Measuring rock strain with distributed acoustic sensing (DAS) and cemented patch cables

Summary A rigid connection between the optical fiber and the rock allows to properly measure ‘rock strain’ with Distributed Acoustic Sensing (DAS). We demonstrate this by running four interrogator units (IU) on a testbed with single-fiber patch cables being cemented into a groove in the concrete floor of Black Forest Observatory (BFO). The recorded signals are compared with the recordings of a calibrated Invar wire strain meter array that has been continuously in operation for the last decades at a distance of about 350 m to the testbed. In this way we compare the ‘fiber strain’ against ‘rock strain’ at frequencies below 0.2 Hz. We investigate this on the background of a previous study with DAS cables unreeled on the floor and loaded down by sand and sandbags, where we found a significant loss of strain amplitude between rock and fiber. Waveform similarity for strong earthquake signals is high with typical values of the normalized correlation coefficient greater than 0.95. The strain amplitude measured by all four IUs is very consistent and by about 10 per cent larger than the strain measured by the strainmeters. We attribute this minor discrepancy to spatially varying strain-strain coupling caused by the local topography. At frequencies up to 14 Hz we make an intercomparison of IUs, showing no significant variation with frequency. The scatter of strain amplitude in between channels which are spatially near to each other in the same fiber route is about ±10 per cent in most cases. The variation of median values in between different IUs and earthquakes is less than 5 per cent. By subtracting the common mode noise, which is coherent along the fiber route, we lower the background signal level to an rms-amplitude of 100 pstrain at 0.1 Hz and 5 pstrain at 1 Hz in a bandwidth of 1/6 decade for the best cases. This allows the detection of the marine microseisms during times of moderate amplitude level.