T-1842 AN ACTIVE SEISMIC RECONNAISSANCE SURVEY OF

T-1842
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AN ACTIVE SEISMIC RECONNAISSANCE SURVEY
OF THE MOUNT PRINCETON AREA
CHAFFEE COUNTY, COLORADO
*•'><
by
James Scott Crompton
ARTHUR LAKES LIBRARY
COLORADO SCHOOL of WINES
On: iv~v COLORADO SOtfr
T-1842
ABSTRACT
An active seismic-reconnaissance survey was conducted of the
Mount. Princeton area, near Buena Vista, Colorado in the fall of 1975.
The survey was designed to monitor large mining blasts from nearby
mining operations at Climax and Monarch Pass, Colorado and Questa,
New Mexico and to interpret these observations using refraction techniques. The results indicate the importance of examining apparent azimuths of energy paths into a local array and comparing these azimuths
with the known source-receiver geometry. Conclusions from this survey suggest the existence of lateral refractions from boundary features
of the upper Arkansas Valley graben, correlating with the extension of
the Rio Grande Rift as far north as Leadville, and a fast time residual
due to a probable normal fault in the subsurface of Chalk Creek, south
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of Mount Princeton.
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This survey demonstrates that a modified version of the crustal
\
refraction technique can be a very cost-effective means of surveying
the velocity and structural character of a large project area.
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TABLE OF CONTENTS
Page
ii
Abstract
List of Figures and Tables
iv
Introduction
1
Acknowledgments
3
Regional Geologic Setting
4
Geologic History
8
Scope of Investigation and Operations
H
Equipment
17
Observations
24
Interpretation
41
Evaluation of Active Seismic Technique
52
Conclusions
56
Recommendations
57
Bibliography
:
iii
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58
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LIST OF FIGURES AND TABLES '
FIGURES
'
Page
1
Location Map
5
2
Geologic Cross Section
7
3
Station Location Map
4
Schematic Diagram of Equipment
17
5
Frequency Characteristics
19
6
WWVB Code
20
7
Teleseism USSR 294-1232
22
8
Local Event 294-2051
22
9
Climax Event 295-2113
23
10
Refraction Profile Teleseism USSR
28
11
Refraction Profile Teleseism Kuril Is '»
28
12
Refraction Profile Teleseism Mexico
29
13
Refraction Profile Elkhead Mtns
31
14
Refraction Profiles Regional WNW
32
15
Refraction Profile Climax Refracted Azimuth . . . .
34
16
Refraction Profile Climax Geographic Azimuth. . .
35
17
Refraction Profile Monarch Pass
37
18
Amplitude versus Distance curve
39
19
Three Station Apparent Velocity Vector
45
20
Lateral Refractions, Buena Vista
46
21
Arrival Time Residual Data
49
22
Velocity and Structure Models
49
.
iv
15
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TABLES
Page
1
Station Coordinates
16
2
Recorded Events . .
25
3
Local Events
40
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INTRODUCTION
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The seismic refraction technique has been used for many years
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to obtain subsurface velocity and structure information. This tech-
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nique was exploited extensively in the 1960's as part of the VELA
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Uniform program of crustal studies in the western United States and
;;
in the design of the large aperture seismic array (LASA) in eastern
Montana. With the availability of nuclear test explosions and large
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mining blasts, large areas were surveyed using equipment designed
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specifically for this purpose (Warrick et al, 1961; Jackson and Pakiser,
1965; Pakiser, 1963; Jackson, Stewart and Pakiser, 1963; Capon, 1974).
Because many different geophysical and geologic measurements
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can be made, exploration decisions must be made based on the strengths
and weaknesses of each technique. The very real limitations of eco•^
nomics , available equipment and time restrict these decisions even
further.
Survey methods
which can investigate a large prospect area
and delineate local features for further study provide an acceptable
compromise between the conflicting demands of data acquisition and
cost. One such survey method, the active seismic reconnaissance
technique, is discussed in this thesis.
MOUNT PRINCETON SURVEY
Between October 20th and October 24th, 1975, an active seismic
reconnaissance survey was conducted of the Mount Princeton area,
near Buena Vista, Colorado. This project was designed as a field
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experiment to test a modified version of the crustal refraction program
used in the VELA Uniform studies by recording mining shots from several locations and interpreting arrival times as a refraction profile.
The independent operations of the microearthquake equipment used
allowed 24-hour continuous monitoring and good areal coverage over
a large project area. Real time interpretation of data allowed for relocation of stations away from problem locations thus improving the
data quality in the field.
The following is a list of the conclusions from this study:
1) The active seismic reconnaissance technique succeeded in
surveying the large Mount Princeton project area by^delineating the
regional structure and by singling out specific local targets for further
investigation.
2) Interpretation of apparent-azimuth data from several sources
indicates that lateral refractions occur from both the east and west
side of the upper Arkansas River Valley.
3) Within the project area, a fast time residual exists, due to
a probable normal fault in the subsurface, corresponding to the Chalk
Creek drainage feature.
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3
ACKNOWLEDGEMENTS
I thank the members of my committee, Dr. David Butler, Dr. Eric
Engdall, and Dr. F. Richard Yeatts, and especially my advisor, Dr.
Phillip Romig for their help and support. This research was made
possible through a contract between AMAX Exploration and MicroGeophysics Corporation to conduct the active seismic experiment at Mount
Princeton. I also appreciate the use of equipment, funding and valuable advice from Art Lange, AMAX Exploration and Dr. Butler and
Paul Larry Brown of MGC. Special thanks to Ed Torrgeson of AMAX
Exploration and Cal Brown at the Climax mine, who were most responsive to requests for information concerning times of blasting at
Climax-and Questa, New Mexico.
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REGIONAL GEOLOGIC SETTING
The Mount Princeton prospect area is located in south-central
Colorado, approximately 150 km southwest of Golden (see figure l^,
location map). Topographically, the area lies on the border of the
Sawatch Range and the upper Arkansas River Valley, between Buena
Vista and Salida in Chaffee County.
The upper Arkansas River Valley, extending north from Salida to
Leadville, is believed to be a continuation of the Rio Grande Rift
Zone (Knepper, 1974). The rift feature, which can be traced from
\
northern Mexico through central New Mexico into central Colorado,
approximately 960 km, has formed since mid-Tertiary time by the
relative westward movement of the Colorado Plateau plate with respect to the Great Plains plate. This created a zone of crustal ex'-^
tension superimposed on the previous tectonic foundation (Grose,
1974). In more recent geologic history, the uplift of the Sangre de
Cristo Range caused a separation of the rift zone in Colorado. The
tilted fault block at Poncha Pass now divides the upper Arkansas
Valley from the San Luis Valley (Knepper, 1974).
The upper Arkansas Valley is a narrow, north-trending, downdropped trough bounded by steeply dipping normal faults. This struc
tural valley is oounded on the west by the Pre-Cambrian igneous and
metamorphic core sequence of the Sawatch anticline. The present
topographic expression of the anticline is the Sawatch Range, which
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includes many of the highest peaks in the Southern Rocky Mountains.
Mount Princeton, elevation 4',328 m (14,197 feet) above sea level, is
one-of-the largest of a series of Tertiary intrusives which cut the PreCambrian rocks in the southern part of the Sawatch Range. The boundary fault on the western edge of the trough is generally a simple, narrow
fault zone. Estimates of the displacement on this fault, based on the
existing topography and the assumption of a late Eocene erosional surface, range from 1500 to 3000 m.
The east side of the trough is bounded by the southern extension
of the Mosquito-Ten Mile Range. In the prospect area, this range degenerates into a rugged highland of predominately Pre-Cambrian igneous
and metamorphic rocks overlain by upper Tertiary volcanic flows and
pyroclastic rocks. The eastern boundary of the trough is characterized
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by a more complex system of parallel normal faults with the major displacement attributed to a fault buried underneath Tertiary and Quaternary sediments approximately paralleling the present course of the
Arkansas River drainage (limbach, 1975; Knepper, 1974) (see figure _2,
geologic cross section).
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EAST
WEST
MOUNT PRINCETON
INTRUSIVE
ARKANSAS RIVER
core of the
SAWATCH ANTICLINE
MOSQUITO-TEN M^LE
HIGHLANDS
(ARKANSAS HILLS)
ALLUVIAL - - FILL
COLORADO PLATEAU
PLATE
FIGURE 2
RIO GRANDE RIFT
GREAT PLAINS
PLATE
GENERALIZED GEOLOGIC CROSS SECTION
UPPER ARKANSAS VALLEY
The following section presents a brief geologic
history of the prospect area with particular emphasis
on the structural and petrographic features which could
have a significant seismic expression and therefore
present keys to understanding the parameters of an active
seismic investigation in this locale.
T-1842
8
GEOLOGIC HISTORY
The Pre-Cambrian history'of the area is separated into four stages
(Knepper, 1974). First, clastic sediments with minor amounts of carbonate material were deposited. Later these rocks were folded and
recrystallized. At the same time, approximately 1.7 billion years ago,
minor basalt flows were extruded. The third stage is characterized by
the intrusion of a fine grained suite of rocks ranging in composition
from granite to quartz monzonite. Finally, regional uplift and erosion
brought a close to the series of events which produced the rigid basement complex.
In early to middle Paleozoic time, the area was part of a structural
low in the Trans-Continental Arch. This structure was between the
active Cordilleran geosyncline to the west and the stable craton to the
east. Geologic input to the region was restricted to shallow marine
sedimentation.
Later in the Paleozoic era, the central Colorado trough, between
the ancestral front range and the Uncompahgre Highland, dominated
the region. In late Pennsylvanian time, an uplift changed the marine
trough into an alluvial valley. In the Mesozoic, the area remained a
positive element subject to erosion until the transgression of the
Cretaceous Sea.
The Paleozoic-Mesozoic sequence of rocks is now either missing
or buried beneath the recent sediments of the upper Arkansas Valley
I
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9
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graben but as much as 3000 m of sediments may have been deposited
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during this time span.
Late Mesozoic to early Cretaceous geologic activity was domit
nated by the Laramide orogeny. Of particular importance to the tectonic framework of the project area was the uplift and folding characteristic of the formation of the Sawatch anticline (72 million years ago).
The uplift created an eastward dipping, late-Eocene, erosional surface,
resulting in the removal of previous sediments and the exposure of the
Pre-Cambrian core of the anticline.
Stages of volcanism and intrusion signalled the evolution of a regional, tensional stress field which opened pre-existing fractures and
created new ones allowing magma to rise into a shallow~crust. The existence of two stages of intrusive activity is indicated by the quartz monzonite batholith of Mount Princeton and the younger granitic intrusive
at Mount Antero. The separate stages resulted
from
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differ-
entiation of a single magma source rather than from two independent
sources. The extrusive sequence in the area (age Eocene to early
Oligocene) is flow and pyroclastic material from a volcanic source to
the southwest, possibly from the Bonanza District (Knepper, 1974).
The development of the Rio Grande Rift and the associated block
faulting and uplift dominated late Cenozoic geologic history of the project area. The northernmost expression of the rifting, the upper Arkansas
Valley graben, developed in five recognizable stages. The following is
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a summary of these stages:
1) mid to late Miocene, the structural outline of
the graben was formed,
2) late Miocene to Pliocene, the graben feature was
enhanced by increased sedimentation,
3) Pliocene, the rifting and uplift continued with
faulting of the sediment section,
4) Pliocene to Pleistocene, the Sangre de Cristo
horst was uplifted and a sediment block was
tilted to form the Poncha Pass topographic feature,
separating the San Luis Valley from the upper
Arkansas Valley,
5) Holocene, minor faulting continuing through
present.
Cenozoic sedimentation includes the Miocene Brown's Canyon
Formation (floodplain and lake deposits), the Miocene to Pliocene
Dry Union Formation (basin fill deposits) and Pleistocene glacial
deposits.
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11
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SCOPE OF THE INVESTIGATION AND OPERATIONS
Velocity information and structural interpretations which can be
obtained from an active seismic survey have become increasingly im-
if
portant in the integrated geologicaland geophysical solution of exploration problems. Economic difficulties, time limitations and
equipment availability are problems faced by all, and serious consideration must be given these parameters when evaluating potential
techniques which could be used to evaluate a prospect area.
The important parameter involved in this technique is the scale
of the investigation involved. Prospecting for targets in the case of
geothermal energy and mineral exploration requires evaluation of large
;
prospect areas. Another study along this line is the environmental
1
analysis of earthquake and other geologic hazards in regard to an
engineering construction project.
Studies of this nature require a
reconnaissance technique which can fulfill the limitations stated
previously.
This survey was designed to take advantage of existing sources
of seismic energy. These include blasting at local mining operations,
in particular: 1) the molybedenum mine at Climax, Colorado, approximately 70 km directly north of the prospect area; 2) the limestone
quarry at Monarch Pass, Colorado, 25 km 30° southwest of Mount
Princeton; and 3) the molybedenum mine at Questa, New Mexico, 275 km
due south of the array.
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Absolute origin times of the man-made events were not known
but approximate times were supplied by the mine operators . The
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events recognized as mining shots were identified on the basis of
this information as well as confirming evidence from apparent azimuths and s-p arrival times.
Each of the ten stations in the survey consisted of a Sprengnether
Instrument Company MEQ-800B microearthquake system. These seismographs are equipped with the capability of smoked paper recording,
comparison of internal time with WWVB and arrival time resolution to
an accuracy of better than -30 ms. The independence of these systems
makes them ideal for use in a reconnaissance, large-scale seismic
survey. The 24-hour recording capability of the equipment allows the
recording of natural as well as man-made seismic events so a passive
microearthquake survey can be conducted in addition to an active seismic
program.
Due to the fact that the mining operations are roughly north and
south of the prospect area, the recording stations were deployed in
approximate north-south lines to allow interpretation of the arrival
times as an in-line conventional refraction profile. Line A was established on October 20-21st in the upper Arkansas River Valley. After
a full day of ten-station operation, the equipment was moved westward
to the mountain front to establish Line B (see figure _3_, page 15). Three
stations were common to both lines and acted as ties between the
lines.
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13
The survey was designed, with the independent recording systems
and the two separate line geometries,to monitor any structural differences'within the graben and horst framework. Geophone plants in
Line A were, by necessity, in unconsolidated alluvium. Although precautions were taken to avoid sources of noise such as wind and cultural activity, low gains were unavoidable because the unconsolidated
subsurface material caused high background noise levels. Stations in
Line B were located on bedrock in locations consistent with the geometrical requirements of the refraction profile and access considerations . The smoked paper records allowed an evaluation in real time
of each station every 24-hour period. Bad stations (noisy, distorted
earthquake signatures, low gain, or ringing frequency response) were
moved to improve the quality of the data. Figure J3 and table_1 show
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station locations and list their coordinates.
A summary of the field operation follows.
October 20th
arrived in.Buena Vista; set up two
stations to calibrate recording parameters to Climax shot, established
three additional stations.
October 21st
set up additional five stations to
complete line A, relocated two
stations for better geometry.
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14
October 22nd
relocated seven stations along the
mountain front to establish line B,
left stations #3, 8, 11 as ties for all
further operations.
October 23rd
relocated station #14 for better gain,
removed station #19 due to operational
problems with snow.
October 24th
picked up equipment and returned to
Golden.
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T-
COTTONWOOD CREEK
FIGURE 1
STATION LOCATION MAP
Mount Princeton area, Chaffee County,Colorado
II
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16
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Table 1
Station Location
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X
Station #
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Y
Z
Line
Geometry
Distance
From Climax
1
2
3
+11.8
+ 6.7
+0.2
A
61.4
+11.2
+ 4.6
+0.2
A
63.5
+14.3
+0.8
+0.0
4
+12.2
+ 2.6
+0.1
5
+12.7
-0.2
+ 0.1
6
+ 6.7
-6.3
+0.2
7
+ 9.2
-3.3
8
9
+16.4
67.2
Base
65.4
-0.2
A
A
A
A
-2.8
-0.1
B
70.8
+15.4
+14.6
+0.1
A
53.4
10
+13.7
+12.2
+ 0.1
55.8
11
+12.2
-0.2
+0.1
12
+10.1
+3.1
+ 0.2
A
A
A
, +15.6
-7.3
-0.3
B
75.3
14
15
+16.6
-4.1
+0.0
B
72.1
+15.5
-1.3
-0.1
B
69.3
16
+18.5
+ 6.6
+0.0
61.6
17
+18.9
+9.1
-0.1
B
B
18
19
20
+19.3
+17.8
-0.2
B
50.5
+18.1
+0.9
-0.5
B
67.2
+17.3
-4.6
-0.1
B
72.7
13
Origin at 38°45'N
106 OO'W
Elevation datum at 8570' (Station #3)
*
+X is west and +Y is north
+Z is down
Coordinates in kilometers
68.2
74.7
71.5
-—-
68.2
65.0
59.1
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EQUIPMENT
Ten independent microearthquake recording systems were employed in the active seismic program at Mount Princeton. Each
system includes a Mark Products L-4C vertical seismometer and a
Sprengnether MEQ-800B visual drum recorder with an internal timing
system synchronized to Universal Coordinated Time (UTC) by means
of the radio reception of WWVB. Figure ^ is a schematic diagram
of the microearthquake recording system. A detailed
explanation
of the individual components is given below.
MARK_PRODUCTS_
1 HZ VERTICAL
SEISMOMETER
SJPRENGNETHER _MEQ^800
AMPLIFIERS
•
FILTERS 1
INTERNAL
TIMING
{
TRUEJTIME
WWVB RECIEVER
VISUAL DRUM RECORDER
FIGURE 4_
SCHEMATIC DIAGRAM of
EQUIPMENT
USED IN SURVEY
Seismometer— The Mark Products L-4C seismometer is a onehertz natural-frequency vertical seismometer. The open-circuit damping is 0.6-critical and the L-4C has an output of 2.7 volts per cm per
second of particle velocity.
Seismograph— The Sprengnether Instrument Company MEQ-800B
is a visual microearthquake recorder. The smoked paper drum recorder
n
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18
was set to a nominal 120 mm per minute rotation speed for this survey,
resulting in a 1 mm spacing between succeeding traces. The amplifier
has-a maximum voltage gain of 120 db and selectable high and low cut
i
filters. Because this survey was designed to monitor mining blasts
from a distance of greater than 50 km, the high cut filters were set at
10 hertz and the low-cut filters were removed. The amplifier gains can
be changed by precise 6 db steps between 60 and 120 db. Individual
station gains ranged between 78 and 96 db depending on local geologic
conditions.
The maximum pen deflection was set at -25 mm. ^though
periods of high winds obscured records at some stations, efforts to avoid
natural and man-made noise were generally successful and relatively
clear records were obtained.
The internal timing system consists of a clock whose drift rate is
stable to less than one part in 10 (about -10 ms per day). The internal
clock was compared with WWVB at the beginning and end of each record
day and adjusted correspondingly in 16 ms increments to agree with
the standard to about -8 ms . Time is displayed on the visual record
by a slight deflection of the pen each second, a two mm long deflection
step each minute, and a four mm long step each hour.
The frequency characteristics of the instrument are shown in figure
j>. From this figure, the essentially flat system response to velocity
over a selected frequency range can be seen.
SENSITIVITY
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WWVB— WWVB is the radio call code for the National Bureau of
Standards GO.kilohertz time-standard broadcast from Fort Collins,
Colorado. This time standard was used to synchronize the internal
timing clocks of the microearthquake recording systems. The WWVB
code (as shown in figure 6) was recorded at the beginning and end of
each record day as an absolute time and date identification for the
record.
-REFERENCE TIME
.m
i
00
10
Figures
000
00
15
-CORRECTION (MSEC. TO UT2 )-
-OAY OF Y E A R -
-HOURS-
-MINUTES-"
20
WWVB
25
TIME
CD <»•
30
+
35
CODE
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hn.
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45
50
55
FORMAT^
The MEQ systems were synchronized daily with WWVB by comparing (on an oscilloscope) the beginning of the WWVB second pulse with
the internal-clock generated second pulse. Adjustment of the internal
s
clock kept agreement with the standard to within acceptable limits.
Significant deviations with standard time were noted and appropriate
corrections made to arrival times. Common drift corrections were on
the order of 30 ms per day.
Data-- Figures 7_ thru j) are examples of records from the Mount
Princeton survey illustrating the seismic signatures from sources at
three different distances. Figure 7_ is a sample recording of a nuclear
test in the USSR at Nova Zemyla (73.04 N and 54.52 E) approximately
60
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21
7500 km to the north-northeast. A local event, within the prospect
-
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area, is shown in figure _8. Figure _9 is an example of the signature
of a Climax shot, from a distance of about 70 km due north. The
smoked paper records can be picked under magnification to a precision
of better than -30 ms.
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FIGURE 7_
TELESEISM , NOVA ZEMYLA, USSR,
Dist 69°
date 294 , time 1232 , station #5 , gain 84 db , filters 5 -10 hz
nuclear test ,approx. Mag 6.4M, 73.04 N 54.52 E (alluvium)
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2051
FIGURE 8
LOCAL EVENT and CLIMAX SHOT
date 294 ? station #11 gain 90 db ^ filters
Q-10 hz (alluvium)
local event r time 2051 epicenter +12.8, 0.0 t near sta #11
Climax event ^ time 2100 f dist 70 km f mining blast
23
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24
OBSERVATIONS
During the four days of field work, 17 seismic events were recorded. Of these, eleven were of sufficient quality (recognized and
timed on at least five stations) to be used in the interpretation scheme.
Three of these events were identified as teleseisms, three as nearregional events and the remaining five were identified as mining blasts,
Four of the blasts were from Climax and one was from Monarch Pass,
Conspicuous by their absence are the events expected from the
Questa mine. Times for five of these events during the field survey
were known by communication with the mine operator but the events
could not be identified on the records. Table _2 lists the recorded
events.
Figures K) thru 17 represent the time-distance plots of the events
used in the refraction analysis. Normally, the travel time of an event
to each station is plotted against the true or straight-line distance to
the source. For large source-receiver distances, such as those in
this survey, the straight-line distance from the source to the station
may not reflect the true travel path. The ray path azimuth associated
with a wave front into a local array may be considerably different than
the geographic azimuth due to lateral changes in velocity somewhere
along the travel path. These lateral changes could be associated with
regional geologic structures which in effect focus incoming energy into
or away from a local array.
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T-1842
25
TABLE 2
RECORDED EVENTS
date
time UTC
(Julian Day)
293
293
294
294
294
294
294
295
295
295
295
295
295
296
296
296
297
2109
2110
0453
1232
1723
2051
2100
0301
0521
1324
2113
2202
2252
0125
2045
2355
0100
# of sta
in fit
location
dist.from array
origin
2
1
5
5
4
3
7
5
7
1
5
7
7
5
7
9
9
Climax
(unknown)
local (near sta
Nova Zemyla
local (Arkansas Hills)
local (near sta #5)
70 km
Climax
Elkhead Mtns
Kuril Is.
local (near sta #8)
Climax
Elkhead Mtns
Climax
Southern Mexico
Monarch Pass
Climax
Regional WNW
local
69°
local
local
70 km
230 km
21°
local
70 km
230 km
70 km
28°
25 km
70 km
100 km
TELESEISMS
date
294
295
296
^
location
from NEIS*, Golden, Co
dist-from array
73.04 N 54.52 E USSR
47.53 N 148.74 E Kuril Is
15.83 N 94.63 W Mexico
69°
21°
28°
11 59 55.5
05 09 51.6
01 19 58.1
*NEIS - National Earthquake Information
Service
REFRACTION SHOTS
date
Climax, Colorado
time
294 2100
295 2113
295
2252
296
2355
Monarch Pass, Colorado
296
2045
All coordinates are in kilometers unless otherwise noted,
location of local events are referenced to the array origin
at 38 .75° N and 106.00° W
T-1842
26
To investigate the possible significance of lateral refractions,
apparent azimuths were calculated for all of the events used in the
interpretation. The apparent azimuth determination comes from a
simple three-station technique, analogous to the strike and dip problem
in structural geology.After making the assumption of plane waves,
knowledge of the coordinates and arrival times at three stations uniquely determines the direction and apparent velocity of the incoming
wave (see figure _19_, page 45). By taking all possible combinations
of three stations from the available data, the apparent azimuth from an
event can be determined with some certainty.
Taking the apparent azimuth data into consideration, the distances
plotted in the following figures are the relative distances between
stations as projected on the normal to the impinging wave front. The
origin time of the events is not generally known to any degree of accuracy, therefore, the travel times used in the profiles are relative to an
assumed zero time.
The apparent-velocity lines, labelled HT on the refraction profiles
were taken from the 1972 Travel time tables (Herrin, et al, 1972) and
assume a normal crustal model. The lines labelled MP (for Mount
Princeton) are least squares fits to the observed data and are included
where the expected apparent velocity lines do not match the observations
Interpretation of the data is based on deviations, in the form of time
residuals, from the normal crustal model and on comparison of apparent
T-1842
27
azimuths with the known shot-receiver geometry.
The uncertainties in time resolution for arrival picks as well as
difficulties in combining separate shots from the same location, with/
out absolute origin times, are important in the discussion of accuracy
in the interpretation. Estimates of the picking accuracy and multiple
picks of arrivals from duplicate shots indicate the uncertainty in determining the time residuals to be on the order of -50 ms.
The results of each plot are discussed in the following section.
Included with each figure is information concerning the source of the
event, expected phase velocity and true and apparent azimuths.
Teleseisms — Figures 10_ thru 12^ are time-distance curves of the teleseisms recorded during the survey.
Figure]^ shows arrivals from a
nuclear test from Nova Zemyla, USSR. This event corresponds in
amplitude and, therefore, energy release to a natural event with a
body wave magnitude of 6 . 4 M . A phase velocity of about 21 km/sec
fit to the data is higher than the expected (HT) velocity of 17. 7 km/sec.
A significant residual is measured only at station #4. The residual is
on the order of 150 ms and may be due to irregularities in the local
geology and the predicted timing uncertainty.
Figure 11 is a profile from a natural event from the Kuril Islands.
The expected phase velocity of 18 km/sec is shown and fits well with
the observations. A significant residual occurs at station #3 , which
is about 350 ms slow.
o 57
0)
c
•H
w
s
H
EH
56
0
RELATIVE DISTANCE
FIGURE 10
. „
in
Km
REFRACTION PROFILE TELESEISM
USSR
18 km/sec
HT
11
u
0)
w
s
g 10
M
20
RELATIVE DISTANCE in Km
FIGURE ±1
REFRACTION PROFILE TELESEISM
KURIL IS.
29
T-
22
16
21
S. MEXICO
296- 0125
dist 28°
az due S
u
0)
19
W 20
11
H
EH
•4
15
H
K
HT
19
12 km/sec
18
10
20
RELATIVE DISTANCE in Km
FIGURE 12
REFRACTION PROFILE TELESEISM
30
S. MEXICO
T-1842
.
30
The apparent velocity from an event from southern Mexico,
(figure 12) shows good agreement with a phase velocity of approximately 9 km/sec. This velocity is slower than the expected velocity
of 12 km/sec calculated from the normal travel time curves.
Near-Regionals — The next profile, figure _13_, is a combination of
of two events, both identified as coming from the Elkhead Mountains.
No effort, in this analysis, was made to discriminate the source of
these events between possible natural events and large mine shots
in the coal mining district of north-western Colorado. An interpretation using the apparent velocity consistent with the normal crustal
model, (5.6 km/sec) differentiates an area outlined by stations #1,
8, 11, 14, and 15 which are fast with respect to stations #7, 12, 16,
17, and 19. The arrival time at station #18 fits neither trend and is
late by at least 500 ms from the expected curve. Although the source
area is only approximately known, the true geographic azimuth between
the local array and the Elkhead Mountains is roughly N25W. The
apparent azimuth of the incoming wave front determined from threestation fits was N60W.
Figure .14 shows a near-regional event from the west-northwest of
the prospect area. Identification of the epicentral location is only
approximate but correlation with records at the GOL station in Bergen
Park, Colorado suggests that the event may originate from the vicinity
of Ruth Mountain. Fitting the arrivals to a 5.6 km/sec apparent velocity
31
T-
ELKHEAD MTNS
295- 0301
295- 2202
dist
az
HT
N60°W
u
0)
19
w
s
I
H
K
18 +
5.6 km/sec
10
RELATIVE DISTANCE in Km
FIGURE 13
REFRACTION PROFILE
20
ELKHEAD MTNS
32
-T-
13
13
46
v
/
19
A- O
~| Q
/
45
-t-3
u
.5
w
./16
44
H
EH
/h.7
REGIONAL WNW
maybe RUTH MTN
297- 0100
dist 130 km
az N45°W
H
K
43
18+ /
•
/ 5.6
' km/sec
42
rv
0
RELATIVE DISTANCE in Km
FIGURE 14 REFRACTION PROFILE REGIONAL WNW
in
shows late arrivals at stations #15, 18, and 19 and early arrivals at
stations #3, and 8. Time residuals as great as the one at station #3
(almost one second) probably result from misidentification of the re-
ti
fracted arrival phase, p , in the background noise. The scatter on
the plot is a result of the uncertainty in location. Contradictory solutions from the three-station fits prove this out. Results from the
three station technique varied from N45W to due north.
Figures 15_ and 16. show a comparison of the four Climax shots using
the straight-line and relative distances in either profile. Figure 15
shows arrival times plotted against, shot-receiver distances taken
normal to the apparent azimuth of N25E (geographic azimuth is due
north). The expected phase velocity of 5.6 km/sec is contrasted to
the best fit velocity of 4.8 km/sec also shown. The four separate
shots were combined by calculating the best fit apparent velocity,
fitting this slope to each individual profile and combining the profiles
by matching the apparent velocity lines.
This profile resembles the Elkhead Mountains profile (figure 13).
An area, including stations #6, 7, 11, 14, and 15,
appears to fit
a
velocity similar to the regional velocity but arrival times are fast by
as much as 300 to 400 ms (with respect to the other stations). Slow
time residuals at stations #19 and a fast residual at station #18 may be
due to difficulties in identification of first arrivals and in combining
separate shots into one profile without knowledge of the true origin
time. Figure jj5 shows the true geographic distances and the expected
5.6 km/sec phase velocity. This profile shows similar behavior to the
T-
19
CLIMAX, CO
293- 2109
294- 2100
295- 2113
295- 2252
296- 2355
dist 70 km
az N25°E
b!4
11
CHALK CREEK
o
.42
17
w
s
H
8 i
H
K
18
5.6 km/sec
0
- 10
20
RELATIVE DISTANCE in Km
FIGURE 15 REFRACTION PROFILE Climax, Co
CLIMAX, CO
5 shots
dist 70 km
az due N
o
<B
CO
c
•H
w
s
H
EH
H
K
0
20
RELATIVE DISTANCE in Km
FIGURE !£_ REFRACTION PROFILE normal CLIMAX, CO
T-1842
36
adjusted profile, figure Jj>, but valuable information concerning size
and lateral extent of the anomalous zone is masked by the lateral refraction.
The final profile, figure J7, shows the Monarch Pass event which ,
with the Climax events, reverses the refraction profile of the Mount
Princeton area. Using the 4 . 8 km/sec apparent velocity determined
from the Climax profile, an area including stations #8, 15, and 19 is
fast relative to station arrivals further northward.
Fitting the data
points to the 4.8 km/sec velocity, brings out the anomaly at Chalk
Creek from a third direction. Without prior knowledge of the apparent
velocity of the Climax profile, the observations seem to better agree with
an apparent velocity much slower than the velocity shown.
The true
azimuth from the center of the local array to Monarch Pass is S30W,
but once again the incoming wave front seems to be focused in a more
east-west direction, as the apparent azimuth solution was S60W.
Figure JJ3 is a plot of amplitude versus relative station distance from
Climax for seismic arrivals recorded by stations on Line B. Only shots
from Climax, recorded on hard rock sites, were'used. These trace
amplitudes were normalized to the response at station #18 and scaled
for comparison at other sites relative to this station. The line through
the data is meant to demonstrate the amplitude (energy) decay with
distance. Stations in Cottonwood and Chalk Creek display significant
amplification compared to the expected attenuation curve.
37
.T- 1842
16
CHALK CREEK
«?—5-
15
o
0)
s
H
EH
14
MONARCH PASS,CO
296- 2045
dist 30 km
az S60°W
H
K
K
4.8 km/sec
13
0
10
20
RELATIVE DISTANCE in Km
FIGURE 17 REFRACTION PROFILE 'MONARCH PASS, CO.
38
T-1842
A rough estimate of the dominant frequency at each station was
made by a simple measurement of the period of the first few cycles
of trie-wave train. The areas of relative amplitude amplification
(shown in figure .18) appear to correlate with arrivals with a predominance of high frequencies. The most pronounced effect, an increase
in the frequency from 4hz to 8hz, was observed at station #8. The
attenuation and dominant frequency analysis were restricted to the
Climax shots because the other sources failed to provide the necessary redundancy of data needed in a comparative analysis.
Passive Microearthquake Survey
The passive microearthquake equipment used in this survey allowed
24-hour, continuous operation,so a survey to detect natural
seismic
activity could be conducted alongside the active program. During the
survey, three events were recorded and identified as local, natural
events. These three events were timed from the smoked paper records
and located using a least-squares-fit location program based on a constant velocity, half space model.
The table below lists information
about the calculated origin time, location and magnitude of these local
events.
The recorded arrival information is not sufficient for reliable depth
estimates,and x and y locations should be treated as approximate (quoted
locations are within -2 km). For a unique solution in the location program used, four arrivals or three arrivals and an origin time are needed.
39
T00
r-'
r-{
#
§
H
30
(0
o
rH
O
COTTON!FOOD
CUEI K
Q
w
tQ
H
CH7LK CREE
K
O
20
17
18
w
1<
16
L5 I
Q
O
EH
H
w
o
K
EH
in
O
1
0
'10
.
1
2
0
RELATIVE DISTANCE in Km
FIGURE 18 AMPLITUDE VERSUS DISTANCE CURVE
14
13
T-1842
40
To locate the local event recorded on only three stations (294-2051),
an s-p time, read from the record at one station, was used to estimate
a'n origin time by assuming a Poissons Ratio of 0 . 2 5 and using the
Wadati Curve (plot of arrival time versus s-p time for a given velocity
ratio).
Local Events
Table 1
time local
t of sta
T0
X
date
time UTC
294
0453
1053PM
5
12 .81 +11.0 +4 .9
294
1723
1123AM
4
12 .35 -8.7
294
2051
0251PM
3
24 .05 +12.8
Y
Mag
-0. 5M
-7 .5
+1. 25M
0 .0
0. OM
Location coordinates are in km referenced to the origin of the local
array (37.75 N and 108.00 W), magnitude is quoted as body wave magnitude corrected for distance and frequency response.
The upper Arkansas Valley and the San Luis Valley are areas of very
'*-x
little historic seismic activity (Hadsell, 1968). A microearthquake survey of the San Luis Valley (Keller and Adams, 1976) recorded 6 local
events in a three week survey, a very low level of seismicity considering the sensitivity of the recording equipment. Although the level recorded by this.survey, three events in four days, is appreciably higher,
the Rio Grande Rift Zone in Colorado seems to display a low level of
seismicity with respect to the recorded activity from this zone in central
New Mexico and to the abundant geologic evidence of recent activity
in the upper Arkansas Valley (Scott, 1970).
T-1842
41
INTERPRETATION
. An active seismic investigation of a prospect area
provides valuable information on the seismic expression
of the subsurface geologic character.
''
This information
is contained in the interpretation of travel time residuals,
observation of amplitude behavior and comparison of known
source-reciever geometry to the apparent azimuth of waves
observed by the local array.
Mount Princeton Survey
Interpretation of these parameters from the Mount
Princeton survey suggests the following conclusions.
These
are discussed in detail on the following pages.
1)
Interpretation of teleseisms and near-regional
events in a refraction scheme aids in developing a
regional subsurface interpretation by providing travel paths
from different azimuths and from different depths. A
reversed profile, including the events from southern Mexico
and from Nova Zemyla, indicates that the crustal thickness
increases to the north under this profile.
2) -Regional lateral refractions from the boundaries
of the north-trending upper Arkansas Valley graben focus
incoming seismic energy by bending the wave fronts into a
more east-west direction.
These boundaries can be interpreted
as the continuation of the Rio Grande Rift structure
as far north as Leadville.
T-1842
42
3)
The failure to record mining shots from
Questa, New Mexico and the absence of earthquake observations
form the northern extension of the Rio Grande Rift at GOL,
indicates high attenuation of waves through this area and
the need for high-gain recording sites within the
region to more accurately determine the local seismicity.
4) A significant velocity or structural ridge
exists in the subsurface of Chalk Creek, south of Mount
Princeton.
This east-west cross structure to the north-
trending horst-graben pattern of the regional tectonic
framework may be related to the Mount Princeton batholith
and the other Tertiary intrusives in the southern part of
the Sawatch Range.
5)
Although the survey was not specifically designed
to study^attenuation behavior, observations of amplitude
and dominant frequency indicate anomalous behavior for
stations within Chalk Creek.
Amplitude amplification or,
more likely a longer travel path through more competant
material would be consistent with the large amplitude,
high frequency and early arrival times.
Discussion of Results
•
1)
Apparent velocities from the USSR teleseism were
faster than predicted from a normal crustal model, which
assumes horizontal boundaries at depth.
Arrivals from the
southern Mexico event displayed the opposite effect.
These
two observations suggest crustal thickening to the north
over the travel paths involved.
Crustal refraction studies
T-1842
43
(Pakiser and Jackson, 1965), in icated a deeper crust
uderneath the southern Rocky Mountains and the Great Plains
supporting this conclusion.
2)
The comparison of the local apparent azimuths
recorded by the array and the known source-reciever
geometry can provide valuable information concerning the
horizontal velocity distribution and structural character
between the array and the source.
Sources from varying
distances and directions can further define the subsurface
picture by revealing both local detail and gross regional
structure of the project area.
At this point some mention should be made of the
uncertainties involved im measuring apparent azimuths.
Specifically, the question of the significance of the 30 degree
lateral refraction quoted in this paper should be addressed
with regard to the sources of error.
To evaluate this problem, careful analysis was made
of two possible measures of error in the apparent azimuth
determination: 1) agreement between the three-station fits
over the entire array, and 2) the effect of timing errors on
a single three-station
fit.
After removal of solutions which varied widely from the
majority of the fits (these random fits can be attributed
to erroneous signal identifications or local geologic
problems at specific recording locations), three-station
fits agreed to within ± 5° for lateral refractions from
Climax, Monarch Pass and the Elkhead Mountains.
In regard to the second problem mentioned, a sample
three-station
Iff!
' !
T-1842
.
44
solution was modified by introducing a timing error of -25 ms and
-f
+
-50 ms into the calculations. The -25 ms error led to solutions only
-2°'from the initial fit, while the -50 ms error, which is greater than
the expected timing error in the system, led to solutions -5° from the
observed apparent azimuth (see figure Jj} for description of the technique and calculations involved).
Consideration of these major sources of error predicts an error
of less than -10° in this method. While this error can be significantly
reduced by larger-aperture, omnidirectional arrays, the error analysis
performed supports the conclusion that the observed lateral refractions
are indeed real and significant and not a function of random errors.
The mining operations at Climax, Colorado are due north of the
array, however, the apparent wave front was determined to have an
azimuth of N25E. The topography of the valley and the regional geologic structure suggests a possible lateral boundary on the east side
of the valley, near Buena Vista. After assuming a vertical boundary
and a 4 to 3 velocity contrast at the boundary, the focusing effect of
the regional geology can be explained by a boundary striking N30W.
Data from the Monarch Pass and Elkhead Mountains events suggests a second boundary striking north-south corresponding to the
mountain front on the west side of the valley. An interpretative illustration of the lateral refraction is shown in figure 20.
It must be stressed that this interpretation is not unique. Assumption of a velocity contrast and orientation of the refracting horizon were
T-
45
Normal to
Apparent
Wave Front
Apparent Wave Front
STA#3
FIGURE JJ3
THREE STATION APPARENT VELOCITY VECTOR
Method
0 arrange stations in increasing arrival time
2?) find distance a by
T
1
2- 1= a
T
3-T1'
^ draw apparent wave front between STA#2 and a
draw normal to apparent wave front
\apparent velocity equals
J
L
46
BOUNDARY
V1=0.75
V2
APPARENT AZIMUTH
A:
T : TRUE AZIMUTH
TO
MONARCH
CO' C'" •"•'' ; ..'•
•
' -•GOLDLN. COLOIiADO 50401
v^x^. „ -_/ - .... ,j - - -•
FIGURE 20
LATERAL REFRACTIONS at MOUNT PRINCETON
dashed lines represent wave fronts from Climax and solid
lines are wave fronts from Monarch Pass, stations in
local array shown for scale
T-1842
ff
47
made to honor the probable geology in the area. The boundary on the
west is placed with greater control as this structure must pass between the array and the Monarch Pass quarry. In contrast, the boundary on the east can'be placed as close to the array as the Arkansas
*
River drainage or as far away as the highlands near Trout Creek Pass.
3) As a side issue, but an interesting observation nonetheless,
is the failure of the array to record arrivals from the blasting operations
at Questa, New Mexico, 275 km due south of the project area. The
question is whether normal attenuation would account for the absence
of measureable seismic energy or whether the Rio Grande Rift Zone,
which any arrival from Questa must traverse, acts as an energy barrier
creating a shadow zone for arrivals from this azimuth.
Mining blasts are designed more to break rock at the source than
to enhance efficient transmission of seismic energy. However, two
shots, October 20th (51,400 Ibs. , 18 delays) and October 22nd (44,000
Ibs., 16 delays), were of sufficient size to warrant further consideration.
A crustal refraction survey (Jackson and Pakiser, 1965) of the southern Rocky Mountains used blasts from Climax to investigate the crustal
structure from North Park, Colorado to Casper, Wyoming. In this survey, a blast of 25,500 Ibs. was observed at distances out to 200 km
using recording equipment with less sensitivity than in the present survey. These facts indicate that attenuation effects above and beyond that
expected from a normal travel path may exist between the survey area
and Questa.
T-1842
48
4) After correction of the arrival times for the apparent azi-
muths, the time advances observed from stations in Chalk Creek are
confirmed by profiles from Climax, Monarch Pass and Elkhead Mountains. The travel time residuals can result from either a velocity
change along the travel path, a structural change at depth, or both.
Figure 21 shows an illustration of the time-residual data. Figure 22
demonstrates two possible models for the observed residuals. The
two models cannot be distinguished using the data available from this
survey.
Model A is a fault model with the head wall of the normal fault
to the north of Chalk Creek. The second model is based on a subsurface velocity contrast under Chalk Creek. Both models are consistent with the amplitude and dominant frequency measurements as they
l
'-x
involve a longer travel path in more competent material compared to a
path outside of Chalk Creek.
If the 300 ms residual for the Chalk Creek Stations is to be explained from only a structural viewpoint, assuming a velocity of 5.0
km/sec, displacement on the fault in Chalk Creek is on the order of
1.5 km. This is consistent with the observation of displacement on the
main boundary fault at the mountain front which is at least 1.5 km and
maybe as much as 3 km.
A high velocity ridge could be interpreted as a fault zone which has
undergone extensive alteration which changed the physical properties
J '
MODEL A
0
Surface
KM
Surface
V.
MODEL
FIGURE 22.
FIGURE 21
ARRIVAL TIME RESIDUAL DATA
PLOTTED IN MAP VIEW VERSUS
STATION LOCATIONS
V.
B
Velocity and Structure
Models
jfl
T-1842
50
of the zone. An approximate velocity contrast of 40% would be necessary to explain the observed time residual without considering any
structural change in the travel path. A more logical interpretation
/
would include a combination of both explanations. An east-west
cross structure would be consistent with the local geology, and it
would be logical to assume that alteration associated with fault movement would be restricted to a localized zone.
It should be emphasized that the anomaly is a time residual
anomaly and interpretation of velocity and structure without independent
confirmation from additional measurements is not unique. Additional
geophysical and geological investigations could shed more light on
the nature of the subsurface of Chalk Creek.
5) The amplitude and dominant frequency observations can be
explained by several different models. As well as the non-unique
character of any interpretation, this survey was not designed specifically to study attenuation. Despite this design problem, the survey
did include the following necessary elements of an amplitude study:
1) the recording systems displayed similar response to ground motion,
2) particular care was taken to locate seismometers on hard rock sites
(only stations from Line B were used to compile the data shown on
figure 18), 3) straight line profiles were used to minimize the effect
of inhomogeneities, and 4) multiple shots from the same location were
used to look at only comparative results. However, the source spectra
was not known, the charge weights were not known, so the only scaling
•possible was the adjustment of the observations to the response at a
T-1842
51
chosen site (station #18).
As much as uniform geology
was not present, the effect of the response of very local
geologic conditions cannot be ruled out as a contributing
factor.
Possible explanations for the phenomena, assuming a
coherent anomaly exists, include the hypothesis of a longer
travel path through more competent material as mentioned
previously and the possible focusing effect of the Chalk
Creek structure.
The detectors were all vertical
seismometers, sensitive to ground motion in a vertical
direction only, thus amplitude amplification could result
from a local structure steering seismic energy into a
more vertical direction
(bending the emerging wave front
toward the normal to the surface).
Actual variations in
physical properties could also affect the transmission
characteristics in a local area.
Interpretation of the amplitude data beyond the general
conclusions stated here is not warranted without better
control on the remaining variables.
The important conclusion
drawn from these observations is, while the structures
proposed from other observations do not uniquely explain
the attenuation effects seen, the models presented are a
possible explaination and the amplitude and frequency
measurements do not disprove the conclusions of this paper.
The apparent agreement between the time residual and the
attenuations observations point to interesting speculation
and deserve further study.
T-1842
52
EVALUATION OF THE ACTIVE SEISMIC TECHNIQUE
The most important conclusion of this paper is the evaluation of
the apparent azimuth technique used to display the arrival time observations.
The significant observation of the regional focusing of seismic
energy into or away from a local array leads to the
question of
where is this technique preferable, and in some cases, necessary,
over the conventional straight-line profiling method used by previous
investigators.
I believe that, based on the conclusions of this paper, the apparent
azimuth technique should be considered and, at the very least, be compared to the conventional method when the following conditions exist.
The importance of lateral refractions in interpretations drawn from a
large scale refraction survey in these cases cannot be discounted without further investigation.
These conditions are: 1) source-receiver dis-
tances such that arrivals are either from the direct path or are critical
refractions from shallow horizons; this method would not be directly
applicable to investigations of the Moho discontinuity as the regional
features projected as the cause of the lateral refractions probably do not
extend to this depth and wo uld add only minor deviations to arrivals from
deep refractions or reflections, and 2) the existence of regional geologic features such as, boundaries of readily discernable geologic provinces, rift zones, large horst-graben features and major tectonic features
such as ridges or subduction zones associated with large changes in the
T-1842
53
crustal structure, which could cause lateral steering of seismic energy
crossing such a feature.
•The concept of lateral changes affecting seismic arrivals of an in('
line profile is expressed in the migration of seismic reflection data to
account for the influence of structures horizontal to the time section
being examined. The apparent azimuth technique is analogous to this
correction as it attempts to correct distances to display the true direction
of energy propagation and take into account the effect of lateral features.
The following conclusions are directed to the evaluation and optimization of the active seismic reconnaissance method.
1) The apparent azimuths into a local array can be significantly
different than the geographic azimuth between source and receiver, therefore, better control of the azimuth is critical.
2) The uncertainty in the station residuals are less than -50 ms
in this survey. This error can be attributed to near surface geologic conditions or small changes in the source location. Better knowledge of
source location, shot origin time and avoidance of site problems could
add more accuracy and precision to the technique.
3) Arrival time residuals and attenuation are a function of arrival
azimuth and emergence angle. Data received from many azimuths and distances would provide a more complete picture of the regional and local
**
seismic structure.
To use this survey method at another prospect area, the following
points should be considered:
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1) Several stations should be placed outside the area of in-
terest for the duration of the survey. These stations will act as control'for identification and independent confirmation of regional seismic
structure.
2) Several stations inside the project area should remain stationary for the duration of the survey to act as local control of arrival
azimuths and tie points for roving stations.
3) Station locations should be positioned to avoid site geologic problems (near-surface amplification and slow time residuals).
The array geometry should be omnidirectional and cover a large area
(refraction interpretations are still useful assuming plane waves and
measuring distances perpendicular to the apparent azimuth). In placement of stations, special attention should be given to specific targets
'••^
of interest.
4) Roving stations should be left in one position until a sufficient sampling of available sources, considering both distance and azimuth, is obtained.
5) The density of stations must be high to insure statistical significance of the obtained data.
Combination of profiles from a single
source should be made with knowledge of origin times.
6) Origin times should be obtained from each source to provide
control for calculation of absolute velocities, depth to refractors and
arrival residuals.
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7) Local sources of seismic energy could provide detailed
measurements of near-surface structure underneath the array and
complete the picture of the seismic expression of features in the
area.
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CONCLUSIONS
1) The active seismic reconnaissance technique conducted
•
.
at the Mount Princeton project area succeeded in surveying
a large area by delineating regional structures, specifically
the boundaries of the Rio Grande Rift Zone, and by singling
out a specific local target for further investigation. This
technique proved to be a cost-effective means of obtaining
valuable information with a minimum amount of time, equipment and cost.
2) Interpretation of apparent azimuth data from several sources
indicates that lateral refractions occur from both the east
and west side of the upper Arkansas River Valley graben.
The existence of these boundaries supports the interpretation
that the upper Arkansas Valley is a continuation of the Rio
Grande Rift Zone of central New Mexico and southern Colorado,
3) Within the prospect area, an advance time residual exists
corresponding to the Chalk Creek drainage topographic feature.
A probable normal fault in the subsurface, downdropped to the
north, is interpreted as the source of this time anomaly. This
east-west feature is normal to the north-trending regional
structure and may have formed in response to the intrusion of
the Mount Princeton batholith or may be associated with the
more recent rifting activity.
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RECOMMENDATIONS
The active seismic technique can be applied whenever subsurface
velocity or structural information is needed over a large area. This
survey technique can be especially useful when man-made sources of
seismic energy (such as blasting from mining or construction operations)
are readily available. Information from teleseisms and regional events
can be used in the same manner to provide better azimuthal coverage of
the prospect area. Interpretation of these events is aided by assuming
a normal crustal model and interpreting residual observations from the
expected arrival times.
The structural feature of the Chalk Creek fault and the boundaries of
the upper Arkansas Valley graben should be investigated by independent
geophysical and geologic techniques. Detail refraction work (profile
lines on the order of 1 or 2 km long with station spacing of 40 to 80 m)
should be run in a north-south direction over Chalk Creek to further define this structure. Lines both close to the mountain front and on the
valley floor would help to map the lateral extent of the feature and provide data to help interpret the significance of the structure. Detailed electrical soundings should also be effective as the alteration associated with
the fault should provide a measurable contrast in the physical properties
of this zone.
More observations from distant events recorded on an array designed
to look for lateral refractions would better delineate the boundaries and
produce additional data for this effect.
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BIBLIOGRAPHY
1)
Ackermann, Hans D., 1975, Seismic Refraction Study in the
Raft River Geothermal Area, Idaho (abst); 45th Annual
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2)
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3)
Capon, Jack, 1974, Characterization of Crust and Upper
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4)
Caton, P. W. , 1975, Plane Wave Apparent Velocity Vectors:
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v
^
Sciences, Inc.
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59
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16) Pakiser, L. C. , 1963, Structure of the Crust and upper Mantle
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•
-
vol 68, no 20, p 5747.
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'^.
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23) Tweto, Ogden and J. E. Case, 1972, Gravity and Magnetic
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