Abstract
The purpose of the study is to show the effect of the probe dimensions on the display of the low-frequency dispersion of the geological formations’ electromagnetic properties in transient measurements by electric lines in the axial area of the source for the water areas up to 100 m deep. The study analyzes the change in the transient signal, the finite difference, and the transform (the ratio of the above two) as a function of the length of the source (a horizontal grounded electric line (AB) 50 to 2,000 m), the receiver (a three-electrode electric line (MON) 50 to 2,000 m), and the distance between their centers (spacing) 100 to 4,000 m. The values obtained from the conductive and conductive polarizing models are compared for the identical probes installed at the same depth. The grounded electric line is located within the conducting medium with a conductive polarizable base. The conducting medium is associated with the seawater thickness in the marine shelves up to 100 m deep. The conductive polarizable base is a geological environment (earth) covered with a layer of water. The polarizability of the base is registered by introducing frequency-dependent electrical resistivity by the Cole-Cole formula. The calculations show the display of different transient components associated with the transient buildup and the earth’s low-dispersion properties caused by both galvanic and eddy currents. These components manifest themselves differently for the probes with different dimensions of the source line, receiving line, and spacing. Based on the calculations, it can be argued that in the time range from 1 ms to 16 s, at the probes that have different dimensions and are immersed in the water layer up to 100 m thick, the signal changes depending on the immersion depth for “small” installations (AB of 50 and 100 m), while there is no such dependence for the rest of the probes used in the calculations (AB of 250, 500, 1,000, and 2,000 m).
Highlights
Науки о Земле и недропользование / ISSN 2686-9993, 2686-7931 Earth sciences and subsoil use / ISSN 2686-9993, 2686-7931 between their centers 100 to 4,000 m
Calculations for the probes submerged at a depth of 50 m, with the water layer thickness of 100 m, for the conductive model with a conductive polarizable base: a – transient signals ΔU(t); b – transient difference signals Δ2U(t); c – transform P1(t); d – transform difference ΔδP1(t)
Calculations for the probes submerged at a depth of 0 m, with the water layer thickness of 100 m, for the conductive model with a conductive polarizable base: a – transient signals ΔU(t); b – transient difference signals Δ2U(t); c – transform P1(t); d – transform difference ΔδP1(t)
Summary
В основе исследования лежат результаты решения прямой задачи от одномерной проводящей поляризующейся среды для горизонтальной электрической компоненты неустановившегося электромагнитного поля. Авторами использовался метод линейной фильтрации решения задачи электромагнитного становления [17]. ВП среды учитывалась частотно зависимым удельным электрическим сопротивлением (УЭС) по формуле Коула – Коула (1): ρ(ω) = ρ0 (1 − 1η+((iωiωττ))cc),. Для численного эксперимента была выбрана простая среда – двухслойное полупространство. Проводящая геологическая среда – с УЭС 1,5 Ом·м, свойственным морским осадкам. Поляризуемость геологической среды изменялась и была задана 0 или 15 %. Расчеты сигнала переходного процесса ΔU(t), конечной разности сигнала переходного процесса Δ2U(t) и трансформанты P1(t) проведены для ряда установок Расчеты величин ΔU(t), Δ2U(t) и P1(t) проведены при различных глубинах расположения источника и приемника. Переходный процесс на интервале времен от 1 мс до 16 с рассчитывался после бесконечного импульса тока (импульс возбуждения – функция Хевисайта)
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