WELLOG                      Single Point Resistance (SPR)

# REVISED 05-07-2007

What is Single Point Resistance?

Single point resistance is based on the electrical resistance of the earth. The concept follows the rules of Ohms law. Ohms law states that the rate of current flow, I, through a conductor is proportional to the potential difference, E, causing the flow. Another parameter, resistance, R, determines the rate of flow. Ohms law is expressed as the formula:

E = I x R

Where:            E is voltage in volts

I is current in Amperes

R is resistance in ohms

The resistance of a conductor depends on its cross sectional area and length. Resistance logging devices measure the resistance of the earth between a down hole electrode and a surface electrode or between two down hole electrodes (differential resistance). The actual measurement is a measurement of voltage between the electrodes using a constant current, resistance is obtained using the calculation:

R = E/I

Note: if current is constant, resistance is directly proportional to voltage.

Single Point Resistance logging:

Single Point Resistance (SPR) logging tools are available for the purpose of logging Single Point Resistance. In one case, The tool contains a spring loaded motorized caliper with electrode. The electrode maintains direct contact with the borehole. A constant current ac source at the surface generates a constant current though the earth with return to the surface electrode.

Typical instrumentation:

The Gearhart-Owen RSM-204 uses a 60 Hz constant current source created by a zener regulated 100 volt transformer secondary. The regulated transformer secondary voltage is applied through a 6000 ohm resistance. The result is a constant current source of 16.7 milliamps. Voltage is measured directly from the down hole electrode with reference to the surface electrode, rectified by a synchronous electro-mechanical chopper and filtered to produce an output proportional to single point resistance. The panel also measures spontaneous potential and applies the DC component (SP) to another output for recording by the logging system

In another case which is limited to fluid filled holes, a tool having a single inert lead electrode is used to measure resistance. Resistance of mudcake, borehole diameter and borehole fluid resistance can affect accuracy. SPR logs are often corrected to compensate for borehole effects.

You can contact for answers to your single point resistance logging questions.