T.426

GEOLOGICAL SURVEY CIRCULAR 426

THE SEISMIC METHOD IN SUBSURFACE 

EXPLORATION OF HIGHWAY AND 

FOUNDATION SITES IN 

MASSACHUSETTS

Cooperative Geologic Program

of 

U. S. Department of the Interior, Geological Survey

and 

Commonwealth of Massachusetts, Department of Public Works
U|TCTED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

GEOLOGICAL SURVEY CIRCULAR 426

THE SEISMIC METHOD IN SUBSURFACE EXPLORATION

OF HIGHWAY AND FOUNDATION SITES IN

MASSACHUSETTS

By L. W. Currier

Cooperative Geologic Program

of 

U. S. Department of the Interior, Geological Survey

and 

Commonwealth of Massachusetts, Department of Public Works

Washington, D. C. 1960

Free on application to the Geological Survey, Washington 25, D. C.
THE SEISMIC METHOD IN SUBSURFACE EXPLORATION

OF HIGHWAY AND FOUNDATION SITES IN

MASSACHUSETTS

By L. W. Currier

In 1938 the Massachusetts Department of 

Public Works entered into a cooperative 

agreement with the U.S. Geological Survey to 

prepare an up-to-date geologic atlas of the 

Commonwealth by quadrangles on a scale of 

2 inches equal 1 mile. The maps were designed to show not only the distribution and 

nature of the bedrock formations but a set 

was also to be prepared to show the distribution, materials, and forms of the loose surficial, the engineering soil, materials and the 

exposures of bedrock ledges. In part of this 

program geologic information was provided 

that related to highway and other engineering 

projects.

First, let us understand what geology is 

and then what it can furnish in the way of 

data related to engineering projects. Geology 

is the science of the earth its materials, its 

structure, and its history. It embraces a 

study of the natural materials that make up 

the terrain, both at and below the surface, and 

the forms and structures of these materials. 

Inasmuch as engineers and contractors work 

with geologic materials, it follows that a 

knowledge of their nature, behavior and structure is of practical value. This knowledge can 

be obtained through geologic field techniques.

Obviously only those mate rials that are exposed to view may be studied by direct observation. Those that are covered may be 

interpreted only from data obtained by indirect methods, that is, by inferences based 

on general knowledge of the geology of the 

region, or by some means of subsurface exploration. Let it be understood that a geologist does not have X-ray eyes, a crystal 

ball, or extra-sensory perception. He draws 

upon his basic geologic knowledge and his

Presented before a meeting of the New England Road Builders 

Association at the Statler-Hilton Hotel, Boston, Mass., Nov. 18, 

1959.

knowledge of the local and regional geology 

to interpret the probable or possible subsurface conditions at a given site. That, of 

course, lies within- his particular field of 

scientific competence. The degree of his 

success depends upon his background of 

training and experience. But it is truly and 

strictly indirect and interpretative. In many 

instances he can give a shrewd guess what 

we might call an educated guess as to what 

lies below the surface of the ground. These 

interpretations, or trained guesses, are, of 

course, in no sense quantitative, though a 

geologist may often reasonably suggest that 

a bedrock ledge is either close to the surface 

or relatively deep.

For engineering purposes there is needed 

some way of measuring with a reasonable 

and practical degree of accuracy, such things 

as the thicknesses of various soil materials, 

the depths to bedrock, and the variations in 

depth of the bedrock surface along the line 

of excavation. How can this accuracy be 

attained?

There are three principal methods of approach to quantitative depth data, no one of 

which is infallible. These are: (1) wash 

borings, (2) core drillings, and (3) physical 

tests or what are called geophysical methods. Borings and core drillings give positive 

results up to a certain point. They are direct, but they are very limited, as you well 

know. Geophysical methods are indirect 

they do not directly identify materials. Rather 

they provide data based on certain physical 

properties of the subsurface materials, and 

these data must be interpreted in terms of 

materials known to exist in the region.

Now let us consider the advantages and the 

limitations of these three ways of solving the
problem of what lies below the surface, at 

what depths, and in what forms.

First, let us consider wash borings. These 

are very helpful in many places. They are 

also very much limited as to what they indicate, particularly with respect to depths to 

bedrock in a glaciated region like New England 

where most of the soil materials are not derived in place from the underlying bedrock 

and the thickness of the soil cover may vary 

considerably within a few feet. Wash borings 

give information only at a single point. With 

them, when you reach refusal, you do not know 

whether or not it is due to ledge, to a boulder, 

or to a detached block of bedrock. You may 

make several wash borings within a few feet 

and interpret the refusal as a boulder, but it 

may be a pinnacle or steep-sided knob of 

bedrock, such as is indicated in figure 1.

The sludges from wash borings, if properly 

taken and studied, will, of course, commonly 

disclose the general nature of the soil material, but the structure and grain relationships 

are totally destroyed; moreover, the sludges 

are mixed, so that to identify different soil 

layers accurately requires additional and 

most careful sampling. Drive sample borings 

will, of course, provide relatively undisturbed 

soil samples and permit a more accurate 

identification of materials, textures, and 

structures; these, too, increase costs and 

time. Such soil studies, too, are significant 

to all of us only if we use a common classification and terminology. A geologist might 

identify material as glacial till or "hardpan," 

which commonly contains boulders, and so 

give a shrewd guess as to whether or not the 

refusal is a boulder. By the same token, to 

make several borings in order to identify a 

refusal increases costs and time. It may be 

argued that a single core drilling will serve 

to identify a refusal in this case. This is true 

if the drill hole goes deeply enough into the

_____ __ Surface

Boulder

Pinnacle oulder

Figure 1. Diagrammatic section to show a common relationship 

between soil cover, bedrock pinnacle, and boulders in Massachusetts.

rock, but even then it does not measure the 

size of the boulder it gives but one dimension. In New England we have boulders and 

blocks of rock in excess of 25 feet through; 

certainly a conventionally limited penetration 

of 5 or even 10 feet is not sufficient to prove 

"ledge" rock. Some surface boulders are so 

large that they have been given contours on 

the topographic maps'.

Core drillings are more satisfactory than 

wash borings for identifying rock materials 

but are much more expensive and time consuming. To be adequate in the majority of 

cases they must penetrate in excess of 10 

feet, and as I have indicated, require much 

deeper penetration for absolute identification. 

Like wash borings they give information at a 

single point only. To truly prove up aterrain 

by enough closely spaced core drillings is 

prohibitive in time and money, except in some 

instances, such as sites for heavy bridge 

piers, where very accurate subsurface data 

over a small area are needed almost regardless of cost. However, core drillings should 

be used more than they are in conjunction 

with other methods of subsurface exploration.

We come now to a discussion of geophysical 

methods. Two methods are used at engineering sites; these are refraction seismic testing 

and electrical resistivity. We have used both 

in highway exploration work in Massachusetts 

and have found the seismic method to be, by 

far, the most satisfactory. The seismic 

method, like other geophysical methods, is 

indirect insofar as identifying materials is 

concerned. The results are, therefore, interpreted from local and regional geologic data 

into materials known to exist in the specific 

area. The results are quantitative to some 

degree, but by no means do they approach a 

slide rule or 'engineering accuracy. Sometimes they depart very widely from desired 

or practical accuracy. The degree of accuracy depends upon such things as homogeneity 

and uniformity of the materials, and the simplicity of their structures, surfaces, and interfaces. Nowhere do these ideal conditions 

exist, so that interpretation of seismic results depends all the more on the geologist's 

terrain knowledge and experience.

Let us now examine the refraction seismic 

test procedure. The test measures the speed 

of certain elastic waves or impulses sent 

through the materials. The speed (sometimes
incorrectly called the "velocity of the material") at which the impulse, or wave,.travels 

through a given material depends upon its inherent elastic properties and its bulk density, 

or compactness. Of most rocks involved in 

highway construction problems, it may be 

said that fhe more compact or denser the 

material, the faster the wave travels through 

it. By sending an impulse through the ground 

and re cor ding it for definite successive points 

on the ground surface, we can measure the 

time of travel through the material, providing 

we also have an adequate time-recording instrument, or seismograph. Thus, the seismic 

instrument consists of a time-recording device and a number of seismometers also 

called pickups or "jugs." The pickups are 

placed at measured intervals along the 

ground, and each is connected electrically to 

the recording instrument. The impulse is 

provided by exploding dynamite in the ground 

at one end of the line. As the impulse reaches 

each pickup it is recorded separately on a

moving photographic ribbon in the recording 

instrument. The ribbon and timing mechanism are, of course, properly synchronized 

and calibrated. We can then plot the distances 

and arrival times for each pickup, and so 

determine the speed index for the material. 

The plot looks like the solid line in figure 2. 

Obviously, the slower the impulse travels, 

the greater will be the times plotted on the 

graph while the horizontal distances will remain the same. This will give a steeper 

slope to the speed (so-called "velocity") line 

on the graph, as illustrated by the dotted line 

of figure 2.

Suppose we have two materials, one on top 

of the other, and that seismic waves travel 

faster in the lower one, as in figure 3. Pickups numbered 1 to 5 receive the waves from 

the upper material, the "hardpan," of lowwave speed. Thereafter pickups 6 to 12, the 

first waves received, are those refracted 

from the lower, denser, higher-wave-speed

Figure 2. Time-distance graphs (seismic speed graphs) for single, uniform, subsurface material. Speed of wave is indicated directly by 

the slope of the speed line, which is measured by X. Wave paths shown for first four'detectors. Dashed line represents speed line for 

medium of lower wave speed.
bedrock. So we get a speed line for each of 

these two materials. They intersect at point 

d and the length of Ad'ls a mathematical quantity with which the depth to the bedrock surface at the shot point A can be calculated according to an established formula. This is 

the theoretical situation for the simple case 

of two uniform, homogeneous materials whose

surface of contact is parallel with the surface 

of the ground.

Now, the seismic record may be read directly to the one-hundredth of a second, and 

interpolated to the one-thousandth of a second. Accordingly, if the wave speed in the 

upper material in figure 3 is, for example,

LJ

2

r456 

DISTANCE

B

Ground surface 

+-B

Soil 

3000 ft per sec

* \

\

\

\ 

V

/ / /

/ I t

1 1 1

1

.'

1

Bedrock //,\5,000 ft Per

Figure 3. Diagrammatic seismic speed curves for soil cover and bedrock where the bedrock surface is parallel with the ground surface. 

Soil and bedrock speeds assumed. Slopes of speed graphs are proportional to assumed speeds and are drawn to scale. Abscissa Ad' is a. 

mathematical function of the depth to bedrock at point A, according to an established formula. Wave paths for first seven detectors 

("pickups") indicated by dashed lines. Note that wave paths entirely in soil for five and beyond would not register owing to earlier arrival of higher speed waves from the bedrock.
3,000 feet per second which is a common 

value for some engineering soils such as dry, 

bouldery sand and gravel, it would appear 

that the theoretical mathematical accuracy 

of the depth calculation is approximately 3 

feet. However, since the length of the critical 

wave path includes return to the surface, or 

approximately twice the depth to the lower, 

denser material, the maximum theoretical 

accuracy of the depth determination would be 

approximately li feet. With surface materials 

of greaterwave speeds the degree of accuracy 

is correspondingly less; for example, for a 

6000 feet per second speed, the theoretical

accuracy could not be better than 3 feet. In 

no case would the theoretical accuracy of a 

single seismic test be better than 1/2000 of 

the wave speed of the upper material. The 

theoretical accuracy is, indeed, rarely attained, because of material variations in the 

soil layer, of irregularities in the ground 

surface and the interface, of difficulties in 

reading and interpolating the record, and of 

other operational factors.

Let us assume now that the bedrock surface rises from point A toward point B, as in 

figure 4. The impulses that pass along the

UJ 

2E

H

DISTANCE

Shot point 

XI 

Figure 4. Seismic graph (speed curves) for soil and bedrock where bedrock surface rises relative to topographic surface, from shot point. 

Wave paths for first seven detectors indicated by dashed lines. Note flatness of bedrock speed line as compared with figure 3, indicating 

abnormally high wave speed. See also explanation of figure 3.
bedrock and are refracted from it, are hastened because they pass through less and less 

low-speed soil on their return to the pickups. 

The plotted speed line for the bedrock then 

has a flatter slope in the graph. This abnormally high speed for the bedrock tells us that 

from point A toward point B, the rock surf ace 

is rising, but it does not tell us by how much 

it rises. By "shooting" from point B back 

toward point A, however, we can obtain abedrock depth for point B as we did for A and 

so tie down the bedrock line at both ends of 

the profile, if the bedrock surface is not irregular. Incidentally we always "shoot" the 

reverse profile as a check of the interpretation of bedrock slopes. Moreover, the total 

elapsed time for both profiles should theoretically be the same, or rather, within one onethousandth of a second.

All this works out very nicely for the 

ideal case we have chosen, namely two uniform and homogeneous materials, a uniform 

ground surface, and a uniform surface of 

bedrock. But these conditions are found 

rarely. The ground surface is not always, 

uniform and flat, the materials are never 

uniform and homogeneous, and the bedrock 

surface is generally irregular with knobs, 

depressions, pinnacles, etc.; moreover, even 

the bedrock material may not be homogeneous. The result is that the points plotted 

on the time-distance graph do not fall exactly on a single straight line. Moreover a 

depression in bedrock will displace a segment of the velocity line above the rest of 

the line, and a knob or pinnacle will throw a 

segment of the line below, as shown in figure 

5. We must judge whether or not a displaced

LU -KDISTANCE

Ground 

surface 

'____L

Figure 5.- Magrammatic seismic graph to illustrate effect of changes in slope of bedrodk surface on continuity of bedrock speed graph, d'

has same significance as in figure 3.
point on the graph is anomalous or signifiT 

cant.

Now you can see how complications arise 

in the interpretation of seismic data and in 

the determination of depths to bedrock. But 

this complication of irregular bedrock surfaces is only c^ne of several. Others include: 

variations in the internal structure of the 

materials, which, let me repeat, are rarely 

uniform; relatively low speed values of some 

kinds of rock, such as shale and some mica 

schists, that may not be appreciably different 

from the true speed values of some very compact soil material;^ and partly disintegrated 

upper parts of bedrock, causing a lower speed 

value than is normal for the fresh, solid bedrock. Also, the speed value across the layers 

of schist-rock may be much lower than that 

along the layers. Finally, large blocks of 

rock or nests of boulders in the soil material 

are not certainly detectable, although they 

sometimes affect the apparent speed value of 

the soil to a minor degree.

The effective depth of penetration of the 

impulse and hence the depth from which a 

bedrock surface may be detected, is afunction 

of the length of the seismic traverse line. 

To record depths of 100 feet will ordinarily 

require a traverse line about 330 feet long. 

This permits a 30-foot spacing for the 12 

pickups. A knob or depression of appreciable 

size would not be detected by such spacing 

unless one or more pickups happened to rest 

directly above it. These features can be found 

by placing the geophones closer together and 

shooting segments of a traverse. This, of 

course, increases the time and expense involved in making a single traverse. It is 

clear, therefore, that the length of the seismic traverse should be chosen according tp 

the approximate depth to bedrock, for best 

results. Inasmuch as the depth is what we 

are trying to determine, it is sometimes necessary to repeat a traverse at a site, using 

different spacings, and perhaps a different 

orientation.

The contractor has to consider large or 

nested accumulations of boulders as bedrock, 

or to use the engineering term, "ledge," in 

estimating excavation costs. The geologist 

cannot yet identify hidden large boulders or 

boulder concentrations, though in some instances he may suspect nests of boulders 

from anomalies in the seismic record. We

are working on this problem. So there is a 

difference in definition between the geologist's 

"bedrock" and the contractor's "ledge." In 

subsequent comparisons between seismic 

predictions and actual excavations, this difference in definition must be recognized. It 

probably represents the area of greatest and 

most common discrepancy between subsurface seismic profiles and what is actually 

found as "contractor's ledge" when the excavation is made.

Obviously, also, when you try to draw a 

subsurface profile for a section line that is 

situated away from a seismic traverse, or 

that lies between two seismic traverses, you 

introduce another possible error, for the 

seismic profile indicates the probable conditions only along the seismic traverse line. 

Unless you know that the bedrock surface is 

regular, you cannot assume that the nearest 

seismic profile or profiles can be safely interpolated to the offset or intermediate line. 

A depression in the bedrock surface, or a 

knob may intervene. By the same token a remote wash boringmay reach an unrelated refusal or reflect only a strictly local variation 

in soil materials.

So far I have considered only two-layer 

problems. When there are three layers of 

wave speeds increasing with depth such as 

dry sand, "hardpan," and solid rock there 

are three speed lines on the graph to be interpreted. A corresponding complication is 

introduced into the interpretation. The principles involved are, however, similar.

You may ask, then, and quite properly, if it 

is worth while to make seismic tests, when 

such inaccuracies and difficulties are inherent in the method. We would reply that seismic tests provide the best available practical 

guide considering the time and expense involved. We have made many comparisons 

between seismic profiles and the actual conditions disclosed by excavations. We have 

found that approximately 85 percent of the 

profiles have been satisfactory. This includes 

the places where the bedrock surface clearly 

lies below the proposed depth of the excavation. We have found also that most of the 

serious errors result where the bedrock surface lies at a very shallow depth. As a result 

of our experience we do not recommend the 

seismic method for depths under, say, 10 to 

15 feet, despite the fact that we have had some
very good results with such shallow depths. 

That the refraction seismic method is of 

practical value in subsurface exploration of 

highway and bridge sites is attested by the 

high incidence of satisfactory results as compared with wash borings and site "guess 

works." It is also attested by the increasing 

use of the method elsewhere. The seismic 

method is not new; the reflection method has 

long been used in the petroleum industry for 

determining deep structures, and both reflection and refraction methods have been 

used in studies of the earth's crust. Use of 

the refraction method for shallow depths and 

short traverses in highway engineering projects and for foundation sites is a comparatively recent development. In Massachusetts 

we started to use it on highway sites in an 

experimental way in 1945. So far as I know 

this was essentially a pioneer effort. Several 

states now use it extensively.

A seismic profile permits an interpretation 

of the bedrock surface along a line between 

shot points, whereas a wash boring or a core 

drilling provides no data beyond the point at 

which it is made. It would take many drillings, 

at much greater expense, to give the same 

amount of information as a single seismic 

traverse. Though appreciably less accurate,

the seismic traverse generally gives practical results within a reasonable range of accuracy. But the method must be used with a 

complete understanding of its limitations as 

well as of its advantages. A line that is designated as a seismic bedrock profile should 

be considered only as a guide. In many places 

it needs support by surf ace geologic surveys, 

wash borings, core drillings. The data must 

be interpreted by a geologist who is familiar 

with the regional as well as the local geology. 

Everything considered, the method is the 

most satisfactory procedure for preliminary 

subsurface exploration of highway sites. As 

you have seen, it is not infallible but it is 

highly practical considering the time and 

money involved, especially when supported 

by collateral data.

Improvement is clearly needed in seismic 

technique and interpretation procedures used 

in solving shallow exploration problems. That 

fact does not vitiate its present usefulness, 

but it does point up a field of needed research 

directed to greater overall accuracy of interpretation and correlation of abstract seismic data with specific geologic materials and 

structures. Our long-range plans include 

such research.

INT.DUP..D.C.60-