Analogue, numerical and field site studies of EM induction in the China-Korea-Japan region

dc.contributor.authorMeng, Zhiwei
dc.contributor.supervisorDosso, Harry William of Physics and Astronomyen_US of Philosophy Ph.D.en_US
dc.description.abstractElectromagnetic induction in the continental Bohai Bay coastal region of China and the island region of Japan is studied with the aid of laboratory analogue models. Detailed model measurements of the electric (Ex, Ey) and magnetic (Bx, By, Bz) field components are presented for an approximately uniform overhead horizontal source field for E- and B-polarizations. With the aid of 2D numerical models, criteria are developed for permitting approximate removal of the coast effect responses in field site measurements in coastal regions. For the Bohai Bay laboratory analogue model, large anomalous in-phase and quadrature model magnetic fields are observed over the Korea-Japan strait for E-polarization, and over the Bohai strait for B-polarization, due to current channelling through the straits. Large responses over the peninsulas in the shallow coastal areas occur at short periods but decrease abruptly with increasing period. The model induction arrows show that the induction in the local Bohai Bay is important primarily at short periods. At long periods, induction in the distant deeper Yellow Sea must be considered in any interpretation of field site measurements. In general, the analogue model results indicate that the effects of peninsulas, straits, bays and the irregularities in the coastlines play an important role in determining the electric and magnetic field responses both on-shore and off-shore for this complex coastal Bohai Bay region. For the Japan laboratory analogue model, electromagnetic field measurements are examined in detail for a series of traverses over Japan, the Korean peninsula, and the coastal region of China and the U.S.S.R. Large anomalous in-phase model magnetic fields are observed over the Korea-Japan strait for E-polarization and over Bohai strait, Tsugaru strait, and La Perouse strait for B-polarization due to off-shore current channelling. The significant responses observed at short periods over the peninsulas in the shallow coastal areas decrease with increasing period. Large gradients in the in-phase BZ are observed over all regions of Japan for E-polarization for both short and long periods due to the effects of induced currents in the surrounding oceans. Thus, induction arrow responses over all regions of Japan show the dominant effects of the ocean. The 2D numerical calculations of EM induction in continental and island coastal regions for an anomalous conductor in the form of an upwelling or a depression in the conductive substratum, show that if the anomalous conductor - ocean separation distance is at least as great as the coast effect response range YR (defined in the present work to be the range where the coast effect |Bz/Byn| has decreased to a value of 0.2), then the coast effect can be removed by vector subtraction to yield a response, approximately that of the anomalous conductor alone. For a given period (in the range 5 -120 min), YR is found to increase with increasing ocean depth, conductive substratum depth, and island width. Further, the on period is found to vary from model to model, but the general trend is for YR to decrease with increasing period, on account of the increasing importance of the underlying conductive substratum through the skin depth effect in the host. Empirical curves are presented showing how the response range depends on the ocean depth, the conductive substratum depth, the island width and the period. Coast effect response values for 3D laboratory analogue models are employed to approximately remove the geomagnetic coast effects in field measurements for some coastal sites in the Bohai Bay continental region of China and the island regions of Japan. The validity of the subtraction is examined for several models of conductive anomalies at sufficiently large anomalous conductor - ocean ranges to satisfy the response range criteria developed for 2D numerical models. The resulting interpretation of field site measurements in complex coastal regions is discussed. With the coast effect removed though subtraction of the model arrows from field site results available in the Bohai Bay region, the resulting difference arrows indicate a N-S striking conductor to the west of Bohai Bay. These difference arrows, as well as the 2D numerical calculations, support the premise of such a conductor, in the form of an upwelling in the conductive substratum (with conductivity 0.5Sm-1 at 80 km depth), situated at about 150 km from the Bohai Bay coastline to account for the field site observations. A comparison of laboratory analogue model and field site MT results at two sites west of Bohai Bay shows that the analogue model apparent resistivities are about an order of magnitude greater than the field site apparent resistivities. This result also supports the model of a conductive anomaly, in addition to the conductive substratum at 80 km depth. Laboratory analogue model measurements are employed to subtract the ocean effect in field measurements to yield difference arrow for these complex island regions of Japan ( the Kii Peninsula region, the central Honshu region and the regions of northern Honshu, Hokkaido and Tsugaru strait). These difference arrows as well as the 2-D numerical model result for the Kii Peninsula region, the central Honshu region, and the northern Honshu region support the premise of two conductive anomalies (with conductivity 0.5Sm-1), one beneath the Pacific Ocean and one beneath the Japan Sea at a depth of 30 km. Further, the difference arrows over the entire Japan region suggest that the two conductors roughly follow the general trend of the island arc, and eventually may be connected by an E-W striking conductor beneath Tsugaru strait to the north.en_US
dc.rightsAvailable to the World Wide Weben_US
dc.subjectElectromagnetic inductionen_US
dc.subjectElectromagnetic fields, China, Po Hai Regionen_US
dc.subjectYellow Seaen_US
dc.subjectJapan, Sea ofen_US
dc.titleAnalogue, numerical and field site studies of EM induction in the China-Korea-Japan regionen_US


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