This document discusses a case study using distributed acoustic sensing (DAS) to detect small earthquakes in the Bōsō Peninsula in Chiba, Japan. It finds that DAS installed in wells can detect earthquakes as small as magnitude 0.8. Straight fiber optic cables provided clearer signals than helically wound cables. Detectability depended on the direction of the earthquake, with amplitude varying based on crustal velocity structure. Cement quality in the wells also impacted noise levels and detectability. Overall, the study shows the potential for DAS to detect very small seismic events.

1. How Small Can You Detect with
DAS: A Case Study in the Bōsō
Peninsula
Yohanes Nuwara
13 August 2021

2. Outline
● Why detect earthquakes using DAS?
● Field setting of our instrument in Chiba
● Waveform response of DAS
● Event detection
● Spectral content
● Detectability dependence on azimuth

3. Why DAS?
Igel, 2019
Hammond et al, 2019

4. 4
Lead line
~80m
Helical Wounded Cable (HWC）
(2 OF lines are spliced at the well bottom)
Straight OF Cable（STC）
Seismometer
880m well (OF cemented
behind casing) （Sokaku well）
iDAS
Fusion splice
E2000
connector
Surface
Seismometer well
(250m)
250m well
(OF cemented)
Straight cable
Installed in 2016
Room
Wells
HWC
STC
(Miyoshi, 2020)

5. Waveform response of DAS to natural earthquakes

6. What’s different?
● Helically wound cable (HWC)
provides less noisy data but weaker
amplitude response
● Straight cables (both STC and BC)
provides stronger and clearer
response
● BC shows less multiples

7. STC250
HWC250 BC880
P-arrivals

8. STC250
HWC250 BC880
S-arrivals

9. Event detection

10. Near surface noise
Stripe pattern
(poor completion)
Noise-free
(good completion)
Horizontal pattern of
optical noise
Stripes pattern

11. Detectability dependence on azimuth

12. Amplitude dependency on azimuth
● Amplitude dependency on
azimuth (or back azimuth)
mainly depends on the crustal
velocity (and attenuation)
structure
● Epicentre of event 3 is located
at Southern Boso Peninsula,
forming azimuth of 354° (or
back-azimuth 174°)
BOSO
ICHIHARA
KUJUKURI

13. Conclusions
● Helically wound cable (HWC) provides less noisy data but weaker amplitude
response
● Straight cables (both STC and BC) provides stronger and clearer response
● Significantly affected by cement integrity (poor completion creates a kind of
“stripes pattern” noise)
● DAS is still able to detect earthquakes down to 0.8
● Detectability of DAS depends on azimuth (not merely epi-distance,
magnitude, etc.)

14. Thank you
e-mail: ign.nuwara97@gmail.com
yohanesnuwara