Reconnaissance Shallow Seismic Investigation of Depth-to-Bedrock and Possible Methane-bearing Coalbeds, Galena, Alaska 

by 

W. J. Stephenson, R. A. Williams, J. K. Odum, 

C. E. Barker, D. M. Worley, A. C. Clark, and J.G. Clough 

Open File Report 02-450 

Galena from an airplane 

Yukon River at sunset 

Shotpoint “blowout” during 

seismic acquisition 2002


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

editorial standards nor with the North American Stratigraphic Code. Any use of trade names in 

this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government 

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 
RECONNAISSANCE SHALLOW SEISMIC INVESTIGATION OF DEPTH-TOBEDROCK AND POSSIBLE METHANE-BEARING COALBEDS, GALENA,

ALASKA

by 

W. J. Stephenson1, R. A. Williams1, J. K. Odum1, C. E. Barker

2,

D. M. Worley1, A. C. Clark3, and J.G. Clough4

1U.S. Geological Survey

Geologic Hazards Team, Central Region

Box 25046, MS 966

Denver, Colorado 80225-0046

2U.S. Geological Survey

Energy Team, Central Region

Box 25046, MS 977

Denver, Colorado 80225

3U.S. Geological Survey

Water Resources Discipline, Central Region

Box 25046

Denver, Colorado 80225

4Alaska Division of Geological and Geophysical Surveys

794 University Ave., Suite 200

Fairbanks, AK 99709-3645

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

editorial standards nor with the North American Stratigraphic Code. Any use of trade names in 

this publication is for descriptive purposes only and does not imply endorsement by the U.S. Gov­ ernment 
ABSTRACT 

A reconnaissance shallow seismic reflection/refraction investigation in and around 

the city of Galena, Alaska suggests that Tertiary and/or Cretaceous bedrock, and possible 

coalbeds within the Cretaceous, is at least as deep as 550 feet in the immediate vicinity of 

town. Rock could be deeper than 1000 feet under alternate interpretations. Reflections 

recorded in these data are believed to be from the sediment/bedrock interface. Analysis 

of these reflections and associated refractions indicates that this interface, interpreted at 

most of the six profile locations, has a high seismic velocity, possibly indicating nonsedimentary rock (e.g. volcanic or igneous). 

INTRODUCTION 

The City of Galena, Alaska, has interest in obtaining a local methane gas supply to 

supplement their heating and electrical needs. It has been proposed that Cretaceous 

coalbeds, observed in outcrop at Hartnet Island, roughly 12 miles east of Galena and 

roughly 20 to 30 miles west of Galena near Koyukuk and Nulato, might provide a source 

of methane gas. However, the depth of these possible coalbeds beneath the city is critical 

to determine their viability as an economic methane source. Without knowing the total 

thickness of the late-Tertiary (?) to Quaternary section, the total depth to Cretaceous strata 

and thus a drilling method cannot be determined. To help estimate the possible drilling 

depth to Cretaceous and older rock, we acquired high-resolution seismic 

reflection/refraction data at six sites in and near the city of Galena (Figure 1). These data 

provide information on depths to reflecting geologic boundaries that may be the 

sediment/bedrock boundary. 

1

Profile 4 

Profile 5 

Profile 6 

Profile 2 Profile 3 

Profile1 

Galena 

Red lines are approximate locations of 60 receivers 

Stars are locations of seismic shot points off profile 

1 mile 

Map base is USGS 1:63,360 Nulato Quadrangle. 

Map of City of Galena, Alaska, and seismic profile locations. 

for each of six profiles (receivers spaced 10 m apart along each profile). ends, which were needed for deeper coverage. Figure 1. 

2

GEOLOGIC SETTING 

Based on published geologic information (Bradley, 1938; Pewe, 1948; Cass, 1959; 

Patton and Hoare, 1968; Nakanishi and Dorava, 1994) and field evidence (C. Barker, 

unpublished data), we make the following observations: 1) The Cretaceous section in the 

Galena area, based on aeromagnetic response, is on the order of several thousand feet 

thick; 2) Regionally, the Upper Cretaceous strata contain approximately 800 ft. of coalbearing strata. As this unit is poorly exposed in outcrop, the cumulative thickness of 

coalbeds in this unit is unknown; 3) Much of the Cretaceous section is non-coal bearing 

and significant erosion of the upper part of this section would likely remove the coalbearing unit; 4) Coal-bearing rocks are mapped as exposed in the banks of the Yukon 

River on either side of Galena, at Hartnet Island some 12 miles east of Galena and some 

20 to 30 miles west of Galena near Koyukuk and Nulato; 5) Coals in the Koyukuk and 

Nulato areas exist as discontinuous pods rather than as laterally extensive seams; 6) The 

coal bed exposed at Hartnet Island is approximately 9 feet thick and dips about 70° to the 

southeast, away from the city of Galena; 7) There is also the possibility that coal deposited 

in mid-Tertiary grabens is present beneath or near Galena. 

From these observations, it is likely that Cretaceous or Tertiary rocks underlie 

Galena beneath the near-surface late Tertiary(?) to Quaternary fluvial plain deposits of the 

Yukon River. It is unknown whether the Cretaceous or Tertiary strata beneath Galena 

contain coal. Because the coal-bearing section is in the uppermost Cretaceous section, it 

is possible that it has been eroded away, possibly by the Yukon River in the late Tertiary(?) 

to Quaternary. At least 360 feet of soft, water-saturated, fluvial and swamp-derived 

sediments were penetrated without hitting the underlying Cretaceous unit in a water well 

drilled by the City of Galena in 1998. No other deep drill hole information is known to 

3

exist within the immediate area. 

SEISMIC DATA ACQUISITION AND PROCESSING 

The signal we use in the seismic data to extract information, such as the depth of 

the sediment/bedrock interface as well as general stratigraphic layering, is in the form of 

reflected and refracted sound waves that are directly dependent on velocity (and density) 

variations in the subsurface. The reflection amplitude is proportional to the velocity (and 

density) contrast across the boundary. A diagram showing basic reflected and refracted 

signal travel paths for a single seismic wave initiated at the surface is shown in Figure 2. 

Depths are estimated by identifying the signal type and by analyzing signal travel times. 

Inherent error in interpretation occurs because of our lack of knowledge of the velocity 

structure and geometry of geologic layering. Noise recorded in the data from wind, 

traffic, and electrical sources (60-Hz transformers, radio transmitters) also degrade data 

quality and therefore the interpretation accuracy. Two types of seismic waves are 

important for this study: compressional (or P)-waves, and shear (or S)-waves. S-waves 

typically travel at 60% the velocity of P-waves. We designed our study to focus on Pwaves, and these are primarily what was recorded by our instrumentation. Unless 

otherwise stated, all reference to seismic waves in the text will refer to P-waves. 

The seismic reflection/refraction data were acquired at six sites over a five-day 

acquisition period in September 2000 (Figure 1). The profiles are labeled from 1 to 6, and 

were acquired at the following locations: 1) north of the Galena airport; 2) along the bike 

path through the city of Galena; 3) roughly midway between Galena and the old Campion 

air base; 4) old Campion air base; 5) north of the AM radio tower and south of Profile 4; 

and 6) southwest of Profile 5 ending near the Yukon River. The general seismic 

4

f

Shot point 

e av

d w 

a te

e r c

Direct wave R 

Reflected waves 

ground surface 

V1 

V2 

V3 

V4 

Figure 2. Simplified diagram of selected travel paths for a single seismic wave from a 

shot point at the ground surface. Velocities increase with depth (V4>V3>V2>V1) and no 

refractions from the two deeper interfaces are shown. Seismic reflections are generated 

where velocity and material density changes across a boundary. Both P- and S-waves 

generate direct waves, reflections, and refractions 

acquisition parameters for these profiles are listed in Table 1. Topographic survey data 

were acquired by GPS. The accuracy of these survey data was poor, typically no better 

than ± 15 ft, due to thick tree cover generally in the vicinity of the profiles (hence poor 

satellite coverage). 

Estimating the amount of Kinestik® seismic explosive used per source point in an 

ad hoc manner was a primary acquisition difficulty. The farther from the receiver 

locations, the more explosive was used, but sometimes the amount was underestimated 

because of unknown local site conditions, making the seismic signal weak and therefore 

5

difficult to analyze at farther receiver distances. Because of the reconnaissance nature of 

the investigation and the limited budget, the explosives had to be carefully rationed and 

therefore we rarely had the luxury of re-shooting a source point. Another major problem 

we encountered was the source hole depth and diameter. The drilling rig provided by the 

City of Galena drilled an excellent hole for charges of 1-pound and less. But for larger 

charges, a smaller diameter hole and/or a deeper hole would have been advantageous 

because the higher energy shots often “blew out” the recently excavated back-filled dirt. 

The energy released during these blowouts dramatically decreased the energy sent into the 

ground as seismic signal. 

Table 1. Generalized Acquisition Parameters for Galena Seismic Data 

Parameter Description 

Source type Kinestik® seismic explosive 

Source size Single shot hole 6-8 feet deep with 1/3 to 4 

pounds of explosives (dependent on distance 

to receivers) 

Source point interval Relative to receiver spread: in the 

middle,16.4 ft off each end, 985 ft off each 

end, and 1970 ft off each end (when 

possible) 

Receiver type 8-Hz resonant frequency, vertical component 

geophone 

Receiver array Single geophone per station 

Shooting geometry 60 channels, deployed out as linearly as 

possible. Sources as in-line as practical 

Station interval 32.8 ft 

Field filters 60 Hz notch 

Recording system Geometrics 60-channel StrataVisor 

Sampling rate 1 millisecond 

Recording time 2 seconds 

Kinestik® is a registered trademark of Kinepak Corporation 

Data processing was limited only to steps necessary to maximize coherent 

reflection and refraction signals (Table 2). CDP stacking (i.e. to make a stacked seismic 

section), a technique universally used in the oil industry, was unsuccessful at enhancing 

6

these data. This was primarily because the very limited number of shot records obtained 

did not yield adequate coverage for this technique. Some additional signal enhancement 

was achieved by combining traces at similar source and receiver offsets from all shots at 

each profile location (‘sort and stack’ step in Table 2). Shot records prior to this final 

processing step are shown in the Appendix. 

In the following section, the data are presented after all of the above steps have 

been performed (Table 2). The data are shown as recording time versus distance from the 

receiver spread midpoint. Interpretations are made using industry-standard algorithms for 

estimating depths of reflecting and refracting boundaries (sometimes referred to herein as 

interfaces; e.g. Telford et al., 1990). As will be noted, the earliest (in time) seismic events 

are often difficult to differentiate and thus are often referred to as “direct 

arrival/refraction,” indicating this ambiguity. 

Table 2. Generalized Data Processing Steps for Galena Seismic Data 

Processing Step Comment 

Data Reformat

Geometry

Trace Edit

Bandpass Filter (Hz)

Automatic Gain Correction

Deconvolution

Elevation Statics

Sort and Stack

Convert field data to processing format

Install topographic coordinate data into trace headers

Omit bad traces and change incorrect trace polarities

Limit frequency range to optimize signal

(40-to-500 Hz pass band).

Adjust amplitudes using 500 ms gain window

Compress wavelet and attenuate reverberation using

adaptive algorithm with 200 ms operator length

Time-shift traces based on station elevation

differences to better align recorded signal from

receiver to receiver

Sort all traces at each profile location by offset

(distance from source to receivers) and add these

common-offset traces together. Display as a single

ensemble centered at the midpoint of the receiver

stations

7

GEOLOGICAL INTERPRETATION 

Based on limited knowledge of the depositional environment, it is believed that 

the Yukon Valley basin sediments are relatively homogeneous in terms of their seismic 

characteristics. Lateral variation in deposition and limited compaction time (few million 

years) often make seismic reflection/refraction signal small across sedimentary layer 

boundaries. It is reasonable to infer that one of the first continuous and observable 

seismic contacts is between the Quaternary/Tertiary saturated sediments and 

Tertiary/Cretaceous bedrock. In the following interpretations, we thus assume one of the 

first continuous and observable reflections/refractions at farthest source-receiver offsets 

on each profile are from this sediment/bedrock interface. It should be remembered, 

however, that both observable reflections within the sediments will occur and that the 

sediment/bedrock velocity (and density) contrast may not be high enough to detect. 

Permafrost was present in almost every drilled shot hole. It appeared to be thick 

(based on the seismic data, on the order of 100 feet or greater) and continuous at all of the 

sites we investigated. It had been hoped that this high-velocity layer would be thin to 

non-existent at some locations, allowing the use of seismic refraction modeling 

techniques that utilize direct and first-refraction arrivals. However, the direct and 

refraction arrivals in these data have traveled almost entirely from permafrost, as 

suggested by their arrival velocities between 11400 and 13120 ft/s, which rendered this 

part of the data unusable for refraction analysis of bedrock depth. Therefore, this left us 

with the analysis of reflection events and later refraction events within the seismic data. 

The disadvantages of analyzing these later events on data acquired in reconnaissance 

mode such as these include: 1) it is more difficult to accurately define boundary depths; 

8

and 2) layer dip is not easily resolvable. In general, all depths estimated from these data 

have an accuracy of ± 100 ft. 

Numerical Simulation of Shot Record for Assumed Flat-Layer Sub-Surfaces 

We utilized a numerical modeling technique (elastic two-dimensional finitedifference modeling; Larsen, 1992) to investigate the effect bedrock depth beneath 

permafrost and saturated sediments had on the recorded data. Modeling was performed 

to better qualify our interpretation. The velocity structure used in the finite-difference 

modeling is somewhat constrained by the seismic data. The models were comprised of 

four layers that represent: 1) unfrozen surface soils and mud; 2) permafrost sediments; 3) 

Qua t e rna ry/l a t e Te rti a ry unfroz en and unconsolida t ed deposits; and 4) 

Tertiary/Cretaceous bedrock. The layers were all assumed to be horizontal. Dipping 

layers would yield different results depending on the steepness of dip. 

Direct-arrival, reflection, and refraction measurements of the field data helped 

determine seismic velocities in the upper three layers. Velocities in the bedrock layer 

were inferred from comparison of outcropped rock with typical velocity ranges for these 

rocks. P-wave velocities ranged from 2640 ft/s to 13200 ft/s across the four layers, with 

corresponding S-wave velocities ranging from 1320 ft/s to 7920 ft/s. Densities varied 

from 2.2 gram/cm3 to 2.6 gram/cm3, and were derived from published values for the 

assumed materials (e.g. Telford et al., 1990). No attenuation was incorporated in the 

simulations, and only event arrival times were used to assess modeling results. Because 

the goal of this modeling was primarily to determine bedrock depth under permafrost and 

saturated unfrozen sediment, no attempt was made to model thin coal deposits within the 

bedrock. 

9

Several of the simulated shot records created for this study are shown in Figure 3. 

Layer 1 (surface soil/mud) was set at 6.5 ft thick, and layer 2 (permafrost) was set at 100 

ft thick. The thickness of layer 1 was determined based on drilling, while that of layer 2 

was estimated from the seismic data and from previously existing drilling information. In 

general, these thicknesses permit a reasonable match of the direct and refracted arrivals 

within the actual seismic data. Bedrock depth and layer velocities were varied between 

each model. As bedrock gets shallower and/or sediment velocity changes, the arrival time 

of the bedrock reflection changes accordingly (Rb in Figure 3). Similarly, an event 

labeled Rf (Model B), which is the refraction off bedrock, appears sooner in the record as 

bedrock becomes shallower. If bedrock is as shallow as 200 ft (Model E), both Rb and Rf 

become masked by the direct and refracted arrivals from the permafrost layer. Modeling 

such as this is important to better qualify what seismic interfaces may be within the 

complex shallow velocity structure around Galena. 

Profile 1 

Profile 1 was acquired north of the Galena airport. The first events on the seismic 

record are direct-and-refraction arrivals with an average velocity of 11500 ft/s (Figure 4). 

By 1980 ft offset from the profile midpoint, the direct-arrival/refraction has died out. 

This is probably caused in part by use of undersized seismic charges at far offsets, as well 

as by the permafrost thickness along the profile and the decrease in velocity with depth 

beneath the permafrost. At offsets between 2000 and 2970 ft and after 0.25 s recording 

time, a coherent event is observed that is labeled as R1. Unfortunately, after exhaustive 

analysis, it was not clearly resolvable as either a reflection or refraction. If it is a 

reflection, then it moves out at a velocity of approximately 8250 ft/s and would emanate 

10

Recording Time (seconds) 

A B C D E 

0.0 

0.2 

0.4 

0.6 

Rb Rb 

Rb 

Rb 

Rb 

P -wa ve

d irect arriv al/ 

refr a c tion 

S-wave

direct

arrival Rf 

Rf 

Rf 

Figure 3. Synthetic seismic shot records generated with finite-difference numerical modeling. All models consist of four layers, as discussed in text. Bedrock reflection is 

labeled Rb on each record. Refraction from bedrock is labeled as Rf

. Model A has bedrock at 550 ft depth and has 100 ft of permafrost. Model B has bedrock at 550 ft depth 

and permafrost velocity at 92% of model A. Model C has bedrock at 550 ft depth and 

saturated sediment velocity at 110% of model A. Model D and E are also similar to 

model A but with bedrock at 1100 ft and 200 ft depths, respectively. As bedrock gets 

shallower and/or velocity of sediments increases, bedrock reflection arrives earlier in 

recording. Even in synthetic shots such as these, with simple flat layer geology, complexity of seismic wave field is apparent. 

11

from a reflector estimated at about 1100 ft ± 100 ft depth. If event R1 is a refraction, then 

it emanates from a seismic interface that dips to the southeast and ranges in depth from 

550 ft to 800 ft, ± 100 ft, along the profile. The coherent event arriving just after R1 is 

possibly another reflection, R2, moving at a velocity of about 6270 ft/s. If R2 is a 

reflection, then it would be from a reflector at about 650 ft depth. Given the assumptions 

discussed in the previous section, we believe that R2 is the reflection and R1 is the 

refraction from bedrock beneath Profile 1, at an average depth of 650 ft. The bedrock 

velocity of approximately 11150 ft/s is higher than one would anticipate for young 

sedimentary rock, suggesting that the reflector may be a volcanic or igneous rock. 

Profile 2 

Profile 2 was acquired along the bicycle path on the north side of the main road 

through Galena (Figure 1). These data are similar in overall appearance to those acquired 

along Profile 1. The first arrivals at this site average 12200 ft/s, slightly faster than at 

Profile 1 (Figure 5). Just as observed on Profile 1, the first coherent event at offsets 

beyond 2000 ft is not clearly resolvable as either a reflection or refraction. If event R1 is 

a reflection, then it moves out at a velocity of approximately 8250 ft/s and would be from 

a reflector estimated at about 900 ft depth. If it is a refraction, then it is from a boundary 

that dips gently west and ranges in depth from 510 ft to 575 ft. The coherent event R2, 

may be a reflection that moves out at a velocity of about 6600 ft/s. As in the case of 

profile 1, if R2 is a reflection, then R1 is its corresponding refraction from a boundary at 

roughly 550 ft depth. The estimated bedrock velocity beneath Profile 2 (assuming the 

reflection/refraction are from bedrock) is also higher than anticipated for sedimentary 

12

0.0 

0.5 

NW SE 

R1? R1? 

Direct Arrival/ 

Refraction 11500 ft/s 

2970 ft 1980 ft 990 ft 990 ft 1980 ft 2970 ft

R2? 

1.0 

1.5 

2.0 

Figure 4. Data from 5 shots recorded on Profile 1, sorted by shot-receiver offset distance. White 

arrow is at the mid-point of receiver spread profile. Refraction and direct arrival energy are the first 

coherent events seen near the start of recording time. A possible reflection or refraction is labeled R1. 

Another possible reflection believed to be from bedrock is labeled R2. If R2 is a reflection, then R1 

is the refraction from the same interface. 

13 

R

e

c

ording 

Time (s

e

c

o

n

d

s) 
rocks, at 10560 ft/s. 

Profile 3 

Profile 3 was acquired about 3 miles east of Profile 2 along the main road between 

Galena and the old Campion air base. These data have the most prominent and fastest Pand S-wave direct and refracted arrivals of all the profiles, as observed on the eastern side 

of the receiver array midpoint (Figure 6). These data are also unique among the profiles 

because of deeper apparent reflections and a series of high-velocity apparent reflections 

seen at less than 0.5 seconds recording time at far offsets on the eastern side of the profile 

(below R1). Analysis of the east-side direct arrival/refraction suggests a change in 

lithology at about 300 ft depth, and this probably indicates a change in frozen material or 

thicker permafrost at this location. The direct and refraction arrivals on the western side 

of the midpoint are similar to those observed on Profiles 1 and 2. However, estimated 

reflection velocities indicate the reflectors are quite deep, over 1100 ft. The event labeled 

R1 is the best-guess bedrock interface; its depth is approximately 750 ft. Overall, the data 

on profile 3 are complex and may be indicative of thick permafrost coupled with geologic 

structure and rapid lateral change in rock type. 

Profile 4 

Profile 4 was acquired along the abandoned landing strip at the old Campion air 

base. Although over 6 miles from Galena, this site was selected to determine if bedrock 

might be shallower underneath the topographic high at the old base (Figure 1) and 

therefore more economic to drill. These data show two events believed to be reflections 

(Figure 7). The velocity of the shallow event R1 travels at roughly 10230 ft/s, sufficiently 

fast to have traveled through a significant section of frozen sediment or bedrock. We 

14

0.0 

0.5 

E W 

R1? R1? 

Direct Arrival/ 

Refraction 12200 ft/s 

2970 ft 1980 ft 990 ft 990 ft 1980 ft 2970 ft

R2? 

1.0 

1.5 

2.0 

Figure 5. Data from 5 shots recorded on Profile 2, sorted by shot-receiver offset distance. White 

arrow is at the mid-point of receiver spread profile. Refraction and direct arrival energy are the first 

coherent events seen near the beginning of recording time. A possible reflection or refraction is 

labeled R1. Another possible reflection believed to be from bedrock is labeled R2. If R2 is a reflection, then R1 is the refraction from the same interface. 

15 

Recording Time (seconds) 
E


0.0 

0.5 

1.0 

1.5 

2.0 

W 

R1? 

R1? 

Direct Arrival/

14

Refraction 190 ft/s 

2970 ft 1980 ft 990 ft 990 ft 1980 ft 2970 ft 

Deeper 

Reflections? 

Direct Arrival S-wave 

Figure 6. Data from 7 shots recorded on Profile 3, sorted by shot-receiver offset distance. White 

arrow is at the mid-point of receiver spread profile. Refraction and direct arrival energy are the first 

coherent events seen near the beginning of recording time. A possible reflection is labeled R1. A set 

of possible deeper reflections are seen below the S-wave direct/refracted arrival. 

R

e

c

ording 

Time (s

e

c

o

n

d

s) 

16

interpret this event to be from a bedrock interface at approximately 720 ft depth. The 

deeper possible reflection event, R2, arrives much slower at 6940 ft/s. Analysis suggests 

it would be from a reflector at roughly 900 ft depth. 

Profile 5 

Profile 5 was acquired southeast of Profile 4, still within the perimeter of the old 

Campion air force station. The data suggest a thinner layer of permafrost than at Profile 4 

and reveal several apparent reflections (Figure 8). Both the direct and first refracted 

arrivals, traveling at about 10230 ft/s, are slower than those observed along Profile 4. 

Event R1 is most likely a reflection that travels at a velocity of approximately 7425 ft/s. 

Event R2 is believed to be either a reflection or refraction from a deeper event. If a 

refraction, the seismic event is from an boundary at a depth of about 600 ft. A weak 

possible reflection correlates roughly with this depth. Event R1 is interpreted as evidence 

of bedrock at about 415 ft, while event R2 is believed to be from an inter-bedrock 

boundary. 

Profile 6 

Profile 6 was acquired along the last accessible stretch of road southwest of 

Campion air base. These data were acquired to image any shallow rock layers near the 

river and off the topographic high of Profiles 4 and 5. Unfortunately, this profile located 

on the most crooked road, had the most topographic variation, and had the poorest 

elevation control of any acquired in this study. These factors contributed to low-quality 

processed data that required the omission of the westernmost shot data (Figure 9; see 

Figure A6 for unsorted shot records). Even after extensive processing, the data are 

difficult to interpret because of the noise introduced by line geometry errors. The non17

SW 

0.0 

0.5 

1.0 

1.5 

2.0 

NE 

R1? 

R1? 

Direct Arrival/ 

Refraction 13200 ft/s 

2970 ft 1980 ft 990 ft 990 ft 1980 ft 2970 ft 

R2? 

S-wav e dir ect/refracte d arri va l

Recording Time (seconds) 

Figure 7. Data from 7 shots recorded on Profile 4, sorted by shot-receiver offset distance. White 

arrow is at the mid-point of receiver spread profile. Refraction and direct arrival energy are the first 

coherent events seen near the beginning of recording time. A possible reflection is labeled R1. A second possible deeper reflection or refraction is seen above the S-wave direct/refracted arrival. 

18

0.0 

0.5 

1.0 

NW SE

R1? 

R1? 

Direct Arrival/ 

Refraction 

10230 ft/s 

2970 ft 1980 ft 990 ft 990 ft 1980 ft 2970 ft

R2? 

1.5 

2.0 

Figure 8. Data from 5 shots recorded on Profile 5, sorted by shot-receiver offset distance. White 

arrow is at the mid-point of receiver spread. Refraction and direct arrival energy are the first coherent 

events seen near the beginning of recording time. A possible reflection, R1, is believed to be from the 

sediment/bedrock boundary. A second possible deeper reflection or refraction, R2, is probably from 

an inter-bedrock boundary. 

R

e

c

ording 

Time (s

e

c

o

n

d

s) 

19

continuous, rough appearance of the direct/refraction arrival is indicative of this problem. 

Nonetheless, a possible reflection is interpreted at offsets beyond 1000 ft. This event, 

again labeled R1, arrives at significantly different times on the western and eastern ends 

of the profile. Even though elevation timing corrections were made (Table 2), much of 

this difference may be due to poor topographic data. This event averages out to be from a 

boundary at roughly 800 ft depth. We do not have much confidence in the accuracy of 

this value. 

DISCUSSION AND CONCLUSIONS 

A direct comparison between the data from the northwestern half of profile 1 and 

a best-fit synthetic model are shown in Figure 10. The model consisted of a combined 

650 ft of soil, permafrost, and sediment over bedrock. The bedrock reflection beneath the 

shot on the synthetic record is clearly visible. On the real data it is not, primarily because 

of coherent and random noise in the real world that is not predicted by the numerical 

simulation. In general, seismic events such as the direct/refracted P- and S- waves match 

up fairly well between the real and synthetic data. Events labeled Rb and Rf on the 

synthetic data are known to be from the bedrock interface from the model. These events 

align with similar events in the real data. We therefore believe these events in the real 

data are from bedrock, as interpreted. 

In the case of Profile 4, at the Campion air strip, synthetic modeling corroborates 

the assumption that permafrost is thicker than at the previous sites (Figure 11). Profile 4 

was selected for this comparison because it is in an area where the seismic events are 

different than at Profiles 1 and 2. The model consists of 450 ft of permafrost compared 

20

0.0 

W E 

1980 ft 990 ft 990 ft 1980 ft

R1? 

R1? 

Direct Arrival/ 

Refraction 12200 ft/s 

0.5 

1.0 

1.5 

2.0 

Figure 9. Data from 4 of 5 shots recorded on Profile 6, sorted by shot-receiver offset distance. White arrow is at the mid-point of receiver spread. Refraction and direct arrival 

energy are the first coherent events seen near the beginning of recording time. A possible 

reflection or refraction is labeled R1. Data quality was lowest on this profile because of 

line geometry problems. 

R

e

c

ording 

Time (s

e

c

o

n

d

s) 

21

R

ecording Time (seconds) 

0.0
0.2
0.4
0.6


Figure 10. Comparison between a synthetic shot and half of data acquired along Profile 1. Synthetic model consisted of 100 ft of permafrost and 550 ft of sediment over bedrock. Both P- and S-wave direct arrival/refractions match reasonable well, as does the interpreted bedrock reflection (Rb) and refraction (Rf) . Bedrock depth in model of 650 ft is consistent with observed events in data. 

Colorbar (seismic wave amplitude) + – 

P-wave 

Direct Arrival/ 

Refraction 

Rb 

Direct S-wave 

D ri ec

St w- a

e v

Rf 

Rb 

Rb 

Shot point Shot point 

Synthetic Real 

22

with 100 ft in the model of Figure 10. Unfrozen sediment thickness is 150 ft below 

permafrost and overlying bedrock. As seen in Figure 11, the general pattern of arrival 

times in the real data matches up with arrivals in the synthetic data. The thicker 

permafrost causes the bedrock reflection Rb to arrive much sooner in time while any 

possible bedrock refraction is masked by the direct and refracted P- and S- waves 

travelling through the permafrost and sediments. 

A cross section of interpreted bedrock depth is presented in Figure 12. The 

interpreted depth is projected on to a cross section line from the midpoint of each receiver 

array for Profiles 1 through 5. Datum for the cross section is set at 150 feet, roughly the 

mean elevation at Profile 1. Bedrock does not appear to get shallower than approximately 

550 feet in the immediate vicinity of Galena, beneath Profiles 1, 2, or 3. It appears to 

shallow somewhat toward the southeast of Galena, beneath profile 5, but the evidence for 

this is not strongly compelling given the complexity of this data set. Even vertically 

exaggerated 4:1, the interpreted surface is quite flat overall. It is important to remember 

that what has been interpreted as bedrock may be simply a high-velocity unit within the 

sediment package or within a sequence of rock. Further, the velocity contrast at the lateTertiary (?)/Quaternary sediment and rock boundary may not generate an interpretable 

reflection or refraction large enough to image. Of potential interest, the bedrock 

velocities appear to be sufficiently high to suggest the existence of a non-sedimentary unit 

such as a volcanic or igneous rock. Both rock types have been mapped regionally along 

the Yukon River (about 12 miles both west and northeast of Galena), so their existence in 

the vicinity of Galena may not be surprising (Bradley, 1938; Cass, 1959). 

Because of the complexity of the near-surface velocity structure (from the 

23

R

ecording Time (seconds) 

0.0
0.2
0.4
0.6


permafrost over 150 ft of sediment underlain by bedrock. Both P- and S-wave direct arrival/refractions match reasonable well, as does a bedrock reflection (Rb). This give us some confidence that permafrost is thicker at this site and model bedrock depth is reasonable. 

Comparison between a synthetic shot and half of data acquired at Profile 4 location. Synthetic model consisted of 450 ft of 

Colorbar (seismic wave amplitude) + – 

P-wave 

Direct Arrival/ 

Refraction 

Rb 

Direct S-wave D ri ec

St w- ave 

Shot point Shot point 

Synthetic Real 

Figure 11. 

24

Cross section showing depth to interpreted bedrock surface projected from midpoint for receiver arrays of profiles 

Cross section has vertical exaggeration of 4:1 and elevations are projected on to a datum of 150 feet elevation. 

Cross section Map View 

150 feet 

Profile 4 

Profile 5 

Profile 2 

Profile 3 Galena 

bedrock sur af ce 

Galena 

cross section line 

Profile1 

Profile 2 

Profile 3 Profile 4 Profile 5 

1 mile 

Profile1 

1 through 5. Figure 12. 

25

permafrost and saturated sediment layers), as well as the unknown contrast between the 

sediments and the Tertiary or Cretaceous bedrock, we believe an oil-industry-style 

seismic reflection survey would be an appropriate method to image the necessary targets 

to answer the problem. However, its cost might be prohibitive given that several 1000-

foot holes could probably be drilled for the cost of an industry reflection survey. 

Acknowledgments 

We thank Todd Dallegge (University of Alaska, Fairbanks) and Valerie Webb 

(student intern with the Alaska Division of Geological and Geophysical Surveys) for their 

help in data acquisition. We gratefully acknowledge Doyon Native Corp. for permission 

to do the survey and for access to their lands. Thanks very much to the City of Galena for 

supplying vehicles, equipment, and personnel for this project. We greatly appreciate the 

efforts of Marvin Yoder in making this investigation come together. Special thanks also 

to Gordie Cruger for his hospitality during our stay in Galena. This work was performed 

in cooperation with the Alaska Division of Geological and Geophysical Surveys. 

REFERENCES 

Bradley, A.J., 1938, Unconsolidated sediments and topographic features of the lower 

Yukon valley: Geol. Soc. of America Bull. v. 49, no. 2, p303-341. 

Cass, J.T., 1959, Reconnaissance geologic map of the Nulato quadrangle. alaska: U.S. 

Geol. Survey Miscellaneous Investigation Map I-291, 1 sheet, scale. 1:250,000. 

Larsen, S. (1992). E3D: 2D/3D Elastic finite-difference wave propagation code, Regents 

of the University of California, 29 p. 

Patton, W.W.Jr. and Hoare, J.M., 1968, The Kaltag Fault, West-Central, Alaska: 

Geological Survey Research, U,S. Geol. Survey Prof. Paper 600-D, D-147-153. 

26

Pewe, T.L., 1948, Terrain and Permafrost, Galena Air Base, Galena, Alaska: U.S. Army 

Corp of Engineers, Permafrost Program Progress. Report No. 7, 52 p. 

Nakanishi, A.S. and Dorava, J.M., 1994, Overview of environment and Hydrogeologic

Conditions at Galena, AK: U.S. Geological Survey Open-File Report 94-525, 18 p.

Telford, W.M., Geldart, L.P., and Sheriff, R.E., 1990, Applied Geophysics, Cambridge 

University Press, New York, p. 16, 237–262. 

27

APPENDIX

Shot records from six seismic profile locations near the City of Galena, Alaska 

28

source on SE source in middle source on NW source 300 m off source 300 m off 

end of spread of spread end of spread SE end of spread SW end of spread 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

590 m (1970 ft.) 

Figure A1. Shot records from Profile 1, north of the Galena Airport. Arrow indicates source location for each record. 

Sixty receivers were spaced every 10 m over 590-m-long profile. Black represents upward ground motions, white are downward ground motions, and gray are motions at or near zero displacement. Data have been filtered and gaincorrected (see Table 2). 

) sdnoces( e mit g nidroceR

{ 

29

source on W source 300 m off source in middle source on E source 300 m off 

end of spread W end of spread of spread end of spread E end of spread 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

590 m (1935 ft.) 

Figure A2. Shot records from Profile 2, along bike path in Galena. Arrow indicates source location for each record. 

Sixty receivers were spaced every 10 m over 590-m-long profile. Black represents upward ground motions, white are downward ground motions, and gray are motions at or near zero displacement. Data have been filtered and gaincorrected (see Table 2). 

) sdnoces( e mit g nidroceR

{ 

30

source on SE source 300 m off source 300 m off source in middle source 600 m off source 300 m off source on NW 

end of spread SE end of spread SE end of spread of spread NW end of spread NW end of spread end of spread 

0.0 0.2 

0.4 0.6 0.8 1.0 1.2 1.4 1.6 

1.8 2.0 

590 m (1935 ft.) 

Figure A3. Shot records from Profile 3, roughly midway between profiles 2 and 5. Arrow indicates source location for 

each record. Sixty receivers were spaced every 10 m over 590-m-long profile. Black represents upward ground motions, white are downward ground motions, and gray are motions at or near zero displacement. Data have been filtered and gain-corrected (see Table 2). 

) sdnoces( e mit g nidroceR

{ 

31

source on N source 300 m off source 300 m off source in middle source on S source 300 m off source 600 m off 

end of spread N end of spread N end of spread of spread end of spread S end of spread S end of spread 

0.0 0.2 

0.4 0.6 0.8 1.0 1.2 1.4 1.6 

1.8 2.0 

{ 

590 m (1935 ft.) 

Figure A4. Shot records from Profile 4, at Campion air strip. Arrow indicates source location for each record. Sixty receivers were spaced every 10 m over 590-m-long profile. Black represents upward ground motions, white are downward ground motions, and gray are motions at or near zero displacement. Data have been filtered and gain-corrected 

(see Table 2). 

) sdnoces( e mit g nidroceR

32

source 300 m off source on S source in middle source 300 m off source on N 

of spread end of spread S end of spread N end of spread end of spread 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

590 m (1935 ft.) 

Figure A5. Shot records from Profile 5, south of Profile 4. Arrow indicates source location for each record. Sixty 

receivers were spaced every 10 m over 590-m-long profile. Black represents upward ground motions, white are downward ground motions, and gray are motions at or near zero displacement. Data have been filtered and gaincorrected (see Table 2). 

) sdnoces( e mit g nidroceR

{ 

33

source in middle source on W source 300 m off source on N source 300 m off 

of spread end of spread W end of spread end of spread N end of spread 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

590 m (1935 ft.) 

Figure A6. Shot records from Profile 6, southwest of Profile 5. Arrow indicates source location for each record. Sixty 

receivers were spaced every 10 m over 590-m-long profile. Black represents upward ground motions, white are downward ground motions, and gray are motions at or near zero displacement. Data have been filtered and gaincorrected (see Table 2). 

) sdnoces( e mit g nidroceR

{ 

34