Electromagnetic Geophone Research Articles

LITHOLOGY DISCRIMINATION USING SHEAR AND COMPRESSIONAL WAVES IN UYO AND ITS ENVIRON, SOUTHEASTERN NIGERIA

Compressional and shear wave velocities reveal vital information about the physical properties of rocks and their lithology contrast. In this study, we have applied the seismic refraction method to investigate the dynamic elastic modulus of subsurface layers in our study area. Electromagnetic geophones were firmly coupled to the ground to transform generated seismic energy at the source to electrical voltage which is a function of velocity. A total of twenty-seven P-wave refraction shootings were performed and measured P-wave velocity ranged from 218.00 ms-1 to 749.30 ms-1. The S-wave velocity obtained in the field ranged from 98.30 ms-1 to 340.70 ms-1. These values indicate that both layers 1 and 2 are weathered zones characterized by fine and lateritic sands which are capable of withstanding little or no mechanically related load or stress / pressure applied on the surface. In terms of Vp-Vs ratio, the first layer indicated a range of 2.2071 to 2.2183 with a mean value of 2.2108 for the area while the second layer produced a Vp-Vs ratio ranging from 2.1993 to 2.2055 with a mean value of 2.2023. The near constant Vp-Vs ratio of 2.2 obtained in the study shows a similarity in the formation grains in the shallow layers penetrated by the seismic waves.

Piezoelectric geophone: a review from principle to performance

Real-time monitoring of seismic information is important, a wide variety of geophones have emerged to achieve this goal. According to the principle of transforming seismic signals, the geophones can be classified into electromagnetic geophones, piezoelectric geophones, electrochemical geophones, fiber optic geophones and so on. Among them, piezoelectric geophones have been widely used in seismic detection. In this article, the piezoelectric material is briefly introduced at first, and then the piezoelectric seismic detector with piezoelectric ceramic and polyvinylidene fluoride as a sensitive material is involved. After analyzing the electrical equivalent model of the piezoelectric sensitive unit, the signal conditioning circuit of the piezoelectric geophone is summarized as well. After that, the promotion of MEMS technology to piezoelectric geophones is discussed. Finally, a summary of all the work listed in this article is given and an outlook of combining MEMS and PVDF to improve the performance of geophones is proposed.

Use of surface waves for geotechnical engineering applications in Western Sydney

Abstract. Current in situ methods used to geotechnically characterize the ground are predominantly based on invasive mechanical techniques (e.g. CPT, SPT, DMT). These techniques are localized to the tested area thus making it quite time consuming and costly to extensively cover large areas. Hence, a study has been initiated to investigate the use of the non-invasive Multichannel Analysis of Surface Waves (MASW) and Multichannel Simulation with One Receiver (MSOR) techniques to provide both an evaluation of compacted ground and a general geotechnical site characterization. The MASW technique relies on the measurement of active ambient vibrations generated by sledgehammer hits to the ground. Generated vibrations are gathered by interconnected electromagnetic geophones set up in the vertical direction and in a linear array at the ground surface with a constant spacing. The MSOR technique relies on one sensor, one single geophone used as the trigger, and multiple impacts are delivered on a steel plate at several distances in a linear array. The main attributes of these non-invasive techniques are the cost effectiveness and time efficiency when compared to current in situ mechanical invasive methods. They were applied to infer the stiffness of the ground layers by inversion of the phase velocity dispersion curves to derive the shear wave velocity (Vs) profile. The results produced by the MASW and the MSOR techniques were verified against independent mechanical Cone Penetration Test (CPT) and Standard Penetration Test (SPT) data. This paper identifies that the MASW and the MSOR techniques could be potentially useful and powerful tools in the evaluation of the ground compaction and general geotechnical site characterization.

Modeling of an electromagnetic geophone with passive magnetic spring

This paper presents an analysis of a novel, patent pending [Magnetic Innovations, www.MagneticInnovations.com], absolute velocity sensor, or geophone. Conventional geophone topologies are discussed and their intrinsic physical limitations, given by the mechanical construction, are shown. Therefore, a new topology with passive magnetic spring is proposed. A unified semi-analytical solution for the magnetic field equations is derived in cylindrical coordinates and used to determine the static response characteristics of the proposed geophone configuration. Furthermore, a prototype is built and measurements are performed and analyzed to obtain the dynamic behavior. A further improvement of the novel geophone is proposed by means of optimization using finite element analysis.

Fiber-Bragg-grating-based seismic geophone for oil/gas prospecting

A new type of fiber Bragg grating (FBG) sensor–based seismic geophone is presented. The major application of this FBG geophone is in the seismic reflection survey of oilfield exploration to detect the seismic waves from the Earth. The customized FBG geophone prototype that is demonstrated consists of two FBG sensor/demodulation grating pairs attached on a spring-mass mechanical system. The sensor performance is characterized in the laboratory in comparison with the conventional geophone. The FBG sensor is resistant to electromagnetic interference and has higher frequency response bandwidth and sensitivity than a conventional moving-coil electromagnetic geophone. The field tests results taken in the oil field show the sensor system frequency response bandwidth at 10 to 100 Hz in Shallow layer and 10 to 140 Hz in medium and deep layer separately.

Novel piezoelectric structures for sensor applications

Piezoelectric ceramic devices have been formed into the helical spring shapes from tubular extrudates. These structures are shown to possess low compliance and low natural resonant frequencies. Equations are presented allowing the prediction of the resonant frequencies of the devices; these are shown to be in good agreement with measured responses. Using the design criteria a device has been constructed to exhibit a low fundamental resonance and a clear spectrum up to 500 Hz. The frequency response is shown for this device and compared to a conventional electromagnetic geophone. The results show that the device acts as a displacement sensor providing an output charge of 0.427 mC/m with no observed spurious resonances.

The effects of spring and magnetic distortions on electromagnetic geophones

The effects of spring and magnetic field distortions on the output voltage of an electromagnetic moving-coil geophone were studied theoretically and experimentally. Spring distortion is caused by vibrations of large amplitudes. Magnetic distortion is due to the nonuniformity of the magnetic flux through the moving coil. The perturbation method was employed to solve the nonlinear vibration equation. The calculated vibration velocity was then combined with the magnetic flux analysis to study the combined effects of spring and magnetic distortions on the moving-coil geophone. It was found that the third harmonic in geophone output is dominated by spring distortion while the second harmonic is caused primarily by magnetic distortion. The behaviours of geophone output predicted by the mathematical model were verified by experiments in which the coil was wound unevenly and the magnetic flux was kept constant.

IMPROVED PHASE‐ELLIPSE METHOD FOR IN SITU GEOPHONE CALIBRATION*

ABSTRACTFor amplitude and phase response calibration of moving‐coil electromagnetic geophones two parameters are needed, namely, the geophone natural frequency, f0, and the geophone upper resonance frequency fu. The phase‐ellipse method is commonly used for the in situ determination of these parameters. For a given signal‐to‐noise ratio, the precision of the measurement off0 andfu depends on the phase sensitivity, f(δφ/δf) For some commercial geophones f(δφ/δf) atfu can be an order of magnitude less than the sensitivity atf0. In this paper we present an improved phase‐ellipse method with increased precision. Compared to measurements made with the existing phase‐ellipse methods, our method shows a 6‐ and 3‐fold improvement in the precision, respectively, on measurements of f0 andfu on a commercial geophone.

Geophone with damping oil

A conventional electromagnetic geophone includes a coil assembly that is spring-suspended in an air gap between concentric pole pieces of a magnet. The coil assembly is wound on the outer wall of a bobbin. The bobbin is typically made from or includes a metallic material to provide a mass for mechanical damping and a conductor for electrical eddy-current damping of the spring-suspended coil assembly. The improvement of this invention lies in mounting on the bobbin, a vernier mechanical damping mass made of a coil of insulated wire. The coil is open-ended and the mechanical damping force is adjustable in accordance with the number of turns in the coil.

Theory and Practice of Geophone Calibration In Situ Using a Modified Step Method

The transfer function of a moving coil electromagnetic geophone requires up to six constants: the suspended mass, the natural resonant frequency, the mechanical and the electrical damping factors, the inductive time constant of the coil-damping resistance circuit, and the coil transductance. If the mass is known, the remaining constants may be satisfactorily determined by displacing the mass with a known dc current, and observing the time varying output potential on removal of the steady current. The techniques outlined are suited to in situ calibration, and have a minimum equipment requirement of a CRO and a small battery. Improved accuracy is possible using a precision signal generator and a quality CRO, if the geophone natural and upper resonant frequencies are measured by the phase ellipse method. Calibration is possible for both under- and over-damped systems, using either the derived formulas or accompanying graphs. The accuracy attainable is dependent on the geophone damping and frequency range of operation, but is typically 2 percent in absolute amplitude and 2° in phase for 0.7 critical damping.

THE DESIGN OF ELECTROMAGNETIC GEOPHONES*

AbstractThis paper summarizes and extends the published information on the theoretical design of electromagnetic geophones. Both moving‐coil and moving‐armature instruments are considered, their behaviour compared and various design criteria established.

THE LIMITING SENSITIVITY OF SEISMIC DETECTORS

The theory of Brownian motion is applied to the problem of determination of the limiting sensitivity of seismic detectors. It is shown that a certain minimum suspended mass is requird for the recording of small ground motions. Electromagnetic geophones are studied in detail, and it is shown how spontaneous thermal fluctuations in voltage limit the performance of these instruments. Under certain conditions, a geophone may be treated as a power generator, and the necessary suspended mass is then determined by the power requirements of the recording apparatus.