Connectors for
the tool are as follows. The PolyGamma probe
top described above is a standard single
conductor probe top. Other variations of
probe tops and wiring can be ordered from
the factory. The connector between the
PolyElectric and PolyGamma probes is a ring
style connector. The numbering of the rings
begins from the inner most ring (ring 1) and
proceeds to the outer ring (ring 6).

The bridle must
be connected between the cable head and the
top of the PolyElectric -PolyGamma Probe
combination as illustrated. The bridle
provides electrical isolation from the
logging cable armor for normal resistivity
logging.

The 2PEA-1000/F has fluid temperature
and fluid resistivity sensors located on the
bottom of the probe. Please call for more
information.

The normal resistivity and single point
resistance measurements are accomplished by
measuring the amount of survey current that
the logger and probe produce between the A
electrode and the mudplug (or armor during
the normal resistivity using armor
operational mode). A voltage is measured for
each resistance or resistivity channel. All
voltage measurements are made with respect
to the armor. The quotient between the
voltage and current for each channel is used
to calculate the reported value.

where r is resistivity (ohm-meters), R
is resistance (ohms), l is the distance the
survey current travels (meters), A is the
cross sectional area that the current
travels through (meters 2 ), V is voltage
(volts), and I is current (amps). The
quantity (A/l) is called the geometric
factor G (meters). The geometric factor is
approximately 12.5 times the AM spacing,
in meters. The survey current leaves the A
electrode in all directions, diverging as it
does so. In a homogenous medium, concentric
spheres centered around the A electrode,
and with radius AM, delineate the volume
of investigation for the normal resistivity
measurement. AM refers to the distance
between the A and M electrodes. The
volume of investigation (in a homogenous
medium) for the 8 inch normal resistivity
measurement is a sphere with an 8 inch
radius; the volume of investigation for the
64 inch normal resistivity measurement is a
sphere with a 64 inch radius. These spheres
are called equipotential surfaces. The
voltage is measured between an equipotential
surface (sphere surrounding the volume of
investigation) and the reference (armor).
This voltage is divided by the measured
value of the survey current, and the result
multiplied by the geometric factor to obtain
resistivity.

The normal resistivity circuits report the
average resistivity of the material in the
volume of investigation and the volume of
investigation may vary for heterogeneous
mediums. Therefore, the measured resistivity
is called the apparent resistivity. Many
computer programs are available to convert
apparent resistivity to true resistivity.
These programs usually require a geologic
model and the apparent resistivity data to
calculate true resistivity. Some programs
calculate synthetic logs such as invasion
profile, synthetic focused resistivity logs,
and porosity logs.

**Single Point Resistance Measurement**

Refer to Ohms law from above for
the explanation of the single point resistance
measurement. As the survey current leaves the A
electrode, the current diverges, and the cross sectional
area A through which it travels becomes very large
compared to l. The quantity (l/A) in the first equation
approaches zero as the distance from the A electrode
increases. Therefore most of the measured resistance is
a result of the survey current near the A electrode
and also at the mudplug where the current converges. The
resistance indicated by the single point resistance
circuit, is the sum if the resistance near the mudplug,
and the resistance near the A electrode. Since the
resistance near the mudplug does not change, any
excursion indicated in the single point resistance log
is a result of the change in resistance near the A
electrode.

When the PolyElectric - PolyGamma probe combination is
operated in R-SP mode, the current generator and all
measure circuits are contained in the logger at the
surface. The mudplug is used as the current return (B)
and reference (N) electrodes. The top electrode on the
probe functions as the current (A) and measure (M)
electrodes. In this mode, the top electrode on the probe
is connected to the cable line center conductor. Since
the probe requires no power, this mode of operation is
sometimes referred to as the passive mode.

**SP measurement**

The SP (self potential) circuits measure the DC (direct
current) voltage between the top electrode on the probe
and the armor. The resistivity circuits utilize an AC
(alternating current) survey current so that the SP
circuits are not affected. When the PolyElectric -
PolyGamma probe combination is operated in R-SP mode,
the current generator and all measure circuits are
contained in the logger at the surface. The mudplug is
used as the current return (B) and reference (N)
electrodes. The top electrode on the probe functions as
the current (A) and measure (M) electrodes. In this
mode, the top electrode on the probe is connected to the
cable line center conductor. Since the probe requires no
power, this mode of operation is sometimes referred to
as the passive mode. This mode may give better SP log
results near the water level in the borehole.

**Fluid Resistivity Measurement**

The fluid resistivity measurement generates a survey
current between small current (A and B) electrodes
located inside the survey tube. Small measure (M and
N) electrodes, located between the current electrodes,
are used to measure the potential difference generated
in the fluid by the current electrodes. The process is
identical to that of the normal resistivity
measurements, except that the volume of investigation is
entirely contained in the survey tube.

**Fluid Temperature Measurement**

The fluid temperature measurement uses a solid-state
temperature-sensing device. The electrical output of
this device is proportional to the temperature of the
fluid. The thermal mass of the temperature sensor is
kept as low as practical so that the time required for
the sensor to respond to a change in temperature is
minimal.

**Derived
Measurements**

Measurements from the PolyElectric probe can
be combined to make derived quantities.
Lateral resistivity logs and synthetic LL7
logs can be obtained from normal resistivity
logs. Mud invasion profiles can be
determined with multiple spaced resistivity
logs. These profiles illustrate rock
permeability. Mud resistivity can be
calculated from the fluid resistivity. Mud
resistivity can then be used to calculate
porosity. Many of these calculated
measurements can be made in real time while
logging the data. For more information about
these and other derived measurements,
consult Aegis Instruments.

**Specifications:**

Length 2PEA-1000 |
74 inches (188 cm) |

Length 2PEA-1000/F |
87 inches (221 cm) |

Diameter |
1.55 inches (40 mm) |

Weight 2PEA-1000 |
16 lbs. (7.3 Kg) |

Weight 2PEA-1000/F |
22 lbs. (10 Kg) |

Operating Temperature |
0 to 70 degrees C |

Storage Temperature |
-40 to 125 degrees C |

Maximum Pressure |
2000 psi (13.8 Pa) |

Low Range Normal Resistivity Measurement |
0 to 250 ohm-meters |

High Range Normal Resistivity
Measurement |
0 to 2500 ohm-meters |

Normal Resistivity Accuracy |
1 % |

Normal Resistivity Resolution |
0.02 % |

Low Range Single Point Resistance
Measurement |
0 to 500 ohms |

High Range Single Point Resistance
Measurement |
0 to 5000 ohms |

Single Point Resistance Accuracy |
1 % |

Single Point Resistance Resolution |
0.02 % |

Self Potential Measurement Range |
-1.5 to 1.5 VDC |

Self Potential Measurement Accuracy |
1 % |

Self Potential Measurement Resolution |
0.04 % |

Fluid Resistivity Measurement Range |
0-100 ohm-meters |

Fluid Resistivity Accuracy |
1 % |

Fluid Resistivity Resolution |
0.02 % |

Fluid Temperature Measurement Range |
-20 to 70 degrees C |

Fluid Temperature Accuracy |
0.5 % |

Fluid Temperature Resolution |
0.05 % |