Seismic instruments: article

Аnnotation: Distributed acoustic or vibration measurements on a fiber-optic line (FOCL) began to develop rapidly in 2005. One of the effects used for this purpose is the Rayleigh scattering effect. Rayleigh scattering is an elastic process caused by local inhomogeneities of the refractive index of a fiber-optic line. Optical pulses with a certain time interval are launched into the optical fiber and a small part of the backscattered light is registered on the detector. Small deformations of the optical fiber associated with vibration (or seismic) events change the intensity of backscattered light. Analysis of the interference pattern for each section of the fiber allows you to identify the characteristics of the vibration effect on a specific section of the fiber. A key feature of the phase-sensitive reflectometer used in the experiment is the use of a two-pulse phase-modulated optical parcel as a probing signal. An experiment was conducted to compare the records of seismic excitations recorded on standard seismic equipment and on the records of a reflectometer. The experiment was carried out on a straight-line single-mode fiber-optic line 200 m buried in the ground to a depth of 30 cm. A 5 kg hammer was used as a source of seismic vibrations. The blows were delivered on a metal substrate lying on the ground. Time synchronization of the seismic data with the reflectometer data was performed based on the detected first shock intakes. The experience of simultaneous recording of low-power seismic excitations has shown that direct comparison of the response of a fiber-optic system and geophones is impossible. The registered data must be converted to the same physical values. In a specific experiment, the acceleration values measured by the seismic system were converted into displacements. Analysis of the transformed data and the reflectometer data showed that for small deformations of less than one strain, the optical channels show good signal repeatability. Optical channel data obtained even with large deformations show that they can be used for engineering seismic purposes and allow us to obtain reliable seismic sections, despite some discrepancies in the signal form. Due to the density of channels, this disadvantage can be compensated for.