XBTs Provide First‐Order Characterization of Seabed Physical Properties

Expendable Bathythermographs (XBTs) are oceanographic instruments that fall through the ocean's water column and measure ocean temperature with depth. In many instances, however, XBTs continue to record temperature after they impact the seabed. Here we show evidence that XBTs produce unique temperature responses when they impact the seabed that depend directly on seabed physical properties. Specifically, standard‐use XBTs (e.g., T‐4s and T‐5s), when deployed above a mud‐rich seabed, require significant time (tens of minutes) to equilibrate to steady‐state seafloor temperatures after seabed impact. In contrast, XBTs deployed above sand‐rich sediments equilibrate to seabed temperatures rapidly (644k XBT measurements exist publicly (via the National Oceanographic and Atmospheric Administration website), and >74,000 XBTs record temperatures post seabed impact, we suggest that XBT data represents a large, low‐cost, and currently untapped data set for characterizing seabed physical properties globally. Plain Language Summary Expendable Bathythermographs (XBTs) are hand‐deployable torpedo‐shaped single‐use thermistors that measure ocean temperature with depth by being dropped from a ship and transmitting temperature values along a thin copper wire that unspools from the XBT as it falls. When the XBT impacts the seabed, it often continues recording temperature, however, researchers focused on ocean temperature‐depth measurements typically ignore these data. Here, we take a first‐look at XBT temperature response after seabed impact for two different seabed settings: (a) a soft muddy seabed where XBTs may penetrate the seafloor, and (b) a hard, sandy seabed where the XBT likely fails to penetrate. We find that XBTs that impact a soft, muddy seabed show significantly slower rates of change in temperature after impact compared to XBTs that impact hard sandy sediment, consistent with numer