Detecting microseismicity using distributed vibration
Distributed acoustic sensing (DAS) and, more generally, distributed vibration sensing (DVS) offer new approaches to the detection of seismic waves, based on interrogation of a fibre-optic cable. We investigated the detection of microseismic events using a heterodyne distributed vibration sensor (hDVS), comparing to a pair of borehole geophone arrays co-located in the monitoring well. The aim was to establish the current performance level of hDVS for microseismic monitoring and to understand any differences between geophone and hDVS responses. We compared the microseismic events detected simultaneously on both systems. Under the assumption that hDVS measures strain in the fibre, the comparison to geophone particle velocity was made in the strain-rate domain. We supported these observations with modelling work. We found the hDVS measurements directly comparable to the geophones, indicating a potentially true amplitude response from hDVS. An anomaly in the hDVS frequency response was a function of apparent phase velocity. Modelling verified the observed effect as consistent with the filtering effect of the light pulse used in fibre interrogation. We find that hDVS detects microseismicity but is not yet a replacement for the borehole array. However, improved microseismic response might be achieved with shorter pulse-length systems in the next generation of fibre interrogators. The heterodyne distributed vibration sensor (hDVS), together with other forms of distributed acoustic sensing (DAS), have been reported as observation techniques for microseismic monitoring (Webster et al., 2013; Li et al., 2015; Molteni et al., 2016). We present further results from the study described by Molteni et al. (2016) and extend the discussion on the differences between observation with a borehole array of geophones and observation on a laser-interrogated fibre-optic cable that is cemented behind casing.