A TEST OF A MECHANICAL MULTI-IMPACT 

SHEAR-WAVE SEISMIC SOURCE 

by David M. Worley, Jack K. Odum, Robert A. Williams, and William J. Stephenson 

Open-File Report 01-0440 

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and 

stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the 

USGS. 

U.S. DEPARTMENT OF THE INTERIOR 

U.S. Geological Survey 

Golden, Colorado 
1/4/02, 3:29 PM 

A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SEISMIC SOURCE 

By David M. Worley, Jack K. Odum, Robert A. Williams, William J. Stephenson 

ABSTRACT 

We modified two gasoline-engine-powered earth tampers, commonly used as compressional-(P) 

wave seismic energy sources for shallow reflection studies, for use as shear(S)-wave energy 

sources. This new configuration, termed “Hacker” (horizontal Wacker®), is evaluated as an 

alternative to the manual sledgehammer typically used in conjunction with a large timber held down 

by the front wheels of a vehicle. The Hacker maximizes the use of existing equipment by a quick 

changeover of bolt-on accessories as opposed to the handling of a separate source, and is intended 

to improve the depth of penetration of S-wave data by stacking hundreds of impacts over a two to 

three minute period. 

Records were made with a variety of configurations involving up to two Hackers simultaneously 

then compared to a reference record made with a sledgehammer. Preliminary results indicate 

moderate success by the higher amplitude S-waves recorded with the Hacker as compared to the 

hammer method. False triggers generated by the backswing of the Hacker add unwanted noise and 

we are currently working to modify the device to eliminate this effect. Correlation noise caused by 

insufficient randomness of the Hacker impact sequence is also a significant noise problem that we 

hope to reduce by improving the coupling of the Hacker to the timber so that the operator has more 

control over the impact sequence. 

INTRODUCTION 

Accurate measurement of shallow S-wave velocity (Vs) in sedimentary basins is crucial for 

earthquake ground-motion models. However, collecting such data in an urban environment 

normally involves restrictions in operations imposed by the local governments, including but not 

limited to: (a) very restricted use of explosives, (b) no large impacts which might damage property, 

and (c) adherence to noise level ordinances. Urban data collection is further hampered by the 

ambient noise of it’s own environment, which requires the use of one or more noise cancellation 

techniques. In an effort to devise an inexpensive and rapid method of determining Vs in the upper 

200 m that complies with the imposed restrictions, improves the signal-to-noise ratio of S-wave data 

used for research and earthquake engineering, and increases depth of penetration, we constructed a 
A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

new non-invasive S-wave seismic source by modifying a pair of Wackers® for horizontal operation. 

Records were digitally recorded with a Geometrics StrataView engineering seismograph using a 

method with similarities to both vibroseis recording and the Mini-Sosie method (Barbier, 1983). 

A variety of non-explosive surface sources are currently used to generate SH waves. These 

include a horizontal timber or similar object, pressed against the ground surface and struck 

manually at the ends with a hammer. This method was introduced by Kobayashi (1959) and is 

uncomplicated, inexpensive, and reasonably reliable. It is limited in effectiveness by a lesser depth 

of penetration than other methods, and the considerable human physical effort required. In a similar 

method, the manually-operated hammer in the first method is replaced by a falling hammer hinged 

at an axis and striking the timber horizontally. The limitations of this alternative are noise 

restrictions, logistical constraints of size, and speed of operation. Typically, a physically larger 

source generates more energy that provides better imaging at depth, but larger sources may be 

restricted for the greater noise they produce or potential environmental damage, and are logistically 

more awkward to use due to their size and weight. Shear-wave vibrators using the vibroseis 

principle, in which a reaction mass is accelerated relative to a shaft fixed to a base plate by 

hydraulic forces are also popular. This method offers superior energy input and repeatability. The 

limitations to this method are that it is very expensive to purchase or lease and maintain, and it is 

often environmentally damaging. Other types of sources use a shuttle mass propelled by 

compressed air to impact an anvil fixed to the base plate (Meissner et al., 1985), or masses affixed 

to an air-actuated cylinder (Liu et al., 1988). The resulting traction exerted on the ground surface 

by the base plate generates the shear-waves. These devices provide excellent energy input and 

repeatability. Generally, a lifting device, such as a manual crane, must be used to move them. A 

large, gasoline-engine-powered air compressor is also required for operation. The limitations are 

the requirement for supporting equipment and difficulty in handling placement of the massive air 

hammer. 

We believe the Hacker has the potential to solve, or ease, many of the limitations described 

above because it is a commonly available device (in vertically-oriented form from construction 

rental agencies), easy to operate, easily handled in small areas, provides more energy than manual 

methods by stacking hundreds of impacts, not excessively noisy, and not damaging to the 

environment or infrastructure. The purpose of this initial report on the Hacker is to summarize: 1) 

the progress made in successfully converting a Wacker® to a Hacker, 2) the success in generating an 

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A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

S-wave seismic record that is comparable to a conventional sledgehammer/timber configuration, 3) 

the limitations and complications encountered in testing this new device that need to be resolved 

before the Hacker is fully proven as an improved S-wave source. 

Source Modifications: Creating a Hacker from a Wacker 

We modified two 68-Kg Wacker®‡ GVR-151Y gasoline-engine-driven mechanical tampers, 

each having a striking force of approximately 1360 Kg in the vertical configuration, for horizontal 

operation by fitting each with a small steel frame bolted to the body of the tamper and affixed with 

four balloon tires (Figure 1). The footplate was replaced with a circular, flat steel plate. For both 

vertical and horizontal configurations, the factory throttle control was replaced with a motorcycle 

handgrip/throttle control to facilitate the requirement of frequent engine speed variation to produce 

the pseudo-random impact series. The end of the 15.24 cm X 15.24 cm X 304.8 cm fir timber was 

fitted with a steel end plate of sufficient width to permit the operation of two tampers side-by-side 

and reinforced with gussets (Figure 1). An impact-sensing device was attached to the base of each 

tamper to provide a reference pulse to the seismograph pilot signal channel (Figure 1). The data 

were correlated to these pulses (Figure 2). Records were made in a variety of configurations and 

compared to reference records made with sledgehammer impacts against a timber or steel cleat 

(designed by Hasbrouck, W. P., personal communication) (Figures 3). The sources were operated 

in a pseudo-random fashion, as used in the Mini-Sosie method, to reduce random noise by varying 

the rate and frequency of the source impacts. For this study the number of impacts from a single 

Hacker during 60-s recording was about 175. This method has been used successfully by the U.S. 

Geological Survey to acquire P-wave reflection data in a vertical impact configuration (e.g.: 

Stephenson, et al., 1999). 

CONFIGURATION AND TESTS 

These tests were conducted on grass at the west end of the Ulysses Park softball field complex in 

Golden, Colorado. Scott (1972) indicates that the site lies on the Denver Formation, which can be 

up to 290-m thick, and consists of claystone, siltstone, friable sandstone, and conglomerate. To 

record the data we deployed a linear seismic receiver line of 60, 4.5 Hz horizontal geophones using 

a receiver interval of 1.5 m. The source timber was placed at the end of the receiver array and 

oriented perpendicular to the receiver line. The front of a pickup truck served as weight on the 

timber to improve seismic coupling to the ground and restrict movement of the timber (Fig. 1). 

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A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

Five-cm-long steel cleats on the bottom of the timber penetrated the sod and helped to minimize 

slippage of the timber. The steel S-wave cleat (Figure 3) was placed at the same location and 

weighted by one person. A 1 ms sample interval was selected. A 60 Hz notch filter was used due 

to the proximity of a power distribution line. A summary of the seismic source tests, corresponding 

to a specific record number, for the data described in this report is listed in Table 1. 

Table 1. Test file summary 

Record 

Number 

Source 

Configuration 

Recording 

Parameters 

1002 Sledgehammer with 

steel S-wave shoe 

1 second record. 

Stack of 4 hammer blows. 

1004 Sledgehammer with 

timber. 

1 second record. 

Stack of 4 hammer blows. 

1005 One Hacker 64 s record, uncorrelated 

1006 One Hacker 1 second record consisting of 

four 24 second records 

correlated, then stacked. 

1007 One Hacker 1 second record consisting of one 

64 second record correlated. 

1009 and 1010 Two Hackers 1 second record consisting of one 

64 second record correlated. 

1020 One Hacker 1 second record consisting of 

stack of 16 hits using a hammer 

trigger and recorded in stacking 

mode, emulating the hammer 

method. 

DISCUSSION AND CONCLUSIONS 

Amplitude and frequency analysis indicate that the timber is generally superior to the steel Swave cleat. Amplitudes of the first-arrival refraction phases at about 220 ms travel time for the 

sledgehammer-timber arrangement on the farthest offset traces are 3 to 6 times greater than for the 

steel cleat. This amplitude difference is seen when comparing seismic records from the two sources 

that are scaled to show their true relative amplitudes (Figure 4). More of the induced energy is 

transmitted the full distance of the receiver line when using the timber. This is an expected result as 

it is possible to strike the timber with greater force than is possible using the steel shoe. All seismic 

sources produced the expected clear first arrivals, followed by, predominantly, surface waves. The 

primary difference in these two configurations is simply a factor of power induced by striking the 

timber more forcefully than the shoe. The dominant frequency peak of the hammer-timber source 

at about 45 Hz is also about 10 Hz higher than the steel cleat (Figure 5a and 5b). For deposits with 

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A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

an S-wave velocity of 200 m/s, this higher frequency peak would allow about 0.4 m improvement in 

resolving a thin bed for the hammer-timber source. Despite the lower amplitude and dominant 

frequency of the steel cleat, we have found this source to be invaluable when space constraints 

preclude the use of a vehicle parked on the timber. For future comparisons against the Hacker 

source described below, the sledgehammer-timber record (file 1004) is considered as the control 

record. 

To isolate the differences between a single Hacker impact and four hammer impacts against a 

timber recorded at this site we compared individual sample amplitudes and whole-record frequency 

analysis. At the time of this writing, we do not have a single hammer impact record to compare 

against the single Hacker blow at this site. For the first-arrival phases, the hammer-timber record 

shows about 1.5 times higher amplitudes at the midpoint of the receiver spread, which suggests that 

an individual Hacker impact is roughly equivalent to or slightly more energetic than a single 

hammer impact (Figure 6). These two seismic sources also have a similar dominant frequency peak 

at about 45 Hz (Figure 5). 

Next, we compared the sledgehammer-timber record (1004) with two different correlated 

Hacker source input-time variations (records 1006 and 1007, Fig. 7; also see Table 1). We recorded 

these data using a single Hacker and found, as expected from our experience using tampers in a 

vertical mode, that a single tamper does not provide a sufficiently random series of impulses and 

results in the appearance of multiple first arrivals or correlation noise (Figure 7). Correlation noise 

appears as a “ghosting” arrival that occurs before time zero, and/or later in the record as repetitive 

first arrivals at regular intervals. In this test, however, a single Hacker also produced strong first 

arrivals that, although somewhat obscured by correlation noise, appear to be more continuous, of 

higher amplitude, and with less interference from ambient high-frequency wind noise than the 

hammer-timber source (Figure 7). Because the true amplitudes of the Hacker data are lost during 

the correlation process, amplitude comparisons between the hammer-timber and the Hacker are not 

possible. However, given the relatively equal amplitudes of individual Hacker impacts and the 

hammer-timber source as described above, signal-to-noise improvements for the 60-s Hacker record 

should be about a factor of 13 for 175 source stacks. It does not appear that 64-s single recording 

has any advantage over a stack of 4, 24-s recordings for a receiver array of this length at this site 

(Figure 7). Whether or not to record a single long record as opposed to a group of shorter records is 

5 
A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

usually site dependent. The ambient noise conditions at some sites may require the use of shorter 

records to avoid some of the noise. 

Record 1009 (Figure 8) was made using two horizontal tampers in an effort to reduce 

correlation noise and to improve the signal-to-noise ratio by increasing the number of source inputs 

without increasing total source input time. Comparing the two-Hacker record (1009) to the control 

record (1004), we find the first arrivals are strong and clear for the full distance of the receiver 

spread, indicating that it is feasible to increase the number of active channels or extend the distance 

of the existing channels. However, it does not appear that correlation noise is reduced by using two 

Hackers instead of one. 

Record 1020 was made using a single Hacker without the correlation process to emulate a 

sledgehammer record (Figure 9). The seismograph was configured for stacking individual blows 

rather than correlation of the Hacker impact time series with the data, and a commercial hammer 

switch was used as the impact sensing device. The tamper was run up to full operating speed, then 

rolled forward on the wheels to contact the timber for one hit, allowed to recoil backward, and 

repeated to accumulate a stack of sixteen hits. This record most closely replicates the 

sledgehammer record. 

If the correlation noise problem can be eliminated or highly reduced, then these results indicate 

that the Hacker is a practical alternative to the sledgehammer for the increased energy input, 

improvement of signal/noise ratio, speed of operation, portability, and reduction in human physical 

force requirements. It is recognized that additional testing at various sites is required for complete 

evaluation and that some adjustments in technique are required to provide the quality and integrity 

of data that has been achieved in P-wave data collection. 

The only observed mechanical limitation to this technique is the apparent tendency of the 

tampers to induce undesirable energy into the ground upon recoil of the tamping mechanism. The 

next phase of testing includes a restraint system to dampen the recoil and reduce this undesirable 

component of energy. 

ACKNOWLEDGMENTS 

This study was supported by funding from the National Earthquake Hazard Reduction 

Program (NEHRP). The authors thank the City of Golden, CO, Department of Parks and 

Recreation, for permission to use their land to conduct these tests. Use of commercial product 

names does not imply endorsement by the U.S. Geological Survey. 

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A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

REFERENCES 

Barbier, M. G., 1983, The Mini-Sosie method: International Human Resources Development Corp., Boston, 

Mass., 186 p. 

Kobayashi, N., 1959, A method of determining the underground structure by means of SH waves, Zisin, 

v.12, p. 12-24. 

Liu, H.P., Warrick, R.E., Westerlund, R.E., Fletcher, J.B., and Maxwell, G.L., 1988, An air-powered 

impulsive shear-wave source with repeatable signals, Seismological Society of America, v. 78, p. 355-

369. 

Meissner, R., Stümpel, H., and Theilen, F., 1985, Shear wave studies in shallow sediments (chap. 5), in 

Seismic Shear Waves, Part B: Applications, G. Dohr, Editor, Geophysical Press, London, England. 

Scott, G. R., 1972, Geologic map of the Morrison quadrangle, Jefferson County, Colorado: U.S. Geological 

Survey Map I-790-A, 1:24000. 

Stephenson, W.L., Odum, J.K., Williams, R.A., Pratt, T.L., Harrison, R.W., and D. Hoffman, 1999, 

Quaternary faulting in southeast Missouri across the Commerce geophysical lineament: Bulletin 

Seismological Society of America, v. 89, p. 140-155. 

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A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

Timber 

Hacker 

Hacker 

Source sensor 

Figure 1-Horizontally configured tampers (Hackers) at field test location. 

8
A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE Time (ms) 

Figure 2. Three pulses from a portion of a typical pulse sequence generated by the impact-sensing device 

(labeled “Source sensor” in Figure 1) attached to the base of the Hacker. These pulses are recorded on 

channel 1 and then used to correlate with the data on channels 2 through 60. 

36 cm 

Figure 3. Underside of Steel cleat showing triangular points for penetrating soil to increase signal coupling. 

9 
T

A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

ime (ms) 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

Hammer - Shoe Hammer - Timber 

File 1002 1 5 M File 1004 

Figure 4. Compares hamme r blows on stee l shoe and timbe r. 

File 1004, on the right, is the control record for 

comparisons in this paper. 

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A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

C h a n n e l 

H a c k e r 1 bl o w f r o m un c o rr e l a t e d d a ta P ercen t Power 

10 

Time (ms) 

1 0 3 0 5 0 

Fr equenc y (Hz ) 

50 100 150 

9 0 

50 

c) 

4 4 0 0 

4 6 0 0 

4 8 0 0 F i l e 1 00 5 

S - w a v e s h o e 1 ha m m e r b lo w Fre q uen c y (Hz ) 

5 0 100 15 0 

Percen t Power 9 0 

5 0 

1 0 

C h a n n e l 

10 30 50 

Time (ms) a) 

1 0 0 

5 0 0 

3 0 0 

F i l e 1 0 0 2 

T i m b e r 4 h a m m e r b l ow s Fre q uen c y (Hz ) 

5 0 100 15 0 

P ercen t Power 1 0 

5 0 

90 

C ha n n e l 

10 3 0 50 

Time ( ms) b ) 

5 0 0 

3 0 0 

1 0 0 

F i l e 1 0 04 

F ig u r e 5 . S p ec tr a l c o m p a ris o n s. E n er g y fr o m t h e tim b e r is n e a r e r 5 0 H z . E n e rg y

f r o m t h e s ho e is n ea r e r 2 5 H z . W it h t he H a c k e r, t h e re is gr e a te r

e n er g y i n t he 7 5 - 1 0 0 H z . r a n g e .

11
A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

Figure 6. 

and 4 stacks of the hammer-timber configuration (bottom). 

source. -stack hammer record probably because of the stacking. 

Frequency (Hz)

20 40 60 80 

300 

400 

500 

200 

100 

Time (ms) 

Frequency (Hz)

20 40 60 80 

300 

400 

500 

200 

100 

Time (ms) 

File 1005 

File 1004 

Relative amplitude and frequency comparison between a single, uncorrelated Hacker impact (top) 

Frequency peaks are about the same for each 

Amplitudes are slightly greater for the 4

12
T

A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

ime (ms)

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

Ha mmer - t imber 

1 ta mper - St a ck 4 

2 4 s ec ond r e cor d s 

1 tamper 

64 s econd r ecord 

fir st ar rival corr elat ion noise

3 0 m 

File 1004 File 100 6 File 1007 

Figure 7. Re cords 1004, 1006, 1007. Comparing hamme r to single Wacker

in two different time configurations. Some correla tion noise

introduced by insufficient randomness of single tampe r.

13
t

A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

ime 

m( s) 

Hamme r - timber 

Sin g le t a m p e r - 6 4 s e c . 

co r r e la t e d 

Two t a m p e r s - 6 4 s e c. 

co r re la t e d . 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

File 1004 File 1007 File 1009 

Figure 8. Files 1004, 1007, and 1009 . The addition of a second tamper 

shows a slight improveme nt in data quality, but retains the induced 

correla tion noise . 

14 
t

A TEST OF A MECHANICAL MULTI-IMPACT SHEAR-WAVE SOURCE 

ime (ms) 

Figur e 9. File s 1004 and 10 20 . Comparing ha mmer blows to stacking 

Hammer - timber 

One tamper - s tack 16 

Emula ting hammer 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

File 1004 File 1020 

tamper impacts. Improved S/N ratio of tampe r record improves 

appea rance of da ta with no introduction of correlation noise. 

15