Field tests of square and double exponential pulses for transient resistivity measurements using Wenner arrays and hemispheres.

• Measured transient resistivity ρ(t) is a continuous function over a wide time range, 100 ns ≤ t ≤ 1 s. • Transient resistivity can be measured with four-probe (Wenner, Schlumberger) methods. • Transient resistivity should be measured using hemispherical electrodes and square waves. • The recommended three-terminal oblique test geometry responds predictably in two-layer soil. • An expected dual relation ρ(t) to ρ(f) is seen in soil of low and high resistivity. Models of frequency-dependent soil resistivity ρ(f) suggest the dual concept of time-dependent resistivity ρ(t) that, for high resistivity soil, increases from 100 ns to 100 μs, a range of time relevant for lightning backflashover performance calculations. Measurements of ρ(t) can be made by impressing impulse or step current and monitoring voltage rise. Time-dependent resistivity can be established with Wenner or Schlumberger probe arrays and also with oblique fall-of-potential measurements from buried hemispheres. Tests show that rectangular and double-exponential pulse currents both have value. Specialized evaluation using "Drained Net Charge" concepts may yield estimates of low-frequency resistivity ρ o. [ABSTRACT FROM AUTHOR]