Propagation characteristics of electromagnetic waves generated by an electric dipole are used for a variety of applications, including geological mapping of mineral deposits on the ocean floor, cellular or mobile communications, and detection of unexploded ordnance. Mapping of the regional geology in deep-ocean, near- bottom locations has led to discovery of poly- metallic sulfide mineral deposits in the vicinity of hydrothermal vents on mid-ocean ridges potentially worth billions of dollars.

Figure 1: Comparison between analytical result and COMSOL Multiphysics analysis. (Analytical results from D. Margetis, “Pulse Propagation in Sea Water,” J. Appl. Physics, 1995, Vol. 77 (7), No. 1, pp. 2884-2888.)
Conventional frequency domainmeasurements are restricted in theirapplicability to working on the sea floorand, consequently, methods that measurethe shape of the transient timedomain response are under considerationas more effective techniques fordetecting sea floor conductivity. Inaddition, studies of electric and magneticfield interaction with human tissuehave suggested that exposure tocontinuous electromagnetic fields has alesser effect than exposure to low-frequencytransient fields.

Figure 2: 3D electric field at t=0.001s during a transient current pulse applied to an electric dipole.
To support exploration of the effectsof electromagnetic waves produced by atransient pulse, AltaSim Technologieshas developed computational approachesfor treating the transient nature of apulse with asymmetric, non-zero-riseand decay times. The shape of the currentpulse applied to the electric dipolewas represented by a heavy-side exponentialfunction with independent,non-zero rise and decay times. The currentpulse was integrated into COMSOLMultiphysics to provide solutions for theelectric and magnetic response of anelectric dipole.

To ensure accurate solutions, two criticalaspects of the analysis were found tobe significant: first, refinement of theanalytical mesh to a critical level toensure spatial resolution within the timetransient of the current pulse; and secondly, settings of solver parameters wereoptimized to ensure solution time-steppingwas maintained within the requiredtime steps of the rise and decay times ofthe input current pulse.

Figure 1 compares the results for thenormalized electric field in the x-directionas a function of time at a point 50 mfrom the dipole obtained by computationalanalyses, with those from publishedanalytical results for a transientdipole in a conductive media. The widthof the output pulse is a function of theconductivity of space and the time periodof the input pulse. At zero conductivity,the time period of the pulse is theperiod of the input pulse; as the conductivityof the propagating mediumincreases, the time period increasesfrom that of the input pulse and isdefined by the conductivity of the medium.The spatial variation of the electricfield is given in Figure 2.

Application of the solution methodologydeveloped here has enabled wide-rangingstudies to be performed to identifythe electric and magnetic fieldsdeveloped by a transient pulse electricdipole. The results of these analyseshave shown that considerable differencesin the surrounding environmentare expected to arise in response to atransient pulse compared to that from acontinuous applied current.

This work was performed by Dr. S.P.Yushanov, Dr. J.S. Crompton, and Dr. K.C.Koppenhoefer of AltaSim Technologies, LLCusing COMSOL Multiphysics. For more information,visit http://info.hotims.com/22926-126 .


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