity in the measurement time. The need to determine the frequency of the geophone mounting in their initial mounting and periodic control to this parameter for further operation.

Performance Evaluation of a Real-Time Seismic Detection System Based on CFAR Detectors

In this research work, we present a novel application specific Ordered cell averaging-CFAR (OCA-CFAR) detector for anthropogenic seismic events. We investigate the use of geophones and real-time hardware for the detection of moving targets such as vehicle, human footsteps and activities such as hammering on the ground. We present here energy assisted variants of Constant False Alarm Rate (CFAR) detector implemented on a real-time embedded platform to obtain detection cues of the ground seismic source. The OCA-CFAR algorithm has been modified in terms of window length and position of Cell Under Test (CUT), suitably adapted to the nature of seismic events engendered from moving vehicles and humans. The proposed detector has been compared with the popularly known variants such as Cell Averaging-CFAR, Least Of-CFAR, Greatest Of-CFAR and Ordered Statistics-CFAR for probabilities of false alarm ranging from 1e-1 to 1e-5. The proposed method shows an increased detection rate with minimized false alarms on the field trials and a dataset containing the seismic signature of human activities and some of civilian vehicles.

Alternate Method of Pavement Assessment Using Geophones and Accelerometers for Measuring the Pavement Response

Pavement instrumentation with embeddable in-situ sensors has been a feasible approach to determine pavement deteriorations. Determining pavement deflections during the passage of the load is a promising strategy to determine the overall performance of the pavement. There are different devices that apply loads to the pavements and measure the deflection basin, these include static, vibratory, or impulse loadings. Most commonly used are the static loading like Benkelman beam and impulse loading like the Falling Weight Deflectometer (FWD). However, these techniques are costly and the measurements are recorded infrequently, i.e., once per year or two years. This study focuses on the use of geophones and accelerometers to measure the surface deflections under traffic loading. To develop a method to measure pavement deflections, the sensors were submitted first to laboratory tests, and then tested in situ, in a full scale accelerated pavement test. In the laboratory, the sensors were submitted to different types of loading using a vibrating table. These tests were used to determine the noise and sensitivity of the sensors, and then to evaluate their response to signals simulating pavement deflections under heavy vehicles. The sensor response was compared with measurements of a reference displacement sensor. Different processing techniques were proposed to correct the measurements from geophones and accelerometers, in order to obtain reliable deflection values. Then, the sensors were evaluated in a full scale accelerated test, under real heavy axle loads. Tests were performed at different loads and speeds, and the deflection measurements were compared with a reference anchored deflection sensor. The main advantage of using accelerometers or geophones embedded in the pavement is to enable continuous pavement monitoring, under real traffic. The sensor measurements could also be used to determine the type of vehicles and their corresponding speeds. The study describes in detail the signal analysis needed to measure the pavement deflections accurately. The measurements of pavement deflection can be then used to analyze the pavement behavior in the field, and its evolution with time, and to back-calculate pavement layer properties.

Application of a New Geophone and Geometry in Tunnel Seismic Detection.

Seismic imaging is the most effective geophysical method and has been extensively implemented to detect potential geological hazards in tunnels during construction. The coupling of geophones and the design of geometry in tunnels are the two major challenges. To ensure successful coupling, a high-sensitivity semi-automatic coupling geophone with a broadband was designed. In practice, this geophone is attached with a wheel and two springs. Once inserted into the borehole, an automatic coupling action occurs. This semi-automatic coupling design within the geophone not only guarantees good coupling, but reduces the time and costs usually required to install a traditional geophone. In the use of geophones for tunnel seismic detection, we propose two new two-dimensional (2D) seismic geometries based on the two commonly used geometries. A test to assess the effectiveness of the qualities of imaging from four geometries was completed by comparing the results of the forward modeling of sandwich models. The conclusion is that the larger the horizontal offset of the layout geometry, the higher the resolution of the imaging; the larger the vertical offset, the weaker the mirror image. The vertical offset is limited due to the narrow tunnel condition. Therefore, the mirror effect cannot be entirely eliminated; however, it can be further suppressed by constructing 2D geometry. The two newly proposed 2D geometries caused the imaging arc of the inter-layer, but suppressed the mirror image. The mirror image added a significant number of errors to the data, which could misguide tunnel construction; therefore the new 2D geometries are more reasonable than the two most commonly used. We applied one of the two new 2D geometries that was more practical to an actual project, the Chongqing Jinyunshan Tunnel in China, and acquired high-quality seismic data using two semi-automatic coupling geophones. The detection results were essentially consistent with the excavation conclusions.

Full-scale experimental study on screening effectiveness of SRM-filled trench barriers

Vibration induced by different sources can be seriously harmful to structures and occupants and their isolation has gained importance during recent years with rapid urbanisation. Trench barriers (open or filled) have been used for isolating such unwanted vibrations, but few experimental studies have investigated the effects of different parameters on vibration reduction. This paper provides results of a series of full-scale field experiments conducted to investigate the effect of open and filled trenches on vibration screening. A sand–rubber mixture (SRM) was used to fill the trench as a lightweight, high-energy-absorbing and environmentally friendly material. Results confirmed that the SRM-filled trench can be used as an efficient, low-cost and easily constructed system for reducing ground vibration. In some cases, the performance level of SRM-filled trenches was close to that of open ones. The use of geophones that could record vibrations in three dimensions demonstrated that trench barriers can isolate vertical vibrations better than horizontal and transversal ones.

On the Use of Geophones in the Low-Frequency Regime to Study Rail Vibrations

AimFor a long time, our research team has been successfully using 2Hz geophones to measure vertical velocities and displacements of rails vibrating as a consequence of high-speed train traffic (200 – 300km/h). Our aim is to extend the use of geophones to study vertical vibrations of rails also for lower traffic velocities (as low as possible). The lower the train velocity, the more important the low frequencies to describe the rail vibration. Given that below twice the resonant frequency of the geophone (ωn) the rate |output|/|input| differs significantly from 1, the amplitude of the signal in this frequency interval (0 – 2ωn) must be restored in order to obtain reliable measurements. In this contribution we present a method to correct the amplitude of geophone data in the low-frequency regime, as well as the analysis of “in situ” measurements for testing the goodness of the method. MethodDespite the one-to-one relationship between the spectral amplitude of the input signal and the output of a geophone for frequencies above 2ωn, there are significant phase effects up to 10ωn. This is the reason why phase correction must always be applied. We take advantage of the Discrete Fourier Transform (DFT) to convert the signals from the time domain into the frequency domain. Thus we are able to restore not only the phase but also the amplitude by applying the inverse transfer function of the geophone. The restored data are converted into the time domain by using the inverse DFT. We compare the vertical rail displacements obtained by integrating the amplitude- and phase-corrected velocity measurements with those calculated when only the phase-correction is applied for a rail vibrating when a train travels at 5, 50, 100, 130 and 160km/h. ResultsPreviously to this work, we were able to measure vertical velocities and displacements in rails vibrating as a consequence of high-speed train traffic by using 2Hz geophones. Now, with the new-implemented amplitude correction in the frequency domain, we are also able to obtain these quantities for rails that vibrate when trains with velocity as low as 50km/h travel over them, allowing us to perform studies in underground rapid transit systems.

Using Geophones for Nondestructive Evaluation of Full-Scale Bridges

Nondestructive damage identification for civil engineering structures has received the attention of many researchers over the past several years. Vibration-based damage detection is a nondestructive structural health-monitoring approach that focuses on changes in the dynamic characteristics of a structure as indicators of damage. All vibration-based damage detection techniques require high signal-to-noise vibration data for analysis; thus the sensors used to measure vibrations are an important consideration. The objective of this study is to demonstrate the use of inexpensive geophones for determining the modal parameters of bridges to be used with vibration-based damage detection techniques. A geophone is a velocity transducer commonly used by seismologists for subsurface exploration. Researchers typically use accelerometers to measure bridge vibrations. However, compared with geophones, accelerometers are relatively expensive, have lower sensitivity, and require additional electronics for signal conditioning. To validate the use of geophones for modal parameter identification, a simple beam experiment was conducted, and the results were compared with theoretical values and a finite element model. Modal parameters identified from a full-scale bridge test are presented, and the effects of parapet rails and temperature change on the bridge's modal parameters are discussed. For successful implementation of the proposed methodology in a remote, wireless approach, a solar-powered, cell phone modem–based data acquisition system is demonstrated. This study makes an important contribution because the cost of the sensors needed for implementation of vibration-based damage detection on a large scale is substantially reduced with the use of high-sensitivity geophones.

Source bearing estimation in the Arctic Ocean using ice-mounted geophones.

This paper investigates the use of geophones mounted on the surface of Arctic sea ice for estimating the bearing to acoustic sources in the water column. The approach is based on measuring ice seismic waves for which the direction of particle motion is oriented radially outward from the source. However, the analysis is complicated by the fact that sea ice supports several types of seismic waves, producing complex particle motion that includes significant nonradial components. To suppress seismic waves with transverse particle motion, seismic polarization filters are applied in conjunction with a straightforward rotational analysis (computation of particle-motion power as a function of angle). The polarization filters require three-dimensional (3D) measurements of particle motion, and apply theoretical phase relationships between vertical and horizontal components for the various waves types. In addition, the 180 degrees ambiguity inherent in the rotational analysis can be resolved with 3D measurements by considering particle motion in the vertical-radial plane. Arctic field trials were carried out involving two components. First, a hammer source was used to selectively excite the various ice seismic waves to investigate their propagation properties and relative importance in bearing estimation. Second, impulsive acoustic sources were deployed in the water column at a variety of bearings and ranges from 200-1000 m. For frequencies up to 250 Hz, source bearings are typically estimated to within an average absolute error of approximately 100.