comparative analyses of geophysical methods for determining shear wave velocity of soils

In an effort to ensure their geotechnical investigations are as effective and efficient as possible, the SCPT tool and several available alternatives crosshole: CH; multichannel analysis

THESIS

T 8992

COMPARATIVE ANALYSES OF GEOPHYSICAL METHODS FOR

DETERMINING SHEAR WAVE VELOCITY OF SOILS

by THANOP THITIMAKORN

UMI Number: 3229172

INFORMATION TO USERS

The quality of this reproduction is dependent upon the quality of the copy submitted Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction

In the unlikely event that the author did not send a complete manuscript

and there are missing pages, these will be noted Also, if unauthorized

copyright material had to be removed, a note will indicate the deletion

®

UMI

UMI Microform 3229172 Copyright 2007 by ProQuest Information and Learning Company

All rights reserved This microform edition is protected against unauthorized copying under Title 17, United States Code

ProQuest Information and Learning Company

300 North Zeeb Road P.O Box 1346

Ann Arbor, Ml 48106-1346

© Copyright 2006

By

Thanop Thitimakorn All Rights Reserved

1H ABSTRACT

The Missouri Department of Transportation (MoDOT) routinely acquires seismic cone penetrometer (SCPT) shear wave velocity control as part of the routine investigation

of soils at highway structures or other geotechnical sites within the Mississippi

Embayment In an effort to ensure their geotechnical investigations are as effective and efficient as possible, the SCPT tool and several available alternatives (crosshole: CH; multichannel analysis of surface waves: MASW; and refraction microtremor: ReMi) were evaluated and compared on the basis of the interpretation of processed field data acquired

at two test sites in the Poplar Bluff area, southeast Missouri These four methods for determining the shear wave velocity of soils were subsequently ranked in terms of

accuracy, functionality, cost effectiveness, other considerations and overall utility

On the basis of the comparative analyses, it is concluded that MASW data are

generally more reliable than SCPT data, comparable to quality ReMi data and only

slightly less accurate than CH data However, MASW’s other advantages generally make

it a superior choice over the other methods for general soil classification purposes to depths of 100 ft (as per NEHRP recommendations) MASW data are much less expensive than CH data and can normally be acquired in areas inaccessible to drill rigs MASW data are less expensive than SCPT data and can normally be acquired in areas inaccessible to SCPT rigs In contrast to the MASW tool, quality ReMi data can be acquired only in areas where there are interpretable levels of “passive” acoustic energy and only when the geophone array is aligned with the source(s) of such energy One other real advantage the MASW method has over the CH and SCPT methods is that it can be used to map variable depth to bedrock

iv

ACKNOWLEDGMENTS

Special thanks go to the Missouri Department of Transportation for supporting the

project I thank Dr Neil Anderson who served as my advisor I would also like to thank

Dr David Rogers and Dr Richard Stephenson who served on my dissertation committee

I would like to thank Katherine Mattison, the department administrative assistant and

Paula Cochran, the graduate studies assistant Without their help, patience, and

understanding this project would not have been completed without difficulty

My appreciation goes to Dr Richard Rechtien, who served on my committee and

as my mentor during a very long journey in my graduate study He gave me invaluable advice in data collection and offered me several possible solutions to my data processing Without him the data collection and processing in this project would have been difficult

I thank Dr Gerald Rupert who served on my committee He provided me with the support and motivation that made the research a success He also helped me solve many problems during the past two years Thank you very much Dr Rupert

Finally, I would like to thank my mother and father for their support and

understanding during my study period I would also like to thank my girlfriend for her

support from a distance I would like to thank my fellow students in B40 McNutt Hall

who assisted me in the collection of the data: Ahmed Ismail and Oleg Kovin

TABLE OF CONTENTS

PAGE

ACNOWLEDGMENTS .0 cccccccccecscceseecsesseecsesusceceveuereversversesssesrsenseenstens iv LIST OF ILLUSTRATIONS .0.cccccccecesessecseesseeveseeseecssesseeeseetsstinsenteesnass ix

3.1, OVERVIEW oecccccccccccsccssccsecseecssecssensevcrecrseesecssecreevesnecsereeesnn 15

3.3 BOREHOLE DEVIATION SURVEY cào: He 20

3.6 DATA INTERPRETATION HH nen nh ng 24

vi

3.7 SUMMARY EVALUATION OF CH SEISMIC METHOD 26 SEISMIC CONE PENETROMETER TEST (SCPT) c 29

4.2 SCPT: ACQUISTTION, PROCESSING AND INTERPRETATION 30

4.2.1 Acquisition and prOC€SSITE cv nh nhớ 30 4.2.2 ÍnterpretaflO' ST nà kh tà va 34

4.3 SUMMARY EVALUATION OF SCPT METHOD 47 MULTI-CHANNEL ANALYSIS OF SUREACE WAVE(MASW 52

5.2 1-D MASW: ACQUISITION, PROCESSING AND

INTERPRETATIƠN centre nett netted rene eee eeeee eee ea eneenaee ed 53

5.2.1 Acquisition and DrOC€SSInE con ng n nhe 33 5.2.2 1-D MASW data InfterprefatIOn ence eee eeae eee eeaeeuens 56

5.2.2.1 1-D MASW shear-wave velocity profiles of Site #3 56 5.2.2.2 1-D MASW shear-wave velocity profiles of Site #15 56

5.3, 2-D MASW: ACQUISITION, PROCESSING AND

INTERPRETATION 0 0 ccc cececec teen cent ee ee eee e eens eee e eens bene na seen anes 59

5.3.1 Acquisition and DrOC€SSINE SH nh nh ro 59 5.3.2 2-D MASW data Interpretatlon cà cà như 61

5.4 SUMMARY EVALUATION OE MASW METHOD 69 REFRACTIƠN MICROTREMOR (REMI) che 74

6.2 INTRODUCTION TO THE REMI METHOD 74

vii

6.3 INTERPRETATION OF 1-D REMI DATA AT SITES #3 AND #15 77

6.3.1 1-D ReMI shear-wave velocIty profile at SIte #3 77

6.3.2 1-D ReMI shear-wave velocity profile at Site #1Š 80

6.4 SUMMARY EVALUATION OF REMI METHOD 83

7 COMPARATIVE ANALYSES OF SHEAR-WAVE METHODS 87

rm9 2:22 EE EEE EEE EE REE HERE n ea 87 7.2 ACCURACY cece cece cnn renee rE EEE CEE E EEE nàn Bà hết 87 7.2.1 NEHRP Quantitative Comparison Method 88

7.2.1.1 NEHRP Quantitative Comparison of S1te #3 89

7.2.1.2 NEHRP Quantitative Comparison of Site #lŠ 90

7.2.2 Predicted ground motion amplification c.Ă- 91 7.2.2.1 Predicted ground motion amplification of Site #3 94

7.2.2.2 Predicted ground motion amplification of Site #1Š 95

7.2.3 Discussion of Accuracy Comp4r1SOnS c cà se 96 7.3 FUNCTIONALTTY (DATA ACQUISTTIƠN) 07

7.4 FUNCTIONALTTY (DATA PROCESSING) ¬ nen eae 98 7.5 COST EFFECTIVENESS LH TH HH nọ BE Hà tt nà 99 7.6 OTHER CONSIDERATIONS LH nh HH kg 100 7.7 QVERALL UTILTTY TO MODOT cĂààà nhe Hưe 101 8 NEHRP SITE-CLASS MAP OEF THE POPLAR BLUFE AREA 106

8.1.OVERVIEW cà TK kh nh kh 106 8.2 OVERVIEW OF NEHRP SITE CLASSIFICATIONS 107

8.3, MASW DATA ACQUISITIONS on Hình nh nhe nà 1

8.4, RESULTS OF STUDY ccseccccesessttsesseseeseeseeeseeeee as eaeeaeneeenees 109 8.5 CONCLUSIONS oeecccccccccsssseseeeeeeecceeceeeeeeeeseueseneeseeeeeeeeuaeeneeees 110

9 CONCLUSIONS AND RECOMMENDATIONS cào càà: 115

"N9 4:):à4)07) 20005 l15_ 9.2 CONCLUSION OF THE FIRST OBJECTTVE 115 9,3 CONCLUSION OF THE SECOND OBJECTIVE ¬ 117 APPENDIX A: MASW FIELD PARAMETERS EVALUATION 119 APPENDIX B: MASW SENSITIVITY ANALYSES cằằ cà 131 APPENDIX C: REMI RECEIVER ARRAY ORIENTATION TESTING 139 APPENDIX D: SEISMIC CONE PENETROMETER (SCPT) DATA 144 I?11210(016):Ÿ.).4:)<taaddđđdđdidẨẢđíẨẢđiiiầiaááẳa 159

1X

LIST OF ILLUSTRATIONS

Eigure 1.1: Location map of the sfudy af@a con nh nhu 5

Figure 2.1: Poplar BÏIuf study ar€a cnnn nnn HH nh nh nh nh cà 10 Figure 2.2: Poplar Bluff study area physiographic provinces and

Surficlal maf€r1aÌS - SH TH TT KT nà nà ho hy 11

Figure 2.3: Poplar Bluff study area MASW test sites (2-D MASW shear

wave Velocity profiles were acquired at Sites #3, #10, #13 and #31.) 12 Figure 2.4: Poplar Bluff study area SCPT test SIt€S nen à 13 Figure 2.5: Crosshole seismic test SIf€S SH HH nhe 14

Figure 3.1: Borehole lithologic log at S1fe #3 nh nh nh kh kh re 17

Figure 3.2: Borehole lithologic log at Site #ÏŠ uc nh kh net 18 Figure 3.3: Borehole geometry of Site #3 ¬ entree ren ere nee ree need 19

Figure 3.4: Borehole geometry Of Site #TŠ nh nh nh nhớt 19 Figure 3.5: Shows the crosshole selsmic dafa acqu1S1t1OH 21 Figure 3.6: The borehole s€1SmIC SOUFC€ cọ SH HH nhe kh Hệ 22

Figure 3.7: Three-component downhole receiver with vertical sidewall clamp 22 Figure 3.8: Example of composite CH data from Site #3 e- 23 Figure 3.9: Crosshole data of Site #3 and borehole lithologic log 25 Figure 3.10: CH data Site #15 and borehole lithologic log - 25 Figure 4.1: The SCPT rig used by MoDOTT in the Poplar Bluff area 31

Figure 4.2: IIlustration of SCPT method c co nhi 32 Figure 4.3: Example of SCPT data from Site #3 c.ncnn nen nen nhe be 33

Figure 4.4: Site #3 2-D MASW line, SCPT and CHÍ location 34

Figure 4.5: Site #10 2-D MASW survey line and SCPT location 35

Figure 4.6: Site #13 2-D MASW survey line and SCPT location ¬ 35

Figure 4.7: Site #31 2-D MASXW survey line and SCPT location 36

Figure 4.8: Plot of CH and SCPT shear-wave velocity profiles for Site#3 37

Eigure 4.9: Suit of SCPT shear-wave velocity profiles from Site #3 39

Figure 4.10: Suit of SCPT shear-wave veloctty profiles from Site #10 42

Figure 4.11: Suit of SCPT shear-wave velocity profiles from Site #13 44

Figure 4.12: Suit of SCPT shear-wave velocity profiles from Site #31 46

Figure 5.1: MASW field configuration (Park et al, 1999) cu se seo 54 Figure 5.2: General procedure of MASW processing A multichannel record (shot gather) in (a) is transformed into (b) a dispersion image in which the fundamental-mode dispersion is identified and corresponding signal curve is extracted, and then (c) the curve is inverted into a 1-D Vs profile (Park et al, 1999) " cette eee tees tees eee eeeet nae enaeenanens 55 Figure 5.3: Field MASW data and dispersion curve of Site #3 57

Figure 5.4: Plot of MASW and crosshole shear-wave velocity profiles for Site #3 58

Figure 5.5: MASW data and dispersion curve l)(-š:4 h5 Œa - 58

Figure 5.6: Plot of MASW and crosshole shear-wave velocity profiles for Site #15 59

Figure 5.7: Generation of a 2-D MASW shear-wave velocity profile (Park et al, 1999) occ cece nent e teen ence ne eee tne eet e eee neon ene t ees 60 Figure 5.8: 2-D MASW shear-wave velocity profile for Site #2 62

Figure 5.9: 2-D MASW shear-wave velocity profile for Site #10 65

XI

Figure 5.10: 2-D MASW shear-wave velocity profile for Site #13 66 Eigure 5.11: 2-D MASW shear-wave velocity profile for Site #31 68 Figure 6.1: Shows ReMi data processing steps (Modified from Louis (2001)) 76 Figure 6.2: ReMi data from SIfe #Â LH ene ene e kh kh nà khu 78 Figure 6.3: Plot of CH, SCPT, MASW, and ReMi shear-wave velocity profiles

Figure 6.4: ReMi data of Site #15 HH TT nh nh kh và 81

Figure 6.5: Plot of CH, MASW, and ReMi shear-wave velocity profiles for

Figure 7.1: Amplification ratios of Site #3 00 cece cence eee ee entree e neta ene ne enens 95

Figure 7.2: Amplification ratiOS Of S1t€ #ÏŠ HH nh kh va 96 Figure 8.1: Poplar Bluff study area shear-wave velocity test values and surficial

Materials UNITS occ cece eee ne een rene EEE EEE EEE SEE EES EE ee EE ERE 111 Figure 8.2: Values of Vs(100) vs soil units found in Poplar Bluff area (Table 9.1) 112

Figure 8.3: Soil amplification map of the Poplar Bluff area 113

Table 6.1: Summary ofthe ReMI method con nen 84

Table 7.1: NEHRP site classes - Gà Hy HH kho in 89 Table 7.2: Site #3 Vsio0) , WSs) and site class " 90 Table 7.3: Site #15 s(iooy and site claSS - cu HH HH nh nen ho 91 Table 7.4: Ranking of accuracy of MASW, CH, ReMI, and SCPT 103

Table 7.5: Functionality (acquisition) of MASW, CH, ReMi,

and SCPT methods - - con HH HH HH nh nh ng kh nhu 103 Table 7.6: Functionality (processing) of MASW, CH, ReMi,

and SCPT methods HH HH HH nh ng 104 Table 7.7: Cost-effectiveness of MASW, CH, ReMi, and SCPT

shear-wave methods - co HH HH HH nh nà nà hen kế 104 Table 7.8: Ranking of other considerations of MASW, CH, ReMi,

and SCPT shear-wave velocity profile methods 105

Table 7.9: Overall ranking of CH, MASW, SCPT, and ReMi methods

(based on tests of soils in Poplar BIuff study area) 105 Table §.1: Sites and site classifications in the Poplar Bluff area 114

1 INTRODUCTION

1.1 STATEMENT OF PROBLEM

In situ shear-wave velocity profiles are used in a variety of earthquake

engineering applications, including site response studies, liquefaction analyses, and soil structure interaction evaluations (Woods and Stokoe, 1985; Stoke et al., 1988: Stoke et al., 1994) Borehole seismic methods such as crosshole and downhole methods normally have been employed to measure shear-wave velocity since they are direct measurements More recently, a suspension logger has been used for this purpose However, the borehole methods are normally expensive because borehole needs to be drilled for data

acquisitions Until recently, there are many new techniques have been developed

including seismic surface-wave methods and seismic cone penetrometer method These

methods are increasingly used by many engineering geophysicist and geotechnical

engineer to determine in situ shear-wave velocity However their reliability needs to be

established by comparison with more direct methods

Missouri Department of Transportation (MoDOT) is always acquired shear-wave velocity data for their geotechnical site characterization programs For the above reason, MoDOT wanted to evaluate the relative utility of the conventional and newly developed methods in order to ensure their geotechnical site characterization programs are as

effective and efficient as possible MoDOT asked UMR to evaluate four geophysical field methods used to determine the shear-wave velocity of soils to depths of 100 ft The proposed geophysical methods to be evaluated include:

1 Crosshole Seismic (CH)

2 Seismic Cone Penetrometer Test (SCPT)

3 Multi-channel Analysis of Surface Wave (MASW)

4, Refraction Microtremor (ReMi)

The CH method was selected because CH data is generally accepted as more

reliable than shear-wave velocity data derived from other geophysical methods (Kramer,

1996) Consequently, the CH shear-wave velocity data were used as a yard stick for determining the reasonableness of other selected geophysical methods The SCPT was selected for evaluation because it is the current method that MoDOT uses for determining

shear-wave velocity of soil MASW and ReM1 methods were selected because they are

newly developed surface acquisition methods for determining shear-wave velocity

These four geophysical methods were evaluated individually and comparatively

in terms of accuracy, functionality, cost-effectiveness and overall utility This

comparative evaluation of available shear-wave technologies was conducted in Poplar

Bluff area

1.2 RESEARCH OBJECTIVES

The objective of this research consists of two main purposes The first objective was to evaluate four geophysical methods with a view to comparing and contrasting their

accuracy, functionality, cost effective, and overall utility The ultimate goal is to find

better, less expensive and reliable methods to determine shear wave velocity in the upper

100 ft of the subsurface The advantage and disadvantage of the individual geophysical methods were also discussed on the basis of actual field testing

The second objective of this research was to determine the NEHRP (National

Earthquake Hazards Reduction Program) site classification map of the Poplar Bluff area

by acquiring a large number of shear-wave velocities to 100 ft depth (Vsv100)) VS100) is the shear-wave velocity averaged to 100 ft depth and is one predictor of earthquake ground motion amplification and potential hazard in similar alluviaum-filled basins in California Under NEHRP-UBC provisions (BSSC, 1998) sites are categorized for

shaking hazard using Vsv109) To accomplish the second objective, several shear-wave

velocities within Poplar Bluff area were acquired using MASW method Sufficient shear- wave velocity data was collected to obtain a realistic view of the variation in shear wave velocity All shear wave velocity data were used to generate the NERHP soil

classification map (BSSC, 1998)

1.3 STUDY AREA

The study area for this research is the Poplar Bluff area, Butler County Missouri

(Figure 1.1) It includes the area within the four standard 7.5' topographic quadrangles of

Poplar Bluff, Rombauer, Harviell and Hanleyville (Figure 2.1) The city of Poplar Bluff

is centrally located near where these four quadrangles meet The Poplar Bluff area was selected for study because significant basic geologic and earthquake data already exists for the area and the area is currently being investigated for earthquake soil amplification

by the Missouri Department of Natural Resources (MoDNR), Geological Survey as part

of an Association of Central United States Earthquake Consortium State Geologists project Exchange of data between that project and the research here should provide a synergy that will benefit both projects

1.4 DISSERTATION OUTLINES

The dissertation is organized into 9 chapters Chapter | is an introduction to the research, objectives, significance, and overview of the study area Chapter 2 provides the scope of field work program and the detail of the test-site locations The overview of data acquisitions and data processing of individual geophysical methods were discussed in Chapters 3 to 6 These chapters also cover the advantage and disadvantage of the

individual geophysical methods Chapter 7 covers the comparative analysis and ranking

of all four geophysical methods base on their accuracy, reliability, functionality, cost,

utility, and over utility to MoDOT The NEHRP site-class map of the Poplar Bluff area

was discussed in Chapter 8 Chapter 9 is the conclusions of this research and

recommendations

^

#=3ooncr/31177Indeoe ere Ap yncepen JOverland Ba ait les Sum

Atockieh bake,

2 SCOPE OF WORK

2.1 OVERVIEW

The four field methods were tested at selected sites in the Poplar Bluff study area

during summer and fall 2004 Geophysical data collected for this study consists of CH data, MASW data, SCPT data, and ReMi data All geophysical data were acquired within

the MoDOT right-of-way in order to facilitate accessibility for all geophysics

equipments

2.2 GEOPHYSICAL FIELD WORK PROGRAM

Table 2.1 summarizes the geophysical field work plan A total of 40 sites were

selected in the Poplar Bluff study area 1-D MASW data were acquired at all 40 test sites; 2-D MASW data were acquired at 4 test sites Crosshole (CH) shear wave velocity data were acquired at two test sites SCPT data were acquired at 15 test sites (including the

four 2-D MASW test sites) In this study SCPT data were acquired by MoDOT The field

work plan was devised with the expectation that sufficient data were available for the purposes of evaluation and comparative analyses

Table 2.1: Summary of geophysical field work plan

2.3 POPLAR BLUFF TEST SITES

The Poplar Bluff study area was selected to include four USGS 7.5’ topographic quadrangle maps with the City of Poplar Bluff located near where the four maps join The four maps are the Poplar Bluff, Rombauer, Harviell and Hanleyville quadrangles The

study area is about 17 miles north-south and 14 miles east-west

The northwest portion of the study area is within the Ozarks uplands province and

the southeast portion is in the Mississippi Embayment lowlands province (Figure 2.1) The soils, or surficial materials (Figure 2.2), the topography and the groundwater level in these two areas are quite different The lowland are almost flat and have stream deposited alluvial soils composed mostly of sand with some silt, clay and gravel The alluvial soils are 100 to 200 feet thick, except adjacent to the uplands The groundwater level is very shallow in the lowlands, commonly 5 to 15 feet deep In the Ozarks uplands the

topography varies from quite hilly and rugged near stream valleys to gently rolling or

nearly level on the upland drainage divides The residual soils are derived from prolonged weathering of the bedrock upper surface Intense weathering has dissolved the soluble

portions of the bedrock units leaving behind thick deposits of insoluble clay and large

amounts of chert gravel The residuum varies in thickness from about 40 feet to over

200 feet, commonly being about 100 feet thick The groundwater level is usually below the base of the residuum in the bedrock although small perched groundwater zones

occasionally exist in the residuum Alluvial valleys within the Ozarks have some

characteristics similar to the Mississippi Embayment except the alluvial soils are less extensive, more gravelly and usually thinner One unusual area of sand dune soils exists within the Mississippi Embayment area

Test sites for measuring shear wave velocity were selected based on three criteria: 1) sample the range of soil types and conditions in the study area, 2) achieve a relatively

uniform aerial distribution of sites throughout the study area and 3) test locations at or

near where soil borings and geotechnical data already exist, preferably at MoDOT bridge sites with multiple borings Some consideration was also given to how easy it would be for test equipment to access the sites and site ownership Most sites selected are on

MoDOT right-of-way Of the four shear wave velocity testing techniques used in this

study the MASW testing method is the most versatile because it could be used in all

geologic settings Forty sites were selected for MASW 1-D testing with about half in the uplands and half in the lowlands (Figure 2.3 and Table 2.1)

Of these 40 MASW site a subset of 4 sites were selected for 2-D profiles of 17 MASW tests to evaluate the lateral variation in shear wave velocity (Figure 2.3 and Table 2.1) All 4 of the MASW 2-D profile sites were located in the lowlands area so that they

could be compared to the similar SCPT profiles which were only available in the lowland

setting

The SCPT method was not usable in the uplands area because the rocky, gravelly

nature of the residual soils could not be penetrated by the cone Therefore, all of the SCPT tests were sited in the lowlands or in the alluvial valleys within the uplands area Ten sites were selected for the SCPT method in this study Some of the SCPT had been tested during the previous CUSEC-SG study which shown as existing in Figure 2.4 A subset of 4 test sites had profiles of 5 SCPT run at them (Site #3, #10, #13, and #31)

The CH technique is usable in either the upland or lowland setting but it requires the installation of twin boreholes at each site with casings installed in the holes Two CH sites were selected, one in the uplands and one in the lowlands (Figure 2.5 and Table 2:1)

The lowland site was selected to be the same location as one of the sites with MASW and

SCPT profiles so the methods could be compared

Poplar Bluff Study Area

Poplar Bluff Quadrangle Rombauer Quadrangle

Harviell Quadrangle $$ Hanleyville Quadrangle

Saat icdsl Mists iste LMaiEe

12

Poplar Bluff Study Area

MASW Test Sites

Harviell Quadrangle — Hanleyville Quadrangle

owen aor j2, U8 se 6UB47 Co] Peper ci #erfailil Mlsterlale La Ea

MAW 4.0 Tost Ste 3 Afuons Hmmmase Ll] Faywegrephte Provinces Si GE ach nào Soi Khu > a wk

4 t 4 z 3 4 5 a 7 5 5 Miles BHYT m ntwdr+ ch lớn 8 nem

21L, hot mớc 2 ê nh nh recive

Figure 2.3: Poplar Bluff study area MASW test sites (2-D MASW shear wave

velocity profiles were acquired at Sites #3, #10, #13 and #31.)

Poplar Bluff Study Area

Harviell Quadrangie Hanieyviile Quadrangle

Poplar Bluff Quad

Poplar Bluff Study Area

Crosshole Test Sites

Cara TS oe

A us 60 8 Us 67

[ ¬ = Aurela

[HJ Physiegraphic Provinces ga Set lecss & reskdoum

D5) ? Rsiiei:z 5 - mpta nd residue

15

3 CROSSHOLE SEISMIC (CH)

3.1 OVERVIEW

Crosshole (CH) seismic shear-wave velocity data were acquired at two sites, site

#3 and #15, in the Poplar Bluff study area (Figure 2.5) Good quality CH data is generally accepted as more reliable than shear-wave velocity data derived from other methods (Kramer, 1996) Consequently, the CH data set were used as a yard stick for determining the reasonableness of the acquired MASW, ReMi, and SCPT shear-wave velocity data

CH testing in this research was conducted according to ASTM standard: D

4428/D 4428-00 CH seismic testing is a valuable technique for site investigations, since

it can be used to produce detailed compression (P) and shear (S)-wave velocity profiles (Butler and Curro, 1981) Seismic CH tests use two or more boreholes to measure wave

propagation velocities along horizontal paths The simplest CH test configuration consists

of two boreholes, one of which contains an impulse energy source and the other a

receiver By fixing both source and receiver at the same depth in each borehole, the wave

propagation velocity of the material between the boreholes at that depth is measured By

testing at various depths, a velocity profile can be obtained

16

3.2 BOREHOLE PREPARATION

In this research, twin boreholes were drilled at two sites (Sites #3 and #15) by MoDOT during summer 2004 Each borehole was cased with 4-inch D PVC pipe, grouted to the host medium, sealed, and evaluated The borehole lithologic logs of these two sites were shown in Figure 3.1 for Site #3 and Figure 3.2 for site #15 The soil at Site #3 consists almost exclusively of sand, silt and clay The limestone bedrock at Site

#3 was encounter at a depth of ~113 ft The soil at Site #15 consists almost exclusively of residium (gravel, sand, silt, and clay) The bedrock was not encountered at this site

The borehole remained evacuated during all seismic data acquisition activity

Figure 3.3 shows the twin borehole configuration of site #3 At this site, the borehole BH-

AI ¡s the source hole and borehole BH-A2 is the receiver hole The spacing between the two boreholes is about 15 ft Figure 3.4 is borehole configuration of the site #15 In this site, BH-B1 is used as the source hole and BH-B2 is the receiver hole The borehole spacing at this site is 15 ft

Light gray fine clean sand

90 Light gray medium to fine clean sand with some gravel ,

Light yellow silty clay

Coarse sand and fine gravel with some

red silty clay

Mostly tan to light yellow very stiff silty clay and red clayey sand with some fine gravel

Red clayey sand

Brownish red sandy clay with small amount of course

Figure 3.2: Borehole lithologic log at Site #15

18

20 3.3 BOREHOLE DEVIATION SURVEY

The borehole deviation survey was conducted for each borehole at two sites The objective was to determine accurately the horizontal distance between two boreholes The borehole deviation tool employs a combination of precision magnetometers and

inclinometers to measure its orientation with respect to the earth’s magnetic and

gravitational fields This information was transmitted continually to the surface data

acquisition system, as tool was raised from bottom to top inside the borehole

At each point in the borehole, two measurements were taken, normally referred to

as northing or inclination, and easting or declination Inclination refers to the deviation angle of the boreholes with the vertical Declination refers to the orientation of the

borehole relative to azimuth Inclination and declination measurements are used to

calculate the distance between the two boreholes at each station

From the result of borehole deviation survey at both sites, the boreholes at each

site were quite vertical The maximum deviation did not excess 1 ft at both two sites

3.4 DATA ACQUISITION

The acquisition of the CH shear-wave velocity data was relatively

straightforward Figure 3.5 illustrated the CH seismic data acquisition used in this study

In this research the borehole hammer (Figure 3.6) was used as a source to generate a high frequency shear-wave The borehole receiver used in this study was BHG-3 model made

by Geostuff Company This borehole receiver contains a 3-component geophone in an X- Y-Z orientation and was used to detect the signal (Figure 3.7).To acquire the data, source was lowered to the base of the source hole and the receiver was lowered to the same depth in the adjacent borehole The source and receiver were locked in-place At each

21

station, the borehole mechanical source was discharged twice in the upward and

downward directions The source and receiver were raised from bottom to the surface at 5

Figure 3.6: The borehole seismic source

23 3.5 DATA PROCESSING

The seismic data acquired at each depth were gathered together to form the

composite seismic record (Figure 3.8) At each depth the forward and reverse polarity seismic records were plotted on the same depth In Figure 3.8, although the downhole hammer is suitable for generating vertical polarized shear-wave, observable

compressional wave (p-wave) can also be seen on the composite seismic record

The transit time of the shear wave, form source to receiver, was determined for each test depth on the basis of the cross-over time of the reverse polarity seismic records The transit time and borehole separation data were then used to determine the in-situ shear-wave velocity of the soil at each depth tested CH shear-wave velocity profiles for

Site 3 and Site 15 are shown in Figures 3.9 and 3.10

24

3.6 DATA INTERPRETATION

Figure 3.9 shows the shear-wave velocity profile determined from CH testing at site #3 and also accompanied by borehole lithologic log The twinned borehole at this site

was encountered bedrock at a depth of ~113 ft However, either because of obstructions

or the shortness of the PVC casing, CH shear-wave seismic data were obtained only to a

depth of 110 ft The visual inspection of CH profile indicates that the shear-wave velocity

of soil, with minor fluctuations (on the order of + 100 ft/sec), increases gradually with depth of burial (from a low of about 600 ft/sec to a high of about 1000 ft/sec) The soil at

site #3 consists almost exclusively of sand, silt and clay The observed minor fluctuations

in shear-wave velocity are attributed to minor changes in lithology (sand, silt, clay

concentrations) and grain size

CH shear-wave velocity of site #15 is plotted in Figure 3.10 The twinned

borehole at this site did not encounter bedrock The visual inspection of the CH data at

this site indicates that the shear-wave velocity of soil, with minor fluctuations (on the

order of + 50 ft/sec), increases gradually with depth of burial (from a low of about 850

ft/sec to about 1875 fu/sec) The soil at site #15 consists almost exclusively of residium The observed minor fluctuations in shear-wave velocity are attributed to changes in lithology (gravel, sand, silt, clay concentrations) and grain size

Tan silty clay

Brown clean fine to medium sand Mostly clean brown chert gravel with some tan and light gray sand

Mostly light gray stiff clayey sand

Tan/brown gravelly sand Gray and tan clean sandy fine gravel

Light gray fine clean sand

Light gray medium to fine clean sand with some gravel Light gray fine clean sand

Light gray medium to fine clean sand with some gravel

Mostly tan to light yellow very stiff silty clay and red clayey sand with some fine gravel

Red clayey sand Brownish red sandy clay with small amount of course

Figure 3.10: CH data Site #15 and borehole lithologic log

25

26

3.7 SUMMARY EVALUATION OF CH SEISMIC METHOD

The CH method can be used to generate a very accurate shear-wave velocity

profile of the subsurface (Indeed CH shear-wave velocity profiles are generally

considered to be more accurate than MASW, SCPT or ReMi shear-wave velocity

profiles.) The CH method works particularly well in fairly uniform soils and in

reasonably quiet (acoustically) environments The main problem with the method is that

it is very expensive, as twinned (or tripled) PVC-cased, air-filled boreholes are required (Borehole deviation data must also be acquired.) A tabularized summary of the CH

method is presented as Table 3.1

Table 3.1: Tabularized summary of the CH method

Twinned boreholes (PVC-cased, air-filled) were drilled with a separation of

15 ft A shear-wave acoustic source was lowered to the base of one borehole;

a shear-wave acoustic receiver was lowered to the same depth in the adjacent

borehole The source and receiver were locked inplace The borehole source was discharged (in an upward direction); the receiver recorded the arrival time and amplitude of the acoustic shear-wave energy that traveled directly

from the source to the receiver (cross-hole seismic field record) The source

was then discharged in a downward direction thereby generating an opposite

polarity field record The source and receiver were raised to the surface at 5 ft increments Each time the source/receiver pair was raised, the source was

discharged twice (in opposite directions), and cross-hole seismic field records

were recorded A borehole deviation tool was used to meter accurately determine the separation between the twinned boreholes at every depth tested

The transit time and borehole separation data were ultimately used to

determine the in-situ shear-wave velocity assigned to the soil at each test depth

1,2 Field equipment The borehole acoustic data were acquired using a portable equipment

consisting of a borehole shear-wave acoustic source, a borehole receiver (triaxial geophone), source and receiver cables, a source control unit (with trigger switch cable), an engineering seismograph, 12-V battery, and laptop The borehole deviation tool (rental) consisted of a borehole inclinometer, a winch and a control unit

and crew are transported in a single vehicle

27

completed

e Background noise Acoustic noise can degrade the quality of the recorded data, particularly at

shallow test depths If the boreholes are located adjacent to a roadway, the

source should be discharged multiple times (in both directions at each test

depth) and the records should be stacked The source should also be discharged when traffic noise is relatively low

e Anchoring requirements The equipment does not need to be physically anchored or coupled to the

ground surface; however the winch needs to be set firmly on the ground

se Depth of investigation CH data can be acquired anywhere twinned boreholes can be drilled and

completed The acoustic source and receiver can be lowered to the base of any

ait-filled cased borehole

e Proximity to structures

and utilities (buried)

CH data can be acquired anywhere twinned boreholes can be drilled and completed However, background acoustic noise can be a problem particularly at shallow test depths

e - Proximity to built

structures and utilities

CH data can be acquired anywhere twinned boreholes can be drilled and completed However, background acoustic noise can be a problem

1.5 Brief description of field data The field data (one record for each depth tested), consisting of unfiltered

cross-hole seismic field records, are recorded digitally and stored on the laptop coupled to the seismograph Borehole deviation data are also recorded

digitally

1.6 Time required to acquired to

acquire field data at one test site

One set of cross-hole data (twinned boreholes) and associated borehole deviation data can generally be acquired less than six hours (assuming: crew

and equipment are on-site)

1.7 Estimated cost to acquire field

data at one test site

Basic field costs include: a) 6 hours of crew time plus travel time; b)

equipment rental and/or depreciation; c) vehicle rental and/or depreciation

plus fuel Note: the cost of drilling and completing the twinned boreholes is not included in this estimate

1.8 Potential for errors

source If the geophone and source are accurately placed (and coupled to the

casing) each time the source is discharged, there is little possibility for human error leading to significant misinterpretation Inasmuch as the source and

receiver are separated laterally by 10 ft, errors in the vertical placement the

source and receiver on the order of less than 6 inches will not be significant However, accurate borehole deviation data must be acquired

e Equipment Equipment problems are unlikely to generate errors that will lead to

misinterpretation

1.9 Reproducibility of field tests Field results are reproducible This is one of the reasons that CH shear-wave

velocity data are generally assumed to be more reliable than SCPT, MASW,

and ReMi data

2 DATA PROCESSING

2.1 Brief overview of data

processing Each pair of opposite polarity cross-hole field records is analyzed visually The transit time of the acoustic shear-wave energy (from source to receiver)

and the physical source-receiver separation (from deviation data) at each test

depth is used to calculate the shear-wave velocity assigned to that test depth The output is a 1-D shear-wave velocity profile of the subsurface (with velocity values at vertical depth intervals of 5 ft)

2.2 Output of data processing The output is a 1-D shear-wave velocity profile of the subsurface with values

at depth intervals of 5 ft (Poplar Bluff data set) This shear-wave velocity

profile constitutes the final deliverable

2.3 Estimated cost to process field

data from one test site Basic processing costs include: a) 2 hours of interpreter’s time; b) hardware/software renta! and/or depreciation