TM 5-3820-256-10
(c)
Turn the electrode selector switch to the 2.5-foot position and repeat Steps (a) and (b). The dial
reading on the ohmmeter now needs to be multiplied by 10 to obtain the apparent earth resistivity for
this electrode spacing.
(d)
Logging notes: The 0.25 and 2.5-foot NORMAL readings are preferred for most wells. If an SP curve
is desired, it should be read with the electrode selector switch in the 0.25 foot position. In some
cases, for example with a large bore-hole or with exceedingly high formation resistivity, it may be
advantageous to use the 10-foot NORMAL position. The instrument is not direct reading; multiply the
reading by a factor of 40.
In logging mud-filled or deep holes, attach a sinker rod to the probe so that there is enough weight that
you can feel the bottom of the hole. Such a weight should be wrapped with friction or electricians
tape; attach it to the probe with a leather thong.
The distance between readings depends on how much detail is being sought. For most situations, a
reading every 2.5 feet is adequate; however, if formations of 1 to 2 feet thickness is sought, take
readings at every 1 or 2 foot distances.
Remember, that in order to obtain a log, the well probe must be below the level of fluid in the well.
When the probe is pulled out of the water, the resistivity instrument will go "dead." A quick means of
checking depth to the fluid level is by turning the function switch to CUR and holding the current
switch in the ON position. When the well probe is pulled out of the fluid, the galvanometer needle will
return to zero.
2-33.
Logging with LATERAL Arrangement.
a. General. In areas with extremely high resistive surface materials or large and varying ground potentials, it may
not be possible to make a NORMAL log. The former condition is found where a large thickness of dry sand is
found at the surface (desert or dune areas) and the latter in high industrialized areas where large DC generator's
are in use. The LATERAL log will generally overcome such conditions.
b. Setting up and logging. Set up the equipment similar to the NORMAL arrangement except that the lead oxide
surface electrode (POT) is not required. Furthermore, the current circuit may be completed by attaching the
surface current wire to the well casing or any other good ground. Set the electrode selector switch in one of the
two lateral positions. The procedure for taking readings is the same as described for NORMAL arrangement,
however the meter is not direct reading. For 0.25-foot lateral the factor is 1.025; for 2.5; the factor is 13.33.
2-103
