# Dictionary:Apparent resistivity

**1**. The resistivity of homogeneous, isotropic ground that would give the same voltage-current relationship as measured. **Direct current apparent resistivity**, ρ_{a}, is an Ohm’s-law ratio of measured voltage *V* to applied current *l*, multipled by a geometric constant *k* which depends on the electrode array: ρ_{a}=*kV*/*l*. Usually has units of ohm-meters. See *resistivity*. **2**. With electromagnetic methods, quantities such as the moduli of the electric and magnetic field intensities (**E** and **H**) are measured at a certain frequency or time. If the subsurface were homogeneous and isotropic, these would yield the true resistivity via a certain equation. However, use of the same equation for a heterogeneous subsurface yields the ‘‘apparent’’ resistivity ρ_{a}:

_{a}=ρ

_{HS}

*F*(

*V*)/

*F*(

*V*)

_{HS},

where ρ_{HS}=resistivity of a homogeneous half-space, *F* is a function of *V*=observed voltage, and *V*_{HS}=voltage for a half-space. Asymptotic definitions such as **late-time** and **early-time apparent resistivity** are sometimes used for apparent resistivity values over a limited range of measurement parameters. For some EM models apparent resistivity may be undefined or multivalued. See Spies and Eggers (1986) and Strack (1992). **3**. The resistivity recorded by an electrical log that differs from the true resistivity of the formation because of the presence of mud column, invaded zone, influence of adjacent beds, etc.