Elsevier

Optical Fiber Technology

Volume 60, December 2020, 102384
Optical Fiber Technology

Simulation and hardware implementation of demodulation for fiber optic seismic sensor with linear edge filtering method

https://doi.org/10.1016/j.yofte.2020.102384Get rights and content

Abstract

The demodulation system is a very critical component of the seismic exploration, which determines the response speed and accuracy of data acquisition of the detection system. Here, we demonstrate a simulation and hardware implementation of demodulation system based on linear edge filtering method for fiber optic seismic sensor. The system is implemented mainly by an edge filter (long period fiber grating) with the linear slope of 1.368 dBm/nm and the sideband width of 6.05 nm, and a homemade circuit board. The results show that the system can be used to demodulate low-frequency (10 Hz–200 Hz) vibration signals, and the best signal-to-noise ratio is up to ~70 dB. Moreover, theoretically, the system could be used to demodulate higher frequency vibration and can be adapted to any vibration measurement, and suitable for engineering application due to the characteristics of simple structure, low cost, stable performance, fast response speed and high precision.

Introduction

Seismic exploration technologies are ones of the most important geophysical techniques for oil and gas exploration [1], [2], monitoring and anticipation of geological disasters [3], debris flow [4], and engineering quality inspection [5]. During seismic exploration, a seismic sensor with high performance is needed to detect the seismic signal induced by the artificial seismic source. Usually, the strain of seismic sensors transduced from mechanical motion, such as displacement, velocity, or acceleration [6], [7], is conventionally detected by some mechanical or electrodynamic sensors (e.g. the coil geophone and the Micro Electro Mechanical Systems (MEMS)) [8], which have inherent deficiencies in response band, spatial resolution, resistance to electromagnetic interference, high-temperature, and high-pressure, etc., as well as the use in downhole harsh environments. By contrast, due to the high performances, including sensitivity, lightweight, compact, capability of multiplexing, and immunity to electromagnetic fields, fiber optic sensors are widely used for the seismic exploration [9], [10]. As one of the most attractive technologies for optical fiber sensing, the fiber Bragg grating (FBG)-based sensor can obtain seismic signal parameters by demodulating the wavelength shift induced by perturbation modulation. At present, the application of optical fiber sensors in engineering has made great development[11], [12], [13], [14], however, in the engineering application of fiber optic especially for seismic exploration, the demodulation is critical role and still need to be developed.

Varieties of demodulation methods have been developed, which can be categorized into two groups according to their implementation [15]. The first category is using interferometric scheme, including familiar Mach–Zehnder interferometer (MZI) [16], unbalanced MZI [17], microwave photonic (MWP) filter [18], and the optoelectronic oscillator [19]. Nevertheless, a demodulation system based on the interference mechanism is sensitive to environmental disturbances, such as temperature change or subtle vibration, thus, the system stability would be obviously deteriorated. The second kind, on the other side, demodulates the signal by non-interferometric scheme, for which an optical component is used to convert the wavelength shift of FBG into an intensity variety, usually, depends on edge filter [20], [21], tunable Fabry-Pérot filter [22], matching grating [23] and array waveguide grating (AWG) [24].

The edge filtering demodulation method is proposed by S. M. Melle et al. for the first time [25]. Comparing to other demodulation techniques, the linear filtering method is an all-fiber method, and is suitable for the demodulation of high-speed transient signals due to its non-mechanical sweep in the system. Moreover, it is based on the functional relationship between intensity and the wavelength of the output light, hence, the system shows good linearity in wavelength-to-intensity conversion. Thus, it has been widely applied in various fields owing the advantages of high resolution, high speed, stable performance, low cost, and a simple structure. Usually, it needs optical elements to provide linear edge for the sidebands, such as AWG, Sagnac loop [26], and many grating-based techniques including FBG filter, tilted FBG, chirped fiber grating, and long-period fiber grating (LPFG) [27], to modulate the signal from sensor probe and then recover the signal by demodulation processing [28].

For practical applications in engineering, a stable and robust demodulation system should have merits of simple structure, high performance, low cost, and being able to demodulate signals in real time and accurately for end-user. The edge filtering method is suitable for sensor engineering application, and has great development in some fields presently. For instance, G. Kouroussis et al. reported dynamically monitoring and accurately capturing the parameters of interest for railway traffic monitoring with the cascaded wavelength-multiplexed FBG [29]; C. A. R. Díaz et al. used the optical fiber damaged by the catastrophic fuse effect as an edge filter to build a cost effective fiber Bragg grating interrogator system [30]; S. Alamandala et al. used a long period grating as an edge filter to achieve real-time load measurement for structural health monitoring (SHM) [31], and so on. However, it is rarely seen in literature that the edge filtering method is used in the demodulation for optical fiber seismic wave exploration.

In this paper, we demonstrate the demodulation system based on linear edge filter with LPFG for optical fiber vibration sensor for seismic exploration application. The edge filter with the linear slope of 1.368 dBm/nm and the sideband width of 6.05 nm, and a circuit board homemade base on theoretical analysis and simulation are employed to realize demodulation. The experimental results show that the system can be used to demodulate low-frequency (10 Hz–200 Hz) vibration signals, and the best signal-to-noise ratio (SNR) is up to ~70 dB. However, the system could be applied to higher frequency demodulation and can be adapted to any vibration measurement if there is no limitation of the seismic sensor, thus, it can be applied in engineering of seismic exploration with the characteristics of simple structure, low cost, stable performance, fast response speed and high precision.

Section snippets

Principle of the edge filtering method

As shown in Fig. 1, an edge filter provides an approximately linear sideband of the transmission spectrum, and the sideband width of the filter is the linear edge part of the filter. When the reflection spectrum of FBG passes through the edge filter, the intensity will be changed linearly with the shifting wavelength of the FBG induced by vibration.

Suppose the slope of the linear edge filter is k, the FBG center wavelength is λ0, and R0 is the peak reflectivity. When seismic wave reaches the

Experiment and discussion

During the experiment, a sensor based on a double-semicircle cantilever beam with symmetrical structure is used to detect the vibration signal, as shown in Fig. 7(a). The sensor is made of beryllium bronze, the left end of which is fixed on an aluminum alloy shell for installation on a vibrating table or inserted into the soil, and the right end can be freely vibrated up and down. The FBG is prestressed and then is glued on the central axis of the double-semicircle by two-point packaging

Conclusion

In summary, we have demonstrated a simulation and hardware implementation of demodulation system based on linear edge filtering method for fiber optic seismic sensor. The experimental results show that the system can demodulate the vibration signal well at frequency range from 10 Hz to 200 Hz, which the best signal-to-noise ratio (SNR) is up to ~70 dB. Theoretically, the system could be used to demodulate higher frequency vibration as long as the corresponding seismic sensor is available, and

Funding

This work was supported by National Key Research and Development Plan, Key strategic advanced electronic materials (No. 2017YFB0405502), National Natural Science Foundation of China (Nos. 61735014, 61927812), National Science and Technology Project, Large Oil and Gas Field and Coal Bed Gas Development Project (No. 2017ZX05019-006), and Natural Science Foundation of Shaanxi Province (No. 2017JM6112).

CRediT authorship contribution statement

Rui Zhou: Conceptualization, Methodology, Writing - original draft. Hong Gao: Writing - review & editing. Zhongyao Feng: Software. Xueguang Qiao: Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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