Geophysical logging of water wells in northeastern Illinois

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URBANA

|[-L|NOIS STATE GEOLOGICAL SURVEY

3 3051 00003 5240

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STATE OF ILLINOIS

DWIGHT H. GREEN, Governor

DEPARTMENT OF REGISTRATION AND EDUCATION

FRANK G. THOMPSON, Director

DIVISION OF THE

STATE GEOLOGICAL SURVEY

M. M. LEIGHTON, Chief

URBANA

CIRCULAR NO. 113

GEOPHYSICAL LOGGING OF WATER WELLS
IN NORTHEASTERN ILLINOIS

By
CARL A. BAYS and STEWART H. FOLK

REPRINTED FROM THE JOURNAL OF THE

WESTERN SOCIETY OF ENGINEERS,

VOL. 49, NO. 3, SEPTEMBER 1944

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS

URBANA, ILLINOIS
1944

248

Journal of the Western Society of Engineers

Geophysical Logging of Water Wells in
Northeastern llinois

Carl A. Bays*
Stewart H. Folk f

Presented November 4, 1943

Paper presented at a general meeting
of the Society arranged by the Program
Committee.

FOREWORD

Because of the many problems of
groundwater supply in northeastern Illi-
nois, ayid particularly those occasioned by
the great increase in industrial and mu-
nicipal demands due to the war, a spe-
cial geological and geophysical study of
the groundwater resources of the region
was undertaken. Although a great
amount of data was available on the deep
wells in this region, it tvas believed that
many of the basic factors controlling the
production of water in any well and the
conditions that are regionally important
were not known. Geophysical and geo-
logical methods for obtaining siynilar per-
tinent information in the oil industry had
already been developed but had not been
generally applied to the water well in-
dustry and practically no applications
had been made in Illinois.

A program, of experimental studies ivas
set up to try the applications and modifi-
cations of these oil-well methods to water
wells and to obtain as much information
as possible on the producing zones and
production conditions of deep wells in
northeastern Illinois. A number of dif-
ferent tools and instruments were run in
icells to obtain different types of data on
the wall rock and fluids. As the investi-
gations progressed there were developed
a number of modifications of the usual
techniques, measurements, scales and in-
terpretative practices. In addition, tools

* Geologist and Engineer, t Associate Geologist,
[llinois State Geological Survey, Urbana, Illinois.

and techniques were developed for par-
ticular water well problems. To records
obtained by tliese methods of logging the
term "geophysical log" has been applied.
Geophysical logs have been made of
more than 20 tvells in northeastern Illi-
nois, and from them many concepts that
are essential to the understanding of wa-
ter production have been worked out.
The geophysical data have been closely
integrated with geological and production
data to give a sound basis for principles
that have wide application and for spe-
cific recommendations on individual ivells.

GEOPHYSICAL LOGGING METHODS
Logging Truck and Equipment

Geophysical well surveys are made
from special trucks which are completely
equipped for this work. They have spe-
cial mono-conductor or multi-conductor
cables for running the instruments into
the well, a winch to spool the cable, and
accurate measuring devices to obtain cor-
rect measurements of depth. All meas-
urements of different conditions within
wells are made electrically and are re-
corded photoelectrically with galvano-
meters as continuous curves on sensitized
film or paper. The trucks carry their
own power supply, usually both batteries
and generator. All of the tools run into
the hole are constructed for electrical
measurement of various parameters to
considerable depths under the hydrostatic
pressure of the fluid column in a well.

Vol. 49. No. 3

Geophysical Logging of Water Wells

249

Measuring Devices

In many wells no correct or accurate
measurements of the casing', liners, pro-
ducing zones, or even of the total depth
are available. Correct measurements play
an important role in obtaining well data,
and the measuring devices are a basic
part of geophysical logging equipment.
There are two types. Both are counter
devices which are motivated by the
sheave or sheaves over which the con-
ductor lowering a surveying device into
a well is run. In one type, the measuring
device is mechanically connected to the
camera in the logging truck and drives
the film so that the correct depths are re-
corded photoelectrically. In the other
type a self-synchronizing electric motor
unit is used to operate the camera in
coordination with the measuring sheave.

Electric Logs

Curves which record the electric po-
tentials and the electric resistivities as
measured in a well-bore together consti-
tute the "electric log." In most cases a
potential curve and a resistivity curve
are recorded simultaneously by means of
a traveling electrode assembly that con-
sists of several electrodes spaced differ-
ently. Uusally they are drawn on the
electric log so that the potential log is
at the left and the resistivity curve is
at the right. The "higher" or negative
potentials are at the left side of the po-
tential log and the positive are at the
right; resistivity values increase to the
right. Measurements indicative of the
formations are obtained only in open
hole. No record other than the presence
of casing and liners is obtained inside
of pipe.

Potential Logs

Measurements of potentials are made
with a single pick-up on the traveling
electrode. The curves indicate the vari-
ous conditions which create potentials
and may usually be attributed to any of
several causes, chief of which seem to
be electrofiltration and several different
types of electrochemical phenomena. The
principal use of potential logs is to de-
termine the relative effective permeabil-
ity of the different zones within wells,

although in some wells the measure-
ments yield considerable data on stray
electrical earth currents which are im-
portant in the study of corrosion prob-
lems.

On potential logs the permeable zones,
usually sandstones, creviced limestones
or dolomites, have negative potentials
which are relatively "higher" than those
of many fresh water wells where there
is little circulation and no difference in
composition between the water in the
well-bore and the water in the porous
zones penetrated by the well. In some of
these wells the addition of salt to the
water in the well-bore will increase the
potential relief so that the relative per-
meability may be recognized and the
curve used. In other wells where a suffi-
cient water volume is available at the
well, a potential log made while keeping
the hole filled with water has been the
only usable curve by which to distin-
guish permeable zones. It is therefore
desirable to run a "hole-filled" potential
log wherever water connections, fire-
hydrants and hoses, or other supplies are
readily available.

Resistivity Logs

There are several methods of measur-
ing the apparent electric resistivity or
impedance of the formations in a well.
That most commonly used employs two
potential electrodes and one current elec-
trode, all combined in a traveling assem-
bly which is lowered into the well, and
another current electrode which is
grounded at the surface. The potential
or measuring electrodes are spaced from
a few inches apart (for detailed logging
of thin-bedded zones) to six feet or more
apart for deeper penetration of the well
rock in order to investigate the character
of the fluids in the porous formations
beyond the zone invaded by fluid from
the well bore. Measurements made with
closely spaced electrodes sometimes do
not penetrate deeply enough into the
formations to reveal their true char-
acter, and the measurements made with
widely spaced electrodes fail to show thin
beds. Another method, and one which
obtains extremely detailed logs of thin-
bedded zones, employs a single traveling

September 1944

250

Journal of the Western Society of Engineers

electrode and one electrode grounded at
the surface. In all methods the resistivity
measurements are affected by the char-
acter of the fluid in the well-bore and
by the hole diameter, as well as by the
character of the wall rocks and the fluids
in them.

On most well surveys two or more
electric resistivity logs are run, usually
a so-called "normal" curve made with
the potential electrodes closely spaced
(ordinarily 18 inches apart) or with the
single traveling electrode to obtain a
detailed log, and a so-called "third"
curve with the potential electrodes from
four to six feet apart to obtain informa-
tion concerning the fluids in the forma-
tions. Where further information is
needed, additional logs can be run with
the potential electrodes only a few inches
apart (the "auxiliary" curve) or with
them more than six feet apart (the
"lateral" or "fourth" curve).

The resistivity curves are used pri-
marily to log the lithology of the wall
rock in wells, but from them the char-
acter of the fluids (fresh water, salt
water, oil, gas) can be surmised, and the
exact location of casing and liner in the
hole can be determined. Non-porous non-
argillaceous materials, such as most lime-
stones and dolomites, have high resistiv-
ity values; shales and other argillaceous
materials have low resistivity values;
the resistivity values of porous rocks
such as most sandstones and some dolo-
mites and limestones depend largely upon
the amount and character of the con-
tained fluids, and generally are interme-
diate between the typically high values
of limestone and the low values of shale.
The resistivity of metallic objects such as
casing is extremely low. Close correla-
tion of well-cuttings and resistivity
curves through any or all zones in a
number of wells gives a sound basis for
interpretation of the curves alone
through equivalent zones in wells where
samples are not available.

Fluid Temperature

Temperature logs are made with con-
tinuously recording resistance thermome-
ters which record the temperature of
water in the well as a continuous curve.

From these logs the temperature of
water from the different aquifers may be
recognized. In most wells surveyed the
temperature does not increase at a con-
stant rate. Most temperature curves are
interpreted as indicating circulation con-
ditions or geological conditions in the
well or producing conditions in adjacent
wells at the time of logging.

In many of the wells some thermome-
ters are affected by "noise" or stray
earth currents so that many minor varia-
tions and irregularities not indicative of
temperature changes in the fluid are re-
corded. These may mask irregularities
which are related to actual minor tem-
perature changes. Therefore, in drafting
and interpreting temperature logs, par-
ticularly in the industrial sections in and
near Chicago, such minor variations are
eliminated or disregarded.

In one well surveyed recently a tem-
perature log was run while water was
flowed into the well from the surface.
From the log, the zone where most of the
added water was leaving the hole was
recognized and was thus identified as the
principal aquifer.

In oil wells the principal use of tem-
perature logs has been to locate the ap-
proximate top of cement behind the cas-
ing. The method is based upon the fact
that the heat generated during the set-
ting of the cement produces a marked
increase in temperature of the fluid
within the casing at the top of the ce-
ment. Temperature logs have been used
for the same purpose in a few water
wells and undoubtedly will be so used to
a greater extent in the future. The ad-
vantages of cemented casing are now
recognized, and more and more casing
strings are being cemented.

Hole Diameter

A continuous record of hole diameter
is furnished by the hole caliper. The
caliper tool consists essentially of four
arms which are extended by springs, and
an electric resistor which is motivated by
the arms. The instrument is run into the
hole with the arms closed, held to the
frame by a small steel band. The arms
are opened on bottom by breaking the
band through detonation of a small shot.

Vol. 49. No. 3

Geophysical Logging of Water Wells

251

Then the average hole diameter is logged
as a continuous graph by recording the
changes in resistance in the circuit due
to the arms moving the resistor as the
tool is drawn up the hole.

Caliper surveys of water wells in Illi-
nois have been of value in analyzing the
effect of shooting, in finding the actual
diameter of wells about which there was
no available information, and in locating
caving zones. They have also furnished
additional important information on the
condition of casing and liners and the
condition of the casing seat, and they
have proved the existence of major crev-
ice systems in some of the dolomite
formations which, contrary to previous
thought, play an important part in the
groundwater supplies of northern Illi-
nois.

Caliper records are also affected by
stray direct currents in some urban
areas, and because these currents cause
many anomalies, some of the records are
of greater relief than is explained by the
variations in hole diameter, especially in
lower shaly formations where such inter-
ference is at a maximum. Where stray
earth currents exist, satisfactory caliper
logs have been obtained only by cutting
out the regular tool-opening and record-
ing circuits and by using instead a rod
affixed to the lower part of the tool which
breaks the band and releases the arms
when the tool is set on bottom, and an
improvised alternating-current circuit to
take a series of readings throughout the
well; of course no continuous record is
obtained but a log can be constructed
from the spot readings.

Fluid-Resistivity Logs

The logs of the variations in resistivity
of the fluid have furnished much cor-
roborative evidence regarding circulation
conditions within wells. Logs have been
run with several different instruments
with closely spaced electrodes inside an
insulated tube, all measuring resistivity
of the fluid in the well-bore.

In most of the geophysical surveys
made, two fluid-resistivity logs have been
run, one under natural conditions and
the other after salt was added to the
well. Originally it was planned to run a

September, 1944

fluid-resistivity survey to check the dis-
tribution of salt in the well, and it was
decided that in order to do so it was
essential to know what variations, if any,
existed before salting. It was found af-
ter a few experimental runs that there
frequently were major variations in fluid
resistivity in wells and that some sig-
nificance could be attached to them. It
was also found that in static wells there
was a tendency to spill salt on top of the
fluid column while loading the Salter so
that the top few feet of water were very
salty but that otherwise the salt was
distributed fairly evenly. However, in
most of the wells surveyed, the resistivo-
meter surveys run after salting showed
even greater variations in fluid resis-
tivity than natural fluid-resistivity
curves. Most of these variations corre-
spond to anomalies on other logs and
therefore corroborate postulated circu-
lation conditions within wells.

Fluid Movement

The current meter, modified from the
ordinary stream gauging meter, has long
been used in water well work. For use
with geophysical surveys an ordinary
stream-type propeller meter was adapted
to run in a vertical plane inside a pro-
tecting housing. Three contact pins on
the gear driven by the propeller are
spaced to create signals from which the
direction of flow up or down through the
instrument can be recognized. The rate
of flow can be calculated from timing the
period between signals. When direct cur-
rent was used there was considerable
electrolysis of working parts, but satis-
factory results have been obtained by use
of low amperage alternating current.
The chief handicap of this instrument is
that it becomes fouled in bacterial
growths, debris, cavings, etc., which in-
terfere with satisfactory operation.

Another type of current meter for use
in wells has been developed but is still
in the experimental stage. It consists of
a counter-weighted vane which motivates
a variable resistor. Moving fluids de-
flect the vane upward or downward
changing the resistance in the circuit.
The instrument is run into a well at a
continuous rate of speed, giving a known

252

Journal of the Western Society of Engineers

value of deflection, and deviations are
interpreted to indicate fluid movement.
Experimental calibration of the instru-
ment and experimental use in several
wells suggest that it may work satis-
factorily under conditions where the pro-
peller-type meter may prove to be unsat-
isfactory. A more sensitive and finished
model as to shop work and mechanical
details is being constructed and will be
tested in the field soon.

Additional Geophysical Methods

The logs described above have provided
much new information and increased our
understanding of groundwater problems
in Illinois. It is apparent that there are
a number of improvements possible in
the technique of operation and in the
interpretation of results from the present
methods. In addition it seems probable
that a number of other geophysical meth-
ods or auxiliary instruments might have
useful application in groundwater prob-
lems in Illinois and elsewhere.

The formation or drill-stem tester is
widely used for testing the fluid content
of individual formations in oil wells. To
date no formation-tests have been run
on water wells in Illinois but valuable in-
formation on the water resources has
been obtained by the use of this tool in
oil wells, and excellent results have been
obtained in water wells in other sections
of the country. Measurement of the static
head or pressure in each aquifer and the
sampling of its fluid content by use of
the formation-tester would give worth
while information about any well.

Various types of fluid samplers, which
take samples at different depths, are in
use in various industries and fields of
investigations. Some samplers have been
used in both oil and water wells in Illi-
nois but there is no sampler at present
available for use with a logging truck or
measuring line which would prove satis-
factory for obtaining samples large
enough for chemical analysis. A fluid
sampler has recently been designed, and
it is planned to use it in detailed sam-
pling of fluids in the well bore to obtain
more detailed information than is now
available and to furnish a check on fluid-
resistivity logs.

The camera has been used successfully
to inspect the lithology and physical
characteristics of the wall rocks in water
wells and is reported to have been used
in oil wells. It is believed that use of a
camera would be invaluable in fishing
jobs and inspecting casing and liners.

Present methods of continuous record-
ing of pH, or hydrogen-ion concentration,
apparently do not lend themselves read-
ily to measurements at depths such as
would be necessary to obtain a pH log
of the typical deep water well. However,
a pH measuring device is under construc-
tion which it is hoped can be adapted to
well work and modified to use through
the recording panel of the regular log-
ging truck.

Magnetic logging to determine the lo-
cation of pipe and tools, etc., in wells
has not been done on any water wells in
Illinois as yet. It is expected that some
situation will arise to permit the use of
this method and a study of its applica-
tion to the drilling, production, and com-
pletion problems of groundwater geology.

Radioactivity surveys (gamma ray and
neutron) are used to log formations in
cased and uncased portions of oil wells.
It seems likely that these methods of
investigation, particularly as applied to
the formations behind the casing, should
prove useful in some water-well prob-
lems. For instance, it may permit the
identification of both glacial and bedrock
aquifers which have been cased off, there-
by giving new information from exist-
ing wells.

Attempts have been made with one of
the available side-wall samplers or coring
devices to obtain samples of the wallrock
from an Illinois water well for core anal-
ysis and examination. It is believed that
valuable information on wells already
drilled and on formations which do not
crop out close to the producing areas may
be obtained from such side-wall samples,
but most of the formations in the deep
wells of Illinois are too hard for satis-
factory use of methods of side-wall sam-
pling hitherto available. Recently devel-
oped side-wall sampling devices designed
to overcome these conditions may prove
successful.

Vol. 49, No. 3

Geophysical Logging of Water Wells

253

COLUMNAR SECTION

FEET

0|-

500

1000

2000

2200-

4300

4500

— C

^

-j—y- i i ■-./■—/ i (■ /■

PLEISTOCENE

NIAGARAN

ALEXANDRIAN

MAQUOKETA

GALENA

PLATTEVILLE

ST. PETER

SHAKOPEE

NEW RICHMOND

ONEOTA

TREMPEALEAU

FRANCONIA

GALESVILLE

EAU CLAIRE

MT SIMON

PRE-CAMBRIAN
GRANITE

ELECTRIC LOG

POTENTIAL

500

1000

1500

2000

:««;<] GLACIAL DRIFT
DOLOMITE

j SANDSTONE
iffiBll SHALE

CREVICE

FIG. 1

Diagrammatic Columnar Section and Composite Electric Log

for Northeastern Illinois

September, 1944

254

Journal of the Western Society of Engineers

UTILITY OF GEOPHYSICAL LOGS

In the course of the Illinois Geological
Survey's geophysical well logging pro-
gram during 1942-43, geophysical logs
were made of wells that ranged in age
from some which had been drilled nearly
half a century ago to some in which drill-
ing was still in progress. Available rec-
ords for the older wells were incomplete,
unreliable, or lacking altogether, but for
most of the newer wells there were care-
fully kept records and sets of cuttings
that were studied in the Geological Sur-
very laboratories.

The geophysical logs have afforded
valuable information about all of them.
In the newer wells they supplement the
drillers' logs, geological studies, and en-
gineering data in accurately determining
the numerous subsurface conditions and
phenomena that affect drilling and op-
eration. By comparison and correlation
of geophysical logs of wells for which
there are poor or no records with geo-
physical logs which have been success-
fully interpreted through integration
with drilling, geological data and pro-
duction data on their respective wells,
the formations present in the "unknown"
wells can be identified, the producing
zones located, and other features deter-
mined. Geophysical logs are therefore
particularly helpful in guiding the re-
habilitation of old wells which is espe-
cially important at the present time be-
cause of the shortage of manpower and
materials for new construction.

In addition to their practical value to
the men who are responsible for the
drilling, maintenance, and operation of
water wells, and consequently to the
many individuals and industries depend-
ent upon the water from these wells,
geophysical logs furnish a wealth of data
that are of scientific value today and
that may prove to be of practical value
tomorrow. This discussion, however, is
limited to the results that repay the cost
of geophysical surveys in dollars and
cents to the well owner and his engineer
or to the drilling contractor.

Identification of Producing Zones

That the Pleistocene sands and grav-
els, Niagaran and Galena limestones and
dolomites, St. Peter, New Richmond,
Galesville, and Mt. Simon sandstones are
important aquifers in various parts of
northern Illinois has been well known for
many years. Fig. 1 shows a diagramma-
tic columnar section and composite elec-
tric log. But geophysical logging has re-
vealed that in some wells there are other
important aquifers, such as the middle
limestone member of the Maquoketa for-
mation, creviced poi'tions of the Trem-
pealeau dolomite, some parts of the
Franconia formation, and some sand-
stones in the Eau Claire formation.

It has not been definitely known here-
tofore what particular zones furnish
water in any one well, and of course each
well is an individual unit, differing in
some respects from all others. With geo-
physical logs the depths, thicknesses, and
characters of the important producing
zones and the principal factors influenc-
ing their respective yields in different
wells have been determined. For example,
geophysical logs have demonstrated that
most of the water produced from the
Galesville (formerly "Dresbach") forma-
tion comes from the lower 40 to 60 feet,
the upper 100 or so feet of the formation
being dolomitic and relatively imperme-
able. It had been supposed that the
whole section was about equally produc-
tive. This and other productive zones in
old wells for which there are no records
can be located with geophysical logs
through correlation with wells for which
records and cuttings are available. Fig.
2 illustrates the correlation of two such
wells.

With the geological and geophysical
data now available, the depths and thick-
nesses and the relative yields of all the
aquifers in the uncased portions of a well
and any appreciable flow from behind
the casing can be determined by making
a geophysical survey of that well. Two
cases will illustrate the economic impor-
tance of this information. In one city
in northern Illinois the question arose as
to whether or not enough water was pro-
duced from the Eau Claire formation to
justify drilling the wells deep enough to

Vol. 49. No. 3

Geophysical Logging of Water Wells

255

WELL "A"

SAMPLE ELECTRIC LOG
STUDY

POTENTIAL IMPEDANCE

WELL "B"
ELECTRIC LOG
POTENTIAL IMPEDANCE

MILLIVOLTS OHMS

(-> C+)

-1520

MILLIVOLTS OHMS
C-) 1+) *■

ERANCONIA

GALESVILLE

MOST PRODUCTIVE ZONE

EAU CLAIRE

FIG. 2
Location of Productive Zone by Correlation of Electric Log a

penetrate it. From a geophysical survey
it was determined that a sandstone in
the lower part of the Eau Claire in that
area is both porous and permeable and
is capable of furnishing large volumes
of water. In another municipality a well
that supposedly had been drilled through
the Galesville, but for which the bottom-
hole cuttings were not delivered to the
Geological Survey, had a surprisingly
low yield, too low in fact to fulfill the
local requirements. On correlation of the
electric log of this well with that of a
well a few miles away (see Fig. 3) it is
evident that the well in question does
not penetrate the lower part of the
Galesville, which is by far the most pro-
ductive part of the formation, and that

a large increase in production may result
by deepening the well by only 50 feet.

Sources of Contamination and Pollution

The widespread occurrence of pollution
and contamination of wells by surface or
near-surface fluids that enter through
holes in casing, through crevices below
the casing, or by way of imperfect casing
seats constitutes a menace to public
health and increases the treatment costs
of water for both public and industrial
use. The place of entrance of such
waters can be detected and the condi-
tions that permit their entrance be deter-
mined by the use of geophysical surveys,
including temperature, current meter,
and fluid-resistivity measurements in

September. 1944

256

Journal of the Western Society of Engineers

WELL "A"

WELL "B"

POTENTIAL

SAMPLE

POTENTIAL

SAMPLE
(+) STUDY

FIG. 3

Correlation of Portions of Electric Logs of Two Wells

conjunction with electric logs and caliper
logs. With that information available it
is possible to determine what remedial
measures — such as replacing the defec-
tive casing, reaming the hole to improve
the casing seat or extending the casing
below the creviced zone, and/or cement-
ing— should be taken to eliminate the
contamination or pollution.

Water from the well illustrated in Fig.
4 was unfit for use as either drinking
water or boiler water in the ordnance
plant by which it was urgently needed.
The casing seat was located by the
electric log and the caliper log. A 120-
foot section of the dolomite under the
casing was shown to be creviced, but the
high negative potentials indicated that

Vol. 49. No. 8

Geophysical Logging of Water Wells

257

SAMPLE
STUDY

ELECTRIC LOG

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TEMPERATURE

Degrees fahrenheil
54 55 56 57 58 59

FIG. 4

Portion of Composite Geophysical Log TJsed to Locate Source of Contamination

258

Journal of the Western Society of Engineers

water was leaving the hole through the
crevices. A distinct "cold" anomaly was
recorded on the temperature log just
under the casing seat, and inasmuch as
the well was surveyed in warm weather
and the fluid column in the casing was
warm, this anomaly was interpreted as
indicative of cold water from behind the
pipe entering the well around the casing
seat. When the current meter was placed
just below the pipe it indicated water
moving downward at such a velocity that
the flow was about 200 gallons per min-
ute. On the basis of the geophysical
survey it was recommended that the hole
be reamed down and the casing be set on
a firm seat below the upper crevice zones
and cemented to the top. All of the pol-
lution and contamination was shut off
by this working-over. The water from
the well had been extremely variable in
composition and had necessitated consid-
erable expense in treatment. By shutting
off harder waters from the glacial drift
and the surface waters, a water of con-
stant composition was obtained and
treatment costs were greatly reduced.

Thieving Zones

Where considerable differences in hy-
drostatic head exist in the various aqui-
fers of multi-zone wells, some zones may
take or "thieve" water from the wells.
By use of temperature logs, fluid-resistiv-
ity logs, and current-meter logs in con-
junction with the electric log such thiev-
ing zones can be recognized. If the thiev-
ing zone is a creviced dolomite, as is fre-
quently the case in Illinois wells, the
caliper log also is helpful in locating it.

In the well illustrated in Fig. 5 large
changes in fluid resistivity and tempera-
ture at the creviced zone indicated on the
caliper log coincide with a major disturb-
ance of the potential curve of such mag-
nitude that it was necessary to introduce
a manual shift to keep the record on
scale in spite of the fact that full scale
is 900 millivolts. The relations of the
curves suggest that large volumes of
water were leaving the hole through
these crevices at the time of logging.
When the head on such thieving zones is
lower than the operating levels in the
well it is desirable to shut off the zones
and thus increase the yield of the well.

Location of Casing and Liners

The location of casing and liners in a
well and the determination of their con-
dition is an important problem. In Fig. 5
the impedance curve showed a liner from
938 to 997 feet and the irregularities in
diameter measured by the caliper indi-
cated it to be in very poor condition.
For another well there was an old set of
samples but no reliable record of the
pipe, or hole diameter below the top.
The length of casing and position of the
top of a liner were determined from the
impedance log, and the diameters of the
casing and of the open hole were meas-
ured with the caliper. The hole was
bridged across the liner at 740 feet so
the log could not be run deeper. This
well was being used as an observation
well, supposedly to record the fluctua-
tions in water levels as indicative of the
water resources of the important sand-
stone aquifer at about 1600 feet through
which the well was reported to have been
drilled. The geophysical survey, by indi-
cating that the hole was bridged, showed
that the water levels could reflect hydro-
logic conditions only in formations above
740 feet. In addition a hole in the sur-
face string of casing was located by the
caliper at a depth between 250 and 260
feet. As the well was logged in the mid-
dle of the winter during its period of
highest water-level, and the top of the
fluid, as indicated by the temperature
and fluid-resistivity curves, so closely
coincides with the hole in the casing, it
appears that the water levels are con-
trolled by the rate at which water can
leave the well through the hole. It be-
came apparent that this well was value-
less as an observation well for the water-
level data.

Effects of Shooting

For better drilling and completion
practice it has been worth while to ana-
lyze the effect of shooting sandstone
zones in wells. A number of 150-pound
shots were used in different spots in a
northern Illinois well (see Fig. 6) with-
out knowing exactly where the producing
water sands were. In the good water
sands the diameter of the hole was en-
larged from 12 inches to 32 inches but in

VoL 49. No. 8

Geophysical Logging of Water Wells

259

SAMPLE
STUDY

ELECTRIC LOGS CALIPER

POTENTIAL IMPEDANCE

TEMPERATURE

FLUID
IMPEDANCE

::

*

ES

2=Z

[7^

FIG. 5

Log Showing Influence at Crevice Zone

September, 1944

260

Journal of the Western Society of Engineers

the tight sands the maximum diameter
obtained was 20 inches.

Caliper logs are valuable aids to re-
working old wells because they locate
tight spots and zones of caving shales,
conglomerate, or soft sandstones in the
wells. The caving shale and conglomer-
ate at a depth of about 460 feet in the
well shown, probably should have been
cased off during drilling. The tight spot,
or place that was not drilled to gauge
between 980 and 990 feet, should be
reamed out to prevent the tools from
sticking while working on the well, or
to prevent subsequent bridging by the
caving of materials which would shut off
the water sand below.

the position of drilling tools, joints of old
air-lift pipe, and other steel or iron arti-
cles that might have been dropped in the
hole. In the bottom of the well illus-
trated in Fig. 7 the portion of a string
of tools lost while cleaning out after
shooting was located by both the electric
log and the caliper log. Inasmuch as
they were below the shot-hole in the
main producing water sand and it is
unlikely that the well will ever be deep-
ened, they were not fished out. The shot-
holes in this well are also of interest in
that they illustrate clearly the differ-
ences in the effect of shooting on the
loose water sand and the tight dolomitic
sand.

Location of "Fish"

In addition to the location of casing
and liners, it may be important to know
the location of lost tools or junk in the
hole. As has been shown, the electric
log gives good measurements of the cas-
ing and liners and in some wells locates

Stray Earth Currents

A byproduct of the geophysical well
surveys is the location of areas and sub-
surface zones in which stray electric cur-
rents are most intense. These stray cur-
rents probably account for much of the
corrosion of liners, casing, pumps, and

SAMPLE
STUDY

ELECTRIC LOG

POTENTIAL IMPEDANCE

ST PETER

_

TREMPEALEAU

/ /

/ /

FRANCONIA

. " .

-

GALESVILLE

EAU CLAIRE

-

....

MT SIMON

-

- 600 -

*-

BO" 30

°

normal — -.

third curve — --«

bottom of pipe
~) at 32?

{

-400 -

I

cavng
zone

j

- 600 -

.1501b
^ shot

I

- 800 -

,'5011)

/'SO lb
J sfot

j

-lOOO-

not to
gauge

<J,5?',b

i

-1200-

CALIPER TEMPERATURE FLUID

RESISTIVITY

Degrees Fahrenheit

44 52 60 65 76 *.

FIG. 6
Portion of Log Showing Effect of Shooting

Vol. 49. No. 3

Geophysical Logging of Water Wells

261

SAMPLE
STUDY

ELECTRIC LOG

POTENTIAL IMPEDANCE

(-1 Millivolts (♦) NORMAL CURvt

_ 24 ohms 46
NATURAL — < U^J 96 144

CALIPER TEMPERATURE FLUID IMPEDANCE

' Zs (Third Cur,.)

\

J 1

1

I .

' . .

2

'.' / ■

'• .

.'•

V

2

■/ 1

/ /

1 1

»>'■ 1

1- / .

■, '•

' , 1

/ /

/■>■/

dr

O

•1 al.

1

s

'/ 1 .

1 I

II,

=

Degrees Fahrenheit
62 64 66 69 TO 72

SALTED--^ NATURAL^^
200 ohms 250 7 75 ohms 800

Fin

Portion of Composite Geophysical Log of an Ordnance Plant Well

September, 1944

262

Journal of the Western Society of Engineers

column pipe in wells, especially in indus-
trial districts. Geophysical methods have
long been used in the study and remedy
of electrolysis and corrosion of pipelines,
underground telephone and telegraph ca-
bles, and other buried metal objects. It
is hoped that as additional data on stray
electric currents and their relation to
corrosion are collected by geophysical
surveys and other methods some practi-
cal suggestions can be made toward the
solution of these problems in water wells.

INTEGRATION OF GEOPHYSICAL
AND GEOLOGICAL DATA

Through compilation and comparison
of geophysical data derived from a num-
ber of methods of investigation, supple-
mented with all available geological and
production data from a specific well and
nearby wells, a reasonably good picture
of subsurface conditions can be obtained
and many of the problems of ground-
water production can be solved. To in-
tegrate all such information, a composite
log of each well is prepared, on which is
displayed all the phases that have been
investigated. The composite logs now
made by the Illinois Geological Survey
present the following:

1. A detailed log of the lithology of
the uncased portions of the well
based on microscopic study of cut-
tings from the well or nearby
wells and the electric log and a
sample study of the cased por-
tions of the well.

2. Exact measurements of the cas-
ing and liners, both as to depths
and inside diameters, and some
knowledge of their condition.

3. Location, thickness, and relative
importance of the water-produc-
ing zones.

4. Location of "thieving" zones.

5. Approximate salinity of the water
in the well-bore, and probable
zones of production of waters of
different salinities.

6. Temperature of the water in the
well-bore and the approximate
temperature of water from differ-
ent zones.

7. Caving zones that have not been
cased off.

8. Circulation conditions under non-
operating conditions.

9. Critical production or well con-
ditions such as collapsed or cor-
roded liners, poor casing seats, lo-
cation of iron or steel "fish," etc.

10. Effects of shooting, acidizing, cav-
ing, and other special conditions
within the well.

CONCEPTS BASED ON GEOPHYSICAL

SURVEYS PERTINENT TO WATER

RESOURCE DEVELOPMENT

From the geophysical logs made in Illi-
nois, and correlation of the data there-
from with all the available geological, en-
gineering, and production records, it has
been possible to formulate a number of
concepts that are believed to be widely
applicable in development and production
of the water resources of deep wells in
northern Illinois. These concepts merit
consideration in laying plans for the re-
habilitation of old wells and for drilling
new wells.

Conservation of Drilling

Examination of the zones producing
large quantities of water in the deep
wells has shown that in addition to the
main sandstone zones, there are numer-
ous crevice systems with high specific
capacities. In many wells sandstones and
crevices act as thieving zones which take
considerable volumes of water from the
wells under both producing and static
conditions. Because the role of the
crevice systems was not understood,
many wells have been drilled through
several zones, each of which was capable
of producing the needed water supply.
Usually the specifications for a well re-
quire drilling to a specified depth or pro-
ducing zone because it is generally under-
stood that most of the water in the area
is produced from a certain aquifer or
above the specified depth.

Drilling costs and time could be re-
duced if well drilling could be done with
the geological conditions in mind. In
many areas, crevice systems in the Ni-
agaran or Galena-Platteville formations

Vol. 49. No. 3

Geophysical Logging of Water Wells

263

are quite capable of producing as much
water as would be obtained from wells
drilled to the Galesville or Mt. Simon
zones where under typical producing con-
ditions one or two zones wrill produce
considerable water but where losses are
high in the thieving zones. In many
areas careful testing of each possible
producing zone during drilling will lead
to considerable saving if drilling is
stopped where adequate water is ob-
tained. Wherever there is evidence that
a crevice system is intersected by a hole,
investigations should be made to see
whether the crevices are capable of sup-
plying the required demand. Several
wells nearly 2000 feet deep which have
been surveyed obtained most of their pro-
duction from the first few hundred feet
and have serious losses of water into
some of the lower zones. In the Chicago-
Joliet area there is evidence to suggest
that wells drilled to crevices in the Trem-
pealeau formation might yield as much
water as deeper wells drilled through the
Galesville.

Running a pumping test on each indi-
vidual zone is not usually considered a
feasible means for testing because of the
high cost involved. However, another
rather simple method may be used in
water wells. The technique is a result
of observations by the writers when run-
ning water into wells to obtain hole-filled
potential and temperature logs. It was
noted that wells of high capacity took
large volumes of water readily and that
high inputs were necessary to keep the
wells filled while logging, whereas wells
with smaller capacity took proportion-
ately smaller volumes. Wells capable of
making 1200 gallons per minute (gpm)
took as much as 600 gpm to keep them
full, and smaller-capacity wells took
much less. While no directly proportional
or mathematical relationships have as yet
been worked out, it is safe to conclude
that a well that requires comparatively
large volumes of water to stay filled will
also produce large volumes. If a nearby
water supply, such as a fire-hydrant, is
available, it will provide a ready and in-
expensive "rule-of-thumb" method of
testing the capacity of a well during
drilling.

Cementing of Casing

In most wells in northeastern Illinois
a string of pipe is set through the glacial
drift into bedrock. Another string of
casing is usually set inside this "sur-
face" string running from the surface
through the Silurian limestone and dolo-
mites, through the Maquoketa shale and
for some short distance into the under-
lying Galena dolomite. This may be re-
ferred to as the "long" string. Usually
no other casing, except liners to case-out
caving shale, is set in these wells.

The surface string is either driven
through the glacial drift, with a forged-
steel or similar shoe on the bottom of
the pipe, or it is set on bedrock after the
hole is made. The long string is usually
set on a shoulder in the dolomite near
the top of the Galena formation. In
nearly all of the wells surveyed ineffec-
tive casing seats were found. Most dolo-
mites, particularly where they are cre-
viced and fractured, do not permit a
satisfactory shut-off without special
means of sealing being used. A nearly
standard recommendation following a
geophysical survey is for the long string
to be cemented or pressure-grouted from
bottom to top. The best known method
of cementing pipe is to bridge or plug
the well immediately under the casing,
pump a cement slurry of proper weight
and composition down the casing and up
around the outside until the pipe is com-
pletely enveloped in cement. Such ce-
menting practice is standard in oil wells.

Proper cementing effectively seals the
casing seats of wells and thereby pre-
vents pollution and contamination of the
well by surface or near-surface waters,
and also may be expected to reduce pipe
corrosion by the protection of the casing
and to prevent circulation of water be-
hind the casing between cased-off aqui-
fers, thus protecting important gravels
and shallow crevice systems from con
tamination.

Plugging of Abandoned Wells

With one exception, all of the geo-
physical surveys have been run in wells
under so called static or non-pumping
conditions. In only rare instances have

September, 1944

264

Journal of the Western Society of Engineers

such wells been actually static; generally
water is moving from one zone to an-
other because of the differential hydro-
static pressures in the different aquifers.
In some cases this circulation is at rates
of more than 100 gpm. There must be
several hundred deep wells in north-
eastern Illinois which are not in use or
for which no further use is planned. A
number of these probably could be re-
conditioned as useful water producers
with proper engineering practice.

It is probable that the majority of such
wells are acting as channels for thieving
between formations and are effectively
reducing the water levels in wells in
their vicinity inasmuch as the levels in
such wells are controlled by the head in
the formation which has the lowest pres-
sure. In addition, many such wells have
poor casing seats and so receive polluted
or contaminated water from the surface.
In many areas wells have been aban-
doned because they were drilled into
zones which produce water too hard, too
salty, or for other reasons undesirable.
Thus every abandoned well is a possible
channel for loss of large quantities of
water from aquifers productive in other
wells, a possible cause of part of the
local reduction in water levels in some
areas, a possible source for introduction
of contamination and pollution into
widely used aquifers, and a possible
cause for high chloride content, hardness,
or other undesirable features of the wa-
ter in nearby producing wells. All such
abandoned wells not in use that do not
readily lend themselves to rehabilitation
should be plugged from top to bottom in
order to eliminate the hazards they
create.

Well Spacing

From geophysical surveys and allied
geological investigations some picture of
the effective permeability of the produc-
ing formations in the deep wells has been
obtained. The magnitude of the crevice
systems which provide rapid water trans-
fer between closely spaced wells which
intersect them has been recognized. From
these data it has become evident that in
many areas there are too many wells for
stable water production under existing

conditions of permeability. For example,
in one urban section where calculations
based on permeability studies suggest an
optimum spacing of one deep well per
900 acres, there is an area where wells
are spaced one to approximately 60 acres.
Rapid recession of water levels is re-
ported during the periods of high pro-
duction.

Inasmuch as the intersection of a
crevice system by a well is largely for-
tuitous, determination of well spacing
should be largely on the basis of the
permeability of the sandstone aquifers
which are everywhere present and have
fairly uniform characteristics. Wells
should be spaced on the basis of obtain-
ing stabilized water levels within a few
feet — both static and operating, a mini-
mum of lift for producing the water, and
a minimum of interference between pro-
ducing wells. The demand, probable
pumping periods, recovery periods, and
much fundamental geological data form
the basis for calculation of the best spac-
ing in any area. Also there are many
other factors of ownership and engineer-
ing which necessitate compromise in de-
termining the actual spacing of wells.
However, it is now possible to make rec-
ommendations of suitable spacing for
new developments on a sound basis. Gen-
erally the tendency has been to put wells
too close together, and thereby to overtax
the local water resources, create a large
local cone of depression, create irregular
non-operating and operating water levels
because of competitive interference with
nearby wells, and thus require continu-
ous lowering of pump settings, giving no
permanence to water-supply installations.
Wider spacing will eliminate these diffi-
culties and obtain much more permanent
development of deep well water resources.

In one area, where about half-mile
spacing for several wells in a war in-
dustry was used, a study of the data sug-
gested a spacing of 6800 feet as optimum
for the area. Later wells drilled at a
spacing of nearly a mile gave higher
yields and much more satisfactory oper-
ation during the heavy pumping season.
Similar calculations can be made for
other areas, and in cases where new de-
velopments are planned or reconditioning

Vol. 49, No. 3

Geophysical Logging of Water Wells

265

of old wells is considered, spacing- merits
consideration as a basis for such plan-
ning.

Operating Level

Analyses of the geological and produc-
tion conditions in wells in northeastern
Illinois show that where there are sev-
eral producing zones in a well the specific
capacity is increased as the operating-
level is lowered. Thus during the first
part of the drawdown from non-oper-
ating to operating level in any well, a
comparatively low specific capacity is to
be expected. As the operating level is
progressively lowered, different zones
with successively lower hydrostatic heads
begin to contribute to the well's yield.
If the head, or the critical operating-
level for any zone can be determined
in any well, then this zone's contribution
can be controlled by controlling the oper-
ating level. This concept has two major
important applications for planning of
water wells.

1. If allowance is made for an increase
in specific capacity with lower drawdown
levels, pump settings can be planned at
levels that are safe for operation under
all conditions, and yet pumps do not have
to operate against such great heads as
might be necessary if the specific ca-
pacity for only the upper part of the
drawdown were used. Setting pumps on
this basis instead of that normally used
will give savings in installation materials
and costs of operation.

2. Certain of the sandstone zones in
northeastern Illinois in the Franconia
and Eau Claire formations are highly
glauconitic. Glauconite is a complex
hydrous ferric-aluminum-potassium-mag-
nesium silicate with high base-exchange
properties which make it ideally suited
for a natural water-softener. Water
from the glauconitic sandstones conse-
quently is not as hard as that from other
formations. A few establishments in
northern Illinois take advantage of these
underground water-softeners by casing
off the overlying formations and produc-
ing water only from the glauconite-bear-
ing zones. Unfortunately the yield of
these zones is low, but it might be pos-
sible to utilize their water-softening

September, 1944

properties and still obtain large well
yields by proper well construction and
production practice.

In one well in northern Illinois, it was
noted that hard water was pumped all
the time when the well was pumped con-
tinuously, but that much softer water
was obtained when the well was pumped
on alternate days. The geological ex-
planation that can be offered is that
when the well was pumped continuously
it produced hard water from the Gales-
ville sandstone, but when pumped in-
termittently the water levels dui'ing the
rest periods were about the head of water
in the Franconia sandstone so that water
from the Galesville was fed into that
zone and softened, and then when
pumped the soft water was produced as
the level was lowered below the head of
the water in the Franconia. When the
pressures of water in sandstones of the
Franconia and Eau Claire formations in
other areas can be determined, it is pos-
sible that by control of operating levels
much softer water can be secured by use
of the natural water softeners of these
formations.

Temperature Gradients

It is well known that in most areas,
except for a relatively shallow zone of
annual temperature variation, the earth's
temperature remains essentially constant
at any given depth at any one locality
and that the temperature increases with
depth. In northeastern Illinois, according
to the best available measurements, the
earth's temperature the year around at
a depth of about 100 feet remains close
to 50 degrees Fahrenheit. Below that
depth the temperature increases at ap-
proximately one degree for each 125 feet,
which accordingly is considered the nor-
mal temperature gradient for the region.
It is unusual, however, for the tempera-
ture log of a well to be a straight line
having that gradient, for in most wells
circulation and other conditions produce
anomalies in the temperature logs. Nev-
ertheless the bottom-hole temperature of
most wells is that which would be ex-
pected for the depth on the basis of the
normal temperature gradient, and the
temperature of the water produced from

266

Journal of the Western Society of Engineers

each aquifer is close to that expected for
the depth from which it comes. The nor-
mal temperature gradient can be used as
a "rule-of-thumb" method for determin-
ing the greatest possible depths at which
water of any desired temperature can be
obtained.

Water for Cooling Purposes

Although it is evident that water from
the shallowest aquifers will usually pro-
vide the most desirable water for cooling
purposes, it is not uncommon to drill to
increasingly greater depths to obtain
large supplies of water for air-condition-
ing and similar uses. Where cooling
water is desired, glacial sands and grav-
els, which are widespread in buried
valleys above the bedrock, should be
sought as the best possible source. Next
the crevice systems in the shallow bed-
rock formations should be tested, and
only finally a deep well considered as a
possible source of cooling water.

Where large quantities of cooling wa-
ter are needed and inadequate sources
are found in creviced or porous shallow
limestones or dolomites, the probable in-
creases in production from acidizing
these zones should be considered. Re-
cently in Kansas it was found that a
shallow well in dolomites and limestone
had its capacity increased four times by
acidizing. Some water wells in Illinois

have been treated with acid, but wider
application of this method of increasing
yields seems merited. Commercial acid-
izing services use non-toxic inhibitors
particularly adapted to use in water
wells, so that they will not react un-
favorably on any of the well casing,
liners, or equipment.

SUMMARY

Geophysical methods have proved suc-
cessful in solving many of the problems
that arise in drilling, completing, and
producing water wells. The geophysical
surveys made of Illinois water wells and
research in groundwater geology have
furnished much factual material that,
when properly integrated, permits sound
interpretation of groundwater phenom-
ena and subsurface conditions. Such
knowledge can be used to correct many
of the defects in old wells and to plan
more intelligent development and con-
servation of groundwater resources in
the future.

As the investigations are continued it
is expected that many of the questions in
geophysical surveys and interpretations
will be answered, and that new tech-
niques and instruments can be developed
for solving other problems in ground-
water work, not only in Illinois but in
many other areas where the same or
similar problems are encountered.

215