Frequency characteristics of electrical conductivity anomalies and the coastal effect

Abstract

In geophysical studies of the electrical conductivity of the Earth's crust and upper mantle, quantitative integral parameters describing the scale/intensity of electrical conductivity anomalies are almost not used yet. A simple and informative parameter is the total lengthwise conductance G = Q∙ϭ_i, where Q is the cross-section area of the anomalous body and ϭ_i is its conductivity. In the Earth's lithosphere, anomalous fields are excited/aroused mainly by the conductive mechanism, for which a theory was developed (Rokityansky, 1972; Rokityansky, 1975a, Rokityansky, 1975b; Rokityansky, 1982). This theory links G to the frequency characteristic of the anomalous field or more specifically with the period T₀ at which the real part of the anomalous field reaches a maximum and the imaginary part is equal to 0, changing sign. The anomalous field is mainly represented by the induction arrow. One of the main objectives of this work is to specify the relationship between the observed value T₀ and the desired G under real Earth conditions, that is, to determine the function G (T₀). Estimating G is important for several factors: 1. Any additional parameter enhances a study's possibilities. 2. A highly reliable quantity G determined by magnetic variation profiling allows using it as a priori information for subsequent computer interpretation of 5-component records. 3. The availability of a quantitative characteristic allows ordering anomalies according to their scale/intensity. The method was first described in 1975 in PEPI, but it was based on simplified Earth models, and the dependence G(T₀) was obtained with an error exceeding half an order of magnitude. This paper presents results of new 2D models calculations which were as close as possible to typical Earth structures and the error was reduced by approximately three times. The main models were deep conductors with a compact cross-section and surface conductors represented by rectangular seawater layer of varying depths, forming a coastal effect. In a complicated model of the deep (4000 m) sea with the shelf (100 km wide, 200 m deep), a previously unknown phenomenon was discovered - a very local increase in the anomalous field in the coastal zone of land, which we termed the resonance-synergistic effect. The paper further presents results of 137 observatories of the INTERMAGNET network processing, covering for the first time all latitudes from the Arctic Ocean to Antarctica. At all observatories, induction arrows were calculated for periods of 225, 450, 900, 1800 and 3000 s. Five coastal observatories with induction arrows exceeding 0.8 are shown on maps along with the bathymetry of the adjacent seas. The analysis demonstrated that the regularities calculated on 2D models explain the main patterns of behavior observed near the seas with explicit 3D bathymetry. The resonance-synergistic effect was found in the data from two coastal observatories. The most fundamental result obtained using the G(T₀) approach is the fact that no anomaly with T₀ greater than 5000 s (and consequently G > 2∙10⁹ S∙m) was detected. Apparently, such and greater conductivities are not present in the Earth's lithosphere.