We demonstrated a down-hole seismic survey system that can be applied in three dimensions vertical seismic profile (VSP) detection in petroleum exploration. The results of experiments show that the system has a dynamic measurement range of 80db (ratio of maximum to minimum value) and the total delay for signal collection, process and communication is less than 200ms @ 2k bit sample rates. An array consisting of six fiber-optic accelerometers (receivers) is applied in this system. Each receiver is comprised of three fiber-optic Michelson interferometers. In order to meet the requirements of high precision and real-time measurement, the high-speed DSP chips are employed to realize the algorithms of signal filters and Phase Generated Carrier (PGC) demodulation to obtain the seismic information. Multi-ARM CPUs are introduced into the system to design the fiber-optic accelerometer array controller and the receiver array local bus that are used for real-time data communication between the multi-level receivers and controller. The system interface for traditional ELIS Down-hole Instrument Bus (EDIB) is designed by the use of FPGA so that our system can attach to EDIB and cooperate with other instruments. The design and experiments of the system are given in this paper in detail.
In this paper, the long period grating (LPG) with a metal outer clad is analyzed. The transfer constant of core mode and cladding mode were derived from numerical method. The complex refractive index of the metal outer clad make the transfer constant to be a complex number. The imaginary part of the transfer constant is representing the loss during the wave's transmission. Theoretical analysis shows that the core mode's loss caused by metal clad is can be ignored when the radius of the clad is 62.5um. The complex refractive index make larger resonance wavelength shift than the refractive index without imaginary part.