Questa Baseline and Pre-Mining Ground-Water 

Quality Investigation. 1. Depth to Bedrock

Determinations Using Shallow Seismic Data 

Acquired in the Straight Creek Drainage Near Red

River, New Mexico 

By Michael H. Powers, and Bethany L. Burton 

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

by the U.S. Government. 

U.S. Geological Survey Open-File Report 2004-1236 

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 
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Contents 

Summary.......................................................................................................................................................3

Site Description.............................................................................................................................................3

Seismic Profiles ............................................................................................................................................7

Line 1 Results .........................................................................................................................................10

Line 2 Results .........................................................................................................................................12

Line 3 Results .........................................................................................................................................14

Line 4 Results .........................................................................................................................................15

Conclusions.................................................................................................................................................17

Acknowledgements.....................................................................................................................................17

References...................................................................................................................................................17
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Summary 

In late May and early June of 2002, the U.S. Geological Survey (USGS) acquired four Pwave seismic profiles across the Straight Creek drainage near Red River, New Mexico. The data 

were acquired to support a larger effort to investigate baseline and pre-mining ground-water 

quality in the Red River basin (Nordstrom and others, 2002). For ground-water flow modeling, 

knowledge of the thickness of the valley fill material above the bedrock is required. 

Geophysical surveys using electrical methods were completed in 2001 (Lucius and 

others, 2001). They were acquired with the hope that the valley fill material would be wetter and 

more conductive than the bedrock. This was not the case. The boundary between the bedrock 

and the valley fill shows little or no contrast in electrical properties, and could not be determined 

with surface electrical geophysical methods. 

Several monitoring wells were drilled and logged through the Straight Creek valley fill 

from November 2001 to February of 2002. The drillers were able to determine when they were 

drilling through bedrock and not fill material, but the boundary was indistinct. Geophysical logs 

(gamma, neutron, and electrical induction) were acquired and show changes due to variations in 

water quality and saturation (Paillet, 2002). The well information provides depth estimates for 

the top of bedrock at point locations. 

When the seismic data were processed for reflection events, no clear interface could be 

consistently determined on any of the four lines. Fortunately, when curved-ray refraction 

tomography was used with the seismic first arrival times, the resulting images of interval 

velocity versus depth clearly show a sharp velocity contrast where the bedrock interface is 

expected. The images show that the interpreted buried bedrock surface is neither smooth nor 

sharp, but it is clearly defined across the valley along the seismic line profiles. The bedrock 

models defined by the seismic refraction images are consistent with the well data. 

Site Description 

The Straight Creek drainage is in the volcanic mountains of northern New Mexico known 

as the Red River district (Roberts and others, 1990). Figure 1 shows the general location of the 

valley. The volcanic bedrock flowed and slowly cooled over Tertiary sediments that existed on 

the surface 25-30 million years ago. The area was later uplifted, faulted, and intruded as part of 

the eastern side of the Rio Grande Rift. The resulting bedrock has areas of economic 

mineralization, along with areas of high pyrite content and high clay mineral content. 

Characteristic “scars” exist on the local slopes (figures 2 and 3). These are natural 

features associated with high pyrite content, steep slopes, no soil or vegetation, and high erosion 

(Meyer and Leonardson, 1990). During heavy rainfall, the clay minerals lose their cohesiveness 

and great volumes of eroded debris flow down the valleys (figure 4). The resulting valley fill is 

poorly sorted material that is somewhat cemented when dry (figure 5). Tests of the surface water 

in the drainage in September 2001 found it to be acidic (USGS, 2001), and the bedrock interface 

is suspected to be rough, variable, and permeable. 

Given these conditions, we were not sure if a sharp seismic velocity contrast would exist 

to define the subsurface boundary between the fill and the bedrock. We were also concerned 

about generating sufficient seismic energy at the source to penetrate the valley fill. The area is in 

an environmentally sensitive part of the National Forest with noise restrictions, among others, 

due to nesting raptors. 
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The Straight Creek drainage is a narrow valley, broadening as it comes down in 

elevation, with steep slopes on both sides. The valley floor was only 60 meters across at our 

highest elevation of investigation and about 250 meters across at the mouth of the creek into the 

Red River valley. 

Figure 1. The Straight Creek drainage runs south into the Red River. It is east of Questa, New Mexico, 

and northeast of Taos. 
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Figure 2. View is southwest across Straight Creek showing a small scar above the valley on the west 

slope. These natural features occur where conditions of steep slopes, high pyrite content, natural 

fracture zones, and sparse vegetative cover combine. 

Figure 3. View is southeast toward the town of Red River from the ridge above the east side of Straight 

Creek. A large feature like this also exists at the top of the Straight Creek drainage. 
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Figure 4. View is south down the valley floor in the dry bed of Straight Creek. Mud on the tree indicates 

the occurrence of high mudflow conditions. The orange pin flag at the base of the tree marks a 

station on Line 1. 

Figure 5. The character of the valley fill material is displayed in this view of the dry creek bed just above 

Line 1. 
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Seismic Profiles 

With this initial acquisition effort, we acquired four lines. The line locations are shown 

in figure 6. For the first three lines, we used 72 live channels with a geophone spacing of 0.75 

meter, 1 meter, and 1.5 meters, respectively. For lines 2 and 3, we hammered between the first 

48 geophones, moved the first 24 geophones to the end of the line, and continued hammering for 

48 more positions, giving us 96 total stations. For the fourth line, we had 96 live channels but 

still jumped an extra 24 stations for a total of 120 stations. This resulted in line lengths of 54, 96, 

145, and 241 meters, respectively. These lengths approximately matched the width of the valley 

floor at each location. At every source position we recorded two separate hammer blows. With 

post-acquisition processing, we could add the two shot records of any source position to increase 

the signal-to-noise ratio if desired. Figures 7 and 8 show the hammer source and the character of 

the valley floor for Line 1, and the steep slopes at the ends of the line. 

Our intention was to process the data to create reflection profiles. In particular, we hoped 

to image the bedrock reflection from its known shallow depth from one bedrock slope, under the 

valley fill material to the other slope. We were disappointed in this effort at the lack of an 

identifiable bedrock reflection and by abundant reflection arrivals from off-end or off-line 

positions. While analyzing the data, we did notice good first arrival energy that clearly showed 

evidence of a high-speed layer at depth. This led us to consider simple refraction analysis. In 

searching for a modern program to do this analysis, we became aware of commercially available 

packages for refraction tomography. 

Refraction tomography algorithms have evolved over the last decade. They are based on 

classical seismic refraction analyses (Thornburgh, 1930; Hagedoorn, 1959), combined with more 

recent developments in inversion theory and numerical modeling (Aki and Richards, 1980; 

Vidale, 1988; Aldridge and Oldenburg, 1992; Aldridge and Oldenburg, 1993). They have 

improved in the last few years with the most recent procedures for considering Fresnel zones 

while solving for a good fit of traveltimes and raypaths (Schuster and others, 1999; Watanabe 

and others, 1999; Lecomte and others, 2000). 

After running a freely available demo version of one package, using only 10 source 

positions of our 96 available on Line 3, we were impressed with the result and purchased the 

package. We learned that our acquisition configuration was excellent, and even excessive, for 

refraction tomography. This allowed us to perform the imaging process on various subsets of 

each profile dataset and to compare the results. The images from two separate subsets of each 

profile are presented below (figures 9, 12, 14 and 16). The degree to which the images are alike 

gives us confidence in the results. Differences between the two images of each profile give a 

sense of the resolution of the process and the results. One should not interpret a small feature 

that appears in only one of the two images. Common features are probably real. 

Our acquisition geometry was such that our first-arrival data do not image the bedrock 

wall rising at the ends of the profiles. Instead, the known location of the bedrock off the end of 

each line was forced into the data prior to gridding the result. For this reason, the position and 

nature of the bedrock as it is shown rising to the surface should be considered only an estimation. 
8

Figure 6. The locations of four seismic profiles and nearby wells are shown in the Straight Creek 

drainage. Straight Creek meets the Red River about 3 miles above the MolyCorp mill site. 
9

Figure 7. Along Line 1 the valley floor is fairly level. Note the steep slope in the background marking the 

sharp bedrock rise. The source was a 20 lb. handheld sledgehammer striking a metal plate. 

Figure 8. From the same place as the photo in figure 7 above, but now looking the other 

direction along Line 1, the steep slope on the other side of the valley is visible through 

the trees. 
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Line 1 Results 

Figure 9. Two independent velocity depth models have been created with a refraction tomography 

algorithm. Different source-receiver travel time data, subsets of the total data for this line, were 

used. The yellow colors represent velocities typical of dry unconsolidated fill. The blue colors 

represent velocities typical of saturated unconsolidated fill or semi-consolidated dry material. The 

red to black colors represent velocities typical of consolidated bedrock. 

Line 1 had 72 receiver stations and 73 source stations spread across 54 meters. In the 

images of figure 9, the lateral extent has been extended at both ends to the evident bedrock of the 

valley walls. The colors are as described in the figure caption, but it is important to note that 

blue does not always indicate saturated fill material. The images are gradational, such that if 

loose, dry sediment were directly against bedrock, blue would show between the low and high 

velocities but would not indicate wet material. In general, when the velocity is lower than 1500 

meters per second (m/s), as in the yellow colors, the material cannot be fully saturated. 

However, velocities greater than 1500 m/s are not always due to saturated sediment. Partially 

cemented but dry sediment can have the same velocity as loose, wet sediment. Bedrock in an 

area is often defined as the first highly consolidated material encountered under the soil and 

unconsolidated material. Material with a seismic velocity of over 3000 m/s can be considered 

bedrock by definition. One notable feature of these models is the rapid velocity variation from 

less than 2000 m/s to over 3000 m/s, strongly suggesting a clear bedrock interface. 
11 

The two images of figure 9 were created using different source-receiver combinations. 

Model A used 19 of the 73 source positions, and Model B used 17 source positions. Only the 

first hammer blow at each station was used for both models. Other model images were created 

using only the second hammer blow, and all show the same degree of variation in detail and the 

same common features. Details that vary between the two images should not be strictly 

interpreted. Common features can be interpreted with reasonable confidence. In this case, even 

though the current streambed exists in the topographic low at about 40 meters on the horizontal 

scale, the deeper part of the valley is clearly between 20 and 30 meters horizontally. Also, from 

25 to 35 meters horizontally, the bedrock interface is rough and indistinct as it rises from near 27 

meters depth to near 15 meters depth. 

Line 1 was acquired near two wells drilled early in 2002. One of the wells was 

completed to monitor the bedrock (Well SC-1B), and the other to monitor the alluvium (Well 

SC-1A). Both wells are north of Line 1. Well SC-1A is about 15 meters north of horizontal 

distance 15 on Line 1, and SC-1B is about 6 meters north of horizontal distance 21 meters on 

Line 1. Figure 10 shows results of several logs run in Well SC-1B at different stages of well 

completion. For more information about the design, development, and logs of the wells drilled 

as part of this study see Naus and others (in press). 

Figure 10. Well SC-1B was drilled into bedrock and completed with a screen from 39.6 to 42.7 m depth 

(see Naus 2004). From this data, hydrologists and consultants estimated depth to bedrock at 

25.6 meters. The inverted black triangle indicates water level in the well. 

The logs of SC-1B indicate saturated sediment starting at about 10 meters depth and a 

bedrock interface at about 26 meters depth in this hole. Figure 11 shows the results of two logs 

acquired in hole SC-1A. The drillers estimated depth to consolidated bedrock in this hole at 

about 20.5 m depth. The log data suggest saturated alluvium starting at about 8 meters depth. 

Most important, the data from both holes correlate well with the refraction tomography image of 

Line 1 (figure 9). The velocity images of Line 1 also show the potential difficulties of estimating 

a single, clear depth to bedrock with well data, depending on the well location. 
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Figure 11. The induction log varies according to the electrical conductivity of the material around the 

borehole. Because of the poor water quality in this area, wet sediment is likely to be more 

electrically conductive than dry sediment. The change in saturation near this hole probably 

occurs at about 8 meters depth. The change from dry to wet sediment is also commonly 

associated with an increase of seismic velocity from less than 1500 m/s to over 1600 m/s (yellow 

to blue on the color scale). (Figure from Paillet, 2002. See Naus, in press, for well design.) 

Line 2 Results 

Line 2 was acquired about 250 meters south (downslope) of Line 1. This part of the 

valley has an elevated debris fan of material on the west side associated with erosion from the 

small scar on the slope above. We situated the line such that the western portion of it ran along 

the surface of this debris fan, down to the road in the center of the valley, and across the almost 

level valley floor to the east slope. In particular, we wondered what the shape of the bedrock 

interface might be under this debris fan. 

Figure 12 shows the refraction tomography velocity versus depth models created from the 

data of Line 2. Model A used 20 of the 97 shot files, and model B used 26 shots. Both models 

used first-arrival times from only the first hammer blow for each included shot station. Depth to 

bedrock here is about the same or even a bit less than the deepest part at Line 1. However, the 

bedrock channel has now broadened to almost the complete width of the valley floor. It also 

appears to be separated into two channels nearly equal in width and depth, with just a slight 

bedrock rise dividing them. The bedrock slope under the debris fan on the west half of the 

profile is well defined. Only the portion from about 5 meters on the horizontal scale to the west 

end has been approximated. From 5 meters to 35 meters on the horizontal scale, this image gives 

a good depiction of the buried bedrock slope on the west side of the valley under the debris fan. 
13

Figure 12. Two images created from separate subsets of the data acquired along Line 2 show similar 

subsurface features. The bedrock channel has broadened from that of Line 1, but it has not 

deepened. The west side of this image shows the slope of the bedrock below the debris fan of 

erosional material from the small scar located just above the line on the west slope. 

The rough nature of this slope is not unlike the exposed bedrock on the slopes above the 

current valley floor. Figure 13 shows a picture of bedrock features on the east slope just above 

Line 1. It is possible to imagine that the buried bedrock slopes include such features. 

Figure 13. This bedrock feature is currently exposed on the east slope above Line 1. Similar features 

may exist in the buried bedrock slopes further down in the valley. This can account for the rough 

appearance of the bedrock interface in the models. 
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Line 3 Results 

Figure 14. The velocity-depth models created from the data of Line 3 show the broad bedrock channel at 

about 25 meters depth, but here a central channel exists where the depth is about 40 meters. 

The two models were again created with separate subsets of the data. Well SC4A is just off the 

line at about 105 meters on the horizontal scale. 

Line 3 was located to coincide with several wells. It crosses the road about 250 meters 

south (downslope) from Line 2. Line 3 crosses the valley from west to east with a slight 

southward trend, such that the west end is higher in elevation than the east end. The current dry 

streambed is on the far east side of the valley. Figure 14 shows the velocity-depth models for 

Line 3. Only the first hammer blow at each of 20 shot stations for Model A and of 34 shot 

stations for Model B was used. 

Well SC-4A was drilled and completed as an alluvial well. The bottom of the well was 

meant to stop at bedrock and be screened to intercept water from the alluvium for monitoring. 

This well was just a meter or two off of Line 3 at 105 meters on the Line 3 horizontal scale. 

Figure 15 shows the gamma and induction log results from this well. The images from the 

seismic data show water saturated sediment at about 20 meters depth near the well and bedrock 

at about 28 meters depth. This correlates well with the two logs assuming the wet sediment is 
15 

more electrically conductive than the dry sediment, and the wet bedrock is a little less electrically 

conductive (probably a little less porous) than the wet sediment. These are both reasonable 

assumptions. The gamma log shows only a slight change at the bedrock interface, which is 

difficult to interpret without other information. The seismic images allow this slight change to 

be interpreted as bedrock. Because the valley fill and bedrock in this area have essentially the 

same mineral makeup, a lack of gamma log variation at the bedrock interface is not surprising. 

The decision to plug this alluvial well at 28-30 meters is consistent with our models. 

Figure 15. The logs from well SC-4A, located just off Line 3 at 105 meters on the horizontal scale, are 

reasonably interpreted based on the images from the seismic data. They suggest dry fill to about 

20 meters, and wet fill below this to bedrock at about 28 meters. (Figure from Paillet, 2002) 

Line 4 Results 

Line 4 was located another 250 meters south down the road from Line 3. Line 4 is above 

the sewage treatment facility and the opening of the Straight Creek drainage into the Red River 

valley. Because the recording spread for each hammer blow was longer in this survey than the 

others, the first arrival energy at the far ends of the spread was weaker. This made the traveltime 

picks less accurate, which in turn means the depth to bedrock in the deepest part of the valley is 

computed with less certainty. Figure 16 shows the refraction tomography images for this line. 

Even though the data contributing to these images are somewhat noisier than that of the 

other lines, the two images created from separate subsets still look similar. Model A was created 

with data from 25 of 120 shot points, and model B was created with data from 31 shot points. 

Figure 17 shows the coverage in the subsurface of velocity values computed by the refraction 

tomography imaging algorithm. For model B, 2976 first arrival traveltimes were picked and 

used to calculate velocities. The velocities shown in the color images where no coverage exists, 

mostly deep depths at the ends of the lines, are extrapolations. The coverage section confirms 

that the important features showing the wet/dry and bedrock interfaces are well constrained by 

the velocity data. 
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Figure 16. Two velocity depth models created from separate subsets of the Line 4 data show similar 

features. The deepest bedrock channel (55 meters deep) is 30 meters east of the road, which 

crosses this line at about 95-100 meters. A flat wet-sediment contact is at about 25 m depth. 

Figure 17. This image shows the subsurface coverage of velocity values computed by the refraction 

tomography algorithm to create model B above. Only 31 of 120 shots were used, but coverage is 

good over most of the image. Velocities in areas with no coverage are extrapolations. 
17

Conclusions 

Refraction tomography images, created with first arrival traveltime picks of seismic data 

acquired in the Straight Creek drainage near Red River, New Mexico, show depths to water table 

and bedrock with values that are consistent with information from wells in the area. The 

tomographic images show subsurface velocity versus depth. Standard correlations of velocity 

with lithology and saturation lead to interpretations of subsurface features that are supported by 

independent well information. 

Given the geologic and geomorphic conditions at this site, and the information from the 

wells, we believe our seismic models are reasonable estimations of subsurface features. The 

color scheme we have presented allows reasonable interpretations of bedrock as red on the 

images. The yellow color can be interpreted as dry to under-saturated alluvium. Blue, however, 

can be either fully-saturated alluvium or partially consolidated but less than fully saturated 

alluvium. We present two separate images of each model so that common features can be 

confidently interpreted, and variable features can be identified as questionable. 

Acknowledgements 

A joint U.S. Geological Survey Geologic and Water Resources Disciplines project with the New 

Mexico Environment Department funded this work. Jeff Lucius and Jared Abraham helped 

acquire the data, and provided useful input throughout the survey effort. Reviews and comments 

by David V. Smith, Cheryl Naus, and Jared Abraham are much appreciated. 

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