Planting a hydrophone anchor with the cable attached.
On the GUIDE.
Hydrophone would pick up sound from RAR TNT explosion.
Image ID: theb2243, NOAA's Historic Coast & Geodetic Survey (C&GS) Collection
Location: Oregon, Cape Blanco area
Photo Date: 1924
Credit: C&GS Season's Report Luce 1924-49
Category: Charting/Navigation/Electronic/Radio-Acoustic Ranging/
• High Resolution Photo Available