While traditional seismic methods measure the impedance (and therefore density) of the subsurface being surveyed, EM methods give information about the resistivity. In practical terms, this means that while seismic work can contribute to determining whether there is liquid underlying the survey area, EM measurements give information that can be used to distinguish types of liquid from each other, e.g. salt water from oil. In addition, EM provides sensitivity in regions such as sub-basalt and sub-salt that have proven difficult to image with standard seismic methods. Traditionally, EM measurements were conducted by drilling an exploratory hole and inserting a device into it. However, with oil production moving more into the deepwater arena, a need arose to remotely survey potential sites, as drilling of exploratory holes at extreme depth is very costly. Thus the technique of remote EM surveys came into usage in the resource exploration industry.