Patent ID: 7982465

Claim:
A method for processing and analysing acquired electromagnetic measurements (R) made at or in a sea ( 4 ) over a seafloor ( 1 ) with rock formations ( 3 ) having relatively low resistivity (ρ 3 ) for detecting a possibly underlying petroleum bearing reservoir formation ( 2 ) having relatively high resistivity (ρ 2 ), wherein a low frequency electromagnetic transmitter ( 5 ) is arranged in the sea ( 4 ) and emits an electromagnetic field (F) propagating in the sea ( 4 ), in the rocks ( 3 , 2 ) and in the air ( 0 ) above the sea; wherein electromagnetic sensors ( 6 ) are arranged with desired offsets (x) in the sea ( 4 ) for measuring the electromagnetic field (P(x)) while the field propagates, characterized in that one or more component of the electromagnetic field (F) is measured at at least one large offset (x L ) from the transmitter ( 5 ) where the field (F) essentially only has its origin from the field propagating as a field (P 0 ) through the air ( 0 ); that the one or more components of the electromagnetic field (F) measured at the large offset (x L ) is calculated back to a recalculated field (F 0 (x)) to one or more offsets (x) being shorter than the large offset (x L ); that the recalculated field (F 0 (x)) is subtracted from the field (F(x)) for possibly highlighting a field which has its origin in resistivity anomalies in the underground such as the possibly petroleum bearing reservoir formation ( 2 ) having relatively high resistivity (ρ 2 ).