Optical Vertical Seismic Profile on Wireline Cable | Schlumberger
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Optical Vertical Seismic Profile on Wireline Cable


Vertical seismic profiles (VSPs) are routinely acquired by deploying downhole seismic sensors in a borehole with wireline logging cable and then triggering a seismic source. Rig time has always been a limiting factor in VSP acquisition. Although multilevel receivers were developed to speed-up acquisition and are now the standard practice, simple operations such a zero-offset VSP remain the norm. The value of complex operations such as multi-offset or walkaway VSPs is often questioned because of long acquisition time.

Distributed vibration sensing (DVS), also known sometimes as distributed acoustic sensing (DAS), on optical fiber is proposed to speed-up VSP acquisition. An optical interrogator unit is deployed at surface and connected to the downhole fiber. DVS is a coherent-light version of the well-known optical time-domain reflectometry (OTDR) technique. In essence, pulses of narrowband laser light are launched into the fiber and the Rayleigh backscattered light is analyzed on its return to the interrogator and provides a local measure of the dynamic strain of the fiber, measured every few meters along a cable. Successful vertical seismic optical profiles have been published for deployments with fiber either cemented behind casing or strapped to the outside of completion tubing.

A prototype for an optical telemetry system that was based on a heptacable (a steel-armored wireline cable containing seven conductors) incorporating single-mode optical fiber was developed in the mid-2000s. The cable is otherwise fully compatible with electrical wireline cable. In recent experiments, this hybrid heptacable was deployed to record several VSPs. Both impulsive and vibrator seismic sources have been successfully tested, and their results compared with conventional VSP acquisition. Data quality is sufficient to meet most of the operator’s needs, and acquisition time is a fraction of the conventional operation.

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