1. A human-portable self-standing locator system for locating a buried object characterized by an electromagnetic (EM) field emission, the system comprising: at least three foldable support elements adapted to support the locator system on a substantially horizontal surface; within at least one of the foldable support elements, an EM sensor array; a gradient processing circuit coupled to the EM sensor array for producing a signal representing the B-field gradient at the EM sensor array as a function of time within at least one frequency band; a locator processor coupled to the gradient processing circuit for producing signals representing target locations responsive to signals accepted from the gradient processing circuit; an accessory mounting interface coupled to the locator processor and disposed to accept temporary attachment of an accessory sensor system; a user interface coupled to the locator processor for accepting user commands and for indicating the buried object location relative to the locator system.