Linear Filtering of VLF Dip‐angle Measurements*

Abstract

KAROUS, M. and HJELT, SE. 1983, Linear Filtering of VLF Dip-Angle Measurements, Geophysical Prospecting 3 1, 782-794.The suggested interpretation technique is based on discrete linear filtering of VLF data. The output of the described filtering results is expressed in terms of an equivalent current density at a specified depth that would cause the measured magnetic field. The most practical six-point filter gives an accuracy of 8%. The filter is an extension of the commonly used Fraser filter to process VLF dip-angle data.Filtering the same data set for various depths gives an idea about the change of current density with depth. Areas with high current-density correspond to good conductors. The conductor dip can also be determined.The use of the technique is illustrated on theoretical and field examples. In all cases a good correlation with original models and other types of geophysical measurements was obtained. As shown in the last example, the filtering technique is also applicable in interpretation of other electromagnetic methods.

Comparison of geophysical investigations for detection of massive ground ice (pingo ice)

“…We used the software package RAMAG to process data from the WADI and transform field data. The linear filtering process used is based on the Karous and Hjelt algorithm [Karous and Hjelt, 1983]. This filtering algorithm has been combined with a weighted running average smoothing function.…”

Groundwater Potential and Vulnerability Assessment from Integrated Geophysical Methods, Borehole Logging, and Hydrogeological Measurement

“…The quantitative analysis enabled the identification of profiles where positive amplitude of filtered real crossover the inflection points of the raw real as points of anomaly for vertical electrical Granite AKOKO AREA sounding [29]. The VLF-EM sections are plotted as Karous-Hjelt filtered real component [30]. The VLF electromagnetic profiling data are presented as plots of filtered real and filtered imaginary (in %) against station position.…”

Hydrogeophysics Techniques to Aquifer Locating and Monitoring for Industrial Zone North Thanglong and Quangminh, Hanoi

“…Bằng cách đo các điểm trên từng tuyến, chúng ta thu đƣợc giá trị về độ lớn của trƣờng và sự lệch pha của nó gây ra bởi một thành tạo địa chất bị nứt nẻ, dập vỡ có chứa nƣớc ngầm [25]. Chuỗi số liệu đo đƣợc xử lý loại bỏ nhiễu trên bề mặt bằng phép lọc Fraser và Karous-Hjelt, sau đó đƣợc minh giải để tiếp cận đến nguồn gây ra dị thƣờng VLF bằng các mặt cắt mật độ của dòng [21,32]. Hình 5c dƣới đây trình bày mặt cắt mật độ dòng của tuyến VLF đo tại OW1 với bƣớc đo 5m.…”