Paper Borehole Shear-Wave Surveys

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Borehole Shear-WaveSurveys for Engineering

Site Investigations

Doug Crice

Geostuf f

19623 Via Escuela Drive

Saratoga, CA 95070 USA

phone 408-867-3792

fax 408-867-4900

http://www.georadar.com/geostuff

February, 2002


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Introduction

There is a trend in building codes to require shear wave velocity studies as part of foundation investi-gations for major structures. Shear wave velocitymeasurements are an important tool in designing

buildings for site specific conditions such asground-spectral earthquake response. And, they area much more diagnostic tool for engineering proper-ties than P-wave velocities. Consequently, engineersthat are more comfortable with soil mechanics thanthe wave equation are taking an interest ingeo physics.

Since most practicing geo physicists seldom get anopportunity to conduct one of these surveys, itseemed worthwhile to document some of the meth-ods used. The techniques are seldom covered in text-

books or papers, the former because they’re tooesoteric and the latter because they’re common

knowledge. Of course it isn’t common knowl -edge—geo physicists that know how to actuallyconduct shear wave surveys are few and widelyscattered.

What are shear waves and why do we care?

In com pressive waves, the ground vibrates in thesame direction that the wave travels (see Figure 1).In shear waves, the ground wiggles transversely tothe direction that the wave is propagating.

The P-wave velocity in a material is mostly depend-ent on com pressive strength. Experience (with a lit-tle common sense and some helpful tables) allows us

to guess something about the material once thevelocity is known. For exam ple, if the P-wave veloc-ity is 2000 ft/second (600 m/s), then we know thatthe material is proba bly a com pacted soil. A suddenincrease to 5000 ft/sec (1500 m/s) suggests that wehave hit the water table. A velocity above 10,000ft/sec (3000 m/s) is almost certainly a fairly com pe-tent bedrock. A refraction analysis will tell us thedepth from the surface to each of these materials andthis result is adequate for many applications such as

finding the depth to groundwater or the excavationcosts.

Now sup pose that instead of digging a swimming pool, we want to put something a little more substan-tial on this site—a nuclear power plant perhaps, andthat substantially all of the foundation will go in thatlayer with a P-wave velocity of 5000 ft/sec. No lon-ger can we assume that this is just a saturated alluvialmaterial. Consider some of the materials that mightexhibit this same compressional wave velocity: satu-rated gravels, clay deposits, weathered rock, coal, or even quicksand. It looks like we really don’t knowwhat’s down there, only that it has approximately thesame com pressive wave velocities.

Reasonable values for shear wave velocities are listedin the following table. While there will be wide vari-ations in the velocities in these materials, clearly the

shear wave velocity tells us a lot more about the char-acter of the material in-situ. Shear wave velocities aredependent on the shear strength of the material andshear strength is what sup ports buildings and pilesand keeps a rip ping tooth from cutting rock. If we

know the shear wave velocity, add some geologicalknowledge, a little common sense, and maybe a fewdrill holes, you can know a lot more about the mate-rial. Empirical studies by OYO Corporation

1 showed

that shear wave velocity correlates very well with blow counts, one of engineering geology’swell-established measurements.

If you know the velocities of the P and S waves andthe density of the material, you can calculate theElastic properties that relate the magnitude of thestrain response to the applied stress. These elastic

properties include the following:

(1) Young’s Modulus (E), stress/strain, is the ratio ofthe applied stress to the fractional extension (or short-ening) of the sam ple length parallel to the tension (or com pression). Stress is force/unit area and strain isthe linear change in dimension divided by the originallength.

(2) Shear Modulus (G) is the ratio of the appliedstress to the distortion (rotation) of a plane originally

per pendicular to the applied shear stress; it is alsotermed the Modulus of Rigidity.

Shear Waves Page 2

Figure 1, Particle motion and wave propagation.

Material Vp ft/sec (m/s) Vs ft/sec (m/s)saturated gravels (clean) 5000 (1500) 1000 (300)

saturated gravels (dirty) 5000 (1500) 2000 (600)

clay de posits 5000 (1500) 3000 (900)

weathered rock 5000 (1500) 2000-3000 (600-900)

coal 5000 (1500) 3000 (900)

quicksand 5000 (1500) 0 (0)


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(3) Bulk Modulus (k) is the ratio of the confining pressure to the fractional reduction of volume inresponse to the applied hydrostatic pressure. Thevolume strain is the change in volume of the sam pledivided by the original volume. Bulk Modulus isalso termed the Modulus of Incompressibility.

(4) Poisson’s ratio (F) is the ratio of lateral strain(per pendicular to an applied stress) to the longitudi-nal strain (parallel to applied stress).

For elastic and isotro pic materials, the elastic prop-erties can be calculated from the seismic velocities.For exam ple:

Poisson’s Ratio F p = (Vp/Vs)2 - 2

2(Vp/Vs)2- 2

Shear Modulus G= d Vs2

Young’s Modulus E= 2G (1+F p)

Bulk Modulus K= 1 . E3 1-2F p

Vp = P-wave velocityVs = shear wave velocity d = density

These are of course elastic linear dynamic charac-teristics and are not always applica ble to static per -formance of the foundation material when sub jectedto large static or dynamic loads.

How are shear wave velocities measured?

The sim plistic answer is that shear waves are col-lected and measured just like P-waves. An energysource is used to generate elastic waves in theground, and these elastic waves are detected at mul-ti ple locations by vibration sensors. The signals arecollected and dis played on a seismograph. There isone major problem. Shear waves travel slower thanP-waves and thus will be imbedded in the com plexwavetrain somewhere after the first arrival.

In a normal refraction survey, identification of theP-waves is sim ple since they arrive first in therecord. After the “first arrivals”, many other waveswill be buried in the later part of the seismic record.In addition to the shear waves, there are P waves

Page 3 Shear Waves

Figure 2. Basic field procedure for recording clean shear-wave records.


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with different refraction paths, reflections, surfacewaves and various converted waves. Occasionally,you will see exam ples of com posite records withalleged shear wave arrivals identified by the ampli-tude or frequency, but as a practical matter it isimpossi ble to reliably pick a shear wave out of anormal refraction record.

So what’s the solution? The answer is to use a seis-mic energy source that generates mostly shear waves, and vibration sensors sensitive to shear waves. Consider Figure 2. which illustrates the

basic field procedure. One extremely effective and popular mechanism to generate a clean shear waveis sim ply a wooden plank weighted down with avehicle. By hitting the end of the plank with a ham-mer, a shearing stress is applied to the ground. Theshear wave propagates in the direction per pendicu-lar to the plank towards the geo phone.

Geo phones are available with different sensitiveaxes, usually horizontal or vertical. In this illustra-tion, we are using a horizontal geo phone (Horizon-tal geo phones are often mistakenly called “shear

phones” because they are commonly used for shear-wave surveys, but shear waves can be ori-ented in any direction depending on the polarizationof the source). The geo phone is oriented parallel tothe plank, in the same axis as the particle motion. Itwill be quite sensitive to the shear waves, and rela-tively insensitive to any com pressive waves.

The seismic record from this survey will resem blean ordinary seismic record and an illustrative exam-

ple is shown in Figure 3A. There is a classiczero-phase wavelet with a strong first arrival fol-lowed by larger excursions which die down after afew cycles. In a properly done survey, a good shear wave record will be less com plex than refractiondata, because we aren’t dealing with mode conver-sions and because the survey geometry is chosen tominimize multi ple arrivals. To confirm that wereally have a shear wave, take another record byhitting the other end of the plank. It should look like the one in Figure 3B. The first break is in theopposite direction, which is your confirmation thatthe arrival is most likely a shear wave. Many ana-lysts like to superim pose the records as shown in

Figure 3C. for better com parison, either by makinga trans parent copy of one record, or with a com- puter plotting program.

Of course this same vehicle-on-a-plank source can be used with multi ple geo phones in a line on thesurface (i.e. a refraction survey) and the refractionformulas can be used to analyze the data. The prob-lem is that refraction analysis relies on the assump-tion that the wave velocities increase with depth. Inthose situations where determining the shear wave

velocities is an important objective of the survey, itis more common for this assumption to be invalid.Besides the proba bility that there will be layers withlower velocities than those above, it is also likelythat there will be thin beds that are not resolved bystandard refraction analysis. For these reasons, sur -face shear-wave refraction surveys are not generallyused for analysis of layered alluvial materials, or any other situation where there is likely to be alow-velocity layer sandwiched between high-veloc -ity layers (i.e. a velocity inversion).

Field Techniques

Because subsurface shear wave velocities cannot bereliably measured on the surface, the normal proce-dure is to conduct the surveys in boreholes. Thereare two princi pal methods: cross hole and downhole.

The cross-hole survey is shown in Figure 4. Two(or three) holes are drilled side-by-side, typicallywith 10 foot (3 m) spacing. A vertical geo phone isclamped in one hole at some depth. A down-holeshear-wave hammer is clamped at the same depth inthe adjacent hole. The hammer is a special tool witha sliding weight that can bang downward (as the

weight is allowed to drop) or upward (as the weightis pulled upward with a cable) to generate a pair of shear waves of opposite polarity. A pair of recordsis taken as shown earlier in Figure 3. The procedureis repeated at different depths until a com plete setof measurements has been taken. The shear wavevelocity for each geologic layer is calculated fromthe distance between the holes and the travel time.

Shear Waves Page 4

Figure 3. Shear waves reverse polarity when the source polarity

is reversed.


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Down hole is a sim ple extension of the surface sur-vey described earlier, with a setup as shown in Fig-ure 5. A borehole is pre pared and the plank-vehiclecom bination located near the top of the hole. A hor -izontal geo phone is clamped in the hole (actually atri-axial geo phone) and the data is acquired by

collecting recordsfrom impacts on bothends of the plank. Athird record of P-wavevelocity data is col-lected by hitting astriker plate on theground surface to gen-erate com pressivewaves (which isdetected and recordedfrom a vertical geo-

phone). The tri-axialgeo phone package ismoved a short dis-tance and the wholesequence repeateduntil records have

been obtained at inter -vals from the surfaceto the depth of interest.

There are significantlogistic and technicaldifferences betweenthe two methods.Typically, practitio-ners will have astrong preference for one method over theother, often approach-ing a religious fervor.

The cross-holemethod was popular-ized earlier than downhole, and is an ASTMstandard1, so it willoften be specified on

bid documents, andthere will not be a freechoice in that case.Occasionally, bothtypes will be con-ducted to determine

anisotropy, since cross-hole surveys measures Vsv(the velocity of a vertically polarized shear wave)

and down hole surveys measure Vsh (the velocity ofa horizontally polarized shear wave). There areadvantages and disadvantages to either method:

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Figure 4, Cross hole shear wave measurements.

1 The ASTM standard is D4428/D4428M-00 Standard Test Methods for Crosshole Seismic Testing. It can be

purchased and downloaded from the ASTM web site at http://www.astm.org


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Cross-hole surveys require two (or three) boreholesinstead of one, and the holes must be surveyed to

determine the distance between them at each depth.Depending on your driller, the holes can deviatefrom vertical (or rather, equal spacing) by a sub -stantial distance. A not-uncommon variation of 1meter in spacing between two boreholes 3 metersapart can affect your results enough to make fieldinter pretation and quality control difficult.

A down-hole hammer is required for cross hole; anextra piece of equipment which is not widely avail-able. There are practical limits to the maximumdepth the weight-and-cable hammer mechanismscan be used, and most require dry holes.

Shear waves traveling horizontally between bore-holes can refract down (or up) and travel throughadjacent, higher-velocity layers, just like surfacerefraction surveys, taking the quickest path betweenthe holes. This limits the ability to resolve thin lay-ers, or even to determine their dimensions. Therefraction problem is the reason that cross-hole sur-veys must be conducted with small distances

between boreholes.

Down hole surveys eliminate most of these prob-lems. Only one hole is needed, the energy sourceis the surface plank-hammer com bination, andsince the waves travel nearly vertically, there arefewer ambiguities about the path. The main disad-vantage of downhole surveys is that attenuationand natural filtering by the earth rounds off theseismic arrivals so that it can be difficult to pick the first breaks with the necessary precision.

Data Analysis and Interpretation

As a general rule, the person acquiring the datawill pre pare a report that describes the field pro-cedures, shows the raw data, and tabulates theresults in the form of Vp and Vs for various layersand depths. Raw data should include both P-waveand S-wave waveforms (in both polarities, seeFigure 6.) in sufficient detail to allow inde pendentreview. Tabulated data should be presented as a

stacked section showing the P-waves and shear waves dis played side-by-side (see Figures 7 and 8for exam ples of P-wave and shear wave sectionsof data taken at the same site).

Analysis of the stacked section can take twoforms. One approach is to superim pose “best fit”lines of the data arrivals to make a general inter-

pretation of the geologic structure. The secondapproach is to look specifically at the intervalvelocities between adjacent traces. The preferredmethod depends on the pur pose the data will

serve and the structure, and also depends on thequality of the downhole signals. Waveforms con-taminated by noise or interference may precludecom puting reliable interval velocities.

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Figure 5, Down-hole shear wave configuration

Figure 6, Sample shear wave data showing stacked traces with

opposite polarities.


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Cross-hole data is relatively sim ple to inter pret anddis play, since there is only one record at eachdepth, and the velocity, Vs, is calculated from thearrival time and separation.

In the case of down-hole data, there will be twohorizontal geo phones that may be oriented in a ran-dom azimuth. If the geo phones orientation is off theazimuth of the plank, then the shear wave data willappear on both horizontal geo phones. Worst case is70% of the waveform amplitude, and at least onewill have a decent signal. The reverse polarity test(by recording data from the other end of the plank)must be made before the geo phone is re-positionedso that valid com parisons can be made.

Equipment

The Seismograph

Modern exploration seismographs are ideal for shear-wave surveys. They offer precise time syn-

chronization with the hammer switch, fast datasam pling, and summing of multi ple hammer blowsto allow surveys to greater depths and in the pres-ence of cultural noise. Floating point amplifiers (or 24-bit systems) eliminate the need for operator gainadjustments regardless of the distance between the

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Figure 7, Composite down hole P-wave section Figure 8, Composite down hole shear wave section, in same

borehole as Figure 7 (time scale on record is changed).


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hammer and geo phone, ensuring maximum resolu-tion of the signal. Analog and digital filters reduceor eliminate cultural and system related noise. Digi-tal recording in PC com pati ble media allows later

play back, analysis or processing. Digital plotterswith precise time scales allow control of the dis play

parameters and appearance for record com parison.

Hammer

There is nothing special about the hammer, exceptthat an impact switch (normally sup plied as anaccessory to the seismograph) is attached to thehandle near the head with electrical tape. Of coursethey are available in different weights from four lbs. (2 Kg) to 20 lbs. (9 Kg). Different users prefer different weights and the best choice is not obvious.The impedance match between the hammer and the

plank affect the dominant frequencies and energytransfer (though not to the extent that soil condi-

tions affect them). A 16-lb hammer will push moreenergy into the ground at lower frequencies, and itwould be the obvious choice for deep, down holesurveys. Some users would argue that with an 8-lbhammer, they can swing it at a higher velocity, andmore often, without tiring the hammer operator.

Down-hole hammer

Cross-hole surveys require a down-hole hammer,which is sim ply a device that can be clamped intothe borehole and then banged up or down, impact-ing a shear force against the borehole wall. Some

practitioners construct their own and some are

available commercially2. The usual approach is tostart with a metal pipe, closed on each end, with amoving weight inside. The moving weight isattached to a cable (fed through a hole in the end) tothe surface. The operator pulls up on the cable tocreate an upward impact, then releases the cable tolet the weight drop and create a downward impact.An impact switch is attached to the pipe to providea precise zero time.

The hammer is clamped in the hole with an inflat-able bladder or some mechanical mechanism. Theamount of energy from these down-hole hammersis limited, but generally adequate for the small

separations between holes in a cross-hole survey.Falling weight hammers don’t work underwater, sowet holes must be pumped dry before the survey.

Plank

The plank can be an ordinary fence post or railroadtie. It should be long enough to protrude from bothsides of the vehicle used to weigh it down. Theground should be pre pared by scraping the surfacewith a flat shovel to expose bare, undisturbed soil to

provide a good friction contact. It is not necessaryor desirable to excavate and backfill the site

because that will tends to spread the energy andconvert the shear stress into com pressive stress.

Some users like to enhance their plank. Oneimprovement is to put metal plates on the ends toreduce the wear and tear from extended pounding.Another improvement is to bolt short pieces of

channel iron to the bottom of the plank, transverseto the long axis, to provide a better grip ping sur-face. For air-mobility, buy a fence post at a localhardware store and nail metal joint brackets to the

bottom.2

Typically the plank is located some distance fromthe hole, 5 to 10 feet (2 to 3 meters). This offsetmust be corrected for when com puting travel timesfor shallow depths, and the separation between the

plank and the borehole should be kept small enoughto allow accurate measurement of near-surfacearrival times. Some practitioners locate the plank away from the borehole as much as 1/3 the total

depth of the borehole, presumably to avoid tubewaves. The author disagrees strongly with thisapproach, since it destroys your ability to measurenear-surface interval velocities

3 The best offset dis-

tance seems to be determined by the distance between the vehicle’s wheels and the adjacent bumper, which should be clear of the top of the borehole to provide access.

Geophones

The basic sensing element is the geo phone, chosenwith a frequency low enough to capture the signals.Shear waves frequencies are lower than those of

Shear Waves Page 8

2 Other types of shear-wave energy source mechanisms are possible, though few can match the simplicity and

effectiveness of the weighted plank and hammer. Because the plank and hammer provide particularly clear shear

wave arrivals, this is the preferred source for less-experienced users and the choice of experts. One substitute–

hammering the side of a trench–is particularly challenging for the uninitiated, because the plastic deformation of the

trench wall produces strong P waves that make proper identification of the shear wave arrivals difficult.


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P-waves. Looking at some of the data in this report,the lowest frequency seen is about 20 Hz, whichsuggests using geo phones with a natural frequencyof 10 or 14 Hz. Much lower frequency sensors will

be tilt sensitive and may have spurious resonancesin the bandpass of interest. 28-Hz sensors may losesome of the information but have the advantage of operating in any position, such as in horizontal drillholes. Higher frequency geo phones are not desir-

able except for tomographicapplications in rock.

A down-hole geo phone is con-structed with three geo phoneelements in an X-Y-Zorthogonal configurationsealed in a cylindrical pack-age. The geo phone must befirmly clamped against theside of the hole so that it fol -lows the ground vibrationsexactly. Geo phones are typi-cally clamped with either inflatable bladders or mechan-ical arms, home made or pur-chased from a commercialsource. The clamping devicemust be located on just oneside of the geo phone so thatthe housing is firmly presseddirectly against the wall andmust be located near the centerof the assem bly.

Home-constructed systemsusually employ a bicycle innertube attached to the geo phone

package. Pressure tubing isrun to the surface, where a

bicycle pump or gas cylinder is used to inflate the bladder.Such systems perform theclamping function adequately,

but lack a certain robustnesswhich hinders productivity.The systems tend to leak,

bladders pop, are inconvenientto re-position, and increasing

pressure is required as water depth increases. Theyoften don’t fit into existing boreholes. Nonetheless,they do the job and are economical. Every geo-

phone manufacturer has a down-hole, 3-com ponentgeo phone assem bly available as a building block.Commercial bladder-clamped systems are alsoavailable, and are more reliable because of the extraengineering effort, but they tend to suffer frommany of the same problems.

Mechanical arm systems are more com plex andthus difficult to construct and more costly. How -ever, they are generally more satisfactory to use.They are reliable, operate at greater depths, and areeasy and quick to re-position. Battery-poweredunits are easier to handle since there is no air line.One such unit is shown in Figure 9, available com-mercially for purchase or rental.

Preparing the hole

We have so far carefully glossed over the details of the borehole, using the construct of a sim ple hole inthe ground. In fact, you can use a sim ple hole in theground and it will work very well unless there arewashed-out segments where the tool can’t makedecent contact.

In practice, holes in alluvial materials have a ten-dency to close in, leaving your geo phone perma-nently “secured” in place. To prevent collapse or washouts, bore holes are normally cased with plas-tic PVC pipe. The space between the outside of the

pipe must be backfilled with pea gravel or

low-strength grout to ensure that the pipe followsthe motions of the adjacent soil exactly. Any voidsoutside the pipe will allow the pipe to shake inresponse to vibrations above or below the tool loca -tion, and mode conversions between P and S-waveswill occur. Bad data is normally caused by bad

backfilling in an otherwise properly conducted sur -vey. Do not backfill the hole with pure concrete or cement. The preferred recipe is 10 gallons of water

per bag of cement, diluted with 5 to 10% bentonite by volume.

Shear Waves Page 9

Figure 9. Typical

mechanical wall-lock

geophone

3 When the plank is an appreciable distance from the borehole, at shallow geophone depths, the shear waves will

travel nearly horizontally toward the geophone. That means that the waves will arrive at the borehole at virtually the

same time for the first few locations (the reader should sketch the geometry to see the problem). Since the interval

times will be close to zero, it will not be possible to calculate velocities. Even worse, it is likely that waves will

refract down, across, and back up to the geophone location. The interval velocities will be negative. Even at

appreciably deeper locations, the waves will tend to refract rather than taking a direct path, distorting the velocities.


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If pre paring a new borehole, put a cap on the bot -tom of the pipe to keep mud and debris out and toallow pumping the pipe dry if necessary. Avoidconnecting pipe segments with anything that pro-

jects into the hole (like pop rivets) to interfere withmovement of the tool. Have a piece of foam rub ber to stick in the top of the hole to keep the sound of the hammer from going down the pipe.

Field procedures

Cross-hole surveys don’t require much discussion beyond that already mentioned. The only common problem encountered is not with the shear waves, but with the P-waves. In a good system, the energythat radiates normal to the down-hole hammer is

practically free of com pressive waves. The com- pressive waves radiate vertically from the source(straight up and down). So, even though your hori-zontal geo phones will point directly at the source,

the signal may be very small or even undetectable.One solution is to use a source rich in com pressivewaves, such as a blasting cap. Some users suggestchanging the depth of either the source or receiver so that some fraction of the vertically radiatingcom pressive waves propagates between them. Afield procedure could be developed collecting data

between sequential repositioning of the tools.

As discussed earlier, data should be collected fromimpacts in both directions (down-hole hammer or ends of the plank) to confirm that you are looking atshear waves and not P-waves, or some odd reflec -tion, or tube waves, or some other problem.

For down-hole surveys, start with the geo phonenear the top of the hole and take three separaterecords—one on each end of the plank and one ver-tical to get the P-wave information (or two for cross-hole). Look at the records and see if theymeet the test of reasonableness: shear waves thatreverse and a P wave that arrives in about half thetime with higher frequencies. Next, position thegeo phone at the next lower depth and repeat the

process. Continue until you have records availablefrom a few depths, then tape the records onto theside of your truck, stacked vertically, to see howyour section looks. Plot up a section on some graph

paper. If the data looks reasonable, continue thesurvey until you reach the required depth.

Now, while still in the field, plot your com pletesection, P-waves and S-waves, and look at it again.Does it look reasonable? Are the P-wave velocitiesreasonable for the types of materials? Are the shear wave velocities lower than the P-wave velocities?Do the plots follow a reasonable progression downthe hole? Are any of the interval velocities

unusually fast or slow or even negative? Some vari-ation is normal because of the short time intervals,

but if arrivals come in sooner as you go down thehole, that’s a sign of a serious problem. Be ready torepeat some of the points or the whole survey whileyou are at the site. It’s expensive to go back again(or to never get another job from that client). Whileit’s tempting to just save the data on some digitalmedia and go back to the office to work up thereport, it’s crucial to do enough data analysis in thefield to know that the data is good, especially in thelearning period.

After you gain some skill, experience, and confi-dence (prefera bly in that sequence), some modifica-tions to the field procedure can be made to speedthings up and sim plify the process. If your system isdesigned so that you can select which channel onthe seismograph is connected to which geo phone,then the stacked section can be acquired directly inthe instrument. Selecting the seismograph channelcan be done with rotary switches in a separate box,or by using standard geo phone clip connectors anda conventional spread cable with multi ple takeoutsgathered up on the surface. Just connect the geo-

phone to the takeout which matches the desiredchannel. After you acquire 12 or 24 channels of data, you can save the digital data and plot out the

paper copy. This procedure was used for the composite records shown earlier in Figures 7 and 8.

If you start at the bottom of the hole and drag thegeo phone up (without unclamping), it will notrotate significantly—another key to an attractivestacked section. The reverse impacts don’t have to

be plotted (other than a representative sam ple),since com paring the data on sequential levels can

provide quality control. Of course conducting thesurvey with this procedure cuts the field timerequired substantially. The P-wave section can beacquired as a separate traverse or at the same timeas desired. When starting at the bottom, the recordswon’t be as clean and easily picked. For that rea -son, take a few test shots on the way down the holeto confirm that things are working properly andyour data is pickable.

Down hole surveys can be conducted to consider-

able depths. Shear waves are body waves whichtravel a relatively direct path between source andreceiver. Thus, they don’t suffer the attenuationcharacteristic of waves that must refract down,across and back up as in the refraction method. Youmay see some attenuation when traveling through ahigh-impedance contrast interface, such as glacialtill over limestone. Shear waves are generally fairlylow in frequency, and suffer less from the high-cutfiltering that the earth applies to seismic waves.Excellent data can be acquired as deep as 600 feet

Shear Waves Page 10


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(200 meters) or more by stacking just a few ham-mer blows.

Shear-wave stacking, can theoretically clean upyour records. The procedure is to connect a polar -ity-reversal switch to the geo phone, and then, usinga stacking seismograph, apply blows to oppositeends of the plank while reversing the geo phone

polarity. The shear waves will stack, while wavesthat don’t reverse polarity will tend to cancel out.Despite the theoretical benefits of this procedure,shear wave stacking is not widely practiced becausethere are no obvious benefits and because the addedcom plexity leads to confusion and errors. Somehave been known to hit one end of the plank,reverse the polarity of the geo phone, and hit thesame end of the plank under the mistaken idea thatthis constitutes a shear wave reversal.

Remem ber to put a little slack in the cable after clamping the geo phone to prevent the waves run-ning down the cable from shaking the geo phone.

Measurement Intervals

The depth intervals at which the geo phone is posi-tioned in the borehole is a matter of judgement andof site conditions. Closer intervals will result in

better resolution of the subsurface layers and moreaccurate velocities, but will require more time toconduct the survey. When in doubt as to the mostappro priate measurement interval, it is better to usesmaller intervals always. As an exam ple, if a surveyis required to a depth of 220 ft (70 m), a reasonable

and prudent sequence would be 3 ft (1 m) intervalsfrom the surface to a depth of 100 ft (30 m) and 5 ft(2 m) intervals below that depth.

Picking arrival times

Arrival times (or transit times) are plotted againsttheir respective depths. The data points will gener-ally plot as a sequence of nearly straight-line seg-ments representing the various subsurface layers,and the velocities are the slopes of the least-squareor visual straight-line fits to these segments.

The first breaks from down-hole shear waves are

very gradual, particularly as the geo phone getsdeeper, and it is difficult to precisely pick the “firstarrivals”. To get more precise time interval mea -surements, it is common to pick the first large peak in the wavelet.

At the shallower depths the times must be correctedfor the horizontal offset of the energy source(plank) from the collar of the borehole. The signaltravels along a slant path from the source to the

receiver, and the travel time must be multi plied bythe cosine of the angle between vertical (i.e. the

borehole) and the slant path before plotting itagainst its corresponding depth. This is an approxi-mate way of converting the time spent travelingalong the slant path to the time the signal wouldhave taken if it had traveled a vertical path down tothe receiver. At some depth (typically about 10times the offset) the cosine correction becomesinsignificant and the raw times can be used.

If the onset of the signal is used (first break), thenthe cosine correction can be performed as justdescribed. However, if a later portion of the signalis used, such as the first peak (and correspondingtrough from the reverse blow on the plank), then thecosine correction must be performed as follows.

Tvert = to cos 2

Tvert = (t p - )t) cos 2

Where )t is determined from the near-surfacerecords where the frist breaks are easily picked. As

previously mentioned, the onset of the shear wave,to, becomes increasingly obscure with increasingdepth, but the first peak can be followed down withreasonable accuracy. Whichever portion of the

record is picked, it must be used from top to bot-tom, just as the same filters must be used from topto bottom, to maintain accurate time relationships,and the time difference between it and the first

break used in the above equation. If the cosine cor -rection is applied without taking account of )t, thenthe near surface velocities will be too low.

When picking later arrivals, we are only interestedin the time difference between levels, and the equa -tion for the correct time becomes:

Tcorrect = [ (t p - )t) cos 2] + )t

The time interval and velocity from the ground sur -face to the first depth point are ignored.

Some users like to put multi ple geo phones in thehole. By having, say two geo phones separated by a10 ft (3 m) interval, the pair of arrivals can be more

precisely measured. Since they record the vibra -tions from a single hammer blow, any timing errorscan be eliminated and the close similarity betweenthe two records allows more precise time

Shear Waves Page 11


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com parisons. The geo phones can be connected witha flexi ble non-rotating mechanism such as a motor -cycle chain. The near-surface interval times muststill be corrected for the offset as discussed earlier.

Most seismographs are equipped with filters, ana-log or digital or both. Filters are generally used toremove geo physical noise from the data that mightobscure the signals of interest. Exam ples arelow-frequency vibrations from wind blowing ontrees or the sound of the hammer hitting the plank.All analog filters (and many digital filters) intro-duce some phase shift into the signal. So, if you usefilters, use the same filter for the whole survey.

Orientation

There are two horizontal sensors in a down-holegeo phone, which will be oriented at random if leftto their own devices. The maximum signal is the

vector sum of the out put from both geo phones.Since most seismographs have digital recording, itis not too difficult to make this calculation later andit is often done in an academic environment. Your data will be just as good if you take the signal fromthe geo phone most closely aligned to the plank, andadjust the trace size on the dis play to normalize theexcursions.

Better surveys and data do result if one of the geophones is continuously aligned with the plank. Theother horizontal geo phone can be ignored, the sur -vey goes much faster, the stacked section is easy to

plot and inter pret, and problems with anisotropy are

eliminated. One way to accom plish this is to casethe hole with grooved (slope indicator) pipe andmodify the geo phone to track in the groove. It isalso possi ble to purchase commercial geo phoneswith automatic orientation systems that will alignthe geo phone in the azimuth of your choice.

The issue of signal polarity is one that is seldomaddressed in discussions of shear wave surveys, andyet it can be critically important in identification of the shear wave. The ability to maintain a consistentsignal polarity, i.e. To know in which direction theseismograph trace will deflect for a given directionof hammer blow, has proven quite helpful in deep

surveys. This issue has not been considered impor -tant because it was a moot point when using a ran -domly oriented downhole sensor. Aligning thesensor with the plank can be critically important inthe presence of noise, interference, multi ple signal

paths or weak signals.

Tube waves

A tube wave is a pressure pulse that propagatesnearly unattenuated down (and up) the fluid col-umn. The velocity of a tube wave is a function of the bulk Modulus of the fluid, the elastic Modulus

of the casing, and the shear modulus of the sur-rounding material. They can resem ble shear wavesand can even reverse polarity as seen in Figure10A. Experienced skillful geo physicists have beenknown to conduct an entire shear wave survey pick-ing only the tube wave arrivals. As a general rule,they are less of a problem for shallow surveys(


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casing. Yet another reason to plot com pos-ite sections while stillin the field.

Data Processing

Since modern seismo-graphs have digitaldata storage, com put-ers can sim plify the

processing and dis playof data. At this time,no general pur poseshear wave processingsoftware is commer-cially available

because of the limitednum ber of potentialcustomers. Programs

designed to dis playseismic reflection datacan plot shear wavedata using their rudi-mentary editing rou-tines—a shear waverecord is similar to acommon offset gather.Some users have writ -ten cross-correlationroutines to get a more

precise measurementof delta-time and thusinterval velocity for

sequential depths. F-Kfilters could be used toremove tube waves.

Anisotropy

The shear wave veloc-ities are seldom thesame in the vertical(Sv) and in the hori-zontal (Sh) plane. Shvelocities may alsovary in different azi-muths, a situation

known as horizontalanisotropy, usually asa result of regionalstresses, local massmovement or specialdepositional situa-tions. In this case, theground spectralresponse will be dif -ferent, depending on

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Figure 11, Composite downhole P wave record showing tube waves.


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which way the plank and geo phones are oriented.Anisotrophy in alluvial-type deposits can be as highas 1.4X (e.g. a N-S polarized shear wave may havea velocity 40% greater than an E-W polarized shearwave), although this would be exceptional. Moretypically 5 to 15% horizontal anisotrophy will beobserved.

The devil is in the details

From the preceding, it would appear that measuringshear wave velocities is a trivial exercise. To thecontrary, like many skills, it is difficult for theuninitiated. In fact, the author suggests that the

beginner try this alone before imposing his or her learning experience on a paying client. If you feelthe desire or opportunity to conduct your own shearwave survey, it might be wise to hire the services ofsomeone actually experienced in the practice, thengo along and swing the hammer for them. Look at

real records Learn the tricks. Especially learn torecognize what good data looks like.

Acknowledgment

All the data and most of the knowledge in this paper are the work of Bruce Redpath, who proba blydoes more shear wave surveys than any practicinggeo physicist and who provided insightful critiqueof the draft. Any one else who feels motivated tocorrect errors or bring further insight is encouragedto e-mail [email protected]

1 Imai, Tsuneo, Hideo Fumoto, and Koichiro

Yokota, P- and S-Wave Velocities in SubsurfaceLayers of Ground in Japan, 1976 (available fromOYO Cor poration, http://www.oyo.co.jp/eng-lish/index.html)

2 The only known commercial source for down hole

hammers is from Bob Ballard (check with the SEGmem bership directory, http://www.seg.org)

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Paper Borehole Shear-Wave Surveys

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