IlAD -I

0'V-;,.

Report 2403

THE USE OF GEOPHYSICAL SURFACE METHODS FOR

MILITARY GROUNDWATER DETECTION

DTICELECTEKMay 1984 DEB4 985 :3

ApprovId for public rfaem; distribution unlimited.

LIj

United States Army"Belvoir Research & Development Center

Fort Belvoir, Virginia 22060 _

Reproduced FromBest Available Copy

S4~ GI n t

rr-- .'•• - - h; r '*"' '" "' '' ' '-' *°" ' . ....- "-. . ... x . •' ' , . -*..*..,...v:.,. * .....,:.o.

7/

Destroy this report when it is no longer nedadDo not return it to the originator.

The -tation in this report of trade nmess of'commwrercially availabie products does not const! tutofficial endlorsemenot or approval of the use of suchproducts.

% 1

* '-"* O.o

- * ~ .. *..~.\-,. ,

UNCLASSIFIED89 UOUTY CLASSIFICATION OF THIS PAGE (Whom. Dots ffnsvad)

READ INSTRUCT1ON54REPORT DOCMENTATION PAGE BEFORE. COMaPLE~TING FORMI:OgPORT NUMUDER 12. GOVT ACCESSION NO. A. RECIPIENT*S CAtALOG NUMBER

2403 1 ) 1 .q C 73S

OL TITLE (sutd S"aHeu. S. -Type OF REPORT 4 PERI00 COVEREDTHE USE OF GEOPHYSICAL SURFACE METHODS FOR FnlTcmclRprMILITARY GROUNDWATER DETECTION Finl80 chica9Reor

6. PERFORMING ORG. REPORT NUMBER r

7. AUTHOR(s) S. CONT RACT OR GRANT MUMSER4'a)

C"T Robert J. Thompson

9. PERFORMING ORGANIZATION NAME AND ADDRESS SO. PROGRAM ELEMENT. PROJECT. TASKPetroleum and Environmental Technology Divison; Logistics Support AREA 6 WORK UNIT NUMBERS -

Laboratory; ATTN: STRBE-GS;. Belvoir R&D Center; ~ll23~2Fort Belvoir, VA 22060-5606 Poet L673M

11. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATS

May 1984

12. NUMBER OF PAGES

IS. MONITORING AGENCY NAMIE &AODRESS(II difteent hom Condralh.4 0111.) IIL SECURITY CLASS. (of this WWIV)%%

Unclassified

III.. DEC'-ASSI F1CATION/ DOWNGRADINGSCHIEDUJLE

14. DSTRIBUTION STATEMENT (of £%Ia Repor)

Approved for public release; distribution unlimited.

.7. DISTRIBUTION ST ATEOMENTE (of die ahaboal mnlwd be Black 2.It 1la.1 borngha Repoit)

OIL. SUPPLEMENTARY NOTES

19. KEY WORDS (Comtl"arn mveres,~ off" It ftacosayan d Id~0&l by Week .. Mber)Groundwater DetectionGeophysical ExplorationSeismic RefractionElectrical Resistivity

2L AMYNAC? (Cb~ - ar N em, bbsfeawcmmormd I~ blrr"ok .i

--4usn~out summarises information developed from 91960.1 on the use of suface-deployed geophysicalmethods for military groundwater detection. Thec literature surveys and field testing investigations conducted duo.ing this period indicate the following. (1) Electrical resistivity and seismic refraction are the two geophysicaltechniques with the greatest ne-term potential for suecems (2) complementary seismic refraction and electricalreistivity sumvyis generally can he used successfully for groundwater, detection when the water table occur in Out-consolidated sedlinenta and generally can ntbe used successfully for dietection of groundlwater in confined rock*f

(continued)-

DO ~ 103 ~ -I ~ ~UNCLASSIFIEDS&ZOt1mt" cL-ASSInCTtOf Of THIS PAGE (ft" va-to Infwa4 ,

UNCLASSIFEDSI[CUM iv CL.ASNP"C^A11ON OF THIS PA0e35bw -~ M~

,.apuifen (3) the ammn Pigniicant fawnec affectinig the probability of ddeecting gmumidwater are a copiex-lty*prv*mioiknwledg Of endutig geologica conditions. ,kill of operatodinseqwetor. depth of aquifer. and

shikne ofaqife; () rggd. u~ blemiic efrcton and electsical resstvity equipmient is commerciallyavail"bl which would-rquir wroy little adaptation for military groundwater detecton appication; (5) ime,.pisetaio of the field data is often a complex pocemss equiring an individu'tI with uigificau backgroud,and .

Wrinning in the survey Wechniquesk (6) ruggd field muesocomputer oystem aie commesciafly available- whithaesuitble for poesming and muag in the interpreatio of survey daWa (7) compuste nefwae ezist for bobh

mtrei..ic -nd electrical tsvsrnsy, but it is only quadi user-friendly and mnqmze expesthe to mialm co--%otm prt os,

- 3.

p,&-;i...

UNCLA4

S ME.

S 4W m P.%wwo

%n-..

• -j

PREFACE

The information contained in this report was developed under the gu-dance of the Petroleum

and Environmental Technology Division; Logistics Support Laboratory; US Army Belvoir

Research and Development Center, Fort Belvoir, Virginia, during the period 1980 to 1983.

The preparation of this report was accomplished under the supervision of Gerald 1L'

Eskelund, Chief, Environmental Technology Branch; William F. McGovern, Chief, Petroleum

and Environmental Technology Division; and John A. Christians, Chief, Logistics Support

Laboratory.

AocessiOfl Ior.

NtIS ORA&I

,-0'N -IS C"

DTIC DtIC. TAB 0ELECED nannounced 0

Tustif'; ".n:l

Distribut icaifBAAvailability Godcoa

DIst special.

RE: Classified Reference, DistributionUnlimitedNo change in distribution statement per Hr. ___________

WlimF. McGovern, ABRDC/PETD . *

colo,_ _

DTIC TB . ] ".:"

. L C E I•.•o.c, [ :-P;-.'

CONTENTS

Section Title Page

PREFACE ii-

ILLUSTRATIONS v

TABLES vi

INTRODUCTION

1. Subject 12. Background 1

OVERVIEW OF GROUNDWATER AND METHODS OF DETECTION

3. Characterization of Groundwater 34. Water Dowsing 35. Assessment of Conventional Geophysical Methods 4

III SEISMIC REFRACTION METHOD

6. Principle 97. Equipment 98. Personnel Requirements 9 '.'-.

9. Interpretation 11'C-'-'10. Limitations 12.

IV ELECTRICALRESISTIVITY METHOD.11. Principle ,. ',* , 1212. Equipment . ' 1413. Personnel Requirements

14

14. Interpretation 1415. Limitations 17 .

,-., - . .• .

V INTEGRATED USE OF, SEISMIC REFRACTION AND ELECTRICALRESISTIVITY METHODS

16. Complemeatary Methods 1717. Results of Field Testing 1818. Computer l~equirements 19

VI CONCLUSIOq-"S"..'19 Conclusions 21

iv

". . .. ."-. * * * ~ ~ . - •t - .5 t# - 5._.

ILLUSTRATIONS

Figure Title Page

1 Schematic of Seismic Refraction Survey 10

2 Digital Multichannel Seismograph I1

3 Two Common Electrode Arrays 13

4 Resistivity Instrument for Measuring Current and Potential Difference 15-

5 Sample Log-Log Plot of Electrical Resisitivty Data 16

6 Completely Self-Contained Field Microcomputer for Geophysical 20Surveys

V0

TABLES

Table Title Page --

Groundwater Detection Matrix 5

2 Advantages and Disadvantages of the Seismic Refraction Method 7

3 Advantages and Disadvantages of the Electricazl Resisstivity Method 8-

4 Speed of Performnihg Seismic Refraction/Electrical Resistivity Surveys 19

viU

4 . . °

THE USE OF GEOPHYSICAL METHODS FOR 0

MILITARY GROUNDWATER DETECTION -

I. INTRODUCTION,

1. Subject. This report summarizes information developed by the Petroleum and x' ")Environmental Technology Division, Logistics Support Laboratory, U.S. Army Belvoir -

Research and Development Center, from 1980 to 1983 on the use of surface-deployedgeophysical methods for military groundwater detection.

2. Background. The 'need for groundwater becomes increasingly important in aridregions where surface water sources are non-existent, inadequate, or grossly contaminated (i.e.,with NBC contaminants). Recent emphasis on desert operations has prompted the Army to in-itiate efforts io develop an integrated groundwater detection system consisting of: (a) ground-water statistical mapping overlays; (b) remote data collection techniques (i.e., satellite imaging

devices); ard (c) surfacs-deployed groundwater detection instrumentation. The mappingoverlays and the remote data collection techniques will be ured to identify areas which poten-tially contain groundwater. The surface-deployed groundwater detection instrumentation willidentify the exact location within a potential area where the highest probability of drilling intoan adequate water source exists. Thus, time and resources consumed drilling dry or low-volumewater wells can be saved, and more adequate water sources can be developed quicker. Thegroundwater detection system will permit locating water resources closer to using units, thereby.., **

significantly reducing requirements for long-line bulk haul of water or large-scale water con-duit systems.

During 1980-1982, an investigation was conducted by the Colorado School of Mines(CSM)' under the direction of the Belvoir R&D Center, for the purpose of summarizing the ap- .

plicability of currently available geophysical methods for detecting groundwater and therelative success one might expect.

ij. L Applexae. L. D. Markiewie. and B. D. Idvrigumr. "G#ephyiaI Dewtiea od Grlundwashe." ColoadO Se&eGl o0 MiW,.Goden., CO 41962). * .

,'* -

.-.'-4...,

"-?i"-

-. 0.".

In 1981, a Defense Science Board (DSB) Water Support Task Force2 concluded thattechnology shortfalls exist in surface techniques f'or the deteetion of groundwater. These short- ,.:.

falls in technology were also recognized in a- Draft Uetter of Agreement (DLOA) for aSubsurface Water Detector (SSWD), written by the U.S. Army Engineer School in I981.a Theconsensus of those who reviewed the DLOA was that the concept was premature. I* recognitionof the groundwater detection technology shortfalls and in response to the questions raised. by'the DLOA, a Groundwater Detection Workshop was held at the U.S. Army Engineer Water-ways Experiment Station (WES) in January 1982.' The workshop was cosponsored by WESand the Belvoir R&D Center.

The conclusions of the Geophysics Working Group at the Groundwater Detection Work-shop were: (a) There are two currently "fieldable" geophysical methodsi, electrical resistivityand seismic refraction, that are a'plicable to the groundwater detection problems and may, of-fer a near-term solution to the identified detection technology shortfall and (b) there areseveral state-of-the-art and emerging geophysical techniques that may have potential in the farterm for. application to the groundwater detection problem. Consequently, in 1962J83 a jointfield testing investigation was conducted by CSM5 and WES.' under the direction of BelvoirR&D Center, to assess the feasibility of using electrical resistivity and seismic refractionmethods for military groundwater detection applications.

2 Defeuse Scenrce Board. "Report of the Defense Sce ewe Bosud Task Form. of Waow Support to US. Favess i sm Add ffaimmasat (W)Offive of the Deputy secretary of Defense. Washington, DC (Secret' (195l1.

3U.S Amy Enoiaer MScool. -Draft L~te of Agreemeat (DLOA) for a Subsurface Water Doteser fMWDV: Depasemet of theAesar, Fort Misear. VA (19811.

4Proveredlug of -Groundwater Detertion Workshop," 12 Jsan 62 to 14 Jane 82. published by US. Army Soonaer Waterways Ezxmerimemstations. Vickbsurg. MS.

P. L. Romig. B. D. Rodriguma sad M. H. Powers. mGeopleystcl Methedology Stodira fo Miiary Canimadwt. Ewhls.Skh. 6Coluradeo Sebsal of Miaest Go~lds. CO 119841

60 D. L 'derasin J. L. L1iop. 'Assessment of Two Caurrently Fieldahie Geophysical Meaheds for Militruy Gemaiaw Domeeess.!. UASAmy Eagiaser Waterways Expsenrime station; .Vikdurg. MS (39633.

V.

2 4.

.. . . . .. .. . . .. . . . .. . . .. ......

II. OVERVIEW OF GROUNDWATER AND METHODS OF DETECTION6

3. Characterization of Groundwater. Groundwater is the water which fills pores orcracks in underground rock or sediment strata. It is recharged by nature according to theclimate and geology of the area and is variable in both amount and quality. Not all rock or . :.

sediment strata are porous and penne&.ble enough to contain a sufficient amount of water to, beof practical use. Those rock or sediment strata that do have useable quantities of water arecalled "aquifers.". Gravel, sand, sandstone, and limestone are among the best potentialaquifers.

The porosity of the rock or sediment determines the str,, capacity,, and thepermeability determines water movement through the strata. These two properties occur invarying degrees and are primarily dependent upon :he following: (a) The number and con-figuration of 'interstitial openings; (b) the number and configuration of fractures, joints, andiaults; and (c) the number and configuration of solution channels.

4. Water Dowsing. Water dowsing refers, in general, to the practice of using aforked stick, rod, pendulum, or similar device to locate groundwater and has been a subject ofdiscussion and controversy for hundreds, if not thousands, of years. One of the first knowndiCining rods was that mentioned in the Biblical passage in which Moses strikes a rock withhis rod and water gushes forth (Numbers 20:9-11).

Although tools and methods vary widely, most dowsers (also called diviners orwater witches) probably still use the traditional forked stick, which may come from a variety oftrees, including willow, peach, and witch hnizel. Other dows'ers may use keys, wire coathangers,pliers, wire rods, pendulums, or various kinds of elaborate boxes and electrical instruments.

Iii the classic method upsing a forked stick,cne fork is held in each hand with thepalms upward. The bottom, or butt, end of the "Y" is pointed skyward at an angle of about45c* The dowser then walks bnack and forth over the area to be tested. When he passes over a

source of water, the butt end of the stick is supposed to tmate or e attracted downward. Ac- te:"cording to dowsers, the attraction of the water may be so great that the bark peels off as the rod.......twists in'the h ands. Some dowsers are said to have suffered blistered or bloody hands from thetwisting..

Case histories and demonstrations of dowsers may seem convincing, but when dowsingis exposed to scientific examination, id' presents'a different picture. The U.S. GeologicalSurvey, after reviewing numerous publications which report on scientifically controlled waterdowsing experiments and investigations, have concluded that the expense of further tests on

S.•..' . 5.-..

wate dwsing iosinot Wajtiferd owigrfrlgnrl oteprcieo sn ,.... .

3niw-

* ,.~* *.*.*.*.*..*< *.*"....*.-.*

* iii.. 7 ii

5. Assessment of Conventional Geophysical Methods. Conceptually, the location -

of groundwater by geophysical methods should be straightforward. The presence of ground-water in rock significantly changes both its electrical and seismic properties. However. the %change of physical properties when the rock is buried in the subsurface proves to be non-unique. Changes in rock properties other than percent saturatin may trigger the samegeophysical anomaly as going from a dry rock to a saturated rock. Hence, there is ambiguity inthe interpretation -of the existence of groundwater.

Gravity, magnetic, radiometrie, and self-potential methods are of limited use formilitary groundwater detection application. The gravity and magnetic methods are potentialfield methods which respond to substantial changes in bulk density and magn,,tie susceptibili- .ty, respectively. Neither of these properties is related to 3mall-scale aquifer ch[aracteristics.Radiometric methods are used principally in borehole surveys, not surface explorations.- Theself-potential method is sensitive to fresh groundwater only, is qualitative, not quantitative,and has a maximum groundwater depth detection of less than 300 feet.

The principal methods for groundwater detection are electrical and seismic methods.These are most applicable because water significantly alters the measured physical properties. 'The general characteristics of several electrical and seismic methods are summarized in Table1. This table supports the conclusion that electrical retistivity and seismic refraction are the -

two geophysical techniques with the greatest immediate potential for success in military " .groundwater detection efforts. Tables 2 and 3 summarize the advantages and disadvantages ofboth methods. Neither method used alone is 100 percent successful. However, the methodscompliment each other and when used in an integrated manner, the success rate improvessubstantially. '.

Other geophysical methods, such as electromagnetic and seismic reflection methods,,.may result in groundwater detection, capabilities greater than those currently available with ".-..electrical resistivity and seismic refraction, but not before Additional developmental advances ,

are achieved.

• ...- •

b : 7:-. °

"44 %''7'7.°-°[

* ~• .- % .P . $~' S~~f*.% *;.o°.pb *.-

EF

UU-

-°

0i

lox•

r%'

s EC N

cs cW. i-

10 0s -

P-0

Ixx

-M

100

Cu -.

do CC.) 4I)

~- -C C.~ I L3- -

0 - 0 F"

lu r

40

12 I I s".:,A. as IL !L s

0 b

A I""- ' "

a .. .. E. 004

C4 0

I iITW. Iw

-0I I :.: - -a

§ .2w 0

m Cc ' 2 Z

-00

0 ~06

h. ... o, .•I% •a. 0 o. 0 0a .. 00- 000

----------F

ILL

tvt go a-

""or

M a la 4' -

0 ~ I'

~~c 0. ca

its Q 4-1 .cc & t r-Q

.~- C j

U a~ AlI

cca~

OU~

~IJ a00-

to Co'.~

ag

a Cl

.. .- .-

III. SEISMIC REFRACTION METHOD

6. Principle. The seismic refraction method consists of measuring the travel timesof compressional and sometim" shear waves generated by an impulsive energy source to pointsat various distances along the surface of the groLPd. The energy is detected, amplified, andrecorded so that its time of arrival at each point can be determined.,The instant the impulsiveenergy source is released, the "zero time" is recorded along with the ground vibrations arrivingat the detectors (geophones). The raw data consist of travel times and distances, the travel timebeing the interval between the zero time and the instant that the detector begins to respond tothe disturbance. This time-distance information is then processed to obtain an interpretation inthe form of velocities of wave propagation and structure of the subsurface strata. The processis illustrated schematically in Figure 1. All measurements are made at the surface of the 6ground, and the subsurface structure is inferred from irterpretation methods based on the lawsof wave propagation.

Generally, when depths to interfaces determined by the seismic refraction method arecompared to "ground truth data" from nearby horeholes, the agreement is within - 10 per-cent.'

7. Equipment. The equipment required for seismic refraction surveys consistsof the following: (a) multichannel seismograph for processing, recording, and storing data(Figure 2); (b) seismic sources; (c) geophones; and (d) seismic cable. The seismic source is usually asmall explosive charge or a sledgehammer blow., Geophones are velocity transducers commonlyused in straight-line arrays of 12 or 24. Seismic cables are multiconductors with geophonetakeouts at constant-spacing intervals along its length. Commonly available seismic cablegeophone takeout intervals for seismic refraction surveys are 10, 25, 50, and 100 feet. Totalequipment weight required for seismic refraction surveying is about 350 lb, and the equipmentis easily transportable in a "jeep-size" vehicle. ,

8. Personnel Requirements. Three field, personnel are required for conducting'seismic- refraction surveys. Nonprofessional personnel can be trained to conduct the fieldsurveys and -lso to process the data using existing, quasi user-friendly computer program&.However, interpretation of the field 'data is often a complex process requiring an indlividualwith significant background and training in seismic refraction techniques.

p.-

9 %%........................... "...; ,* * * * . o

CD~

ca

acca c o :

Sb '3 * ~ , 'p

elM 4c'~

* -a sja

aU c SiE-I-

..

WE3 U6 fu

* aca

Lo 02a...

ca *1.c 3 .

2r U -1('

-a am -a

caa

o a.ý -- eLe

Figure 2. Diginwtamuichannel seesmograph.

9. Interpretation. The seismic refraction method is a survey technique in whichthe source locations and geophones are along'a common line. The length of the line should befrom three to four times the desired depth of investigation. Figure I. illustrates the concept of

• .- • '/-

the seismic, refraction method, where the time-distance plot represents, the arrival times of thefirst event at each geophone location. The first event at'a given geophone will be due to a wavewhich propagates. directly from the source or to a wave which is refracted along an interfacewith a higher velocity material, and the arrival 'time-distance data will generally define astraight-line segment for each subsurface layer. The first-arrival time-distance plot can be in-terpreted to give the velocities of subsurface soil/rock layers and depths to interfaces. Theavailability of digital seismographs (Figure 2) and powerful microcomputers now makes itpossible to automate much of the seismic data processing interpretation procedure and to ac-complish it expeditioiusly in the field;

L49

/. .;o** . * *. .

* '. .* ', * %u. ~

The physical principle involved in detection of the water table by seismic methodsis that the compression-wave velocity of saturated sediments is considerably greater than thesame sediments in dry or only partially saturated conditions. For shallow depths of less than100 ft, the characteristic compression-wave velocity for saturated sediments is 5000 ft/s,although some weathered rocks and massive clay deposits ctn have this velocity also. Fordepths greater than 100 ft. the compression-wave velocity of the saturated sedimUents can be as a "id°,.'e

high as 7500 ft/s. The smallest velocity contract at the water table will occur in very finegrained sediments, where the velocity contrast can be as small as 500 ft/s. When the water tableoccurs as an unconfinei' surface in rock, there will always be a velocity increase at the water _table, but it may be small. Where the groundwater occums in a confined rock aquifer, there willbe little in the seismic data to suggest the presence of groundwater without independent orcomplementary information.

10. Limitations. Limitations of the seizi='c refraction method include the following: g.

a. Insufficient Velocity Contrast. If there is no seismic velocity contrastbetween two adjacent layers, or if the contrast is very slight, the underlying bed will not bedetected.

b. Blind Zone. With a certain combination of bed thicknesses and velocities,the first arrival at the surface from a given layer will be masked by arrival from other layersboth deeper and shallower., .' :-..:

c. Velocity Inversion. This condition exists when an underlying bed below, someoverburden layer has a lower velocity instead of a greater velocity than the beds near the sur-face. The resulting refraction of the wave is deeper into the earth instead of shallower, thus nowaves reach the surface from this low veloc,'ty layer.

IV. ELECTRICAL RESISTIVITY METHOD

11. Principle. Surface electrical resistivity surveying is based on the principle that ihedistribution of electrical potential in the ground around a current-carrying electrode dependson the electrical resistivities and distribu.'on of the surrounding soils and rocks. The resistivityof a material is numerically equal to the resistance of a specimen of the material with 'unitdimensions and is a fundamental or characteristic parameter of the material. Most soils androcks conduct current primarily electrolytically; i.e., through interstitial pore fluid. Thus,porosity, water content, and dissolved electrolytes in the water are the controlling factors indetermining resistivity rather than the soil or rock type. -A major exception to this generaliza.tion is clay, which can conduct current both electrolytically and electronically. The usual prae-tice in the field is to apply an electrical current between two electrodes implanted in theground and to measure the difference of potential between two additional electrodes that donot carry current. Expanding the electrode spacing allows one to investigate deeper within theearth. The potential measuring electrodes are usually held at a fixed spacing, while the outerelectrodes (current electrodes) are expanded. Two of the most common electrode arrays are il-lustrated in Figure 3.

.12

St n n. . . . . ..-,' '.• .':-.; 'a' . .•.:-'. .'.', . "..--.-;.-.-. ... '• ... ', .. . *. .'.'- ,.". . . ,_. -.. , -.-. ,. -** . ,.--. *-. - .'.-.

C1 P C2

aI a

WENNER ELECTRODE ARR~AY

PI P2 C:

Cl v C2

SCHLUMBERGER ELECTRODE ARRAY

Fkgure 3. Two common *kade away,

13

* . * * *. * ,:. * *...*b:5

r..

f,. -r %.

Generally when depths to interfaces determined by the electrical resistivity methodare compared to "ground truth data" from nearby boreholes, the agreement is within d: 20percent.

12. Equipment. The equipment for electrical resistivity surveys consists of the following.(a) power upply (12-volt batteries); (b) instrument for measuring current and potential dif-ferenee (Figure 4); (c) four stainless steel electrodes; (d) cable; and (e) noneonducting tape -formeasuring distances. Total equip--aent weight for the electrical resistivity system is about 110.lb, and the equipment is easily transportable in a single "jeep-sise" vehicle-

13. Personnel Requirements. A minimum of three field persoame are required for .•

conducting on electrical resistivity sounding. Nonprofessional personnel can be trained to con-duct the field soundings and also to process the data using existing, Puasi user-friendly com-puter programs. However, interpretation of the field data is often a complex process requiringan individual with significant background and training in electrical resistivity techniques.

14. Interpretation. The measured apparent voltage at the potential electrodes is usedin conjunction with the input current to compute an apparent resistivity based on the geometric ~factor for the electrode spacing. Standard interpretation schemes require one to compute theresistivity of various spacings. These apparent resistivity values are platted as a function ofthe electrode spacing which is assumed to relate to the depth of investigation. The soundingshould be carried out to an outer electrode spacing of at least twice the desired depth of in-vestigation. Measurements are commonly plotted on a log-log plot of apparent resistivity versusone-half of the current electrode spacing (Figure 5).

Electrical resistivity data can be interpreted using either a cuvematching procedureor a computerized inversion method. In the curve-matching technique, the apparent resistivitycurves are compared to characteristic curves based on relative resistivities to assess the resistivi-"ty and the thicknesses of discrete layers. The computerized inversion method uses the apparentresistivity data to compute in an iterative fashion, the "best fit" resistivities and thicknesses fordiscrete layers. While the inversion method is desirable in terins of cosisteney and ease of " -operation, it also can make many of the standard errors that are inhereft in computer inter-pretation, such as honoring all points of data. Therefore, even if inversims methods are used,one should still plot the data by hand, look at the general shape of the esre, and evaluate theresults as a check.

14

p. O~

•o,

, ... .* % ..... 5 .... .o. ' % * *S .... .S.* .. '.. .; * ... y .- t:.b~ ,$ :; . ~ ~

7wa

Ac.

obe

a6

Fipm, Redvkyltw~wm fr aem~namno wW ot df'u

9:4

ca

ca C0cau

.0 ccC.3

0ILU-

Wm%

il-WO 'Al~lSIS3 iN3~d2

16.'-

The most common and successful use of resistivity sounding is for detecting the freshwater-saltwater interface, which will always be detected by the occurrence of a prominentresistivity decrease. Deerction of the water itself is a more difficult problem. Under a favorableconditions, the water table will be detected as the top of a conductive or less resistive layer,since, except for unusual conditions, even fresh potable groundwater is much lower in resistivi-ty than the dry aquifer material. The most favorable conditions will'be when the water tableoccurs in unconsolidated sediments with little clay content. Dry silts, sand, and gravel wilnhave resistivities of 1000 ohm-ft and greater, for fresh water, the resistivity at the water tablewill decrease to range of 50 to 200 ohm-ft. In sediments with considerable clay content, theresistivity contast will be much smalL," and may be undetectable. At the fresh water-salt waterinterface, the resistivity of the aquifer will decrease considerably, perhaps to less than 1 ohm-ft. ": ;I

15. Limitations. Limitations of the electrical resistivity method include the following.

*. Geometric Problems. The effect of lateral changes in resistivity, either throughdip or through changes in rock type, significantly alters the results since the method does not LAindicate these lateral changes, but rather averages large volumes of earth.

b. Resistivity Contrast. The physical properties or extent of individual layerscannot be defined' when there is a lack of discrete resistivity changes between layers.

c. Conductive Zones. A shallow conductive zone allows most of the current to flowthrough this zone and very little current to penetrate below it, thereby discouraging efforts to ',,

resolve the zones below the conductor. ";-,

V. INTEGRATED USE OF SEISMIC REFRACTION

AND ELECTRICAL RESISTIVITY METHODS

16. Complementary Methods. Electrical resistivity and seismic refraction methods arecomplementary in the sense that they respond to or detect different physical properties ofgeologic materials. In cases where both methods detect the water table, one method serves toconfirm the results of the other method or to resolve ambiguities. Also, certain conditions, Suchas the presence of a fresh water-saltwater interface, can be detected by one method but not theother.

17....... ::..:.:

* -, ,,. . .•. : -. - . . . .. . .-. -. *. *.. *...... ..... - ...*.. .~ ... ... .. - . . ..X..... .. . ..-.... , . . .... ... : .. . ,•. . ..

17. Results of Fieid Testing. During 1982 to 19&3. a joint field tes nig investigationwas conducted by CSM and WES, under the direction of ihe Belvoir R&D Center, to assess thefeasibility of using electrical resistivity and seismic refraction f~r military groundwater detec-tion application. Two field sites were selected, each representing a common groundwater oc-currnce. White Sands Missile Range (WSMR), New Mexico, was the site for an -'Juvialaquifer with an unconfined water table. Fort Carson, Colorado, was the site for a confined(artesian) rock aquifer. Five locations were selected at WSMR with water table depths ranging.from approximately 60 to 450 ft and water quality varying from fresh to brackish. For tb,. 'location selected at Fort Carson, the depth to the top of the aquifer was approximately 270 ftand the thickness was approximately 100Aft.

An assessment of the integrated methodologies used for the field testing revealedthe following:

a. Geophysicists possessing no prior knowledge of the in-situ geology were able to ,:

predict the presence and depth of groundwater mt a fair-to-good confidence level.

b. Geophysicists possessing some knowledge of the in-situ geology were abli to predictthe presence and depth of groundwater at a fair-to-excellent confidence level.

c. Geophysicists possessing a complete knowledge of the available in-situ geologicinformation were able to predict the presence and depth of groundwater at a good.toexcellentconfidence level.

d. Used in concert, seismic refraction and electrical resistivity can he used successfuilyto detect groundwater in alluvial materials. The groundwater assessment is more straightfor-ward for those cases where the groundwater occurs in coarse-grained sediments (sends andgravels) as opposed to fine-grained sediments (silts, clays).

e. Seismic refraction and electrical resistivity techniques are not very usefulin rock aquifers.

f. Both methods are relatively slow and require significant lengths of electrical cablefor field data acquisition (Table 4). ":

18 %.

18 *.,.,'''

• 4 .. ... ., *.. ...-,.- -.... .* ...... *..*..-. ....,, .. . .. , .. , *,. .... , .,...,

Table 4. Speed of Performing Seismic Refraction/Electrical Resistivity Surveys

Estimated Number of Complete"Maximum Depth of Investigation Geophysical Assessments per Day

(ft) for a 3-Man Crew*

30 5-6

100 3-4

600 1-2

>600 1

*Am awrmwat mians to nemuwmm at ow peiuI.

18. Computer Requirement&. The complexity of gathering, processing, and interpretinggeophysical survey data can be greatly reduced via the use of computers. Figure 6 depicts amicrocomputer suitable for field use for promssing electrical resistivity and seismic refractiondata. Generally, a microcomputer with 32 K bytes RAM or greater is required. Computer soit-ware exists for both seismic refraction and electrical resistivity, but it is only qu~asi user-friendly and requires expertise to make competent interpretations. Much of the software willrun only on large mainframe computers. Very little user.friendly software exists for smallmobile microcomputers. A user-friendly seismic refraction processing and interpreting softwareprackage *as developed by CSM as an integral part of their geophysical methodology study.Throughout the program, helpful advice was written in plain English in such a way that theuser would not have to read a lengthy manual in order to run the programs.

19

I.

4

-- A -

p

I

F�Ire & Completely self-contained field microcomputer for geophysical airways.I

i20

- . . -

*.t��t%.S�t*******t'******�** *.b*.. **. � *� *.* *.* ..- . .

VI. CONCLUSIONS

19. Conclusions. Based upon studies conducted under the direction of the Belvoir R&DCenter from 1980 to 1983 on the use of surface-deployed geophysical methods for militarygroundwater detecti-n, the following conclusions are drawn:

a. Electrical resistivity and seismic refraction are the two geophysical techniques withthe greatest near-term potential for success.

b. Complementary seismic refraction and electrical resistivity surveys generally canbe used successfully for groundwater detection when the water table occurs in unconsolidatedsediments and generally can not be used successfully for detection of groundwater in confinedrock aquifers.

"c. The most significant factors affecting the probability of detecting groundwater arecomplexitylprevious knowledge of existing geological conditions, skill of operator/interpretor,depth of aquifer, and thickness of aquifer.

d. Rugged, reliable seismic refraction and electrical resistivity equipment is commer-"cially available which would require very little adaptation for military groundwater detectionapplication.r

i. e. Interpretation of the field data is often a complex process requiring an"individual with significant background and training in the survey techniques.

f. Rugged field microcomputer systems are commercially available which are suitable[ for processing and aiding in the interpretation of survey data.

g. Computer software exists for both seismic refraction' and electrical resistivity,but it is only quasi user-friendly and requires expertise to make competent interpretations.

21

!% DISTRIBUTION FOR BELVOIR R&D CENTER REPORT 2403

* No. Copies Addressee No. Copies Addressee

Department of Defense 1,";:~ Commandmr HQ DARCOM .

"12 Defense Technical Information ATTN: DRCDE-SSC tr5001 Eisenhower AveSCenter

Cameron Station Alexandria, VA 22333

Alexandria, VA 22314I Commander, HQ TROSCOM

"ATTN: DRSTS-TIBDepartment of the Army.. ' ~4300 Goodfellow Rd ;

I Commander, HQ TRADOC St. Louis, MO 63120

ATTN: ATEN-MEFort Monroe, VA 23651 Delvoir Research and

Development Center

1 HQDA (DAMA-AOA-M)Washington, DC 20310 Commander, STRBE-Z

Tech Dir, STRBE-ZT1AAssoc Tech Dir, STRBE-HI HQDA (DAEN-RDL)"-Washington, DC 20314 Chief Engineer, STRBE-HE

"Chief Scientist. STRBE-HS

"1 HQDA (DAEN-MPE-T) Program Planning, STRBE-HP"W n , 2Program Support, STRBE-HR".Washington. DC 20314 Systems Analysis, STRBE.HA

1 Director CIRCULATE

US Army Engineer Waterways.• xpeimet SatinI C-, Combined Arms Support Lab,Experiment Station

"* ATTN: Chief, Library Branch STRBE-XTechnical Information Ctr C. Engr Spt Lab, STRBE-N

Vicksburg, MS 39180 C. Engr Svc Spt Lab, STRBE-E jC, Logistics Spt Lab, STRBEG

1" HQDA C, Matls, Fuels & Lubricants Lab,S~STRBE-VODCSLOG

.-

SDALO-TSE Director, Product A&T Directorate,Rom" STRBE-TRoom I E.5W

Pentagon, Washington, DC 20310 CIRCULATE

:. Director 3 Logistics Spt Lab. STRBE-G• UAryT25 Pet & Env Tech Div. STRBE.GS'..• US Army TRADOC

SSystems Analysis Activit 3 Tech Reports Ofe. STRBE-WPGATTN: ATAA-SL (Tech Lib) 3 Security Ofe (for liaison officers),

"White Sands Missile Range, NM STRBE-S

"88002 2 Tech Library, STRBE-WCI Program Support, STRBE-HR

" 1 Commander 1 Public Affairs Ofe, STRBE-I

US Central Command 1 Ofe of Chief Counsel, STRBE-L

ATTN: CCJ4/7-ED (Engr Opus)MacDill AFB, FL 33608

22

ie.

|. . C. . . . 4- - - "

No. Copies Addressee -

Department of the Navy

Library (Code LO8A)"Civil Engineering LaboratoryNaval Construction Battalion Ci"Port Hueneme, CA 93043

1 Director

Earth Physics ProgramCode 464Office of Naval ResearchArlington, VA 22217

Department of the Air Force

1 HQ USAFIRDPTATTN' Mr. Allan EaffyWashington. DC 20330

1 US Air ForceHQ Air Force Engineering and

Services CenterTech Library FL 7050

Tyndall AFB, FL 32403

Others

HQ Marine Corps (LME-2)Washington, DC 20380

a,o

V.

23

4.

S' ' _ ' s * N- . .a 9 . *S* . . " • ' .-. - '. . • ' • • m •a • a