Novel cable coupling technique for improved shallow distributed acoustic sensor VSPs Jonathan D. Munn a, ⁎, Thomas I. Coleman a,b , Beth L. Parker a , Michael J. Mondanos b , Athena Chalari b a G360 – Centre for Applied Groundwater Research, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada b Silixa Ltd., 230 Centennial Park, Centennial Avenue, Elstree, Hertfordshire WD6 3SN, UK abstract article info Article history: Received 16 February 2016 Received in revised form 12 December 2016 Accepted 6 January 2017 Available online 07 January 2017 Vertical seismic profiles (VSPs) collected using fiber optic distributed acoustic sensors (DAS) are becoming in- creasingly common; yet, ensuring good cable coupling with the borehole wall remains a persistent challenge. Traditional cable deployment techniques used in the petroleum industry are either not possible or do not provide data of sufficient quality for shallow applications. Additionally, no direct field comparison of coupling techniques in the same borehole exists to determine the impacts of poor coupling on DAS VSP data quality. This paper ad- dresses these issues by: (1) presenting a novel cable coupling solution using a removable and relatively inexpen- sive FLUTe™ flexible borehole liner; and (2) presenting field examples of DAS VSPs under different coupling conditions. The proposed coupling technique is analogous to a fully cemented deployment in that the cable is continuously coupled directly to the formation. Field experiments conducted to assess and validate the technique demonstrate a marked improvement in VSP data quality when the cable is coupled with a flexible borehole liner. Without the liner, seismic profiles are dominated by a high-amplitude cable wave and the p-wave arrival is not observed; however, with cable coupling provided by a borehole liner inflated using hydrostatic pressure, the cable wave is suppressed and clear p-wave arrivals are visible. Additional tests examining the influence of fiber optic cable structure on seismic responses demonstrate that tight buffered fibers are more sensitive to dynamic strain than loose tube fibers making them potentially better suited for certain DAS applications. © 2017 Elsevier B.V. All rights reserved. Keywords: Distributed acoustic sensing Cable coupling VSP Seismic Fiber optic Flexible borehole liner 1. Introduction Fiber optic distributed acoustic sensors (DAS) measure the acoustic energy along the full length of an optical fiber, making them versatile tools for a wide range of applications. Borehole deployments using DAS are becoming increasingly common; yet, inherent challenges with coupling the cable to the borehole wall persist. DAS is based on optical time-domain reflectometry where incident pulses of light are sent down a standard single-mode optical fiber and a small amount of light is continuously backscattered towards the source due to the fiber impurities (Rayleigh scattering). Acoustic waves impart localized strain on the optical fiber and alter the optical path, which creates interference effects in the backscattered light. The DAS can continuously analyze these interference effects and relate them to the local dynamic strain along the fiber. Since the speed of light is well known, the location of backscatter can be calculated from the 2-way travel time through the fiber. Sampling at acoustic frequencies for each location of backscatter allows seismic waveforms to be resolved. In practice, these principles enable DAS to be utilized as a continuous array of geophones or hydrophones for recording seismic data such as a vertical seismic profile (VSP). VSPs allow the one-way travel time of seismic waves through geological media to be constrained which, when integrated with borehole geophysics, lithological logs, and surface seismic data provide insights into the extent of lithological contacts and structures such as fracture zones and faults away from boreholes. The first use of DAS for a VSP was presented by Mestayer et al. (2011), and subsequent field studies have demonstrated successful applications of the approach (e.g. Barberan et al., 2012; Cox et al., 2012; Madsen et al., 2012; Miller et al., 2012; Daley et al., 2013; Hartog et al., 2014; Mateeva et al., 2014; Harris et al., 2016). DAS has several advantages over geophones in borehole seismic surveys which have been outlined in recent literature (Mateeva et al., 2014; Li et al., 2015). Of these, one primary advantage is the ability to sample all intervals along the optical fiber simultaneously, allowing data to be collected along the full length of the borehole at once rather than incrementally moving a finite number of geophones on a string up and down the borehole to obtain full coverage. Additionally, fiber optic cables can be deployed in horizontal or slim boreholes, and can in- clude additional fibers for other distributed sensors such as a distributed Journal of Applied Geophysics 138 (2017) 72–79 ⁎ Corresponding author: G360 – Centre for Applied Groundwater Research, University of Guelph, 50 Stone Road E, Guelph, ON N1G 2W1, Canada. E-mail address: jmunn@uoguelph.ca (J.D. Munn). http://dx.doi.org/10.1016/j.jappgeo.2017.01.007 0926-9851/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo