Análise da recepção do sinal VLF da Estação NAA (24.0 Khz) EUA e sua utilização na pesquisa de água subterrânea no Estado do Ceará, Nordeste do Brasil

Abstract

During the last decades, several authors have demonstrated the usefulness of VLF (Very Low Frequency) geophysical surveys as a fast and powerful tool on surveying geological structures (faults, fractures, diaclases, contacts, dikes, etc.). Other applications involve mineral prospecting, civil engineering, contaminants identification and especially groundwater prospecting. During this research, it was verified that VLF NAA station (24.0 kHz) has a stronger, stable and less noise-prone signal even in urban areas and meets all the direction variations of fractured zones in the NW quadrant. However, there are still doubts about the efficient use of this method, provoking questions about the usefulness and reliability of VLF signals transmitted over long distances. In response to that question, the hypothesis was put forward: "Can VLF signals from NAA station (24.0 kHz) transmitted from Cutler, Maine-USA be used reliably in groundwater surveying at Ceará's State? ". Thus, in the attempt to sustain this, results and analysis of signal intensity observations of this station executed at Ceará during the period from January 20, 2001 to December 28, 2012. In total, 768 VLF profiles groundwater prospecting were performed in 78 municipalities of Ceará, totaling 347.61 km of geophysical surveys using WADI equipment built by Swedish company called ABEM. These observations were also compared to data acquired by Moore Observatory through continuous monitoring of NAA station. The literature review on the behavior of VLF waves shows that the propagation of these waves occurs in the confining layer formed by ionosphere and Earth’s surface and that prevents VLF wave signal from escaping into space, making it possible for VLF signals to be captured at higher distances up to 10 thousand kilometers of the transmitters. In relation to what was presented and analyzed it can be concluded that: VLF waves propagate at long distances consistently, with stability and reproducibility of signal amplitude and phase; The VLF signal transmitted from NAA station (24.0 kHz) is stable and reproducible, even at distances exceeding 10,000 km; The values of the relative power indicator of VLF signal of NAA station (24.0 kHz) recorded by WADI equipment Ceará's State are 93.86% ofthe cases within the range of 10 to 35, considered reliable by manufacturer and signal values of NAA station (24.0 kHz) obtained in Ceará are proportional and present strong similarities with those obtained at Moore Observatory, with a frequency higher than 90% in the interval between values 10 and 30. In this way, the reliability using VLF signal from NAA station at groundwater surveying at Ceará's State, based on its application principie, is proved.