Geophone orientation and coupling in three-component sea-floor data: a case study

lWe analyse the geophone orientation and coupling in a data set from the No rth Sea. Based on the polarization of the water-break on the sea-floor, we have derived processing algorithms for determining the receiver orientation for gimballed and non-gimballed geophone systems. For a gimballed system, the problem reduces to a simple horizontal rotation. However, for a non-gim balled system, where all three geophone axes may vary due to varying acquis ition conditions such as dipping sea-floor, twisting of recording cable, et c., the three orientation angles cannot be found directly from the recorded displacement vectors. Using the data redundancy within a common-receiver g ather, a robust two-stage method is derived for the non-gimballed system in which all three orientations can initially be unknown. Testing on the Nort h Sea data set acquired with a gimballed system shows that the three-compon ent geophones in the data set are orientated satisfactorily within an error of 5 degrees. However, there are some undesirable cross-couplings between the vertical and horizontal geophones, which results in leakage of shear-wa ve energy from the horizontal components to the vertical components.