Repeat well logging using earthquake wave amplitudes measured by distributed acoustic sensors

Abstract

Well-based technologies for seismic subsurface monitoring increasingly utilize fiber-optic cables installed in boreholes as distributed acoustic sensing (DAS) systems. A DAS cable allows measuring linear strain of the fiber and can serve as an array of densely spaced seismic receivers. The strain amplitudes recorded by the DAS cable depend on the near-well formation properties (the softer the medium, the larger the strain). Thus, these properties can be estimated by measuring relative variations of the amplitudes of seismic waves propagating along the well. An advantage of such an approach to subsurface characterization and monitoring is that no active seismic source is required. Passive sources such as earthquakes can be utilized. A synthetic data example demonstrates viability of the approach for monitoring of small-scale CO2 injection into an aquifer. Two field DAS data examples based on signal recordings from several distant earthquakes show that the relevant properties of the near-well formation can be estimated with an accuracy of approximately 5%.