METHODS OF MEASURING WATER LEVELS IN DEEP WELLS

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Techniques of Water-ResourcesInvestigationsOF the United States GeologicalSurveyChapter A1 METHODS OF MEASURING WATER LEVELSIN DEEPWELLSBy M. S. Garber and F. C. KoopmanPreparedU.S. AtomicA discussionNevadaon behalfof theEnergy Commissionof the techniquesdevelopedatTest Site and other locationsBook 8INSTRUMENTATIONClick here to return to USGS Publications

UNITEDSTATES DEPARTMENTCECIL D. ANDRUS,GEOLOGICALH. WilliamOF THE INTERIORSecretarySURVEYMenard,First printingSecondprintingDirector19681969Third printing 1978UNITED STATES GOVERNMENTPRINTINGOFFICE, WASHINGTONFor sole by the Branch of Distribution,U.S. Geological1200 South Eods Street, Arlington, VA 22202Survey,: 1968

PREFACEThe series of manuals on techniques describes procedures for planningand executing specialized work in water-resources investigations. Thematerial is grouped under major subject headings called “Books” andfurther subdivided into sections and chapters. Section A of Book 8 is oninstruments for measurement of water level.The unit of publication, the chapter, is limited to a narrow field ofsubject matter. This format permits flexibility in revision and publicationas the need arises. “Methods of measuring water levels in deep wells” isthe first chapter to be published under Section A of Book 8.111

CONTENTSPagePreface . . . . . . . . . . . . . . . . . . . . . . . .Abstract .Introduction.Purpose and scope .Previous work .Methods of water-level measurement studied .Steel t.ape .Electric cable .***111111z226PageMethods of water-level measurement studied-Con.Air line .Recording devices .Surface recording devices .Bottom-hole recording devices .Summary .References 14.15.Photographs of 2,000-foot steel measuring tape .Nomograph for determining thermal expansion of steel surveying tape .Photographs of electric cable well-measuring device .Diggrams of water-level sensing probes used with electric-cable device .Calibration curves for three cable devices .Diagrarms showing air-line method .density for temperature range 70”-190 F .Graph of distilled-waterPhotographs of two types of water-level recorders .Photographs of modified recorders .Diagra.m of system used to measure response of aquifers to nuclear experiment in Mississippi,October 1964 . .Photographs of equipment for measuring aquifer response .Record of early pressure response in well HT-5 to Salmon event, Tatum Dome, Lamar County,Miss .Record taken from a bottom-hole pressure recorder .Diagram of typical straddle-packer installation .Bottom-hole pressure record showing pressure buildup and packer leakage during series of hydraulic tests . .35681012131616181919202023TABLESPage1. Effect of thermal expansion on a 2,000-foot steel tape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. Comparison of water-level measurements made by air line to measurements made by steel tape andelectric cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3. Applications at the Nevada Test Site of methods discussed in this report, and advantages and disadvantages of each method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv41422c

METHODS OF MEASURINGWATER LEVELS IN DEEP WELLSBy M. S. GarberAbstractAccurate measurement of water levels deeper than1,000 feet in wells requires specialized equipment.Corrections for stretch and thermal expansion ofmeasuring tapes must be considered, and other measuring devices must be calibrated periodically. Borehole deviation corrections also must be made.Devices for recording fluctuationof fluid levelusually require mechanical modification for use atthese depths. A multichannelrecording device utilizing pressure transducers has been constructed.This device was originally designed to record aquiferresponse to nearby underground nuclear explosionsbut can also be used for recording data from multiwell pumping tests.Bottom-hole recording devices designed for oil-fielduse have been utilized in a limited manner. Thesedevices were generally found to lack the precisionrequired, in ground-waterinvestigationsat theNevada Test Site but may be applicable in otherareas. A newly developed bottom-holerecordingpressure gauge of improved accuracy has been usedwith satisfactoryresults.IntroductionAs the demand for water increases, particularly in the arid Southwest, deeper wellswill be required so that previously untappedaquifers can be utilized. To evaluate the hydraulic characteristics of these deep aquifersand to accurately record the correspondinglydeep water levels in the aquifers, new techniques and new equipment will be needed.Much of the now standard equipment of theground-water hydrologist either will be completely obsolete or will require modification.Since 1958 the U.S. Geological Survey, incooperation with the U.S. Atomic EnergyCommissjon, has been studying the hydrologyof the Nevada Test Site. The problem as origi-andF. C. Koopmannally defined was to determine the hydrologicregimen as a basis for ground-water safetyevaluations. Exploratoryholes have beendrilled to depths greater than 10,000 feet inmedia whose hydrologic properties differgreatly. The procedures described in this report are primarily based on the hydrologictesting program at the Nevada Test Site.The Survey ground-water program hasnow expanded in two principal areas of investigation : evaluation of hydrologic conditions in new testing areas and media, withspecial attention to problems of chamber construction relative to ground water, and detailed studies of hydraulic aquifer responseto underground nuclear detonations.One aspect of the overall problem, that ofobtaining accurate and meaningful measurements of borehole fluid levels at great depth,has been explored by the Geological Surveystaff at the Nevada Test Site and in the NewMexico district. This report discusses someof the techniques and instrumentation developed.Purpose and scopeWater levels in aquifers at the Nevada TestSite range in depth from a few hundred toover 2,000 feet, but most are deeper than1,000 feet. The hydraulic gradients in someof these aquifers are as low as 0.5 foot permile. The determination of such low gradientsrequires an accuracy of water-level measurement not ordinarilyattainable at thesedepths ; therefore, special equipment andtechniques must be used. Great precision isalso required in measuring small diurnalchanges in water level and small changes inwater level during pumping tests. Measure1

2TECHNIQUESOF WATER-RESOURCESment of water-level changes to the nearest0.1 foot at depths greater than 1,000 feet isrequired in many water-resources investigations. This 0.1 foot tolerance at 1,000 feetbelow land surface represents 0.01 percentof the total measured depth.In areas such as the Nevada Test Sitewhere depth to water is great and hydraulicgradient is low, corrections for hole crookedness must be made for accurate point-to-pointcomparisons. Data from numerous boreholedirectional surveys at the test site indicatea difference of 0.5-7 feet between true andmeasured depth to water level. Discussion ofthis factor is beyond the scope of this report,and methods of measuring borehole deviationhave been adequately discussed in the drilling-technology literature.This report discusses available methods ofwater-level measurement, advantages anddisadvantages of each method, and the development of new water-level measuring andrecording equipment. Because of the unusualfield problems encountered, the GeologicalSurvey’s electric-cable device is discussed indetail. A. similar discussion appears in thisreport for the air-line method of measuringwater levels.Previous workVarious water-level measuring techniqueshave been in use for many years. Several ofthe metbods discussed in this report are notnew but are merely modifications of standardprocedures.The steel-tape method was described byWenzel (1936, p. 31; 1942, p. 115) and morerecently by Kazmann (1965, p. 145). Methodsof correcting steel-tape measurements forstretch and temperature also are not newand can be found in surveying handbooks.Because no reference of such correctionsbeing applied to deep-water-level measurements could be found, the procedure is described in some detail in this report.The use of an electrical measuring line forwater-depth measurements is a well-established technique. Several devices utilizing thisprinciple are available commercially. Addi-cINVESTIGATIONStionally, the water level is routinely detectedby most of the sondes utilized in boreholegeophysical logging. Brief general descriptions of the technique are given in Kazmann(1965, p. 146) and Anderson (1964, p. 150).The air-line method is routinely used inindustry, and descriptions of this method arealso given in Kazmann (1965) and Anderson‘( 1964).Several electromechanical water-sensingdevices and pressure-sensing transducerswere described by Shuter and Johnson(1961). A multiwell transducer system wasutilized in studies at the National ReactorTesting Station in Idaho (Keys, 1961, p. 11’7).Mechanical recording devices are discussedin Stevens (no date).Met’hods of Water-LevelMeasurementStudiedlThe graduated steel tape, air line, and electric sounding cable are the three principalmeasuring devices used at the Nevada TestSite. Also, a variety of remote recording devices have been used with varying degrees ofsuccess. The methods employing each of thesemeasuring devices are examined in the discussion that follows.Steel tapeSurveyor’s tape has been used for manyyears by the Geological Survey as the principal water-level measuring device. It is available in lengths up to 1,000 feet. Because mostof the water levels at the Nevada Test Siteare deeper than 1,000 feet, a specially made2,000-foot tape was obtained for the project(fig. 1). Coefficients of stretch and temperature expansion for this tape were providedby the manufacturer.The tape reel is motor driven and hasmeans for both mechanical and electricalbraking. The wellhead mechanism is equippedwith a spring balance that enables the operator to maintain tape tension within desiredlimits and, thus, to prevent damage or loss0

METHODSFigurel.-2,000.footOFMEASURINGsteel measuringWATERtape.of the tape. The total weight of the tape is16 pounds, and maximum tension is neverallowed‘to exceed 35 pounds. The tape is always spooled directly from the reel into thewell and back onto the reel and never allowedto accumulate on the ground. In this way,kinking and wear are minimized.The water level in a well is measured bysuspending a known length of tape below adatum mark so that the lower few feet oftape are below water level. The lower portionLEVELSINDEEPWELLS3of tape is usually coated with blue chalk orsome other substance that exhibits a markedcolor change when wetted. In deep holes,wetted chalk tends to dry out as the tape isremoved from the hole, and the result is anerroneous reading or none at all. To preventthis condition, the chalked tape can be protected by a perforated tygon tube, or a pastethat exhibits a permanent color change oncontact with water can be used in place ofchalk. The water-level measurement is obtained by subtracting the length of the wettedportion from the total length suspended belowthe datum mark.The errors in this method were not evaluated by comparison with errors in othermethods because no more accurate fieldmethod was available at the Nevada TestSite. Certain instrument-errorcorrectionsmay be employed, however, such as those foreffects of thermal expansion and of stretchproduced by the suspended weight of the tapeand the plumbing weight. Though theseerrors are small and may be neglected inmany well measurements, they become significant at high temperatures and for measured depths in excess of 1,000 feet.If the tape is free of kinks, has not beenpermanently stretched (calibration not disturbed), and is hanging as freely as possiblein the well, the principal instrument errorsresult from thermal expansion and stretch.Small random errors may be caused by slightdifferences in the tape’s position in the well,twists in the tape each time the tape is suspended into the well, capillarity at the fluidcontact mark, and slight differences in theoperator’s technique. Several successivemeasurements will rarely differ by more than0.1 foot even at depths of over 1,700 feet.This gives repeatability of more than 1 partin 17,000, or about 0.006 percent.The largest error results from thermal expansion of the tape. The temperature coefficient of linear expansion is 63 x 1O-7 footper foot per degree Fahrenheit where expansion is zero at ‘70 F. As shown in table 1, theeffect of thermal expansion on a 2,000-foottape is significant.

4TECHNIQUESTableI.-Effectof thermal‘I’rllclwratuTeexpansiononOF WATER-RESOURCESCI el. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .cINVESTIGATIONStapec c xg!?,,,, in lenutk(ft)-0.26- .130 .13 .26 .39f .52 .65 .78 .91whereC, stretch correction, in feet,L length of suspended tape, in feetThe coeffkient of stretch supplied by themanufacturer is 1.75 X lo-: foot per foot perpound of applied tension. Stretch resultsfrom the tension applied to the tape by itsown susplended weight and from the plumbing weight. The formula for the stretch correction flollows (R. W. Stallman, writtencommun., 1963) :(corrected for thermal effects) ,W weight of tape per foot 0.008 lb,5’ coefficient of stretch 1.75 x lo-”ft per ft per lb, andP weight of plumb bob in pounds(usually 0.25-0.50 lb).The following example shows the mannerin which these corrections may be appliedand also provides some indication of the orderof magnitude of these corrections. A nomograph for calculating thermal expansion ofthe steel tape is presented in figure 2.ExampleMeasured depth 1,751.23 ft (average of 3 readings)Temperature 85 F at surface; 104 F downhole; andaverageTemperature coefficient 63 x 1O-7ft per ft per “FDepth corrected for thermal expansion measured depth - [measured depthtemperature - 70 F) temperature 1,7X.23 - [1,751.23 (94.5 - 70) (63 x 1,751.23 - 0.2704 1,750.96 ftC, stretch correctionL 1,750.96 ftW 0.008 lbS 1.75 X lo-” ft per ft per lbP 0.50 lb94.5”Fx(averagecoefficient]lo-‘)]c 8 L"WS- PLS2 (1,750.96) 2 x 0.008 x 1.75 x lo-” 2(0.50 Stretch correction Corrected depth 1,750.96 x 1.75 x 10-5)0.215 ft 0.015 ft0.23 ftdepth corrected for thermal expansion - stretchcorrectionL750.96 ft - 0.23 ft1,750.73 ft (this is 0.50 ft less than the uncorrectedvalue.)xc

METHODSOF MEASURINGWATERLEVELSINDEEP5WELLSlnstructlons1. Enter125-120-suspendedaveragewell6 scales.2. 8 112-lengthtemperaturepointsby a straightexpansionof63Xin columnthermal10e7of tapeon Aft perandandline.C.expansionft‘F-70’110 108 -06106 104k102-g20005-y100 - oz98 -zY96 - :sng4-I z1500iI92ir3Z90 - g89-Za88-287-IE86-g85-2wa4-83-82-81-0.1n0 092i5t729000 08800800 06700-78 ionofsteelsurveyingtape.1

TECHNIQUES6OFWATER-RESOURCESINVESTIGATIONScBecause of its accuracy and because nomore accurate field method could be found,the steel tape is used as a standard of comparison for calibrating other measuring devices at the test site. In its use as a secondarystandard., the steel tape is not routinely corrected for temperature and weight becausethe calibration is normally performed in atest well specifically set aside for calibrationpurposes;; the temperature in this well varieslittle below 100 feet, and the same. weight isused on the tape during each calibration run.The tape is seldom used as a routine measuring tool in the field because it is a delicatedevice requiring considerable time and carein its use. The tape reel and wellhead deviceshown in figure 1 were designed by A. C.Doyle (1J.S. Geol. Survey, Mercury, Nev.) .ElectriccableThe elquipment used in this method wasoriginally constructed in the Geological Survey equipment-development laboratory at Columbus, Ohio. The device is versatile andperformsseveral functions : water-levelmeasuring, well sounding, fluid sampling,and vertical fluid-velocity measuring. Thecable, winch, and depth indicator (fig. 3) arecombined in a single unit. The motor assembly slides in place at the rear of the unit andis secured by a single bolt on the underside.The depth indicator, depth-measuring wheel,and water-level indicator are mounted on theend of the boom at the front of the device.For storage, this boom may be removed andthe top plate containing the winch may beremoved and inverted so that the cable reelis inside the unit. Wing nuts are usedthroughout.A single-conductor armored cable havinga diameter of 0.087 inch is used. The armoredshield serves as a ground conductor. Themeasuring wheel has an effective circumference of 1.500 feet and is connected to thedepth indicator by gears. When measurin

and methods of measuring borehole deviation have been adequately discussed in the drill- ing-technology literature. This report discusses available methods of water-level measurement, advantages and disadvantages of each method, and the devel- opment of new water-level measuring and recording equipment.

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