Norskhydro Asa

Multifield Geophysics As

Brian Anthony Farrelly

Svein Erling Johnstad

Harald Westerdahl

A method for electromagnetic geophysical surveying of rock formations () under a sea-floor () comprising the following steps: *&#x2014;towing first and second alternating field (E, E) emitting sources (s, S) in first and second depths below the sea surface, said first field (E) having a first phase (&#x3a6;);&#x2014;said second alternating field (E) given a second phase (&#x3a6;) different from said first phase (&#x3a6;), said sources (S, S) constituting a phased array emitter antenna with directivity for transmitting a major proportion of the combined electromagnetic energy downwards;&#x2014;said first and second fields (E, E) for propagating partly down through the sea-floor () and being reflected and/or refracted through said rock formations () and partly propagating back through the seafloor ();&#x2014;said first and said second fields (E, E) for merging to a total field and being measured by electromagnetic receivers (r, r, . . . , r) recording corresponding field registrations (E(t), E(t), E(t), . . . , E(t)).

Method for electromagnetic geophysical surveying of subsea rock formations

A method for analyzing acquired electromagnetic measurements (R) made at or in a sea () over a seafloor () with rock formations () having relatively low resistivity (&#x3c1;) for detecting a possibly underlying petroleum bearing reservoir formation () having relatively high resistivity (&#x3c1;), wherein a low frequency electromagnetic transmitter () arranged in the sea () emits an electromagnetic field (P) propagating in the sea (), in the rocks () and in the air () above the sea; wherein electromagnetic sensors () are arranged with desired offsets (x) in the sea () for measuring the electromagnetic field (P(x)) while the field propagates, characterized in that one or more component of the electromagnetic field (P) is measured at least one large offset (x) from the transmitter () where the field (P) essentially only has its origin from the field propagating as a field (P) through the air (); that the one or more components of the electromagnetic field (P) measured at the large offset (x) is calculated back to a recalculated field (F(x)) to one or more offsets (x) being shorter than the large offset (x); that the recalculated field (P(X)) is subtracted from the field (P(x)) for possibly highlighting a field which arising due to the possibly petroleum bearing reservoir formation () having relatively high resistivity (&#x3c1;).

Electromagnetic method on shallow water using a controlled source

A method for determining electrical properties of a submarine petroleum fluid-bearing formation () under overburden geological layers () of thickness (s) under a seafloor () of a sea. The method includes: *arranging one or more transmitter antennas () in the sea, and arranging one or more electromagnetic receivers () in the sea, *the sea having a shallow sea depth (d) of 50-350 meters; *using the transmitter antenna () transmitting electromagnetic signals () of one or more pulses () having frequency of very low frequency between 0.01 Hz and 0.10 Hz; *using the receiver antennas () having an offset from the transmitter (), for receiving propagated electro-magnetic signals () due to the emitted electromagnetic signals (); *analyzing one or more components of the received signals () from receivers () having offsets between about 1 km and 12 km, whether the signals () differ significantly from a corresponding signal that would occur from a water-bearing formation (), indicating a petroleum fluid-bearing formation ().

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Rådal, Norway

Brian Anthony Farrelly