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.

Abstract: A visual monitor including a single light indicator for HID lamps (not needing a starter) and a visual monitor including two light indicators for an HID lamp, such as a high pressure sodium lamp, operating in conjunction with the starter. When the lamp is out, the presence or non-presence of an ignited first light indicator indicates lamp failure or ballast failure, respectively. The blinking or nonblinking of the second light indicator indicates whether or not the starter is producing satisfactory starting pulses.