Aplikasi Metode Very Low Frequency Electromagnetic (VLF-EM) untuk Karakteristik Bawah Permukaan di Daerah Kapur  Desa Melirang Kecamatan Bungah Kabupaten Gresik (Halaman 38 s.d. 41)

Metode Very Low Frequency Electromagnetic (VLF-EM) merupakan metode geofisika yang cepat, ramah lingkungan dan metode elektromagnetik pasif yang bekerja pada pita frekuensi 15-30 kHz. Metode ini digunakan untuk mengkarakteristik bawah permukaan dan menentukan sebaran dolomit di daerah kapur Desa Melirang Kecamatan Bungah Kabupaten Gresik. Pengambilan data dilakukan pada enam lintasan dengan panjang 550 m dan spasi setiap lintasan sebesar 5 m. Hasil akuisisi data VLF-EM berupa nilai inphase, quadrature, tilt-angle, dan total field. Data tersebut diolah dengan filter NA-MEMD dan diinterpretasikan secara kualitatif dan kuantitatif. Analisa kualitatif menggunakan filter fraser dan karous-Hjelt untuk menentukan bentuk, jalur, dan kedalaman dolomit dengan tepat (nilai resistivitas 100-104 Ωm), sedangkan analisa kuantitatif dihasilkan nilai resistivitas 2-D dari hasil inversi data tripper (inphase dan quadrature) dengan software Inv2DVLF. Analisa kuantitatif menghasilkan kedalaman dolomit antara 0 – 30 m dan 50 – 70 m dengan nilai resitivitas antara 110 – 600 Ωm. Hasil estimasi ini menunjukkan jalur dolomit dari arah timur laut-barat daya, timur-barat, dan barat laut-tenggara.

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Very low‐frequency electromagnetic interpretation using tilt angle and ellipticity measurements

Geophysics ◽

10.1190/1.1441166 ◽

1981 ◽

Vol 46 (11) ◽

pp. 1594-1605 ◽

Cited By ~ 32

Author(s):

A. Sacit Saydam

Keyword(s):

Tilt Angle ◽

Host Rock ◽

Strong Dependence ◽

Low Frequency ◽

Appreciable Change ◽

Host Medium ◽

Very Low Frequency ◽

Anomalous Field ◽

Insight Into ◽

Complex Manner

During this study, a plane‐wave computer modeling approach was used to gain more insight into the response of vertical sheet conductors at very low frequency (VLF). The medium which surrounds the conductors is assumed to be a dissipative one. Magnitude and shape of the anomalous field caused by a vertical “thin” conductor at VLF is found to be dependent mainly on (1) conductivity‐thickness product of conductor, (2) resistivity of host medium, (3) depth to the top of conductor, and (4) overlying conductive overburden. Other variables being the same, a shift in frequency in the 17 to 25 kHz range does not produce an appreciable change in the overall response of modeled conductors. The lateral distance between the maximum and minimum on a tilt angle profile is related to conductivity and thickness of conductor, resistivity of the host rock, and depth to the top of the conductor in a complex manner. An interpretation scheme has been proposed to determine the conductivity‐thickness product and depth of vertical sheet conductors from tilt angle and ellipticity measurements at VLF. Resistivity of the host medium had to be incorporated in the scheme due to strong dependence of response on this parameter. Five characteristic diagrams have been constructed corresponding to host rock resistivities of 50, 250, 800, 2500, and 8000 Ω-m, which cover the most commonly encountered resistivities in the field. Conductivity‐thickness product [Formula: see text] and depth (D) of conductor can be read directly from the appropriate diagram when peak‐to‐peak tilt angle and ellipticity values are entered.

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Very low‐frequency electromagnetic response of a perfectly conducting half‐plane in a layered half‐space

Geophysics ◽

10.1190/1.1441369 ◽

1982 ◽

Vol 47 (7) ◽

pp. 1059-1067 ◽

Cited By ~ 11

Author(s):

M. Poddar

Keyword(s):

Integral Equation ◽

Half Space ◽

Half Plane ◽

Tilt Angle ◽

Low Frequency ◽

Method Of Successive Approximations ◽

Very Low Frequency ◽

Depth Of Burial ◽

Layered Half Space ◽

Conductivity Contrast

Following Dmitriev (1961), a rigorous theoretical solution for the problem of scattering by a perfectly conducting inclined half‐plane in a layered half‐space in a plane‐wave field has been obtained. The solution is in the form of a Fredholm integral equation of the second kind, where the unknown is the Laplace transform of scattering current in the half‐plane. The integral equation is solved numerically by the method of successive approximations. The scattered fields at the surface of the half‐space are found by integrating the half‐space Green’s function over the transform of the scattering current. The effects of depth of burial and inclination of the half‐plane, conductivity contrast between the overburden and the substratum, and thickness of the overburden are studied in some detail. As expected, the tangent of the tilt angle and the ellipticity of the ellipse of magnetic polarization decrease rapidly with increasing depth of burial, conductivity contrast, and thickness. Inclination introduces asymmetry in the anomaly profile besides affecting its magnitude. Depth of exploration is greater for the ellipticity than for the tilt angle. A target depth equal to half of the skin depth in the substratum is the limiting depth of detection in the very low‐frequency, electromagnetic (VLF-EM) method. An interpretation scheme for VLF-EM field data is presented, based on peak‐to‐peak separation and difference between peaks of the two polarization parameters.

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The Attenuation of Very Low Frequency Brain Oscillations in Transitions from a Rest State to Active Attention

Journal of Psychophysiology ◽

10.1027/0269-8803.23.4.191 ◽

2009 ◽

Vol 23 (4) ◽

pp. 191-198 ◽

Cited By ~ 26

Author(s):

Suzannah K. Helps ◽

Samantha J. Broyd ◽

Christopher J. James ◽

Anke Karl ◽

Edmund J. S. Sonuga-Barke

Keyword(s):

Brain Activity ◽

Sampling Rate ◽

Low Frequency ◽

Future Research ◽

Adhd Symptoms ◽

Default Mode ◽

Very Low Frequency ◽

Wide Range ◽

Interference Hypothesis ◽

Mode Interference

Background: The default mode interference hypothesis ( Sonuga-Barke & Castellanos, 2007 ) predicts (1) the attenuation of very low frequency oscillations (VLFO; e.g., .05 Hz) in brain activity within the default mode network during the transition from rest to task, and (2) that failures to attenuate in this way will lead to an increased likelihood of periodic attention lapses that are synchronized to the VLFO pattern. Here, we tested these predictions using DC-EEG recordings within and outside of a previously identified network of electrode locations hypothesized to reflect DMN activity (i.e., S3 network; Helps et al., 2008 ). Method: 24 young adults (mean age 22.3 years; 8 male), sampled to include a wide range of ADHD symptoms, took part in a study of rest to task transitions. Two conditions were compared: 5 min of rest (eyes open) and a 10-min simple 2-choice RT task with a relatively high sampling rate (ISI 1 s). DC-EEG was recorded during both conditions, and the low-frequency spectrum was decomposed and measures of the power within specific bands extracted. Results: Shift from rest to task led to an attenuation of VLFO activity within the S3 network which was inversely associated with ADHD symptoms. RT during task also showed a VLFO signature. During task there was a small but significant degree of synchronization between EEG and RT in the VLFO band. Attenuators showed a lower degree of synchrony than nonattenuators. Discussion: The results provide some initial EEG-based support for the default mode interference hypothesis and suggest that failure to attenuate VLFO in the S3 network is associated with higher synchrony between low-frequency brain activity and RT fluctuations during a simple RT task. Although significant, the effects were small and future research should employ tasks with a higher sampling rate to increase the possibility of extracting robust and stable signals.

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