Electromagnetic 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 dependan.ee 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. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/9542 |
| Date | 27 June 2018 |
| Creators | Meng, Zhiwei |
| Contributors | Dosso, Harry William |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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