## 2 Setup and Methods  
DAS traces were recorded using a commercial laser with a wavelength of 1552.5 nm and a homemade radiation source based on a telecom laser with a wavelength of 1548.5 nm. The experimental setup previously used in [33] is shown in Figure 1a. The homemade source bandwidth was significantly reduced, reaching a value of about 6 kHz, using an external fiber resonator and the effect of its frequency capturing by the laser. This mode operation was achieved with two polarization-maintaining fiber splitters and a circulator. The splitters formed an annular fiber filter, and the circulator closed the laser output to itself through a 3.75 m long fiber-optic resonator. The bandwidth of the commercial DAS laser--Kohereas Adjustik, NKT Photonics--was 100 Hz. A sensing element made of 4 km SMF-28 optical fiber was interrogated by rectangular pulses with a power of about 100 mW and 100 ns duration, formed by an acousto-optic modulator, EDFA, 2 GHz bandpass filter, and each of the investigated radiation sources in turn. The sensing element impacts were performed utilizing a shaker through 1.5 m of a plastic tube at a distance of approximately 3400 m, with a frequency of 500 Hz and a piezoelectric transducer (PZT) at a distance of about 1800 m, with the frequency of 8000 Hz. The studied optical fiber was divided into three spools (Figure 1b). The fiber on the Spool 1, as well as its separate fragment, which was placed in a plastic tube 1.5 m long, were located in the acoustically isolated room 1. The PEACO-GDK125 DC motor (Peaco Support, Ltd., Shenzhen, China) was also located there. A piston mechanism was attached to the shaft of its rotor, driving a small hammer. The hammer head moved reciprocally, perpendicular to the axis of symmetry of the plastic tube and the optical fiber located in it. At the highest point of the movement, the hammer struck the plastic tube. The fundamental tone of the hammer strokes had a frequency of 500 Hz, while the noise of the motor had a much wider range, also due to frequencies above the fundamental tone. These sound waves affected all objects in the isolated room 1, including Spool 1. The remaining spools were placed in the isolated room 2. The fiber output of Spool 2 was fusion-spliced to the fiber tip of Spool 3 using a Fujikura 80S arc welder. The first 40 m of the Spool 3 fiber was wound onto a piezoelectric ceramic transducer (PZT) producing an 8 kHz frequency vibration (Spool 4). Thus, two events should be distinguishable on the trace: a pointwise event (in isolated room 2) and a combined one--a pointwise event together with a distributed one (in isolated room 1). The fusion splice location is also viewed as a pointwise event. At the same time, practice has shown that connections between fibers of almost any type show phantom peaks at frequencies that, in fact, are not present in the signal. A set of such frequencies can be seen in the middle of a trace or heatmap. The acoustic vibrations emitted in these two isolated rooms do not mix, which is also visible on heat maps. The variation in the intensity of vibrations along the length of the fiber on Spool 1 can be explained by the fact that one side of the spool is closer to the sound source. It is rather difficult to reliably determine the period of these variations from the heat map, since the line was probed with a pulse whose duration was much greater than the circumference of the spool neck. As expected, the commercial DAS laser provided the higher SNR (Figure 2, Table 1). The DAQ card sampling rate was 200 MHz. Thus, one period between the pulses was  
Figure 1: (**a**) The DAS traces’ acquisition setup, applying the commercial and the homemade laser. AWG—arbitrary waveform generator; DFB—distributed feedback; DAQ—data acquisition system; MOS—manual optical switch. (**b**) The schematics of the experiment and fiber optic sensing element. (**c**) The phenomena that took place and were detected. sampled by 8000 points, ensuring that about 25,000 DAS traces could be recorded each second.