**WELLOG **** ****Single Point Resistance (SPR)**

**© 2003 - 2007 ****WELLOG**

**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**** info@wellog.com** **for answers to your
single point resistance logging questions.**