Fourth-generation models add hydrographic monitoring capability to complement the acoustic sensor suite, and are designed to carry out measurement profiles to depths exceeding 4,000 m. MERMAID is a freely-drifting float equipped with a hydrophone to collect hydroacoustic data for the study of earthquakes worldwide.

A pressure sensor monitors descent depth, and a GPS receiver provides location and time corrections during the brief intervals that MERMAID surfaces (on average less than one hour).

Fourth-generation models are multidisciplinary and carry a conductivity-temperature-depth sensor to collect hydrographic profiles of ocean temperature and salinity (similar to those from the Argo program) during their voyages.

MERMAIDs first-generation model (2003-2005) retired after gathering about 120 hours of acoustic pressure data from a depth of around 700 m offshore from La Jolla, California.

[16] Sixty-seven third-generation MERMAIDs (2018-now) were launched in the Pacific Ocean, the South China Sea, and the Mediterranean, from a variety of international (French, Japanese, Chinese) research vessels.

Data segments triggering transmission mostly contain pressure-wave signals from particular earthquakes worldwide,[18] but also noise generated by a variety of sources (e.g. microseisms or volcanic eruptions[19][20]).

[24] Seismic waveforms are released to the public through the IRIS Data Management Center, after a rolling embargo period of typically two years.