EXPLORING VARIABILITY OF TIPPERS AT COASTAL AND ISLAND OBSERVATORIES DUE TO TEMPORAL VARIATIONS OF OCEANIC ELECTRICAL CONDUCTIVITY

Abstract

Geomagnetic field variations recorded at magnetic observatories contain information from both inducing (external, or source) and induced (internal) components. Such data is traditionally used to investigate electrical conductivity of the Earth’s mantle. Furthermore, recent studies have shown that tippers calculated from short-period magnetic data measured at island observatories constrain electrical conductivity of the oceanic litosphere and upper mantle. These are due to conductivity contrasts between resistive continental landmass and conductive seawater. Moreover, tipper components present small but systematic seasonal variations whose origin is believed to be associated to external magnetic source fields variations. Taking both phenomena into account, in this study I performed numerical modelling of the oceanic electrical induction effect (OIE) on tippers estimated in island observatories. I modelled the part of tippers that is due to depth-varying oceanic electrical conductivity. Furthermore, I modelled tippers using time-varying oceanic conductivity to analyze whether part of their temporal variability is due to temporal variations of oceanic conductivity. I created 3-D conductivity models by discretizing bathymetry data. These models incorporate realistic oceanic conductivity, which are calculated from global temperature and salinity fields. To account for three-dimensional (3-D) conductivity effects along coastlines, I modelled electromagnetic (EM) fields using a Cartesian integral equation solver. I defined the OIE by the difference between tippers calculated from models incorporating depth-varying constant (with respect to depth) oceanic electrical conductivity. I also investigated the seasonal variations of experimental tipper components calculated from a global distribution of INTERMAGNET observatories. The results from the modelling part of this study show that differences in tippers due to depth-varying oceanic conductivity are observed in most of the magnetic observatories located in low to mid latitudes, regions where oceanic electrical conductivity is larger than 3.2 S/m. Amplitude of this effect and period ranges where it may be observed vary depending on local oceanic conductivity and bathymetry. However, modelled temporal variations of tippers due to time-varying oceanic conductivity are negligible in comparison with observations. Nevertheless, in general tippers modelled incorporating depth-varying oceanic conductivity reproduce observations better than those calculated from constant conductivity models. Thus, it is recommended to accurately incorporate oceanic electrical conductivity when modelling tippers in oceanic sites, if this data is available and trustworthy. The experimental part of this dissertation was the first study to analyze seasonal variations of tippers in low latitudes and equatorial regions. I calculated experimental tippers from monthly XYZ magnetic observatory data using a robust section-averaging approach. To account for seasonal effects, their corresponding variabilities were grouped into the three Lloyd Seasons of the year. For the northward tipper components, seasonal variations have implications for electromagnetic (EM) inversion only in latitudes larger than 35o and for periods larger than 1200 seconds. Furthermore, in these regions there are large amplitude differences between northward tipper components measured in December and June solstices that need to be accounted for when comparing responses measured during different seasons. For the real eastward tipper component, seasonal variations can be relevant for EM inversions in a larger latitude range, from -35o to 60o, and for periods larger than 2400 seconds. For the imaginary eastward component, seasonal modulations are observed in all latitudes at 2400 seconds, but they are negligible for EM studies. For both components, variations of tippers measured during the equinox are negligible. Thus, measuring tippers during this season can be effective to minimize seasonal source effects in the aforementioned regions. Therefore, results from both parts of this dissertation highlight the necessity of taking into account anomalous geomagnetic inductive effects, either arising from the ocean or from external fields, when modelling and inverting tippers. Geomagnetism; Magnetotellurics; Tippers.