Optical Fiber Vertical Seismic Profile using DAS (Distributed Acoustic Sensing) Technology

Abstract

Distributed optical fiber sensing technologies have been evolved over more than 30 years starting with DTS (Distributed Temperature Sensing). DTS is now commonly used for well monitoring in the oil and gas business. The subsequently developed DAS (Distributed Acoustic Sensing) technology was introduced more than 5 years ago to meet the demands of pipeline monitoring and intrusion detection. The latest optical fiber sensing technology now allows DAS to record borehole seismic signals including VSPs (Vertical Seismic Profiles). This system is called hDVS (heterodyne Distributed Vibration Sensing) to distinguish it from the pipeline monitoring system. Unlike conventional VSP recording tools, which usually use electromagnetic accelerometers or geophones, hDVS/DAS uses optical fiber as a vibration sensor. Because optical fiber can be deployed along the entire well depth as a sensor, either permanently (e.g., using control line) or temporarily (hybrid logging cable), the acquisition time required for hDVS/VSP can be as short as a few minutes, which is essentially the time required for firing of the seismic source, compared with several hours to days for conventional VSPs, including the time required for multiple tool settings. The core part of the optical fiber is made of high-silica glass, so the high-temperature version of optical fiber is widely available rated to more than 200 °C and even above 500 °C in specialized cases. Hence, an optical fiber sensor can be deployed on a permanent basis in high-temperature environments, where conventional geophones cannot be used. This is an absolutely new way of acquiring borehole seismic data by using fiber-optic technology.