Electromagnetic Distortion Research Articles

Near-surface and topographic distortions in electromagnetic induction

The most revealing description of electromagnetic (EM) distortions due to near-surface inhomogeneities and topography is in terms of galvanic and inductive effects. In either case, the distorted electric and magnetic fields can be best visualized as a vectorial sum of primary and secondary fields. Secondary electric fields due to electric charge build-up in the galvanic case persist to the longest periods. In contrast, the secondary electric and magnetic fields due to inductive, vortex currents disappear at long periods. The static shift of magnetotelluric (MT) apparent resistivity sounding curves is a classic example of the galvanic effect. Methods to correct for unwanted distortions such as the static shift can be classified into six categories: use of invariant response parameters, curve shifting, statistical averaging, spatial filtering, use of distortion tensors, and computer modeling. Although invariant impedance calculations are simple to make, they cannot, in general, recover the undistorted impedance. Short period curve shifting is best done with auxiliary soundings such as time domain EM; however, this requires multiple surveys. The shifting of long period MT sounding branches is useful if a standard curve is known and can be matched. Statistical averaging of neighboring MT soundings that are conformal but static shifted has proven very effective at removing random distortions if adaquate data are available. The new EMAP (Electromagnetic Array Profiling) method combats the inherent spatial high pass characteristics of EM distortions by low pass operations in data collection and processing. EMAP proposes the continuous, in-field measurement of electric field dipoles to avoid spatial aliasing. Distortion tensor stripping of topographic distortions is possible since terrain is deterministic but stripping the effects of uncertain subsurface inhomogeneities may be misleading. A new decomposition of the MT impedance tensor under the assumption of surficial three-dimensional (3-D) galvanic effects imposed on a one- or two-dimensional (1-D and 2-D) regional setting promises a way to recover the regional structure. There is a continual need for 3-D computer modeling to test new methods and to calculate topographic and regional effects. Computer modeling has established the value of 2-D modeling of the data identified as transverse magnetic (TM) in some 3-D environments. Ideally, EM distortion correction requires continuous, or at least many, data and the application of more than one correction-modeling scheme.

Pion spectra and the geometry of nuclear collisions

The ability of low-energy charged pion spectra to probe the space-time evolution of high-energy nucleus-nucleus collisions is investigated in a classical picture. The time-dependent nuclear charge density is calculated with a Monte Carlo model of the collision. For the mass-symmetric systems studied here, it is describable by a sum of three expanding gaussian functions, corresponding to one participant and two spectator components. Characteristic expansion velocities are ∼0.4c and ∼0.16c for the participants and spectators, respectively, and the participant charge for central collisions is less than that expected from a clean-cut geometry. For pions emitted in both thermal and direct processes, we calculate the electromagnetic distortion due to the time-dependent nuclear charge distribution by solving classical equations of motion. Many features of the available π-inclusive data can be understood in this model; in particular, the 0π π − π + ratio for Ne + NaF collisions is very well reproduced and is insensitive to the details of pion source. Mid-rapidity spectra for p ⊥ ≲ m π c are calculated for several types of sources. The magnitude of the electromagnetic distortion of the protons emitted in Ne + U collisions is estimated.

Propagation of electromagnetic pulse having a Gaussian envelope in longitudinally inhomogeneous anisotropic ionized media

For radio-wave communication with ionospheric propagation, it is useful to study the distortion of signals propagating through a plasma medium. The problem is discussed of finding the distortion of a pulse after it has propagated through a longitudinally inhomogeneous anisotropic ionized medium whose electron density varies linearly in one direction. As a model of the incident pulse source, the Gaussian envelope carrier pulse is considered. Dependence of the electromagnetic pulse distortion upon the duration of the pulse, the gradient of the electron density, the magnetostatic field, and the carrier frequency is mathematically studied in detail.