Microtime Measurements in 

Aquifer Tests on Open-Hole 

Artesian Wells

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1545-A

Prepared in cooperation with the 

Board of Commissioners, Martin County, 

North Carolina

of

GROUND V/AI6R
Microtime Measurements in 

Aquifer Tests on Open-Hole 

Artesian Wells

By GRANVILLE G. WYRICK and EDWIN O. FLOYD

METHODS OF AQUIFER TESTS

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1545-A

Prepared in cooperation with the 

Board of Commissioners, Martin County, 

North Carolina

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D.C. - Price 15 cents (paper cover)
CONTENTS

Abstract-_-_____-____-_--___--__-___-_------_-

Introduction^ __-_____-__--_______-_-____-_-___

Test sites..___________________________________

Recovery tests________________________________

Methods of photographing recovery measurements. 

Analysis of data._.__-____._________._____._-__

Summary and conclusion____________^__________

Reference _____________________________________

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ILLUSTRATIONS

FIGURE 1. Map of Martin County, N.C., showing location of wells used 

for microtime-measurement tests._______________________

2. Piezometric map of test area, November 1959____-_- ________

3. Analysis of data from well 69 (first test)_____-_-------------

4. Analysis of data from control well 311_ _ _ __-_____--_-_-____

5. Photographs of equipment used in microtime measurements. _

6. Hydrograph and corrected curve for first part of second test 

at well 69_______--____-______-._-___-----_---_------_

7. Analysis of data from well 69 (second test)_________________

in

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METHODS OF AQUIFER TESTS

MICROTIME MEASUREMENTS IN AQUIFER TEST ON 

OPEN-HOLE ARTESIAN WELLS

BY GRANVILLE G. WYRICK and EDWIN O. FLOYD

ABSTRACT

Water-level and time measurements made during the early seconds or minutes 

of drawdown or recovery tests on a pumped or flowing well may be used to 

determine the transmissibility of an artesian aquifer. This report describes 

a photographic method, using a motion picture camera, stopwatch, and manometer tube, of recording water-level measurements and microtime measurements (time measurements at intervals of about %5 second) in a well penetrating a leaky artesian aquifer. The results of the test are compared with the 

results obtained from conventional measurements at a control well. Further 

tests are necessary to determine the utility of this method under different field 

conditions.

INTRODUCTION

There are few satisfactory methods for determining the coefficient 

of transmissibility of an artesian aquifer from water-level measurements made at pumped or flowing wells unless data from nearby 

observation wells are used. One of the problems involved in using 

a single well for a drawdown or recovery test is that the water level 

changes so rapidly in the initial part of the test that it cannot be 

measured accurately, and it follows that the plot of the first part 

of the drawdown or recovery curve is only an approximation. The 

inaccuracy can be an especially serious problem if leakage or barrier 

effects exert an effect early in the test. When a single well is used, 

a rapid and precise method for obtaining time and water-level 

measurements in the early part of drawdown or recovery tests 

is needed. This paper describes such a method which may supplement the conventional types of measurements, especially when observation wells are not available.

The method was developed in the course of a detailed investigation 

of the ground-water resources of Martin County, N.C., being made 

by the U.S. Geological Survey in cooperation with the Martin County 

Board of Commissioners.

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A-2 METHODS OF AQUIFER TESTS

TEST SITES

Two wells in Martin County, each fully penetrating the same 

aquifer, were selected for recovery tests (fig. 1). Well 69 1 is about 

10 feet from the Roanoke River in an area where the piezometric 

map indicates considerable upward leakage from the artesian aquifer 

(fig. 2). Well 311 is about 4 miles from the Roanoke River in an 

area where the piezometric map indicates no leakage or recharge. 

Well 311 was selected as a control well for the tests because of the 

lack of indications of leakage and because the transmissibility determined at the well compares well with values determined in other tests 

on the same aquifer.

Both wells are finished as open holes in a porous limestone aquifer. 

This aquifer is about 17 feet thick in well 69 and 12 feet thick in 

well 311. The aquifer at both locations is overlain by clay and underlain by silty, clayey sand. The overlying clay has been thinned and 

possibly breached by the channel of the Roanoke River near well 69. 

Both .wells had flowed freely for several months prior to being tested.

Martin County

FIGURE 1. Map of Martin County, N.C., showing location of wells used for microtimemeasurement tests.

1 Well numbers conform to the consecutive county numbering system used by the 

U.S. Geological Survey in Martin County, N.C., in order that the wells used in these 

tests may be identified in other published reports.
MICROTIME MEASUREMENTS IN ARTESIAN AQUIFER TESTS A-3

FIGURE 2, Piezometric map of test area, November 1959. 

RECOVERY TESTS

A conventional recovery test was made at each well; the measurements of head were made by measuring the rise in water level in a 

clear plastic hose that was connected to a tap in the well casing. 

Figure 3 shows a semilog plot of the data from the test at well 69. 

The semilog plot (Cooper and Jacob, 1946, p. 526-534) was selected 

for analyzing these data because the radius (r) was sufficiently small 

to permit use of this method and because a break in the curve due 

to leaky-aquifer conditions is more noticeable on a straight-line plot 

than on the Theis curve. Both wells had flowed freely for several 

months, so that the artesian head was declining very slowly. Therefore, it was considered unnecessary to correct the recovery measurements for the pretest water-level trend as the tests were short. The 

only part of the plot (fig. 3) that can be used to calculate the coefficient of transmissibility (T) of the aquifer occurs after about 

2 minutes of recovery. From this straight-line part of the curve, 

T is calculated as 80,000 gpd (gallons per day) per foot. As this 

is about 7 times the value of 12,000 gpd per foot for T in well 311 

(fig. 4) and as the value of 80,000 is inconsistent with values determined for the aquifer elsewhere, it is assumed that the part of the 

recovery curve used was already affected by leakage.
A-4 METHODS OF AQUIFER TESTS

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A-6 METHODS OF AQUIFER TESTS

METHOD OF PHOTOGRAPHING RECOVERY 

MEASUREMENTS

A second recovery test was made on well 69 during which the water 

level was measured by means of a mercury manometer. The test 

setup and a typical frame from the film strip are shown in figure 5. 

The manometer tube and a stopwatch were photographed with an 

8-mm. motion-picture camera that could be adjusted to nominal values 

of 12, 16, 24, and 32 frames per second. The camera was focused to 

include both the manometer tube and the stopwatch in each frame 

and was set at the position for 24 frames per second. However, after 

the film was developed it was found, by counting the number of 

frames for each 1-second interval on the stopwatch, that the camera 

had actually operated at 25 frames per second. Thus, the change in 

pressure in the manometer tube was recorded at intervals of %5 

second.

The camera and stopwatch were started before the valve on the 

discharge line of the well was closed. The valve was closed as rapidly 

as possible, within 3 to 4 seconds, and the manometer tube and stopwatch were photographed at intervals of y25 second for the first 

minute and at intervals of 30 seconds thereafter for the next 8 minutes 

of the recovery period. For the first minute on the graph (fig. 6) 

only the measurements at about 14-second intervals were used. The 

data were taken from the film by projecting the film with a still 

projector, so that the projected image of the manometer tube was 

almost 1/1.1 natural size. At this magnification a standard foot scale 

could be used as a vernier with the projected manometer scale. Thus, 

changes in the manometer tube could be measured with a precision 

equivalent to a few hundredths of a foot of water.

ANALYSIS OF DATA

When the test was completed and the data were taken from the 

photographs and plotted on a hydrograph, the first 14 seconds of 

the hydrograph shows fluctuations which resulted from surges due 

to closing the valve. These fluctuations do not appreciably affect 

the solution of this test since they lasted for only about 10 percent 

of the critical part of the curve. However, since there might be 

longer periods of fluctuations under different hydrologic conditions, 

the fluctuations were corrected, and the corrected data were used in 

solving this test.

The correction was made graphically by tracing the hydrograph, 

inverting and matching the traced hydrograph to the original hydrograph, and picking the corrected curve from the points where the
MICROTIME MEASUREMENTS IN ARTESIAN AQUIFER TESTS A-7

FIGURE 5. Photographs of equipment used in microtime measurements. 

A, Typical equipment setup. B, Typical frame from film strip.
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MICROTIME MEASUREMENTS IN ARTESIAN AQUIFER TESTS A-9

two hydrographs intersected. Figure 6 shows the measured and corrected hydrographs from 2 seconds to 14 seconds after flow stopped.

A semilog plot of the data from the second test at well 69 is shown 

in figure Y. As may be seen, the plot of data, from about 6 seconds 

to about 2 minutes after flow stopped, falls in a straight line 

which when analyzed gives T= 18,000 gpd per foot (solution 1). The 

part of the curve preceding this segment is not valid because of the 

3 or 4 seconds required to close the discharge valve completely. The 

part of the curve following this segment reflects the apparent change 

in transmissibility due to the effects of leakage from the aquifer to 

the river (solution 2). The value for T in solution 2 compares very 

well with the value for T determined in the first test of this well. 

The second tesf (fig. Y) at well 69 was continued for 9 minutes, which 

was about 6 minutes after the first test (fig. 3) indicated that leakage 

had started. The semilog plots of the first and second tests at well 

69 (figs. 3 and Y) may not be compared directly for two reasons. 

First, the time measurement in the first test may be in error as much 

as 15 or 20 seconds which would result in considerable error in the 

semilog plot between 0.1 and 1.0 minutes. This source of error has 

been eliminated by photographing the measurements in the second 

test. Also, the possible number of measurements made during the 

first minute has been increased from 6 or Y questionable measurements in the first test to about 1,500 accurate measurements in the 

second test. The second reason that the two semilog plots may not 

be compared directly is that Q (yield in gallons per minute) decreased about Y percent between the first and second test. Thus, the 

amount of water-level recovery for any given time during the early 

part of recovery would also decrease from the first to the second test.

The value of T (solution 1) was converted to the field coefficient 

of permeability (Pf} for comparison with Pf at the control well (fig. 

4). Pf at well 69 was about 1,100 gpd per square foot, and Pf at 

well 311 was 1,000 gpd per square foot.

SUMMARY AND CONCLUSION

A comparison of the results of the recovery test at control well 311 

and with those of the second test at well 69 indicates that the method 

of photographically recording the early measurements of a drawdown or recovery test gives accurate results and can aid considerably 

in determining the correct coefficient of transmissibility of an aquifer 

if certain conditions are met. The conditions are as follows: (a) The 

method is restricted to use in artesian aquifers, and (b) the effective 

radius of the well tested~must be considered in the calculations. Only
A-10 METHODS OF AQUIFER TESTS

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MICROTIME MEASUREMENTS IN ARTESIAN AQUIFER TESTS A-ll

in an open-hole (unscreened) well can it be assumed with any confidence, in the absence of correlating data from other wells, that 

the effective radius of the well is substantially equal to the actual 

radius.

This method of measuring the early part of the drawndown or 

recovery has not been used in tests on fully developed screened wells. 

It is anticipated that such tests will be made, and that a later paper 

will discuss more fully the advantages and limitations of this method 

in tests on other types of wells and aquifers.

REFERENCE

Cooper, H. H., Jr., and Jacob, C. E., 1946, A generalized graphical method for 

evaluating formation constants and summarizing well-field history: Am. 

Geophys. Union Trans., v. 27, p. 526-534.

U. S. GOVERNMENT PRINTING OFFICE : 1961 O - 592394