1. A logging system comprising: a resistivity tool configured to be eccentered in a wellbore filled with oil-based drilling mud at least when the wellbore is at least partially deviated, to rotate within the wellbore, and to obtain at least two resistivity measurements at different corresponding angles of rotation within the wellbore; and data processing circuitry configured to determine an electrical mud parameter by comparing the at least two resistivity measurements obtained at the two different angles of rotation within the wellbore.