Ionospheric induced very low‐frequency electric field wavetilt changes

Abstract

Very low‐frequency (VLF) electric field wavetilt measurements were made continuously over a period of four months at a fixed location. Daytime and nighttime values of the wavetilt were found to be relatively constant. During sunrise and sunset the wavetilt decreased to 20 percent of the daytime value, making wavetilt measurements meaningless for two three‐hour intervals during a 24 hour period. A mechanism involving modal interference is postulated.This phenomenon may be a problem in other passive electromagnetic (EM) prospecting systems including audio‐frequency magnetic and audiomagnetotelluric.

Electromagnetic induction studies

“…Hermance [1982d, 1983a] has used DC thin sheet models to model telluric distortion effects from surface inhomo-geneities. Park et al [1983] and Park [1985] have identified three distortion mechanisms caused by 3D media: coupling between the upper crust and mantle across a resistive lower crust, resistive coupling of conductive features within the upper crust, and local induction of current cells within finite-size, good conductors. The first two of these produce telluric distortion that is frequencyindependent at low frequencies, while the latter is usually frequency-dependent.…”

“…A fluxgate instrument would also be easier to deploy in the field. Theoretical analyses that elucidate some important design criteria for sensitive, low noise fluxgates are given by Russell et al [1983] and Narod and Russell [1984]. Three companies are now manufacturing ring-core fluxgates similar to those flown in MAGSAT.…”

“…The 1D Fr6chet derivatives of the fields are especially useful in this context. The use of Fr6chet derivatives as sensitivity functions is discussed by G6mez-Trevi•o and Edwards [1983], Chave [1984a], and Edwards et al [1984].…”

“…It is especially well-suited for treating isolated bodies embedded in a simpler substrate, since the numerical complexity is limited to the body itself. Hohmann [1983] reviews the formulation of IE problems and computational procedures for their solution. Wannamaker et al [1984a] describe an IE algorithm for MT problems that can handle a 3D body in an arbitrary layered 1D medium.…”

“…Dawson et al [1982] treated TM mode induction with two thin sheets over a halfspace, where the thin sheets represent respectively a conducting ocean adjoining a continent and a resistive crust. Dawson [1983] extended this to include the TE mode. Both of these models are substantially more realistic than earlier ones.…”