Also called RF detectors and depth multipliers.  Invented in the 1930's by Gerhard Fisher (founder of Fisher Research Labs, now part of the First Texas conglomerate), this type of detector seems to not have changed much in operating characteristics in 80+ years.  From what I've found there are still three competing models in production:  Fisher's current Gemini-3 model (now 30 years old) https://www.detectorprospector.com/metal-detector-database/fisher-gemini-3-r27/, Garrett's Treasure Hound Eag