On Geoelectric Potential Variations Over a Planetary Scale
Fujii, I. & Utada, H.
Abstract
Planetary-scale geoelectric potential variations observed with four submarine cables in the Pacific at periods from seconds to DC were studied. The planetary-scale voltages are produced by geomagnetic field fluctuations of the external origin, sea water motions through a steady geomagnetic field, and secular variations of the geomagnetic field at the Earth's outer core. The externally and motionally induced fields were mainly examined, and the tidal component and secular variations in the cable voltages were briefly described.
The externally induced field reflects electrical conductivity distribution beneath the sea floor. To investigated spatially averaged conductivity distributions of the Phippine Sea Plate, we used voltage differences measured with the Guam-Ninomiya (GN) and Guam-Baler (GP) segments of the TPC-1 cable together with geomagnetic fields at Kakioka, Guam, and Muntinlupa. The GN and GP cables are about 2700km long in almost north-south and east-west directions, respectively, and three geomagnetic observatories locate near ends of the two cables. The magnetotelluric (MT) responses of the GN and GP cable voltages to the geomagnetic variations spatially averaged over the cable distances were obtained at periods 50 seconds to 2 days. The GN and GP responses show significant discrepancies at all periods, which lead to two strikingly different optimum 1D models. However, the thin sheet modeling and 2D finite difference modeling revealed that (1) the three dimensional land-sea distribution and topography of the area significantly distort the electric field of the GP cable and that is the main cause of the discrepancies between the GN and GP responses, and (2) effects of the subducting Pacific plate under the Philippine Sea Plate are seen at longer periods where the thin sheet modeling was performed. As a result the Philippine Sea plate at the depth down to a few hundred kilometers is well approximated by a stratified model which consists of a low conductivity layer with the thickness of 80km overlying a high conductivity layer. If the low conductivity layer corresponds to the lithosphere, it suggests that the Philippine Sea plate is rather thick.
The motionally induced filed was studied using the voltage difference between Hanauma Bay. Hawaii and Point Arena, California observed by the HAW-l cable with the aid of meteorological data provided by ECMWF. Voltages caused by a sea water motion were extracted from the cable voltage of HAW-l for 3.6 years by referring to the geomagnetic field at Honolulu. The extracted voltage showed seasonal variations and significant coherence with atmospheric variables (wind stress curl, wind stress, and surface pressure) a11 over the Pacific basin at periods from 5 to 133 days. This suggests that the voltage reflected regional barotropic water currents which were directly wind-driven. Contour maps of the squared coherency reveal that (1) coherence peaks are 0.6〜0.8 and are both local and nonlocal, (2) significant coherence is seen around the power spectral peaks of the voltage, and (3) spatial patterns of the coherence vary with the periods. Comparison of the power spectra or the coherence maps for each year showed that the wind driven flows are also temporally variable due to temporal variations of the wind field. These results suggest that the wind-driven flow has large scale components, and that the planetary scale cable can monitor such large scale flows up to much longer periods than other observations if the water current 0n a large scale flows across the cable.
The tide and secular variations of a set of the GN, GP and HAW-l voltages as well as the voltage of the Guam-Midway (GM) cable were estimated by a simple least squares procedure. In the tidal analyses, the HAW-l and GN voltages were found to contain larger signals of the oceanic origin than the others. The DC and linear trend were carefully computed from each voltage data set. Results of the GN and GM cables were less reliable than those of the HAW-l and GP, because of the power supply noise and short data length, respectively. The DC estimates with small error bars were obtained from the HAW-l and GP cables, which were 0.2〜0.3 mV/km. The linear trends were of an order of 1x 10 -4m V/km day for every cable. Both the DC and trend showed observable amounts from an experimental view point.
