Showing papers on "Very low frequency published in 1986"

ELF and VLF signals radiated by the “polar electrojet antenna”: Experimental results

Abstract: The heating facility at Ramfjordmoen near Tromso, Norway, has been used to modulate the auroral electrojet at frequencies in the range 223 Hz to 5.44 kHz. ELF/VLF signals have been received at Lycksele, Sweden, 554 km from the heating transmitter, over the whole frequency range with maximum amplitudes of ∼50 fT. Both azimuthal and radial magnetic field components were recorded and the ratio of these two components, commonly termed the polarization, was determined. The experimental results have been successfully modelled by using waveguide mode theory and assuming the heating source to be a point dipole located in the ionosphere at the height of the maximum ELF/VLF Hall current.

Interpretation of VLF Resistivity Data for Ground Water Contamination Surveys

Abstract: Very low frequency (VLF) military communications systems provide a primary field that can be used for shallow geophysical surveys to locate ground water contamination and vertical geologic contacts. Useful properties that can be easily obtained from the interaction of the earth and the primary field are the magnitude of the vertical secondary magnetic field, the surface impedence, and the phase angle between the electrical and magnetic horizontal components. The variations in the secondary magnetic field can be related to vertical geologic contacts, such as the edges of landfill trenches. The surface impedence yields an apparent terrain conductivity, which can be used to locate low-resistivity anomalies often associated with contaminated ground water. The phase angle gives information on vertical variations in resistivity, phase angles less than 45° indicating increasing resistivity with depth. The depth of penetration of the VLF field is about one skin depth. For a frequency of 20 kHz, the skin depth in meters is approximately equal to 3.67 where p is terrain resistivity in ohmmeters.

Intensity and polarization characteristics along the earth's surface for the ELF-VLF waves emitted from a transmission cone in the high latitude

Abstract: Modele de propagation des ondes EBF-TBF dans l'ionosphere aurorale. On tient compte de la conductivite du sol

A portable local loop vlf transmitter for geological fracture mapping

Abstract: A portable low-power Very Low Frequency (VLF) transmitter using a large square loop antenna has been designed, assembled and tested by the Geological Survey of Canada (GSC) for geological studies of fracture patterns in igneous rock masses. Standard laboratory equipment, consisting of a signal generator, a 1100-W power amplifier and several high-power tuning capacitors, was used for the purpose. Field tests at the Chalk River facilities of Atomic Energy of Canada Limited have demonstrated a remarkable similarity between survey results obtained using the VLF signals from the local loop transmitter and from distant US Navy VLF transmitters. The local loop was used to simulate the fields from navy stations NAA in Cutler, Maine and NSS in Annapolis, Maryland. Conductor axes, mapped by using these navy stations, and by using the loop antenna yielded almost identical results. A survey was also done in the same area with the local loop placed in such a manner that the direction of the VLF field was at 45° to the field directions from NAA and NSS. In this case, the same conductor axes were located with only minor shifts in position, indicating that conductors whose axes lie within 45° of the direction of the primary horizontal magnetic field are mapped. Thus, it is probably sufficient to have two sources with orthogonal VLF fields to map all VLF conductors in an area. Since in most areas at least one navy VLF station can be used, the local loop transmitter can be used to generate a signal at right angles to the direction from the navy transmitter to allow a more complete VLF survey coverage.

Comparison of calculated and measured height profiles of transverse electric VLF signals across the daytime earth‐ionosphere waveguide

Abstract: Airborne VLF antennas radiate energy that propagates via both transverse electric (TE) and transverse magnetic (TM) modes in the earth-ionosphere waveguide. In order to compare the structure of such signals, measurements were made using rocket probes launched from Wallops Island, Virginia. The probes measured TE and TM fields at all altitudes between the ground and the base of the ionosphere. The nearly horizontal airborne transmitting antenna radiated a TE signal that was stronger than its TM signal at altitudes above about 10 km. The signals comprised one or more well-defined TE or TM waveguide modes. Calculated height profiles agree well with the measured ones and correctly reproduce details of profile structure caused by interaction between two or more modes.

Omega navigation signal characteristics

Abstract: A description of Omega/VLF signal propagation is given. Particular emphasis is given to nonstandard signal propagation scenarios including propagation over regions of low ground conductivity, signal spreading and converging, antipodal effects and long-path reception, modal interference (including fast terminator transit and off-path effects), and temporal anomalies (SIDs, PCAs, and magnetic storms). These elements of signal behavior are described qualitatively to aid in understanding the basis for signal selection algorithms employed in conventional Omega/VLF receiving systems. Equipped with this knowledge, the user may invoke manual deselection procedures when the receiver is suspected of processing an undesirable signal, i.e., likely to produce significant navigational error. As further guidance, a table of recommended signal deselections is given for approximately 80 geographic locations around the Earth. Signals are recommended for deselection on the basis of modal interference, long-path reception, and solar proton activity.

Forward modeling computer program for the very low frequency, radio-wave, terrain-resistivity electromagnetic method : VLF.BAS

Abstract: ...................................................

Amplitude probability distributions and impulse amplitude distributions for impulsive noise: Atmospheric radio noise from a near thunderstorm and automotive radio noise from a roadway

Abstract: An attempt has been made to derive the envelope amplitude probability distributions (APD) for atmospheric radio noise near thunderstorms. Formulas for APD's have been derived based upon the model for atmospheric radio noise near thunderstorms: First, elementary formulas for APD's are derived for the case when impulsive noise sources are continuously distributed over a limited range. Next, by taking into account the characteristics of the return stroke and K change pulses, APD curves at VLF are calculated using these formulas for atmospheric radio noise near a thunderstorm. Good agreement is shown between the calculated and measured APD curves. Further calculations indicate that the changes of the measurement frequency, and of the shortest distance to a thunderstorm influence the characteristics of the APD's. At last, similar procedure in analysis is applied to derive APD's for automotive radio noise, and the resulting calculated APD curves are shown for (1) a single vehicle and (2) a large number of vehicles running on a roadway, respectively.