TM 5-3820-256-10
2-34.
Interpreting Electrical Logs.
a. Preparing the Log.
(1)
In order to interpret electrical log data, you must first prepare the graphical log data properly. Any suitable
graph paper may be used.
(2)
The cable markings have been measured from the current electrode. Therefore, when preparing the log,
plot the 0.25 foot reading at the depth as read from the marked cable, however, plot the 2.5 foot readings
about one-foot above this point. (This applies to NORMAL and LATERAL arrangements.) If the 10-foot
NORMAL setting is used, plot its readings about 5 foot above the marked cable reading.
b. Significance of 0.25-Foot Spacing. The reading obtained with the 0.25-foot spacing is heavily influenced by the
fluid in the well bore and therefore reads only a fraction of the formation resistivity. However, the short spacing
enables you to see changes in resistivity with greater detail. With this electrode spacing, formation having a
thickness of about 6 inches or greater can be detected. Because of this ability to see small detail, the 0.25-foot
curve should be used to "pick" formation boundaries.
c. Significance of 2.5-Foot Spacing. The 2.5-foot electrode spacing provides you with very nearly the true
formation resistivity for wells having diameters up to about 16 inches and for formations thicker than about 5
feet. For larger diameter wells or thinner formations, the measured resistivity will depart somewhat from the true.
For qualitative interpretation, this departure is not significant. Because the 2.5-foot curve provides you with the
formation resistivity, it is used to identify the type of material penetrated.
d. Significance of LATERAL Log. The LATERAL Log obtained with the equipment is made by a combination of the
10-foot electrode and either the 0.25-foot or 2.5-foot electrode. Because the 10-foot electrode is at a distance
fairly large compared with either of the other two, the interpretation is essentially the same as for the NORMAL
log after using one of the following correction factors:
(1)
For 0.25-foot LATERAL log: 1.025 meter factor.
(2)
For 2.5-foot LATERAL log: 13.33 meter factor.
e. Interpreting Resistivity Values.
(1)
Clays and shales have low resistive values; sands, gravels, sandstone, and limestone have high resistive
values. Igneous and metamorphic rocks (such as granite and gneiss) have, generally, very high resistive
values.
(2)
The exact range of numerical values depends on the following:
(a) the type of earch material making up the formation,
(b) the degree of cementation of the formation,
(c) the quality of the formation water,
(d) the porosity of the formation,
(e) the diameter of the well bore, and
(f) the resistivity of the fluid in the well bore.
2-104
