Electromagnetic surveying for hydrocarbon reservoirs

A method of analysing results from an electromagnetic survey of an area that is thought or known to contain a subterranean hydrocarbon reservoir, comprising:

• providing survey data indicative of phase difference between first and second components of a detector signal resolved along first and second directions respectively.
• extracting the phase differences from the survey data;
• and
• determining a metric from the phase differences that is predictive of the presence or absence of hydrocarbon.

A method of planning an electromagnetic survey of an area that is thought or known to contain a subterranean hydrocarbon reservoir, comprising:

• creating a model of the area to be surveyed including a seafloor, a rock formation containing a postulated hydrocarbon reservoir beneath the seafloor, and a body of water above the seafloor;
• setting values for depth below the seafloor of the postulated hydrocarbon reservoir and resistivity structure of the rock formation;
• performing a simulation of an electromagnetic survey in the model;
• and obtaining from the model phase differences between first and second components of a detector signal resolved along first and second directions respectively.