Method and apparatus for measuring resistivity of an earth formation

Abstract

A method and apparatus are described for reducing 'Groeningen' effect and other anomaly induced errors in resistivity measurements of an earth formation. Such errors are typically noted during resistivity measurements of an earth layer and are generally referred to as 'Groeningen' effect or 'casing' effect. A sonde having a plurality of voltage measuring electrodes and current emitting electrodes is provided and suspended in a borehold by means of a conductive cable. The sonde is attached to the conductive cable by means of a length of insulated cable and two reference electrodes are disposed above the sonde. A survey current and at least two focusing currents are emitted from the sonde at alternate times or on alternate frequencies and various voltages induced by each current are measured and utilized to calculate a corresponding set of transfer impedances for that current. By utilizing the calculated transfer impedances, the relationships of focusing currents to survey current which is necessary to properly focus the sonde may be calculated. With the relationship of the survey current and focusing currents thus defined, the apparent resistivity of the formation may be expressed as a function of the transfer impedances and the currents without the necessity of actually altering the amounts of focusing current. One of the focusing currents is operated at a very low frequency to minimize the effect of a resistive anomaly and, by measuring the voltages induced by that current between widely spaced electrodes, the sonde may be operated at a faster rate than would otherwise be possible.