Seismic Shear Wave Reflection 

Imaging at the Former Fort Ord, 

Monterey, California

Open-File Report 2007-1068

U.S. Department of the Interior

U.S. Geological Survey
In cooperation with the U.S. Army Corps of Engineers 

Seismic Shear Wave Reflection Imaging at the Former 

Fort Ord, Monterey, California 

By Seth S. Haines, Bethany L. Burton, and Lewis E. Hunter 

Open-File Report 2007–1068 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Suggested citation: 

Haines, S.S., Burton, B.L., and Hunter, L.E., 2007, Seismic shear wave reflection imaging at the former 

Fort Ord, Monterey, California: U.S. Geological Survey Open-File Report 2007-1068, 13 p.,

available only online at http://pubs.usgs.gov/of/2007/1068/. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 

ii
Contents 

Abstract................................................................................................................................................................................1

Introduction .........................................................................................................................................................................1

Field Site...............................................................................................................................................................................4

Shear Wave Surveys .........................................................................................................................................................4

Data Collection................................................................................................................................................................4

Data Processing..............................................................................................................................................................7

Shear Wave Images.......................................................................................................................................................7

P-Wave Surveys ...............................................................................................................................................................10

Discussion and Conclusions...........................................................................................................................................11

Recommendations for Future Work ..............................................................................................................................11

Acknowledgments............................................................................................................................................................12

References Cited ..............................................................................................................................................................13

Conversion Factors 

Multiply By To obtain 

foot (ft) 0.3048 meter (m) 

foot per second (ft/s) 0.3048 meter per second (m/s) 

meter (m) 3.281 foot (ft) 

meter per second (m/s) 3.281 foot per second (ft/s) 

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). 

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 

Altitude, as used in this report, refers to distance above the vertical datum. 

iii
Seismic Shear Wave Reflection Imaging at the Former 

Fort Ord, Monterey, California 

By Seth S. Haines, Bethany L. Burton, and Lewis E. Hunter 

Abstract 

At the former Fort Ord in Monterey County, California, contamination threatens an aquifer that 

provides drinking water for local communities. Assessment and remediation require accurate 

hydrological modeling, which in turn require a thorough understanding of aquifer stratigraphy. In order 

to help guide remediation efforts at the site, the U.S. Geological Survey, in cooperation with the U.S. 

Army Corps of Engineers, has undertaken seismic reflection surveys, testing compressional (P) and 

horizontally polarized shear (SH) waves. Sledgehammer-source SH data show reflections from 

interfaces up to approximately 60 m deep, which correspond with the major boundaries between 

aquifers and aquitards. In contrast, P-wave data show only the reflection from the water table at 

approximately 30 m depth. We collected SH data along two transects and processed these data to 

produce reflection images. The interpreted SH-wave images agree with available well information, 

constrain the geology for ground-water models, and provide guidance for future geophysical studies. 

These favorable results demonstrate the effectiveness of SH reflection methods for imaging 

unconsolidated aquifer layers at the former Fort Ord and at other sites with similar geologic conditions. 

Introduction 

The former Fort Ord (fig. 1) is located approximately 9 km north of Monterey, Calif., and has 

been listed on the National Priorities List of the Superfund program since 1990 

(http://www.epa.gov/superfund/sites/npl/ca.htm). Originally operating as a training and staging facility, 

the Fort opened in 1917 and served various purposes before its 1991 decommissioning. Like many such 

sites, surface disposal of cleaning solvents in the 1950's has led to subsurface contamination that 

threatens important ground water aquifers (MACTEC, 2005). 

1
Figure 1. Air photograph of the Fort Ord site. The Old County Road seismic transect is plotted in red, and 

the Landfill transect in blue. The dashed yellow circle shows the approximate boundaries of the 

suspected hole in the Intermediate 180-Foot Aquitard. 

Subsurface geology at the site is well known from borehole information (MACTEC, 2005) and 

is depicted in figure 2. The A aquifer is the uppermost layer and consists of approximately 35 m of 

clean dune sands. The underlying Fort Ord-Salinas Valley Aquitard (FO-SVA) consists of 

approximately 12 m of dense marine clay. The Upper 180-Foot Aquifer consists of approximately 15 m 

of mixed sand, gravel, and silt. The Intermediate 180-Foot Aquitard consists of approximately 10 m of 

silt and clay. The lowermost layer of interest in this study is the Lower 180-Foot Aquifer that consists 

of sand, gravel, and silt. The water table in the A Aquifer is at a depth of 25 to 35 m, depending on 

location. The Upper 180-Foot Aquifer is a confined aquifer in most places, but may have localized air 

pockets during dry periods. The Intermediate 180-Foot Aquitard is the layer of most interest in this 

investigation, because borehole logs and hydrological modeling suggest that the layer is discontinuous, 

and that it may contain a large hole. 

2
Figure 2. Schematic cross section at Fort Ord showing major aquifer units, the suspected hole in the 

Intermediate 180-Foot Aquitard, the A-Aquifer water table (black triangle), the Fort Ord-Salinas Valley 

Aquitard (FO-SVA), and a representation of the carbon tetrachloride (CT) plume that threatens the Lower 

180-Foot Aquitard. 

Contaminants (carbon tetrachloride (CT) and trichloroethene (TCE)) have migrated from the 

surface, through the A-Aquifer sands, through the FO-SVA via improperly sealed boreholes, and into 

the Upper 180-Foot Aquifer. The contaminants now appear to be progressing toward the Lower 180-

Foot Aquifer by way of the suspected hole in the Intermediate 180-Foot Aquitard. Contamination of the 

Lower 180-Foot Aquifer is significant because it provides drinking water for many residents of 

Monterey County and irrigation water for some of the Salinas Valley. Approximately 40,000 people 

rely on drinking water from wells that lie within 5 km of the site 

(http://www.epa.gov/superfund/sites/npl/ca.htm), making accurate characterization and effective 

mitigation imperative. 

In order to help guide remediation efforts and to guide future geophysical studies, USGS 

personnel collected compressional (P) and horizontally polarized shear (SH) wave seismic data along 

two survey transects in July 2006 (fig. 1). The purpose of this pilot study was to test the feasibility of 

using these methods to image the Intermediate 180-Foot Aquitard. The shear wave reflection method 

provides good resolution of the upper aquifer layers and successfully illuminates the Intermediate 180-

Foot Aquitard where it is thick and coherent. However, the effectiveness of the method, as it was 

carried out for this study, is limited where the Intermediate 180-Foot Aquitard consists of several thin 

layers. The P-wave reflection method is ineffective at this site, showing only the water table and the 

first multiple of the water table. 

3
Field Site 

Much of the former Fort Ord has been converted to other uses. The area of the suspected 

aquitard hole is now divided between student housing for California State University at Monterey Bay 

(the residences are visible in fig. 1) and the Fort Ord Natural Reserve, a part of the University of 

California Natural Reserve System (the vegetated areas shown in fig. 1). Geophysical survey work was 

not permitted within the Reserve area except along the Old County Road, and would have been 

logistically difficult in the residential area. 

We conducted seismic surveys along two transects that are near to, but outside of, the suspected 

location of the aquitard hole (fig. 1). These transects were chosen for their proximity to the suspected 

aquitard hole, accessibility, and proximity to boreholes with well log information. Both transects are 

along dirt roads, which simplifies survey logistics and improves data quality because compacted soil 

reduces the amplitude of problematic Love waves (for example, Haines and Ellefsen, 2006). The Old 

County Road transect (fig. 1) lies northeast of the suspected aquitard hole, along the so-called “Old 

County Road” that is a seldom-used, one-lane, unimproved road. The survey transect is adjacent to 

borehole MW-OU2-66-180. The Landfill transect lies southwest of the suspected hole (fig. 1), along 

the unimproved road that runs north-south along the east side of the old Fort Ord landfill. Well MWOU2-62-180 is near the north end of the Landfill transect. 

Shear Wave Surveys 

Data Collection 

A series of tests conducted along the Old County Road transect demonstrated that sledgehammer 

impacts on an aluminum source (Haines and Ellefsen, 2006) produce the best high-amplitude coherent 

reflections, and that horizontal geophones at a spacing of approximately 3 m are the best receiver 

spacing for the survey. We also tested a 90-kg accelerated weight drop seismic source, which is 

configured such that impacts can be tilted 45 degrees from vertical in order to produce shear waves. 

Data from the weight drop are compared with sledgehammer-source data in figure 3, and the superiority 

of the sledgehammer source is evident in figure 3a, where several reflections are clearly visible. The 

weight drop data (fig. 3b) show strong reverberations that obscure the reflected arrivals. 

4
Figure 3. Shear wave data shot gathers from Old County Road transect. a) Data collected with the 

sledgehammer-source, with 4 reflections marked. R1 is from the top of the Fort Ord-Salinas Valley 

Aquitard (FO-SVA), R2 is from an interface within the FO-SVA, R3 is from the base of the FO-SVA, and R4 

is from the top of the Intermediate 180-Foot Aquitard. b) Data collected with the weight drop source, with 

reflection R3 labeled. The other reflections are masked by the strong, reverberatory, source-related 

coherent noise that is labeled “Reverberation”. 

Impacts perpendicular to the seismic transect create SH waves propagating along the transect. 

By stacking data from opposing impacts after a polarity reversal, we minimize unwanted P- and 

Rayleigh-wave energy. For each shot location along each transect, we recorded four impacts, two of 

each polarity. We perform stacking in the office rather than in the field to enable both the rejection of 

noisy shots and any adjustments needed to correct trigger errors. Specific survey parameters are shown 

in table 1. 

Table 1. Seismic data recording parameters. 

Old County Road Line Landfill Line 

Number of recording stations 96 72 

Geophone spacing 3 m 3.5 m 

Shot spacing 3 m 3.5 m 

Line length 289 m 256 m 

5
Shear wave geophones 10-Hz horizontal 10-Hz horizontal 

Shear wave source 5.4-kg sledge on aluminum source 5.4-kg sledge on aluminum source 

P-wave geophones 10-Hz and 40-Hz vertical 10-Hz and 40-Hz vertical 

P-wave source 90-kg accelerated weight drop 5.4-kg sledge on steel plate 

Record length 1 s 1 s 

Sample rate 0.0005 s 0.0005 s 

A representative shear wave shot gather from the Landfill transect is shown in figure 4a. Several 

reflections are apparent, and the four strongest are marked. The reflection R4 appears to be from the top 

of the Intermediate 180-Foot Aquitard, although no well information exists to verify this interpretation. 

Reflections from at least the base of the FO-SVA are clearly visible in shot gathers along both transects, 

indicating that data quality is overall quite good. 

Figure 4. Shot gathers from Landfill transect. a) Shear wave data, with 4 reflections marked. R1 is from 

the top of the Fort Ord-Salinas Valley Aquitard(FO-SVA), R2 is from an interface within the FO-SVA, R3 is 

from the base of the FO-SVA, and R4 is from the top of the Intermediate 180-Foot Aquitard. b) P wave 

data, with the water table reflection (WT) and refraction (WT refr.) marked. The reflection is obscured 

by surface waves at near offsets. Red lines show arrival times for the direct wave (DW), the water table 

reflection, and the water table refraction predicted by numerical modeling. 

6
Data Processing 

In order to produce seismic reflection images, we processed the shear wave data using the 

processing flow shown in table 2. 

Table 2. Seismic data processing flow. 

Processing Step Purpose Comments 

Stack data from separately recorded 

hammer impacts Create shot gather for each shot point In-office stacking allows for quality control 

Apply elevation statics Account for elevation changes along 

transect Not necessary along Landfill transect 

Sort to common mid-point (CMP) 

gathers 

Arrange data by reflection point, for 

stacking 

Apply frequency-wavenumber (f-k) 

filtering Remove surface wave energy 

Has only minimal effect on final 

images, but facilitates velocity 

analysis 

Conduct velocity analysis Determine stacking velocities Conducted using constant-velocity 

NMO-corrected CMP panels. 

Apply normal moveout (NMO) 

correction 

Flatten reflectors in preparation for 

stacking 

Apply top mutes 

Remove surface wave and refraction 

energy, and reflections overstretched by NMO correction 

Stack CMP gathers Create one trace per CMP 

In addition to being essential to create 

a seismic image, stacking is the most 

effective signal/noise separation 

technique 

Apply minimum-phase Butterworth 

bandpass filtering 

Minimize high- and low-frequency 

noise 15 or 20 to 100 or 120 Hz 

Apply lateral smoothing Enhance coherent reflections, 

minimize noise Option, but valuable 

Apply automatic gain control (AGC) Equalize amplitudes Largely for display purposes 

Apply depth conversion Convert vertical axis from time to 

depth 

Using laterally-averaged NMO 

velocities 

This is a typical, basic, seismic reflection processing flow (see, for example, Yilmaz, 2001, p. 

90-150). We have tested a number of more sophisticated processing steps, most notably the surface 

consistent statics algorithm described by Ronen and Claerbout (1985). However, we do not find that 

any of these more sophisticated processing steps significantly improve the images. We present those 

images that represent our best results in terms of both reflector clarity and interpretation confidence. 

The presence of obvious reflections in the shot gathers facilitates data processing, particularly velocity 

analysis, and also builds confidence in the resulting images. Because the data show little lateral 

heterogeneity, the choice of depth-conversion methods is appropriate and migration is unnecessary. 

Shear Wave Images 

The shear wave seismic reflection images in figures 5 and 6 show coherent reflections 

corresponding with the interfaces between the major aquifers and aquitards. In both figures, part (a) is 

the time section, and part (b) is a depth section, with depth conversion performed as described above. 

7
Simplified well log information is shown for the boreholes adjacent to each transect. Fit between the 

seismic data and the well information is quite good. 

Figure 5. Shear wave reflection image for Old County Road transect. a) Time section. b) Depth section, 

with borehole information and interpretations overlain. Depth is relative to ground surface elevation of 

45 m above sea level. 

8
Figure 6. Shear wave reflection image for Landfill transect. a) Time section. b) Depth section, with 

borehole information and interpretations overlain. Depth is relative to ground surface elevation of 66 m 

above sea level. 

Starting from the surface, both images show reflectivity within the A Aquifer corresponding 

with compositional stratigraphy in that layer. Both images clearly show the top and bottom of the FOSVA, along with layering within it. Reflections from within the FO-SVA likely correspond with the 

minor compositional changes that are identified in the driller's logs. The pattern of reflectivity from the 

base of the FO-SVA is particularly interesting. Along the Landfill transect (fig. 6), several reflection 

arrivals occur very close together at a time corresponding with the base of the FO-SVA. Borehole 

information from well MW-OU2-62-180 shows a 3.4-m thick sand layer that is 3 m above the base of 

the FO-SVA. The interlayering of clay/sand/clay is almost certainly responsible for the observed 

reflectivity pattern. This reflectivity pattern broadens in time (depth) to the south along the transect, and 

the individual reflections can be separately observed. We interpret that this reflectivity pattern 

represents the southward thickening of either the sand layer or the basal clay layer. Along the Old 

County Road transect, the reflection pattern corresponding with the base of the FO-SVA is different; the 

interface is seen as a single, low-frequency reflection. Borehole information shows a set of very thin 

9
layers at the base of the FO-SVA. Thus, we interpret that the reflections from the many thin layers 

interfere to result in a single broad reflection. In this case, the relatively high amplitude of the reflection 

would be an example of tuning (for example, Yilmaz, 2001). 

In well MW-OU2-66-180 the Intermediate 180-Foot Aquitard is an 8-m silt layer, and the upper 

surface of this aquitard is visible as a clear reflection in the image between 160 m and 260 m horizontal 

position (fig. 5). Wells located off the ends of this transect show the aquitard as a set of very thin layers 

of silt, sand, and clay. In the image, evidence of the aquitard is not as clear at the ends of the transect as 

it is at the center. The lateral progression of the aquitard from a single homogeneous layer to a set of 

thin layers is likely responsible for the absence of, or faint appearance of, the reflection at the ends of 

the transect. In well MW-OU2-66-180, the base of the Intermediate 180-Foot Aquitard is not a clearly 

defined interface. Instead, the clay content decreases with depth, and numerous thin layers of silty and 

clayey gravel and sand represent the transition between the Intermediate 180-Foot Aquitard and the 

Lower 180-Foot Aquifer. Whether the lack of a clear reflection from the base of the Intermediate 180-

Foot Aquitard is due to the lack of a distinct interface, or simply to insufficient seismic energy 

penetration, is unclear. 

The borehole adjacent to the Landfill survey transect extends only to a depth of 72 m, which is 

above the Intermediate 180-Foot Aquitard (fig. 6). No other nearby wells extend deeper, so we lack 

independent information about the deeper part of the reflection section. The clear reflection at a depth 

of approximately 75 m between horizontal positions 25 and 112 m is likely the expression of the 

Intermediate 180-Foot Aquitard. However, without well information, this reflection can not be 

interpreted with certainty. A separate reflection, at a depth of approximately 65 m and visible between 

75 and 150 m horizontal position, may also be the Intermediate 180-Foot Aquitard or may be a layer 

within the Upper 180-Foot Aquifer. These aquifers and aquitards are not very distinct from one another 

at all locations, and some well logs show them to consist of many layers of various compositions. 

P-Wave Surveys 

We conducted P-wave surveys along the same two transects that were used for the shear wave 

surveys, and we used the same acquisition geometries (table 1). We created the P waves using the 

weight drop source in its vertical-impact position for the Old County Road transect and using a 

sledgehammer impacting a steel plate for the Landfill transect. In the P-wave data (for example, the 

shot gather shown in fig. 4b), only one reflection is apparent. In order to determine whether this 

reflection is from the water table (at a depth of 33 m for the data in fig. 4b) or from the top of the FOSVA (at a depth of 42 m), we conducted forward and inverse modeling using the methods described by 

Zelt and Smith (1992). Because the geology is nearly one-dimensional, we picked arrival times for only 

5 shots interspersed along the Landfill transect; the arrival times are for the direct wave, the reflected 

wave, and the refracted wave associated with the reflected wave (see fig. 4b). Inverting simultaneously 

for velocity and layer boundary positions, we determined that the reflection is due to an interface at 33 

m depth. We interpret that this interface is the water table, not the top of the FO-SVA. Some P-wave 

shot gathers, particularly along the Old County Road transect, show a second reflector. With twice the 

travel time of the water table reflection and opposite polarity, it is almost certainly the first multiple 

reflection of the water table. 

Although the P-wave data do not include any reflections of interest, the water table refraction 

does have an interesting character. As can be seen in figure 4b, the initial onset of the water table 

refraction is diffuse, and it has a very reverberatory appearance. As Bradford and others (1998) point 

out, a thin embedded layer (high-velocity in their case) can trap seismic energy, resulting in arrivals 

with a reverberatory character. For these Fort Ord data, the embedded layer could be the saturated 

10
portion of the A Aquifer, which is beneath the unsaturated portion of the A Aquifer and above the FOSVA. 

Discussion and Conclusions 

The shear wave reflection method is favorable for imaging stratigraphy at the former Fort Ord 

because shear waves are very sensitive to compositional contrasts such as the interfaces between the 

sand aquifers and clay aquitards. In contrast to P waves, shear waves are largely insensitive to pore 

fluids, and thus are not affected by the deep water table at the site. The shear-wave reflection method 

provides good images of aquifer stratigraphy to a depth of at least 60 m. The shear-wave images 

presented here provide the thickness of the A Aquifer and FO-SVA along the survey transects and 

indicate that there is only minor lateral variation within these layers. The images suggest that the 

Intermediate 180-Foot Aquitard shows greater lateral heterogeneity than do the overlying layers, in 

agreement with available borehole information. The images also provide specific information regarding 

the locations of heterogeneities in the Intermediate 180-Foot Aquitard. All of this information can 

provide valuable constraints for hydrogeological modeling. Because the Intermediate 180-Foot 

Aquitard may be coherent enough to prevent water flow even in locations where it is composed of 

layers too thin to image, results such as these would tend to overestimate the size of the hole in the 

aquitard. The difficulty of imaging thin layers, which is more pronounced for deep targets, is one that 

this method shares with virtually all geophysical methods. 

The P-wave reflection method (at least as it was carried out in the present study) cannot 

effectively image aquifer stratigraphy at the former Fort Ord due to the strong reflection from the water 

table. Because any lithologic reflections are much weaker than the water table reflection, they are 

masked by it. Also, the strong impedence contrast of the water table traps much of the seismic energy 

above this reflector, leaving very little energy to image deeper layers. This phenomenon has been 

observed at other sites (for example, Goforth and Hayward, 1992): the reflection coefficient of the water 

table is significantly greater than that of the target, making the target nearly “invisible”. A previous Pwave reflection study conducted elsewhere at the former Fort Ord was able to image the top of the FOSVA using a higher-frequency source (a seis-gun fired 0.6 m below the surface), but the energy could 

not penetrate to deeper layers (Merey and others, 1992). 

Our P-wave data are an example where correct interpretation of the data is best achieved through 

combined inversion of reflected and direct/refracted arrival times (for example, Zelt and Smith, 1992). 

Because the water table refraction is reverberatory, its arrival time is difficult to pick on many shot 

gathers. But combined with the easily-pickable reflection arrival, the first break picks yield a selfconsistent velocity model that fits with the known geology and water table depth. Inversion of only first 

break picks, without the reflection picks, does not yield such a clear result. 

Recommendations for Future Work 

Based on the results of this study, future seismic reflection studies should be limited to only 

shear waves because the P-wave data do not provide useful information regarding aquifer stratigraphy. 

In order to maximize their value, future survey transects should be longer than those in this pilot study, 

as long as 1000 m or more. If possible, each transect should be positioned to pass near multiple 

boreholes and to intersect other survey transects. Such surveys will provide an improved understanding 

of regional trends (stratigraphic continuity versus layer pinch-outs, etc.), characterizing areas between 

boreholes and extending this information into areas where boreholes do not exist. The geophone 

spacings used in the present study were optimal for providing good data quality even for shallow layers. 

A wider geophone spacing could be used for future surveys, particularly if information on the shallower 

11
layers is not considered essential to the work. Wider spacings would speed data collection and reduce 

associated costs. 

Seismic surveys are generally conducted along straight-line transects, though intersecting 

straight-line surveys could be conducted along a jagged transect. Though it would be possible to 

conduct a survey without vehicle access to the transect, logistics and efficiency would be significantly 

reduced. In addition, problematic surface waves are minimized by working on the compacted soils of 

roadways, resulting in better data quality. Ideally, future surveys would be conducted along developed 

routes, whether out-of-use dirt roads such as the Old County Road, sidewalks, pathways, or streets. Any 

surveys extending into the undeveloped area northeast of Abrams Road will likely require the clearing 

of brush in order to create a straight-line transect along which to work. The same is true for any surveys 

in the Fort Ord Natural Reserve (a part of the University of California Natural Reserve System) that 

surrounds the Old County Road. Future survey transects must be positioned according to the 

aforementioned criteria and also according to the needs of the hydrogeologists studying the site. The 

identification of suitable transect locations will be a key step toward establishing the value and 

feasibility of larger-scale seismic surveying. 

This pilot study shows that the primary survey target is nearly at the limit of resolution of the 

shear-wave reflection methods employed for this study. Thus, future shear wave reflection studies must 

employ equal, or preferably improved, data acquisition techniques in order to ensure an equal or 

improved signal-to-noise ratio. Though noise levels could perhaps be reduced by nighttime data 

collection or other means, seismic signal improvement is the more promising way to improve the data 

signal-to-noise ratio. One obvious signal improvement would be the use of a more powerful seismic 

source, such as a vibrator similar to the IVI MiniVib. In addition to providing more total energy than a 

sledgehammer, a vibrator would use a swept signal that would help to overcome cultural noise and 

would also result in a higher frequency (higher resolution) seismic wavelet. Other possible seismic 

sources include accelerated weight drops; however, our results using such a source (see fig. 3) are not 

promising. An improved weight drop source could prove useful, but testing at Fort Ord is not justified 

unless a more effective source is definitively identified elsewhere. The sledgehammer proved effective 

in the pilot study, and could be the best option for future surveys if a vibratory or weight-drop source is 

unavailable. If a sledgehammer is used, it is essential that the signal-to-noise ratio be maintained as 

high as possible through careful survey design and execution. 

The only other geophysical method that could provide useful information regarding the 

Intermediate 180-Foot Aquitard is the time domain electromagnetic sounding method (TDEM). This 

method will have difficulty imaging the layer of interest where it is a set of thin layers, but may be able 

to detect the layer where it is a thick coherent unit, providing lateral resolution on the order of 50 m. 

The first step toward applying the TDEM method to the site on a large scale would be the calculation of 

numerical or analytical models to better assess the chances of success, and then, if warranted, to conduct 

a small TDEM pilot study at the site. 

Acknowledgments 

This work was conducted as a cooperative venture between the USGS, the U.S. Army Corps of 

Engineers (USACE) and the Army Base Realignment and Closure Program (BRAC). David Eisen 

(USACE liaison with BRAC) coordinated the BRAC side of the project and was an essential contributor 

to all phases of the work. Support was provided by BRAC and by the USGS Mendenhall Postdoctoral 

Fellowship Program. The authors are grateful to Paul Wigton for invaluable assistance with field data 

collection and equipment maintenance and repair. Karl Ellefsen and Mike Powers have provided 

valuable guidance from the inception through completion of this project. This manuscript has been 

improved by the careful reviews of Karl Ellefsen and Jeff Lucius. 

12
References Cited 

Bradford, J., Sawyer, D., Zelt, C., and Oldow, J., 1998, Imaging a shallow aquifer in temperate glacial 

sediments using seismic reflection profiling with DMO processing: Geophysics, v. 63, p. 1248-1256. 

Goforth, T., and Hayward, C., 1992, Seismic reflection investigations of a bedrock surface buried under 

alluvium: Geophysics, v. 57, p. 1217-1227. 

Haines, S., and Ellefsen, K., 2006, Aquifer characterization with seismic shear wave reflection 

profiling: Proceedings, Symposium on the Application of Geophysics to Environmental and 

Engineering Problems, Environmental and Engineering Geophysical Society. 

MACTEC Engineering and Consulting, Inc., 2005, Operable unit carbon tetrachloride plume 

groundwater remedial investigation/feasibility study, former Fort Ord, California Volume 1 – 

Remedial Investigation: Final draft report prepared for the US Army Corps of Engineers. 

Merey, C., Miller, R., Ticken, E., and Lewis, J., 1992, Hydrogeologic characterization using a shallow 

seismic reflection survey at Fort Ord, California: 62nd Annual International Meeting, SEG, Expanded 

Abstracts, p. 370-373. 

Ronen, J., and Claerbout, J., 1985, Surface-consistent residual statics estimation by stack-power 

maximization: Geophysics, v. 50, p. 2759-2767. 

Yilmaz, O., 2001, Seismic data analysis: Society of Exploration Geophysicists. 

Zelt, C., and Smith, R., 1992, Seismic travel time inversion for 2-D crustal velocity structure: 

Geophysical Journal International, v. 108, p. 16-34. 

13