Cemented fibers as a testbed for distributed acoustic sensing (DAS)

Abstract

A rigid connection between the optical fiber and the rock makes amplitudes of 'fiber strain' measured with Distributed Acoustic Sensing (DAS) equal to 'rock strain'. We demonstrate this by running four interrogator units (IU) on a DAS 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. This way we measure 'strain transfer rate' (ratio of 'fiber strain' over 'rock strain') at frequencies below 0.2 Hz. Waveform similarity for strong earthquake signals is high with typical values of the normalized correlation coefficient greater than 0.95. The 'strain transfer rate' is close to 1 for all four IUs, while it was significantly less in a previous study with DAS cables unreeled on the floor and loaded down by sand and sandbags, only. At frequencies up to 14 Hz we make an intercomparison of IUs, showing no significant variation with frequency. The scatter of 'strain transfer rate' in between channels which are spatially near to each other in the same fiber route is about $\pm$10 % in most cases. The variation of median values in between different IUs and earthquakes is less than 5 %. By subtracting the common mode laser 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.

Figures (14)

• Figure 1: Floor map of the Black Forest Observatory (BFO). The part of the gallery used for the current experiment is shown. The entrance is at the western end of the tunnel. The overburden increases to the east and is about 170m at the strain meter array, which is used as a reference instrument in the current study. The eastern part hosts the majority of observatory instruments and is additionally protected by two air-locks. They are discussed by zuern2007arichter1995 but have been substantially improved since then to act as a low-pass with a corner period of about 2d. The DAS installation and the testbed with the cemented fibers in particular is located in the western part of the gallery. Details are shown in Fig. \ref{['fig:floor:map:test:bed']}.
• Figure 2: A 10mm wide and 1020 deep groove has been cut into the concrete floor of the front part of the gallery (see Fig. \ref{['fig:full:floor:map']}). Eight single-fiber patch-cables, each of them being 0.9mm thick (see Fig. \ref{['fig:fiber:layers']}), run in parallel in the groove. The groove has been filled with non-shrinking cement. The right-most picture shows a cross-section of a preliminary test-installation of 10 cables.
• Figure 3: The structure of the single-fiber cable cemented into the groove. The cables are 0.9mm thick. Eight of them run in parallel in the cemented groove as shown in Fig. \ref{['fig:pictures:testbed']}. The tight buffer provides some protection against mechanical and chemical damage in the groove while still providing a tight strain coupling to the rock.
• Figure 4: Detail of the floor map (Fig. \ref{['fig:full:floor:map']}) focusing on the DAS testbed. The eight red lines schematically indicate the path of the cemented fibers (see Fig. \ref{['fig:pictures:testbed']}). SPP1 to SPP8 are marked points within the gallery, to which the channel offsets are aligned in tap tests. The reference coils are decoupled from the rock, are located in a niche and protected against dripping water, but are not additionally shielded. Due to space constraints only two IUs (A1-R and OptoDAS) could be installed in the electronics vault. The other two units (QuantX and Treble+) were installed in the laboratory building. We analyze signals recorded in the straight section of the 'Vorstollen'.
• Figure 5: Analysis of the background signal on 2024-03-26 12 for recordings of the QuantX: rms amplitude in 1/6 decade forbrigerPSILN2023 and magnitude-squared coherence carter1973 with respect to the BFO strainmeter data (right). The corners of the applied bandpass are at 20mHz and 15Hz and thus lie outside the displayed range. We compare signals from six channels (indicated by their offsets) to the 'rock strain' in N90°E as obtained from the BFO strainmeter array (purple curve). The curve for the latter is truncated at 0.5Hz, because of the parasitic sensitivity to vertical ground motion at higher frequencies. The dashed lines in the left diagram show the signal level of the raw DAS data. The solid lines are the levels after subtracting the average of the signal on the reference coil in the time domain. Signal amplitudes are not further scaled to correct for 'strain transfer rate' (factor: 1.0). They follow the curvature of the purple line ('rock strain') at 0.140.5. The signal amplitude of the marine microseisms is at a rather low level of 0.1n in this example. Nevertheless the signal is detected by the DAS, which is also confirmed by the coherence being raised to 0.8 in the microseism frequency band.