USES OF GEOPHYSICS Ill SUBSURFl\CE SURVEYING

l i I

USES OF GEOPHYSICS IN SUBSURFACE SURVEYING

D. F. Malott Head, Geophysical Unit

Prepared for Presentation at the Fall Meeting and Exhibition of the Society of

Mining Engineers of AIME Minneapolis, September 1968

Testing Laboratory Section "'"I:esting and Research Division

Report No. TG-17

I LIBRARY 1

michiqan department of

L stale high~~~·~sSIN~

Michigan State Highway Commission

Charles H. Hewitt, Chairman; Wallace D. Nunn, Vice-Chairman; Ardale W. Ferguson; Richard VanderVeen; Henrik E. Stafseth, Director

USES OF GEOPHYSICS IN SUBSURFACE SURVEYING

The Michigan Department of State Highways makes extensive use of geo­physics for subsurface surveying which would be applicable for uses in other fields. Examples of resistivity surveys are given which include a proposed large volume excavation with suggestions for land use after excavation, sub­surface evaluation for engineering design, and a subsurface survey for land appraisal. An example of a combined refraction seismic and resistivity survey is shown where the resistivity is used for outlining the unconsolidated soils, while the seismic method is used to outline bedrock and to evaluate it for rippability. A refraction seismic survey at the bottom of bridge cais­sons for evaluation of bedrock uniformity and a relative aid in assessing rock strength is reported.

INTRODUCTION

These past several years have been a period of unprecendented con­struction and commercial expansion. Vast private manufacturing complexes and government road, power, and office projects are being built or are in planning stages. Nearly everywhere, municipalities are expanding outward and upward. Many towns are developing into cities and many cities are dis­playing new skylines; and the building boom shows little sign of slowing down.

One of the facets of this era of building is an increasing awareness on the part of many engineers and architects that subsurface conditions and materials play an important role in design and construction. Newspapers and trade publications commonly print stories of structure failures or con­tractor-designer litigations arising from the effects of unsuspected detri­mental subsurface conditions. Undoubtedly many more examples never reach the press.

Because of this, many consultants, design organizations, and con­tractors are turning to geophysics as an answer to some of their subsurface problems. Some are contracting the work while others are buying their own equipment and doing geophysical surveys themselves. For the latter group, a word of cautionl Most salesmen of so-called "engineering-type" geo­physical equipment, whether refraction seismic or earth resistivity, are usually very glib in extolling the simplicity and ease of operation of their particular equipment; and they are correct. Just about anyone can be taken

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out in the field and in an hour or so be taught to operate the equipment and record the data. However, even a person with a geological background needs several years of intensive experience and study in one method to be­come proficient in interpreting these data.

GEOPHYSICAL SURVEY METHODS

Earth Resistivity

There are two major methods of geophysical surveying used today for shallow engineering work. These are the earth resistivity method and the refraction seismic method. In the earth resistivity method, an electric current is artifically induced into the ground and the resistance the ground offers to the flow of current is measured. The basic Wenner electrode con­figuration is almost universally used Q). This consists of pounding four electrodes into the ground in a straight line and equidistant apart. A sound­ing at a fixed point is taken by starting with the electrodes close together and expanding them by arbitrary increments until the desired depth is reached. The depth is equal to the electrode spacing.

The ease with which the ground passes an electric current is a function of several variables; the more important being moisture, dissolved elec­trolytes in the moisture, and the surface area of the soil particles. Some of these variables change with the subsurface environment and the seasons. Thus, the same material can yield vastly different resistivity values, while vastly different materials can yield similar resistivity values. This is why a background in geology, field experience, and good boring control is nec­essary. Many methods of interpretation and data treatment can be used. The Geophysical Unit 1.\Ses the Barnes Layer-Method @) and the Moore Cumulative Curve Method @\ of resistivity data reduction. The actual com­putations and some of the plotting are done by data processing equipment.

The earth resistivity method yields a relatively large volume sub­surface measurement. Because of this, it is good for subsurface work in­volving sand and gravel exploration, and subsoil materials inventories for large-volume earth-moving projects. Very useful subsurface information can be obtained from surveys of perimeters of large natural earth brine storage ponds or settling ponds. Information can be obtained regarding subsurface uniformity and potential infiltration areas and seepage zones that may require special design and treatment. The same applies to sur­veys across proposed dam locations. Since earth resistivity measures average subsoil conditions over a relatively large volume, it should not be used where pinpoint information is required.

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Refraction Seismic

The refraction seismic method measures the ability of soil and rock to transmit an elasticwave. Only two fundamental measurements are made from which all refraction seismic computations are derived. These are time and distance; that is, the time it takes an elastic wave to travel a cer­tain distance. Depending upon surface and subsurface field conditions, the necessary correction, interpretation, and computation of field data can be extremely complicated. This requires a high degree of experience and competence on the part of those conducting the survey and interpreting the results. In good, first-order seismic surveys the computed depth to rec­ognized contacts should be within five percent of the actual depth.

A note about seismic equipment; the small, single trace, engineering­type refraction seismographs are extremely limited in their application, and when used on this basis--within their limits--can produce satisfactory results. However, any organization contemplating the serious use of the refraction seismic method should consider multiple trace equipment, as they will eventually end up purchasing it, as we did.

The refraction seismic method is most app!icab le for reconnaissance­type surveys where general information is desired in a hurry. Is there rock in that hill? Approximately, how deep? What kind of rock? What kind of soil lies above the rock? The answers to these types of general questions might be obtained from several seismic traverses. The mobility of present seismic equipment enables it to be taken into areas to obtain gen­eral subsurface information where boring equipment could not easily be transported.

EXAMPLES OF RESISTIVITY SURVEYS

Howard Road Area - A Study in Land Use

Organizations in the strip or open pit mining business--whether it be sand and gravel, coal, or other minerals--are finding it increasingly dif­ficult to open up new pits due to population expansion and zoning laws. So much difficulty has been encountered in the vicinity of large metropolitan areas that some of the larger operators in the midwest are approaching zoning commissions with a master plan for land use.

Some of their presentations are quite elaborate. They deal principally with plans for land use after excavations have been completed. Many exam­ples concern areas where lakes will be formed by underwater excavation.

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o RHO SOUNDING

!P HAND A e MDU B UGER BORING

~ CLAYEYORING DEFIANCE MORAINE

Figure 1

GENERAL WCATION p LAN

Resistivitv ' traverse l ayout

ROW

- Howard R oad Area.

The excavated slopes will be well graded and dressed-up and the property can be subdivided. Usually, a series of landscape architect's drawings of the beauty of the finished product are included. When properly designed and carried through to completion, these proposed sand and gravel pits can enhance the beauty and value of a community to say nothing of the resale of the land.

The day is rapidly ending, in many parts of this country, when mining companies can remove minerals and abandon an ugly scar on the surface of the earth as an eyesore for all time. Responsibility to the public and economics are beginning to dictate that plans be made for land use after mining is completed.

An example of a geophysical survey with land planning in mind is the Howard Road area. This area comprises eleven separate parcels of land with a total of 127.2 acres. It' is bounded by highway, an orchard, a golf country club, and private residences and is located near a suburb of Detroit.

These eleven parcels of land are either partially in, or adjacent to, a huge proposed interchange involving several Interstate highways. Con­siderable quantities of sand will be needed to construct fill sections, bridge abutment backfills, drain backfills, and sand subbase. It was proposed that we investigate these eleven parcels to see if the quantity and quality of sand was present for the proposed construction.

A series of thirty-six north-south resistivity traverses were estab­lished as shown in Figure 1. The resistivity stations on the traverses were at 100-ft irutervals and the distance between traverses was 100 ft which resulted in a grid of the area. Depths for the resistivity soundings were established so that the volumes required would be included within survey limits.

Along with the resistivity, an extensive boring and subsoil sampling program was established. Continuous flight auger samples of the dry sand were taken along with split spoon and sand pump samples of the sand below the water table. Detailed boring logs were made for each boring. The sub­soils were submitted to the Testing Laboratory for specification testing.

Cross sections were plotted for each resistivity traverse (Fig. 2) de­picting the ground surface, stationing, elevations, boring logs, specification test results, water table, proposed excavation limit, geophysical inter­pretations, and all other pertinent data. The geophysical interpretations are in the form of a geologic cross section along each traverse from the ground surface to the depth of the resistivity soundings.

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CROSS SECTION FROM RESISTIVJTY PROFILE-CONTOURS AND CCMULAT!VE CI:RVES

----M. D.S.H PROPERTY·----··--·-------1--HUNTER PROPERTY-+----·-··--····

Clay and Clay Loam

STATIONS ON LINE FF

So.nd)'- Loam, L<mmy Sand and Fine Sand with Silt Sand and Gravelly Sand

Figure 2. Subsurface cross section - Howard Road Area.

fl S<:>i! Samples from Borings )feets Sooeclflcations for Granular ~Iarerial Class rr ~lodified

Once a series of subsoil cross sections are constructed, volumes of the different subsurface materials can be computed by the average-end-area method. Thus, a complete subsurface picture is obtained with reference to the different materials present, their relative locations, their specifi­cation use, and relationship to groundwater.

Part of the proposed excavation would form a Jake, the outline of which is shown in Figure 3. Excavated slopes were designed one on six; gently enough that children could wade out into the water without suddenly going in over their heads. The lake would be one-half mile in length, 300 to 900 ft wide and a maximum of 18 ft deep. The lake would be fed by groundwater and have an outlet through Seeley Creek.

Water table in the immediate vicinity of the excavation would be per­manently lowered 6 to 14 ft; therefore, a series of observation water wells was proposed to monitor possible water well claims from adjacent property owners. The excavated area to the west of the lake was designed to be 4ft above lake level so that it could be subdivided. An open perimeter ditch was proposed around this area for the purpose of drawing down the water table, keeping the area dry for subdividing, and affording boat access to the lake.

The survey's results were presented as a formal report which included a written section with estimated subsoil volumes, detailed logs of the bor­ings, laboratory test reports, and the general location plans and cross sections. Thus, before the parcels were purchased and any excavation begun, thorough subsurface information was available.

Engineering Application

For any construction project involving the excavation of significant volumes of soil and rock, it is considered good engineering practice to in­clude provisions in the design for using the excavated materials. This not only lessens the quantities excavated by cutting here and filling there, but also solves the problem and expense of disposing of the excavated materials.

The excavated materials, when placed, should also be manipulated to a stable condition capable of supporting the required loads to be imposed upon them. This involves placement by density control methods which are dependent upon the texture and moisture contents of the subsurface ma­terials. Under these conditions, the subsurface environment and the type of materials to be excavated become an important factor. Depending upon the size and importance of the project, adequate preliminary subsurface

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GENER<\L LOCATION PLAN

ESTIMATED EXTENT OF PROPOSED LAKFi:- ELEVATION: 848.0'1 AREA: 39.3 ACRES--~

~ORAINIC POND, ELEVATION asg.o'

I PROPOSED PERII.IETER DRAIN

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Figure 3. Extent of proposed lake - Howard Road Area.

HALSTEAD RD.

I SCALE

information can greatly assist the design phase, significantly lower con­struction bid prices, and aid in the construction itself.

An example of a resistivity survey for design evaluation is the pre­liminary survey of two proposed roadway cut sections on M 61 relocation west of Harrison, Michigan. The size and location of these proposed cuts placed them at critical places on the earthwork mass diagram. Depending upon the subsoils encountered in these two areas, proposed grades, or even the location of the road itself, might have to be changed.

Three parallel resistivity traverses were run across each proposed cut. The subsoils in the first cut proved to be extremely variable (Fig. 4). The subsoils consisted of a large body of clayey and loamy materials that were capped and interbedded with sandy soils. The natural moisture con­tents of the clay and loam materials were higher than the optimum moisture from the AASHO T-99 Proctor Test. This means that the material would have to be dried before it could be satisfactorily compacted to the required 95 percent maximum density inan embankment. Also, subsurface seepage zones were encountered that would require the inclusion of suitable treat­ment in the design.

The sand overlying the clay and loam body is of specification quality and can be put to a variety of uses calling for clean, well-draining sand. It was hoped that the sand would be more extensive in the cut section but, because it was not, additional granular borrow will have to be provided. The cross section also shows a water filled basin formed by the loam and clay body in the vicinity of Station 1601. This will have to be drained dur­ing construction and provisions made for permanently keeping it dry.

Thus, during the design stages, detailed subsurface information is available allowing for the proper designof drainage and roadway to fit sub­soil conditions. Provisions can also be made for the best utilization of the excavated materials. The surveys are available to the bidding contractors who use them as a guide to the type and physical condition of the materials they will be excavating, hauling, placing, and compacting. This takes much of the guess work out of the e;arthwork which is reflected in more com­petitive bid prices.

The second proposed cut section is adjacent to a swamp and was sur­veyed for the purpose of outlining sand for use as swamp fill material. The cross section (Fig. 5) shows the subsoil to be principally sand, underlaid with a body consisting of interbedded layers of clay, loam, and fine sand.

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CROSS SECTION FROM Rl::S!~'T!VlTY PROFIU::-CO:<TO!'HS

~-·-IOSCO SOIL SERIES -----+-~-MONTCALM SOIL SERIES----~·+·---- '----WEXFORD SOIL SERIES~·· ~------,

1589 1590 1591

&>nd\' L=m, Loam_\' Sand ~nd So.nd "ith Cb\' Lenses

Sond w>th Scat:ered S!ll and Loam u,,,.,

1592 1593 1594 1595 1596 1597 150S 1599 1600

STATION 50 FEET RIGHT OF SURVEY CENTERLINE

II Soil Sampled Fr~m Bonngs ~!ee<s SP<cifkat!ons For (;ranular M~tcrJ~l Cl~ss 11.

NOTL "P" d~nute• AASHO T··99. NOTE Arabk nurnecol> in horin~s denote P<ro•nl M\"rol tnoi;turo

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Figure 4. Subsurface profile showing wet cla,·ey core.

1604 1805 1606

.~ASHO T-99

Sample Xo. 60&·351 Bonng 8-~, 5G' R>ght of SUL !594

Deotho l '' - :!6 ~laxirnum Dens H) 1\' poi Ophmurn ~!O!Sturo·

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1330

1320

1310

1300

1290

1280

1260

C'JiCl,SH FEt"l'lON FIH)M HJ·:SISTI'."ITY P!UWILE-C:ON'!'O\JRS

---ROSELAWN SOIL SERIES -~..;.~l--t---ROSELAWN SOIL SERIES

ECHO

1634 1635 1636 1637 1636 1639

STATIONS ON SURVEY CENTERLINE

~~ :::::I Layers of Clay, Loam, Silt and Very Fine Sand

ft?t .:;,:~.r rw1 Sand with Scattered Silt lenses

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Soil Sampled From Borings Meets Specifications For Granular Material Class II.

Soil Sampled From Borings Meets Specifications For Granular Material Class III.

1640

Figure 5. Subsurface profile of sand cut.

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1641

CENimAL LOl'ATION PLAN

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Figure 6. Plan of resistivity traverses over gravel property crossed by Interstate 96.

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Laboratory tests of continuous flight auger boring samples indicate that the sand is suitable for use as swamp fill material. On the basis of the sur­vey results, the horizontal alignment of the road was changed 100ft to the south so that the sand in the hill could be entirely utilized as side borrow.

Thus, detailed subsurface exploration, made available during design stages, permits intelligent placing of grades, prudent design of the road­way, and the best uses of the excavated subsoil materials.

Appraisal Survey

During the course of large public works projects it is almost inevitable that some land will be crossed or taken that involves the claim of minerals such as sand and gravel. In these instances, surveys are requested to determine quantities present for appraisal purposes. Later, the survey reports may be used in court as evidence to support the appraiser's tes­timony.

The resistivity survey of Parcel C-31 in the Grand Rapids area, where I 96 crosses land containing gravel, is such a case. Eight resistivity trav­erses were established across the Parcel as shown in Figure 6.

The cross section from cumulative curves, Figure 7, shows a layer of gravel capped and underlaid with clay (il. The continuous flight auger correlation borings indicate that the gravel is partially under water. The ground water has a definite gradient from right to left. Mechanical anal­ysis tests were conducted on the gravelly subsoil samples to determine de­tailed gradation information.

The completed report includes the subsoil cross sections, Laboratory Test Reports, field boring logs, and a written section that includes a des­cription of the survey and computed gravel volumes. These reports have been used many times in litigations and have been accepted by the courts as valid evidence.

COMBINED RESISTIVITY AND REFRACTION SEISMIC SURVEYS

All geophysical methods have limitations and sometimes it is neces­sary to use more than one method to obtain the desired results. For ex­ample, the resistivity values obtained between sand and sandstone are so similar that the resistivity method will not differentiate between the two @). The elastic properties between sand and sandstone are often different,

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I CROSS SECTION FROM RESISTIVITY CGMGIATIVE CUHVES

GROUND SURFACE MDU B-7

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2 3 5 7 8 • 10 II 12

STATIONS ON LINE C

Clay Depth of Rho Sounding

Rho contact from Cumulative Curve Boring Refusal - Bedrock

Sandy Gravel & Gravel WL Water Level in Drill Hole ® Soil Sample Taken, Refer to Field Log of Boring

Figure 7. Resistivity cross section from Moore Cumulative Curves outlining grm-el layer.

therefore the seismic method will differentiate between the two; while the seismic method will often not separate various unconsolidated loamy and sandy overburden and the resistivity method will. Thus, the combination of resistivity and seismic methods will give the more complete survey results.

An example of a combined resistivity and seismic survey is shown on an I 194 roadway cut section, Battle Creek (Fig. 8). Earth resistivity was used to outline the unconsolidated overburden while the refraction seismic method was used to outline the sandstone bedrock.

It can be seen that the overburden consists of interbedded bodies of sand, loamy sand, and sandy loam. This material is under laid with sand­stone bedrock that varies between 2640 and 7090 feet-per-second (fps) seismic velocity. It was predicted that the lower velocity rock (2640 to 3420 fps) could be excavated by blading and scrapers while the higher veloc­ity rock (6110 to 7090 fps) would require ripping and possible light blasting.

The rock excavation correlated very well with predictions. The lower velocity rock (Fig. 9) was excavated by scrapers while the higher velocity sandstone (Fig. 10) did require ripping. No blasting was necessary. Road building in Michigan does not involve a lot of bedrock excavation, but seis­mic velocities in the rock have proved to be valuable indicators as to the type of excavation required.

SEISMIC SURVEY TO EVALUATE BEDROCK IN BRIDGE CAISSON

The I 75 crossing of the Rouge River in Detroit was the. largest bridge ever completely designed bythe Department. The Rouge, being anavigable river, had to be crossed by a high-level bridge in order to maintain an uninterrupted flow of traffic.

The main piers were designed to be set on 54-ft diameter reinforced concrete caissons which were sunk 80 ft through soft clays and founded and keyed into limestone bedrock @). Seismic soundings conducted along the survey line on either side of the Rouge River indicated velocities of 17, 000 to 20, 000 fps further back, with a possible lower velocity zone at the river, which would indicate a possible change in rock structure and a decrease in rock strength.

Bedrock cores for grouting to seal sulfide-gas-laden water beneath the caissons gave satisfactory stratigraphic information but lacked the data

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CROSS SECTIOl\ FROM RESISTIVITY PROFJLE~CONTOURS A:-JD SEISMIC DLSCONTINUITIES

838 837 838 839 840 841

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CENTERLINE OF !-194 SOUTHBOUND LANE

Loam.'' Sand and Sandy Loam

Sand

Saturated Sandy and Loamy Soils Average Seismic Velocity: 4, 560 fps

Soft Weathered Gray )..1edium-r;-rained Sandstone Average Seismic veloeitv: 2, 6.!0 fps

liard Cra,· ~ledium-g;rained Sandstone AYerage Seismi~ \-elocit,·: 6,110 fps

Soft w,.athered Green Fine-g;rained Sandstone A,·erage Seismic \'elocity: :J,-120 fps

l!arJ Green Fine-grained Sandstone Averag-e Seismic \'elocity: 7, 090 fps

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Soil Sampled From Borings Fails to ~leE'! Specif­ications for Granular ~!aterial Cbss II Gr Class

Ill.

Soil Sampled From Borings ~leets Specifications FCir Gnu1ubr ).bterial Class II.

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Figure 8. Combined resistivity and seismic cross section of road cut.

Figure 9. Sandstone (2640 fps seismic velocity) being excavated by scraper at Station 839.

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Figure 10. Sandstone (6110 fps seismic velocity) required ripping for removal. Station 841 shown.

for detailed rock structure. The broomed surface of the bedrock showed glacial scour, jointing, and faulting. Because of this structure, it was nec­essary to evaluate the rock bed as a structural unit.

Geophysical Unit personnel descended three of the four caissons with the seismic equipment (Fig. 11). Seismic soundings were made directly on the bedrock during the muckers lunch break when conditions were quiet (Fig. 12).

The bedrock velocities obtained from these seismic soundings ranged from 11,500 to 15,000 fps which were lower than the velocities obtained farther back on either side of the river, confirming interpretations made from the earlier seismic survey on the surface. The individual points on the time-distance curves were also carefully inspected for unusual varia­tions, which would indicate jointing, micro jointing, vugs, solution cavaties, and gouge, that would indicate possible local structural weaknesses in the bedrock. No significant local variations were observed other than the dif­ferences in velocity of the rock bed itself. The lower velocities measured in the caissons indicated that this bedrock as a structural unit was probably weaker; nevertheless, the seismic velocities obtained along with other data indicated that the bedrock was structurally competent to carry the required loads.

SUMMARY

The Michigan Department of State Highways makes extensive use of geophysics to obtain detailed subsurface information at critical locations. The resistivity and seismic surveys, combined with borings and laboratory testing, yield a complete report that can be used for land evaluation and appraisal, engineering design, contractor bidding, construction, and as a permanent subsurface record should litigations or other problems arise after construction.

The field of shallow geophysics is a dynamic and everchanging one. New equipment is constantly being designed and placed on the market. Data processing takes much of the onerous work out of the reduction of field data and makes possible the option of several different methods of interpretation from the same basic data. The final interpretation; however, is the sum total of all available information, and its accuracy depends upon the back­ground, skill, and imagination of those making the interpretations.

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Figure 11. Personnel descending 80-ft into caisson with seismic equipment.

Figure 12. Conducting refraction seismic sounding on top of limestone bedrock at bottom of caisson.

REFERENCES

1. Wenner, Frank, "Method of Measuring Earth Resistivity," U. S. Bureau of Standards Scientific Paper No. 258, Vol. 12, No. 3, 1915-1916, pp. 469-78.

2. Barnes, H. E., "Soil Investigation Employing a New Method of Layer-Value Determination for Earth Resistivity Interpretation," Highway Research Bulletin No. 65, 1952, pp. 26-36.

3. Moore, R. w., "Geophysical Methods of Subsurface Exploration in Highway Construction," Public Roads, Vol. 26, No. 3, August 1950, pp. 49-64.

4. Malott, Donald F., "Shallow Geophysical Exploration by the Mich­igan Department of State Highways," Engineering Bulletin, Purdue Univer­sity, Engineering Extension Series No. 127, July 1967, pp. 104-134.

5. Malott, Donald F., "The Application of Geophysics to Highway En­gineering in Michigan," Highway Research Record No. 81, 1965, pp. 20-41.

6. Malott, Donald F., "Caisson Bedrock Evaluated by Refraction Seismic Method, "Roads & Streets, Vol. 109, No.6, June 1966, pp. 88-104.

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