Active-Source, Near-Surface, Surface-Wave Measurements using Distributed Acoustic Sensing (DAS) and Traditional Geophones

The data archived in this experiment compares distributed acoustic sensing (DAS) with traditional geophone sensors for acquiring active-source, near-surface, surface-wave measurements. The experiment involved the deployment of a 94-m geophone array (48 receivers at 2-m spacing) adjacent to two 200-meter long buried fiber-optic cables. The geophones were from Geospace Technologies (GS-11D) with a resonant frequency of 4.5 Hz. The geophones were mounted in PC21 land cases and coupled to the ground surface with 7.6-cm aluminum spikes. The geophones were oriented in the horizontal in-line direction. Outputs from the geophones were recorded on two interconnected Geodes from Geometrics. Geodes were configured with a 24 decibel gain. The two fiber-optic cables were deployed in the same shallow trench at the site; one from NanZee Sensing Technology (NZS-DSS-C02) and the other from AFL (X3004955180H-RD). The two cables were joined together at the far end of the array by splicing the NanZee and AFL cables to one another, allowing data acquisition to occur on both cables at the same time. On the near-side of the array, the NanZee fiber was connected to the interrogator unit, an OptaSense ODH4, and the AFL fiber was appropriately terminated to reduce end-reflections. The ODH4 was configured by a team from OptaSense to ensure high-quality data acquisition. The ODH4 utilized a 2.04 m gauge length and a 1.02 m channel separation. The geophone array and DAS fiber-optic cables were used to simultaneously record actively-generated near-surface signals (primarily surface waves) from different sources, including highly-controlled vibroseis shaker trucks (Thumper and T-Rex from the NHERI@UTexas experimental facility) and more-variable impact sources (5.4 kg instrumented sledgehammer from PCB Piezotronics). Vibroseis chirps and sledgehammer impacts were performed 5, 10, 20, and 40 m away from the first geophone position. The sources included T-Rex shaking vertically (trex_p), T-Rex shaking in-line (trex_sl), Thumper shaking vertically (thumper_p), and the 5.4 kg sledgehammer striking vertically (hammer_p). T-Rex was used to produce a 12 second chirp with frequencies swept linearly from 3 to 80 Hz. Thumper, was used to produce a 12 second chirp in the vertical direction with frequencies swept up linearly from 5 to 200 Hz. The frequency content produced by the sledgehammer is broadband, but highly variable and is dependent on the operator, the strike plate used, and the material being tested. For the vibroseis sources, three chirps were performed at each source location, whereas for the sledgehammer source five impacts were performed. The data in this project includes the raw seismic waveforms from the DAS and geophone arrays in Seismic Unix (.su) and comma-separated values (.csv) formats. The DAS and geophone recordings are provided in terms of their raw, uncorrected amplitudes as produced by the OHD4 and Geode acquisition systems, respectively.

The experimental dispersion data extracted from the DAS and geophone measurements archived in this experiment have been shown (Vantassel et al., 2022) to be in excellent agreement, demonstrating that when appropriate considerations are made (i.e., proper cable selection, good cable-soil coupling, and sufficiently short gauge length) DAS can be used to measure surface wave dispersion data that is of equal quality to that acquired using geophones. More details about the experimental setup and post-processing are provided in Vantassel et al. (2022). The authors hope by making this data public to allow others to replicate their work and to encourage further study of the use of DAS for near-surface engineering site characterization.