Estimation of helical fiber pitch angle and trace spacing from colocated DAS, accelerometer, and geophone datasets

Abstract

Distributed acoustic sensing (DAS) data recorded on a fiber loop containing straight and helically wound fiber cables with different indices of refraction poses challenges for determining the trace spacing to use for geometry assignment and dataset registration. Assuming the actual helical pitch angle may be different than the nominal pitch angle, we propose a method to estimate pitch angle and helical trace spacing by cross-correlation with co-located datasets. For a cable with a nominal pitch angle of 30.0 degrees (520 traces per 300 m), cross-correlation with straight fiber data gives estimated pitch angles of 29.6 degrees for helical fiber in a vertical well (507 traces per 300 m) and 28.5 degrees for the same helical fiber in a horizontal trench (502 traces per 300 m). Substituting accelerometer and geophone datasets for straight fiber data does not yield valid estimates of pitch angle, but the estimated trace spacings for helical fiber data are within 3 cm (or less) of those obtained from the straight fiber data.