Analogue-model magnetic field responses of an ocean channel, an island and a seamount in the Hainan Island region

Abstract

The electromagnetic responses of a shallow channel (Hainan Strait), an island (Hainan Island) in a shallow coastal sea, and a large flat-topped seamount (Zhongsha Islands) in a deep ocean were studied using a scaled laboratory analogue model. To examine the responses, in-phase and quadrature H_z and H_Y magnetic fields are presented for traverses over the channel, the island and the seamount for simulated geomagnetic variations with periods in the range 5–500 min. The in-phase H_z and H_Y channel and island responses were found to decrease rapidly with increasing period, reaching negligible values at about 60-min periods, while the in-phase H_z and H_Y seamount responses had significant values over the entire 5–500-min period range. The quadrature H_z and H_Y channel and island responses had maxima at a period of approximately 20 min when the 0.25 km depth ocean in the channel and surrounding the island was 0.025 skin depth (δ). The shapes of the quadrature H_z and H_Y seamount response curves showed a transition from a "channel response" to an "island response" at a period of approximately 20 min, the same period at which maximum in-phase responses occur. At this period the surrounding ocean depth is 0.2–0.4 δ and the ocean depth over the seamount is 0.05 δ. The quadrature H_z and H_Y seamount responses were each at a maximum at approximately 100 min, when the surrounding ocean depth was 0.1–0.2 δ and the ocean over the seamount is 0.025 δ. The addition of a conducting plate to simulate conducting mantle structure at a depth of 100 km led to much less attenuation for the case of the seamount than for the channel or island due to the deep ocean surrounding the seamount, effectively shielding the conducting mantle from the overhead primary source field.

          ARK: https://n2t.net/ark:/88439/y035553

Permalink: https://geophysicsjournal.com/article/302