numerous magnetic field sensors are available to the Navy with differing performance and cost. Until their utility is better understood, it makes little sense to devise new ones.

Active source electromagnetic sensing from aircraft is a primary tool in mineral prospecting, and has recently been adapted to bathymetric charting by the Naval Research Laboratory (formerly NOARL), with impressive results. However, only limited attention has been paid to the use of airborne active electromagnetic methods for direct submarine detection, despite the fact that its performance in shallow water is potentially good enough for it to be a practical tool. Further model investigations and some trial experiments should be carried out.

Animal electroreception, especially by sharks, offers much higher sensitivity to oceanic electric fields than can be devised electronically. The physiology of animal electroreception has been only partially explored, and a better understanding could result in the future development of compact and highly sensitive detectors.

Although there is little evidence for strong nonlinear constitutive relations in earth materials, if they were present, detection of anthropomorphic electromagnetic signals modulated by the natural background could be simplified. Further investigation of the physics of earth materials might find sufficient nonlinearity for this to be practical.