United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas IW 89114
Research and Development
EPA-600/S4-84-065 Aug. 1984
Project Summary
Geophysical Methods for
Locating Abandoned Wells
F. C. Frischknecht, L. Muth, R. Grette, T. Buckley, B. Kornegay, and J. van Ee
Field measurements with spontane-
ous potential (SP), electromagnetics,
and magnetometers indicated that a
magnetometer could be used to locate
steel-cased abandoned wells. A
mathematical model using a set of
magnetic pole pairs was developed to
evaluate the applicability of an airborne
magnetic survey to locate abandoned
wells. The modeling indicated that
airborne measurements made at a
height of 200 feet above the ground
with passes spaced at between 300 to
400 feet could detect with a high level
of confidence the steel-cased wells
within the search area. The aerial
magnetic measurements permit a
reduction in the noise from cultural and
geological sources while the magnetic
anomaly broadens; thus, fewer passes
with an airplane are required than with a
surface magnetometer.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory. Las Vegas. NV, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
A provision of the Underground Injec-
tion Control Regulations issued by the
U.S. Environmental Protection Agency
(EPA) establishes a radius of review for
proposed injection wells. The hydrologi-
cal properties of the subsurface are used
to predict the area affected within an
injection zone. Any abandoned wells
within this calculated area could serve as
possible conduits for the transmission of
injected wastes into a potable water zone.
In the United States, it has been
estimated that there are approximately
500,000 municipal, industrial,
commercial, agricultural, and domestic
wells injecting fluids below the surface
and approximately 5,000 new injection
wells are being constructed each year.
Abandoned wells located near these
injection wells must be located to prevent
contamination of the potable water
aquifer; however, records of the locations
of these abandoned wells may be
imprecise, incomplete, out-of-date, and
difficult to locate. Record searches may
prove to be time-consuming and
expensive, and there is some chance that
a record search will fail to show an-
abandoned well that falls within a radius
of review for an injection well. Other
means for locating abandoned wells are
needed.
The EPA's Environmental Photograph-
ic Interpretation Center (EPIC) is
investigating the use of historical photo-
graphic searches to locate abandoned
wells. Aerial photographs of large areas
of the country exist as far back as the
early thirties and these photographic
searches may either supplement or
complement the record search as a
means for locating abandoned wells. An
advantage of this method is the ability to
locate wells in which the casing was
either nonmetallic or removed; however
the method is limited to the quantity,
quality, and timeliness of the
photographs that cover an area.
Geophysical methods offer another
means for locating abandoned wells.
Although some geophysical methods may
detect an abandoned well without steel-
casing when the well is in close proximity
to the instrument, for all practical
purposes, geophysics offers little hope for
detecting wells without casing. The mag-
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netometer is the most practical
geophysical method for locating steel-
cased abandoned wells.
The magnetic method has been the
focus of an EPA-funded U. S. Geological
Survey (LJSGS) study of geophysical
methods for locating abandoned wells.
Two scenarios can be envisioned in
which a magnetic well survey would be
useful. First, an injection well operation
may identify a proposed site for an
injection well and all wells (including
abandoned wells) in the vicinity of the
proposed site would need to be located.
The second scenario involves the
characterization of large areas where
injection wells could be permitted with
little chance of harm occurring. While
aircraft-based magnetometer surveys
may be economical in the first scenario,
the method appears to offer the greatest
potential for the second scenario.
Procedure
Ground-based magnetic measure-
ments were made in the vicinity of
several steel-cased wells in Colorado to
characterize the magnetic anomaly and
to determine if the anomaly could be
detected from an aircraft. To evaluate the
spacings and altitudes required for a
magnetic survey for abandoned wells, a
mathematical model was developed for
18 wells in the Denver area. A finite
number of magnetic dipoles were used in
the model. Surface magnetometry
measurements were made to adjust the
models for the test wells to produce a map
of the magnetic anomaly on the surface
that was consistent with field
measurements. The measurement of the
magnetic anomaly at various heights and
distances above the well could then be
assessed to determine if actual field tests
with an aircraft were practical and
economical.
Electrical geophysical measurements
were also made on the surface in the
vicinity of the steel-cased wells; however,
the emphasis was on the characterization
of the magnetic anomaly.
Results and Discussion
Ground-based magnetic measure-
ments made in the vicinity of the test
wells in Denver yielded total field
anomalies with peak values ranging from
about 1,500 to 6,000 gammas. The
anomalies measured on the ground were
very narrow; and, considering noise due
to other cultural and geologic sources, a
line spacing on the order of BOfeet would
be necessary to locate all casings in the
test area.
By use of a nonlinear least squares
curve-fitting (inversion) program, model
parameters which characterize each test
casing were determined. The position
and strength of the upper-most pole was
usually well resolved. The parameters of
lower poles were not as well resolved, but
it appears that the results are adequate
for predicting the anomalies which would
be observed at aircraft altitudes.
Modeling based on the parameters
determined from the ground data
indicates that all of the test casings would
be detected by airborne measurements
made at heights of 150 to 200 feet above
the ground, provided lines spaced as
closely as 330 feet were used and
provided noise due to other cultural and
geological sources is not very large.
Given the noise levels of currently
available equipment and assuming very
low magnetic gradients due to geological
sources, the detection rangefortotalfield
measurements is greater than that for
measurements of the horizontal or
vertical gradient of the total intensity.
Electrical self-potential anomalies
were found to be associated with most of
the casings where measurements were
made. However, the anomalies tend to be
very narrow and, in several cases, they
are comparable in magnitude to other
small anomalies which are not directly
associated with casings. Measurements
made with a terrain conductivity meter
and slingram system were negative.
The principal effect of varying the
aircraft altitude is to decrease the peak
amplitude and to broaden the anomaly.
By magnetic compensation of the aircraft
and by recording and correcting for the
motions of the aircraft, the noise level of
an airborne system can be reduced to
about 0.2 gamma or better for the total
field as measured by sensors mounted in
wing-tip pods or tail stingers. Total field
anomalies are slightly broader in the
east-west direction than in the north-
south direction. Also, magnetometer
system noise is likely to be slightly less on
north-south lines than on east-west
lines. Therefore, there is a small
advantage in flying total field surveys in a
north-south rather than east-west
direction. If the smallest identifiable total
field anomaly is assumed to be five times
the expected noise level, or 1 gamma,
then the data from the test wells near
Denver indicate a line spacing of 330feet
will detect all the steel-cased wells in a
rural area.
If the density of wells in an area is very
high, it may be difficult to identify individ-
ual wells using airborne surveys. Use of
small spacings between lines will of!
course help to resolve anomalies due to
individual wells. If, for instance, the
density of wells were 2000 per square
mile as exists in the vicinity of Oklahoma
City, the average spacing between wells
would be only about 118 feet. In such an
area, an airborne survey would ha veto be
made at a height of 50 feet or less with a
line spacing on the order of 50 feet or less
to be able to resolve most of the individual
anomalies. If the density of wells is
extremely high and all wells must be
identified and located, it might be more
practical to use ground measurements
rather than airborne measurements. Of
course, if the density of wells is extremely
high, it may not be necessary to identify
separately all of the wells in a cluster.
The cost for a ground magnetic survey
using two-person crew (including sala-
ries, living expenses, vehicle, equipment
rental, supplies, and overhead, but not
including mobilization to the field site)
has been estimated to be at least
$10,000/4 weeks (1983 costs). By
working a reasonable amount of
overtime, this crew might survey,
process, and plot a maximum of 160 line-
miles in 4 weeks. If a line spacing of 50
feet (15.2 m) were used, this would cover
an area of 1.52 square miles (3.94 km2).
This represents a cost of $6000/square
mile (2.59 km2) covered and $62.50/line-
mile ($38.84/line-km). This is less than
one-half the line-mile costs for mineral
exploration, but much mineral explora-
tion is done in heavily vegetated terrain.
Rates of production would be less, and
costs greater, if the crew had to spend
time in obtaining landowner's authoriza-
tion for access to the land or if the crew
interpreted the results and did additional
detailed work to "pinpoint" the location of
suspected casings or investigate ques-
tionable anomalies.
For areas larger than a few square
miles, airborne surveys are likely to be
considerably less expensive than ground
surveys. Airborne surveys are not
practical for very small areas due to the
high costs of mobilization.
Costs for routine aeromagnetic surveys
using small fixed-wing aircraft are on the
order of $8-14/line mile ($5-9/line-km)
including data processing, provided: (1)
several thousand line miles are flown in
one block, (2) the lines are at least 10-20
miles (16-32 km) long, and (3) Doppler
radar and photographic methods are used
for flight path recovery. The costs for
similar work done with rotary-wing air-
craft are about $25-30/line mile ($16
19/line-km). Costs per line mile are much
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greater if the lines are short and the areas
small, and if a microwave navigation
system is required.
Conclusions and Future Actions
The mathematical model of a magnetic
anomaly, developed by the USGS, for a
steel-cased well indicates that an air-
borne magnetic survey can theoretically
locate abandoned wells. Several areas
near Oklahoma City, will be surveyed in
1983 to test the theory. The University of
Oklahoma's Environmental and
Groundwater Institute will select the test
areas from a search of well drilling
records. The EPIC will examine the same
test areas with historical photographs to
further substantiate the location of wells
in the areas.
The cooperative effort between the
EPA and the USGS is aimed at providing
local, state and federal agencies with the
methodology to determine if abandoned
wells exist in an area where the under-
ground injection of wastes is contempla-
ted. Magnetometer surveys will likely
be just one of several met hods that can be
utilized in locating abandoned wells. The
record searches conducted by the
Environmental and Groundwater
Institute and the historical photographic
searches conducted by the EPA's EPIC
may provide alternative approaches to
the problem of locating abandoned wells
in other areas.The costs of conducting an
airborne magnetic survey have been
estimated but the costs must be weighed
against the alternative approaches. An
assessment of the costs and the benefits
of the various approaches will be possible
after the USGS has completed overflights
of those areas near Oklahoma City that
have been examined through record and
historical photographic searches.
F. C. Frischknecht, L Muth. R. Grette, T. Buckley, and B. Kornegay are with the
U.S. Geological Survey. Denver. CO 80225.
J. Jeffrey van Ee is the EPA Project Officer (see below).
The complete report, entitled "Geophysical Methods for Locating Abandoned
Wells," (Order No. PB 84-212 711; Cost: $20.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
P.O. Box 15027
Las Vegas, NV 89114
U.S. GOVERNMENT PRINTING OFFICE; 1984 — 759-015/7762
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