In this paper we report the development and field test of a high sensitivity fiber Bragg grating (FBG) geophone for geophysical imaging and monitoring application. A high sensitivity FBG geophone is designed, and its sensitivity is about 1000pm/g. The wavelength change of the FBG geophone is interrogated by using interferometric demodulation method, and the demodulation system noise is below 10<sup>-3</sup> pm/ √Hz . And the minimum detectable seismic signal is below 1μg/ √Hz . We are presenting field test results for the FBG geophone and comparing its performance with regular exploration geophones. In comparison, FBG geophone has the advantages of higher signal-to-noise ratio and better low-frequency response. This work shows that using FBG technology to develop geophone for oil and gas exploration is both advantageous and feasible.
Ground surveillance system is a kind of intelligent monitoring equipment for detecting and tracking the ground target. This paper presents a fiber Bragg grating (FBG) acceleration sensor for ground surveillance, which has the characteristics of no power supply, anti-electromagnetic interference, easy large-scale networking, and small size. Which make it able to achieve the advantage of the ground surveillance system while avoiding the shortcoming of the electric sensing. The sensor has a double cantilever beam structure with a sensitivity of 1000 pm/g. Field experiment has been carried out on a flood beach to examine the sensor performance. The result shows that the detection distance on the walking of personnel reaches 70m, and the detection distance on the ordinary motor vehicle reaches 200m. The performance of the FBG sensor can satisfy the actual needs of the ground surveillance system.
A fiber Bragg grating (FBG) based geophone is designed for low-frequency signal detection has high acceleration response of about 60 dB re pm/g in a low frequency range of 5 Hz ~60 Hz. To Guarantee normal operation in field test and practical application, an acceleration amplitude restriction is added in the mechanical design of the FBG geophone. Then a series of environmental and reliability test have been proceeded with online or offline monitoring of its working performance, including high and low temperature test, vibration test, shock test and free drop test. All the tests are planned according to National standard or Oil & Gas Industry Standard. And the experimental results indicate that our FBG geophone meet the criterion of oil and gas industry product and is capable of field application.
A two level 3-component distributed feed-back (DFB) fiber laser micro-seismic geophone array based on wavelength/space division multiplexing is developed. High resolution dynamic wavelength demodulation was realized with a coherent detection technology. The geophone array was tested in laboratory and showed that the detection capability of the weak vibration signals between 5-500 Hz was better than conventional moving-coil geophone. A cross-well test of the array was performed in a 100 m depth well in Changqing Oil Field in northwest China. The two level 3-component fiber laser micro-seismic geophone array was compared with the traditional in-well seismic geophone and showed better signal noise ratio (SNR) in the cross-well seismic signal acquisition. This 3C fiber laser micro-seismic geophone array system is promising in the cross-well seismic monitoring.
This paper describes a dense wavelength-division multiplexing scheme with low crosstalk for fiber Bragg grating (FBG) sensors based on interferometric detection. The scheme uses a 100-GHz spacing dense wavelength-division multiplexer with two-level optical filters to separate the returned wavelengths from FBG sensor array, enabling a high-resolution interferometric dynamic wavelength demodulation for each sensor with narrow spacing. An array with three gratings was demonstrated, and a dynamic wavelength resolution of ∼7×10−4 pm/√Hz was tested, with a bandwidth of 1 Hz to 1 kHz for each sensor. The FBGs have an operating range approaching 0.6 nm with a crosstalk level below −80 dB. The multiplexing scheme demonstrates the potential to address up to 50 FBG sensors through a single fiber with a 40-nm bandwidth optical source.