Apparatus and methods for estimating impedance in conductive medium

An apparatus for estimating a measure of the electrical impedance contribution of at least one but less than all current-contributive conductive paths between a first point and a second point separated by a conductive medium offering at least two conductive paths of differing length between the points, comprising:

• a first electrode for making an electrical connection to the first point and a second electrode for making an electrical connection to the second point;
• an electrical source for applying an electrical potential to the first electrode;
• a detector for making a measurement of the current at the second point;
• and
• a computer programmed to estimate a measure of an electrical impedance property from a measurement of the current at the second electrode obtained at a time tS subsequent to a time t0 at which the electrical potential is applied to the first electrode, wherein the ratio of the time interval (tS−t0) to the minimum time interval for an electromagnetic signal to propagate from the first point to the second point via the shortest conductive path is greater than one and less than the ratio of the length of the longest current-contributive conductive path to the length of the shortest conductive path.

A method of estimating a measure of a localized electrical impedance property of a conductive medium offering at least two conductive paths between a first point and second point, the method comprising:

• with the conductive medium in a reference impedance state, introducing an electrical signal between the first point and second point and making a reference measurement of a property of the electrical signal affected by passage through the conductive medium;
• inducing in the conductive medium a locally altered impedance state relative to the reference impedance state, wherein inducing a locally altered impedance state comprises inducing a localized change in an electrical impedance property, wherein the localized change differentially affects an electrical impedance property of at least one conductive path relative to its effect on at least one other conductive path;
• with the conductive medium in the locally altered impedance state, introducing an electrical signal between the first point and second point and making a measurement of a property of the electrical signal affected by passage through the conductive medium;
• and
• comparing the reference measurement and the measurement made in the locally altered impedance state, and, using such comparison, estimating a measure of a localized electrical impedance property of the conductive medium.