Transient Electromagnetic Process in the Waters of the Sea Shelf with Axial and Equatorial Electric Installations and a Field Experiment

A change in the non-stationary electromagnetic (EM) signal over the conducting polarizable Earth covered by sea water on measuring lines located in the axial and equatorial regions of the source – a pulsed horizontal electric line (HEL) – is considered. When the HEL operates under pulsed conditions, it creates a galvanic and eddy current in the medium. If the medium affected by the HEL is heterogeneous, both influences lead to the separation of bound charges. After attenuating the impact of an artificial source, relaxation (depolarizing) processes of various nature appear in such a medium, manifesting themselves, in particular, in the form of an EM signal. As a result, the transient process recorded by the grounded line after the pulsed effect of the HEL is at least a superposition of three components: the transient electromagnetic (TEM) signals, galvanically induced polarization (GIP) and inductively induced polarization (IIP). As the contribution of the TEM field component to the overall signal decreases, the IP signal is manifested in the transient process by a change in the time response of the decay, to the point where the signal reverses polarity. As shown earlier by numerical simulations for the axial region of the HEL, the manifestation of the IIP signal at late transient process times, for most of the geoelectric conditions on land, is invisible against the GIP manifestation (Ageenkov et al., 2020). These calculations also show that in the axial region, the GIP signal manifests itself in the form of a deceleration of the transient process rate, and the IDP signal – an acceleration of the decay rate, to the point where the signal changes its sign. Field measurements performed by the aquatic differential-normalized method of electrical prospecting (ADNME), which uses axial electrical installations, record transient processes with a change in the time response of the decay: it becomes more delayed or, vice versa, runs faster and may be accompanied by a change in the polarity of the signal. In other words, measured signals of different forms are observed, which are presumably associated with the manifestation of the GIP or IIP signals. The relevance of the publication lies in the need to explain the results of field measurements performed offshore, to understand the relationship between the course of the transient process and the geoelectric conditions existing in the water area. And in general, to describe the formation of the transient response of the medium in the axial and equatorial region of the HEL for the conditions of aquatic geoelectrics. The calculated signal for axial and equatorial electrical installations with several spacings under the conditions of the sea shelf water area is studied when the installation is located on the surface of and inside the water layer, and on the seabed of the water area – on geological formations. For axial installations, calculations are made of the quantities used in the ADNME: the transient process ΔU(t), the finite difference of the transient process Δ²U(t) and the transformant P1(t) – the ratio of Δ²U(t) to ΔU(t). For equatorial installations, the signal ΔU(t). is calculated. The signals of a two-layered model of the medium with polarizable and non-polarizable bases are compared.