U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3048

June 2007

Borehole Geophysical Logging of Water-Supply Wells in 

the Piedmont, Blue Ridge, and Valley and Ridge, Georgia

Crystalline and carbonate-rock aquifers in northern Georgia provide water to an everincreasing number of private and public wells in the region. Understanding the depth and 

yield of water-bearing zones in such wells is crucial for the development and long-term 

sustainability of ground-water resources and for keeping wells in good operating condition. 

Portable geophysical logging units are now widely available and have greatly increased the 

ability of geoscientists to obtain subsurface information from water wells.

What Is Borehole Geophysical Logging?

Many different types of tools are used to collect subsurface data 

in wells (Haeni and others, 2001; Keys, 1990, 1997). These 

include submersible cameras and highly sophisticated instruments that measure water quality, detect lithologies, and measure 

formation properties. Geophysical logging tools are lowered 

into a well by a long cable and continuously record the physical properties of the rock and fluid. Resulting data are presented 

in graphs called borehole geophysical logs. A caliper log, for 

example, is a continuous record of the well diameter that can be 

used to detect fracture openings or changes in well construction.

Why Log a Well?

Geophysical logs are extremely important to gain a better understanding of the aquifer system and fractures supplying a well. 

Drilled wells in northern Georgia are almost always constructed 

with an “open hole,” which means the borehole is unlined and 

open to the bedrock below the surface casing (sketch A, right). 

Water enters the well through fractures and other openings in 

the otherwise solid rock. In such open-hole wells, it is important to ensure that water-bearing fractures are not exposed to air 

due to overpumping or declining water levels. Iron fouling and 

water-quality degradation may result if the water level is lowered below a water-bearing fracture and water cascades into the 

open portion of the well and mixes with air (sketch B, below). 

This situation can be avoided by maintaining the pumping level 

above any water-bearing zone (A) or using a casing or liner above 

the fracture (sketch C, below). Although the depths of some 

water-bearing zones are known from the driller’s log, not all 

zones are detected. Borehole geophysical logs help locate all of 

the water-bearing zones in a well. With this information, a well 

can be constructed or refurbished to prevent water from entering 

the borehole from undesirable locations. Logs also are used 

to evaluate casing condition; well construction; grouting seal 

of new, old, or refurbished wells; lithologic or geologic units; 

relations among water-producing zones and lithologic units; and 

water-quality problems as described on the following pages.

Geophysical logs can be used to detect and prevent water-bearing 

zones from being exposed in the open portion of the borehole. (A) shows 

a correct pumping water level, (B) shows an over-pumped well without 

a liner, and (C) shows a liner that seals off shallow-water zones.

Upper

waterbearing

zone

Upper

waterbearing

zone

sealed

off by

liner

Pumping

water level

Pump

A. Correctly

pumped well

without liner

Pump

Liner

C. Well

with liner

Well casing Well casing

Lower

waterbearing

zone

Lower

waterbearing

zone

Lower

waterbearing

zone

Well casing Pump

Cascading

water and

iron fouling

Pumping

water

level

B. Overpumped well

without liner

(A) As the caliper tool crosses a fracture, the arms open slightly, 

creating the inflection on the caliper log (B). This example log 

shows a fracture at about 42 feet below land surface.

Fracture

Caliper, in inches 

6-inch

steel casing

to 28 feet

0

10

20

30

40

50

60

5 6

Fracture

7 8

A.

Casing

B.

G EORGIA

Atlanta
Caliper logging continuously records well diameter and can be used to 

detect fracture openings 

or changes in borehole 

diameter. A log is collected 

by lowering the tool to the 

bottom of a well, opening 

the arms, and then pulling 

it up the borehole, allowing 

the spring-loaded arms to 

open or close as they pass 

borehole enlargements, or 

restrictions (Keys, 1990). 

Changes in the borehole 

diameter may be related to 

fractures, changes in lithology, borehole construction, 

or hole integrity. A caliper 

log mainly is used to locate 

fracture openings in the 

bedrock as shown in this 

caliper log obtained from a 

crystalline-bedrock well.

Natural gamma logging

measures gamma radiation emitted by subsurface 

rock layers and is recorded 

in American Petroleum 

Institute units (APIU). 

This tool can be run in 

either open or cased holes 

and is used to determine 

lithology. Typically, slight 

changes in the APIU baseline may be observed for 

igneous and metamorphic 

rocks. The tool also may 

be used to detect the presence of radioactive zones, 

which may degrade water 

quality in a well.

Long- and short-normal 

and lateral resistivity logging measures the apparent 

resistivity in ohm meters. 

The tool applies a constant 

current across two electrodes while measuring 

the potential between two 

other electrodes. All three 

resistivity logs are collected 

simultaneously and can 

be compared. Long- and 

short-normal logs measure the resistivity over a 

larger area surrounding a 

borehole. Lateral resistivity 

measures resistivity close to 

a borehole. In hard, resistive 

crystalline bedrock, waterbearing zones typically are 

indicated by low resistivity. 

This example lateral resistivity log helps distinguish 

the water-bearing zones in 

a crystalline-bedrock well. 

Fluid logging methods

measure properties of the 

water column in a borehole 

and typically include the 

simultaneous measurement of fluid resistivity 

and temperature. Changes 

or inflections in the fluid 

temperature or resistivity 

are an indication of possible water-producing and 

water-receiving zones in 

a well. Fluid logs also can 

be collected during pumping to identify water-bearing zones. Borehole grab 

samples may be used to 

collect water samples at 

discrete depths.

Open

waterbearing

fractures

0

100

200

300

400

CALIPER, IN INCHES

DEPTH BELOW LAND SURFACE, IN FEET

5.5 6.5 7.5 8.5 9.5 10.5

8.3-inch casing

to 25 feet

8-inch borehole

6-inch

borehole

Biotite Gneiss Schist Amphibolite

GAMMA, IN APIU

0 100 200 300 400 500

Baseline

gamma

response of

amphibolite

Gamma

response

of buttonschist

Gamma

response and

fractures

related to

lithologic

contacts in

the biotitegneiss

Fracture

Fracture

Fracture

Fracture

LATERAL RESISTIVITY,

IN OHM METERS

0

100

200

300

400

Waterbearing

zones

Note: This is from 

the same well as the

caliper log above.

Response

of resistive,

unfractured

crystalline

rock

DEPTH BELOW LAND SURFACE, IN FEET

0 100 200 300 400 0

100

200

300

400

DEPTH BELOW LAND SURFACE, IN FEET

65 66 67 68 69 70

FLUID TEMPERATURE,

IN DEGREES FAHRENHEIT

Potential

waterproducing

zone

Inflection

(potential

waterproducing

zone)
Flowmeter logging measures the direction and 

magnitude of vertical fluid flow within a borehole, 

making it an ideal tool to identify water-producing or water-losing fractures in a well. Flowmeter 

measurements are collected at discrete locations, 

usually above and below fractures identified using 

logs, or as a continuous log in a trolling mode 

(Paillet, 1998, 2000).

How flowmeter logging works. The example to 

the right shows measurements taken at discrete 

locations. The measurements taken above and 

below the bottom fracture (at locations A and B) 

are both near zero, indicating no measurable flow 

from that zone. Measurements taken straddling 

the next fracture (C and D) show an increase 

in vertical flow from 0 to about 2.5 gallons per 

minute (gal/min); therefore, that fracture produces 

2.5 gal/min. The next pair of measurements show 

an increase of about 1.5 gal/min indicating another 

water-bearing zone (between D and E). Flow 

measurements above E indicate no additional 

contribution to flow in the well (F and G).

Flowmeter types:

The electromagnetic flowmeter with a flow diverter 

can measure flows between 

0.1 and 15 gal/min. Greater 

flows (100 gal/min or more) 

can be measured with proper 

calibration of the flowmeter 

while using an underfit flow 

diverter that allows some of the 

vertical flow to bypass the tool 

(Paillet, 2000).

The heat-pulse flowmeter

with a flow diverter can 

measure flows as small as 

0.01 +/– 0.005 gal/min and as 

great as 1.5 gal/min. The heatpulse flowmeter uses a thermal 

trace to measure the direction 

and rate of vertical flow in a 

borehole (Paillet, 2000).

The spinner flowmeter measures vertical flow by recording 

the rotation rate of an impeller. 

The lowest velocity a typical 

spinner flowmeter can measure 

is about 5 feet per minute, 

which limits its use to higher 

flow rates.

The electromagnetic flowmeter log shown below was collected 

under both nonpumping and pumping conditions. Under nonpumping conditions, water enters the borehole through multiple 

fractures between 100 and 250 feet and exits the borehole into 

shallow fractures near the base of the casing (30 feet). Under 

pumping conditions, the flow contribution from various zones 

was determined; however, pumping did not change the flow 

relations in the well and water continued to exit the borehole 

near the base of the casing. A deeper casing was added to 

rectify this problem.

40

80

120

160

200

240

0 2.5 5

UPWARD FLOW, IN 

GALLONS PER MINUTE

DEPTH BELOW LAND SURFACE, IN FEET

A

B

C

D

E

F

G

Tool positioned

for measurement B

Tool positioned

for measurement E

Flowmeter log

Inflow

Measurement E

Measurement B

Fracture

UPWARD FLOW, IN GALLONS PER MINUTE

Electromagnetic flowmeter

DEPTH BELOW LAND SURFACE, IN FEET

0

50

100

150

200

250

300

350

400

Casing to 30 feet, well not flowing

Non-pumping flow conditions

Pumping at 50 gallons per minute

Measured flow

Interpolated

CALIPER,

IN INCHES

Inflow

Outflow

100 20 30 40 50 60 70 80 5 7 9 11 13
Geophysical Log Database

Geophysical logs provide a basis for larger-scale evaluations 

of the occurrence and availability of ground-water resources 

throughout Georgia. Paper copies and digital records of welllog data for wells in Georgia are stored at the U.S. Geological 

Survey (USGS) office in Atlanta. New logs are being collected 

in cooperation with State and local water agencies and communities through the USGS Cooperative Water-Resources Program 

(Clarke, 2006). 

Summary

When planning for the long-term use of a ground-water source, 

borehole geophysical logs are extremely important to gain a 

better understanding of the aquifer system and water-bearing 

fractures supplying a well. Logs are used to

locate and describe fractures and rock units;

evaluate relations among rock types and yield;

measure yield and water quality of producing zones;

identify water-quality problems; and

check casing condition, well construction, and 

grouting seal of new, old, or refurbished wells.

Selected References

Clarke, J.S., 2006, Helping Solve Georgia’s Water Problems—The 

USGS Cooperative Water Program: U.S. Geological Survey Fact 

Sheet 2006-3032, 4 pages, Web-only publication available at

http://pubs.usgs.gov/fs/2006/3032/.

Haeni, F.P., Lane, J.W., Jr., Williams, J.W., and Johnson, C.D., 2001, 

Use of a geophysical toolbox to characterize ground-water flow in 

fractured rock, in Proceedings of the Fractured Rock Conference, 

March 26 –28, 2001, Toronto, Ontario, CD–ROM.

Johnson, C.D., and Dunstan, A.H., 1998, Lithology and fracture 

characterization from drilling investigations in the Mirror Lake area, 

Grafton County, New Hampshire: U.S. Geological Survey WaterResources Investigations Report 98-4183, 211 p.

•

•

•

•

•

Keys, W.S., 1990, Borehole geophysics applied to ground-water investigations: U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 2, chap. E-2, 150 p.

Keys, W.S., 1997, A practical guide to borehole geophysics in 

environmental investigations: USA, CRC Press, Inc., 176 p.

Paillet, F.L., 1998, Flow modeling and permeability estimation using 

borehole flow logs in heterogeneous fractured formations: Water 

Resources Research, v. 34, no. 5, p. 997–1010.

Paillet, F.L., 2000, Flow logging in difficult boreholes—Making the 

best of a bad deal, in Proceedings of the 7th International Symposium on Borehole Geophysics for Minerals, Geotechnical, and 

Groundwater Applications, Denver, Colo., 2000: The Minerals and 

Geotechnical Logging Society, A Chapter at Large of the Society of 

Professional Well Log Analysts, Houston, Tex., p. 125 –135.

Shapiro, A.M., Hsieh, P.A., and Haeni, F.P., 1999, Integrating multidisciplinary investigations in the characterization of fractured rock, 

in Proceedings of the Technical Meeting of the U.S. Geological 

Survey Toxic Substances Hydrology Program, Morganwalp, D.W., 

and Buxton, H.T., eds., Charleston, South Carolina, held 

March 8 –12, 1999: U.S. Geological Survey Water-Resources 

Investigations Report 99-4018C, v. 3, p. 669 –680.

Williams, J.H., and Johnson, C.D., 2000, Borehole-wall imaging 

with acoustic and optical televiewers for fractured-bedrock aquifer 

investigations, in Proceedings of the 7th Minerals and Geotechnical 

Logging Symposium, Golden, Colo., October 24 –26, 2000: Minerals and Geotechnical Logging Society, p. 43–53, CD–ROM. 

For more information on the USGS Geophysical Logging Program

Contact: Lester J. Williams or Michael F. Peck

U.S. Geological Survey, Georgia Water Science Center

3039 Amwiler Road, Suite 130

Atlanta, Georgia 30360-2824

Phone: 404.906.5761 or 770.903.9100

E-mail: lesterw@usgs.gov or mfpeck@usgs.gov

Camera logging records 

both downhole and side-looking views of a borehole and 

permits direct inspection of 

the borehole wall and details 

of the well construction. 

The images can be used to 

identify changes in rock type 

and small-scale geologic 

structures, locate and describe 

fractures, and identify problems with borehole integrity 

and possible signs of contamination (Johnson and Dunstan, 1998). This method of 

borehole imaging is relatively 

cost-effective and logs can be 

collected quickly. However, 

detailed interpretation of 

video logs can be time-consuming.

Optical televiewer (OTV) logging produces highresolution, oriented color digital images of a borehole wall. 

This method of imaging is relatively time-consuming; however, it is preferred over camera logging in open bedrock 

wells because of the ability to map lithologic units and 

fractures using a single log. Because it is an optical tool, 

the log can be only run 

in relatively clear water. 

Acoustic televiewers 

(not shown) can be run 

in both water and mudfilled holes.

This example OTV log 

shows lower angle foliation 

or layering cut by steeper 

joints in the borehole. The 

location and orientation of 

fractures in relation to rock 

fabric can be particularly 

useful in water-resource 

evaluations.

Borehole camera images 

showing views of waterproducing fracture.

Downhole view

Side-looking view

Design and layout by Caryl J. Wipperfurth

Editing by Patricia L. Nobles

U.S. DEPARTMENT OF THE INTERIOR

DIRK KEMPTHORNE, Secretary

U.S. GEOLOGICAL SURVEY

Mark D. Myers, Director

2-D PROJECTION CORE VIEW

311

309

313

315

DEPTH, IN FEET BELOW LAND SURFACE

Steep

joints

Low-angle

foliation

N E S W N

Fracture

Fracture