Cemented fibers as a test-bed for distributed acoustic sensing (DAS)

Distributed Acoustic Sensing (DAS) has become very popular for recording seismic waves in recent years as it provides dense spatial sampling along an optical fiber with only one single interrogator unit (IU) needed for thousands of channels. Fibers can be several tens of kilometers long and in some applications so-called dark-fibers can be used, which were deployed for telecommunication purposes, but are currently not in use. This greatly reduces the necessary effort for field deployment.
Most applications rely only on the phase information in the recorded data. Use cases which rely on amplitude are less frequent, though DAS is very attractive in studies in which strain needs to be measured directly. While the IU is calibrated to record ’fiber strain’ or strain-rate, the properties of the cable and its coupling to the rock control the ’strain transfer rate’ and hence how much of ’rock strain’ is represented in the recorded signal. The ’strain transfer rate’ can be significantly smaller than 1, which also goes along with a reduction of signal to noise ratio, as instrumental noise levels do not depend on the coupling.
At Black Forest Observatory (BFO) we cemented eight optical fibers into a 250 m long groove in the concrete floor of the gallery. The fibers are made up of a 9 µm thick core, 116 µm thick cladding, 125 µm thick coating, and a 650 µm thick tight buffer adding up to a total thickness of 0.9 mm. This type of installation shall provide the best achievable coupling of the fiber to the rock.
We use this installation as a test-bed for DAS IUs and show results from a huddle test run in spring 2024 with four types of IUs. The ’strain transfer rate’ becomes close to 1 for all IUs, making ’fiber strain’ almost equal ’rock strain’ as expected. A comparison with data recorded by the Invar-wire strainmeter at BFO and the intercomparison of IUs lets us further characterize the signal quality and coherent and incoherent background noise.