The fiber optic sensors have grown to a promising technology in the application of oil and gas prospecting. Our group
has embarked on the experimental design of fiber optic seismic geophone based on fiber-Bragg-grating (FBG)
technology recently. This paper provides the detailed investigation and analysis of the sensor field test result in the
seismic reflection survey of the oilfield exploration as a continuing effort to our previous research. Several revisions of
previous sensor head design have been implemented which include the carbon fiber composite material based cantilever,
auxiliary beam mechanical design, and the moving coil electro-magnetic damping design for an improved seismic signal
sensitivity in the field test. The field tests of the redesigned sensing system show that: (1) the FBG seismic geophone has
a higher sensitivity than the commercial geophone between 10-70 Hz and equal sensitivity to commercial geophone
between 80-140 Hz (2) compared to the conventional geophone, the FBG seismic geophone is significantly immune
from the electro-magnetic interference (3) the FBG geophone has a less nonlinear distortion than the conventional
geophone and it is totally compatible with various commercial seismic recorders.
Traditionally, the broadband amplified spontaneous emission (ASE) source is considered to be used as
the light source for the fiber Bragg grating (FBG) sensing technology. However, this kind of light source has some
disadvantages − the huge volume and the high power consumption. These shortages will hamper the development of
FBG sensing technology in some kind of applications such as unattended sensor and space environment. In this
paper, the authors will present a new approach, the usage of the light emitted diode (LED) as the light source. The
LED source is very compact, easy to integrate, and significantly reduced the cost and power consumption. But the
light density of LED is so weak that the useful signal is almost buried by noise. A solution proposed by our group is
to enhance SNR by digital signal processing (DSP) technology, including high frequency modulation, phase-lock
amplifier, encoding on LED and decoding on the synchronistic detection. The experimental results show our effort
could significantly increases the signal noise ratio (SNR) and make FBG sensor practical on application.
KEYWORDS: Fiber Bragg gratings, Sensors, Head, Fiber optics sensors, Seismic sensors, Mechanical engineering, Signal detection, Analog electronics, Photodetectors, Signal to noise ratio
In this paper we will demonstrate our fiber Bragg grating (FBG) accelerometer system in seismic wave detection applications. Optical fiber sensors using fiber Bragg grating have a number of advantages such as immune to electromagnetic interference, lightweight, and low power consumption. Most important, the FBG sensor has high sensitivity to dynamic strain signals and the strain sensitivity can approach sub micro-strain. The basic principle of the FBG seismic sensing system is that it transforms the acceleration of ground motion into the strain signal of the FBG sensor through mechanical design, and after the optical demodulation generates the analog voltage output proportional to the strain changes. The customized FBG seismic sensor prototype is described, which includes the electro-optical design, mechanical design and the hardware and software interface of the sensor system. The laboratory evaluation of the system is performed systematically on a commercial vibration stage. Studies of the sensor properties show that the sensor has a high sensitivity (2500 mV/g at 90 Hz) to the acceleration signal, a large dynamic range (80 dB), the good linearity and stability after device integration and packaging.
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