During the last decade a number of one-dimensional (1-D) conductivity profiles have been constructed for the upper and mid-mantle of the North Pacific Ocean region. These profiles differ significantly, and from our point of view it is still unclear which profile is the best candidate for the upper and mid-mantle conductivity reference model for this region. Keeping the differences in mind, and inspired by recent findings that the ocean effect is a major contributor to the anomalous behaviour of C-responses up to the period of 20 days (especially at coastal observatories), the goal of this paper has been three-fold: (1) to understand, on the basis of systematic model studies using 3-D ocean models, which of the published 1-D upper and mid-mantle profiles is in best agreement with the available observations. (2) To try to reduce the misfit between the observed and modelled responses by using dense grids in modelling, by considering 3-D models which include not only an inhomogeneous surface layer but also inhomogeneous deeper structures. (3) To derive an alternative 1-D upper and mid-mantle section for the North Pacific Ocean by carefully selecting the data for interpretation and by using 3-D models that are as realistic and detailed as possible. In order to perform the simulations using realistic 3-D models on a routine basis a novel 3-D 'spherical' forward solution has been elaborated in this paper. The solution combines the modified iterative-dissipative method with a conjugate gradient iteration and allows one to compute efficiently the electromagnetic fields in full 3-D spherical models with very high lateral contrasts of conductivity and for very dense grids. During the 3-D simulations a systematic shift of observed C-responses at Honolulu compared with those at other observatories was detected. The reason for this shift is still unclear. Even if a very detailed grid of 0.3 x 0.3 is used, the 3-D simulations using a model of the inhomogeneous surface has no notable ocean effect to C-responses at this site. An attempt has been made to reduce the misfit between observed and modelled C-responses by incorporating a 3-D model with inhomogeneous lithosphere and upper mantle. However, this has resulted only in a slight change in responses. The main conclusion drawn from our 3-D model studies is that there exists a significant disagreement between observed and 3-D modelled C-responses if the published 1-D sections with conducting uppermost 400 km are considered as the upper and mid-mantle sections. Our 3-D simulations and reinterpretation of the data also confirm the recent finding that the upper and mid-mantle beneath North Pacific Ocean in the depth range down to 400 km is much more resistive than hitherto assumed.
Geophysical Journal International, 2005, Vol 160, Issue 2, p. 505-526
C-responses; upper and mid-mantle; North Pacific Ocean; 3-D conductivity spherical models; oceans; forward modelling