T-1842
AN ACTIVE SEISMIC RECONNAISSANCE SURVEY
OF THE MOUNT PRINCETON AREA
CHAFFEE COUNTY, COLORADO
by
James Scott Crompton ProQuest Number: 10782043
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A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the require ments for the degree of Master of Science in Geophysics.
Signed: ■i/s * u>./ ames Scott Crompton
Golden, Colorado Dat e: Y , 1976
Approved: Phillip R. Romig, Jr. Thesis Advisor
George V. Keller Head of Department
Golden, Colorado Date: # 1976 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 tech niques . The results indicate the importance of examining apparent azi muths of energy paths into a local array and comparing these azimuths with the known source-receiver geometry. Conclusions from this sur vey 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 of Mount Princeton.
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.
ii T-1842
TABLE OF CONTENTS
Page A bstract...... ii List of Figures and Tables ...... iv Introduction ...... 1 Acknowledgments ...... 3 Regional Geologic Setting...... 4 Geologic History...... 8 Scope of Investigation and Operations ...... 11 Equipment ...... 17 Observations ...... 24 Interpretation ...... 41 Evaluation of Active Seismic Technique ...... 52 C o n clu sio n s...... 56 Recommendations ...... 57 Bibliography...... ' ...... \...... 58
iii T-1842
LIST OF FIGURES AND TABLES
FIGURES Page 1 Location M ap ...... 5 2 Geologic Cross S e c tio n...... 7 3 Station Location M ap . 15 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 U S S R ...... 28 11 Refraction Profile Teleseism Kuril Is * ...... 2 8 12 Refraction Profile Teleseism M exico ...... 29 13 Refraction Profile Elkhead M tn s ...... 31 14 Refraction Profiles Regional WNW ...... 3 2 15 Refraction Profile Climax Refracted Azimuth .... 34 16 Refraction Profile Climax GeographicAzimuth. . . 35 17 Refraction Profile Monarch P a ss ...... 3 7 18 Amplitude versus Distance curve ...... 39 19 Three Station Apparent Velocity Vector ...... 45 20 Lateral Refractions, Buena Vista ...... 46 21 Arrival Time Residual D a t a ...... 49 2 2 Velocity and Structure M odels ...... 49
iv T-1842
TABLES
Page 1 Station Coordinates ...... 16 2 Recorded E v e n ts ...... 25 3 Local Events ...... 40
v T-1842 1
INTRODUCTION
The seismic refraction technique has been used for many years to obtain subsurface velocity and structure information. This tech nique was exploited extensively in the 1960's as part of the VELA
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 mining blasts, large areas were surveyed using equipment designed 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 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 T-1842 2
experiment to test a modified version of the crustal refraction program
used in the VELA Uniform studies by recording mining shots from sev
eral 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 re location 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. T-1842 3
ACKNOWLE DGE ME NTS
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 MicroGeo- physics Corporation to conduct the active seismic experiment at Mount
Princeton. I also appreciate the use of equipment, funding and val uable 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 re sponsive to requests for information concerning times of blasting at
Climax and Questa, New Mexico.
C ; T-1842 4
REGIONAL GEOLOGIC SETTING
The Mount Princeton prospect area is located in south-central
Colorado, approximately 150 km southwest of Golden (see figure X,
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, 19 74). 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 re
spect 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, down-
dropped 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 T - 1 8 ^ 2
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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 Pre-
Cambrian rocks in the southern part of the Sawatch Range. The bound ary 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 sur face, 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 de generates 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 by a more complex system of parallel normal faults with the major dis placement attributed to a fault buried underneath Tertiary and Quater nary sediments approximately paralleling the present course of the
Arkansas River drainage (Limbach, 1975; Knepper, 1974) (see figure _2, geologic cross section). T— lc&2
WEST EAST
MOUNT PRINCETO INTRUSIVE ARKANSAS RIVER MOSQUITO-TEN MtLE core of the HIGHLANDS SAWATCH ANTICLINE (ARKANSAS HILLS) ALLUVIAL - - - + + + + + + FILL
+ + + + + + + + + +
COLORADO PLATEAU RIO GRANDE RIFT GREAT PLAINS PLATE PLATE
FIGURE 2 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, 19 74). First, clastic sediments with minor amounts of car bonate 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 base ment 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 T-1842 9
graben but as much as 3000 m of sediments may have been deposited
during this time span.
Late Mesozoic to early Cretaceous geologic activity was domi
nated by the Laramide orogeny. Of particular importance to the tec
tonic framework of the project area was the uplift and folding charac
teristic 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 re gional, tensional stress field which opened pre-existing fractures and
created new ones allowing magma to rise into a shallow crust. The exist ence of two stages of intrusive activity is indicated by the quartz mon zonite batholith of Mount Princeton and the younger granitic intrusive at Mount Antero. The separate stages r e s u lte d from t^ie 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 pro ject area. The northernmost expression of the rifting, the upper Arkansas
Valley graben, developed in five recognizable stages. The following is T-1842 10 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. T-1842 11
SCOPE OF THE INVESTIGATION AND OPERATIONS
Velocity information and structural interpretations which can be
obtained from an active seismic survey have become increasingly im
portant in the integrated geological and geophysical solution of ex
ploration problems. Economic difficulties, time limitations and
equipment availability are problems faced by all, and serious con
sideration 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 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, approxi mately 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. T-1842 12
Absolute origin times of the man-made events were not known but approximate times were supplied by the mine operators . The events recognized as mining shots were identified on the basis of this information as well as confirming evidence from apparent azi muths and s-p arrival times.
Each of the ten stations in the survey consisted of a Sprengnether
Instrument Company MEQ-800B microearthquake system. These seis mographs 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 estab lished 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. T-1842 13
The survey was designed, with the independent recording systems and the two separate line geometries,to monitor any structural differ ences‘within the graben and horst framework. Geophone plants in
Line A were, by necessity, in unconsolidated alluvium. Although pre cautions were taken to avoid sources of noise such as wind and cul tural 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 geo metrical requirements of the refraction profile and access consider ations . 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 _3 and table_1 show 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 para
meters to Climax shot, established
three additional stations.
October 21st set up additional five stations to
complete line A, relocated two
stations for better geometry. T-1842 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. T - 1842 15
US 24
■®
MT YALE BUENA \ VISTA ■ © 800 '
10001
c o t t o n w o o d creek
'10000
12800
MT
PRINCETON CO
8000
JOQOO
MT 1 ANTERO
12800
KM
FIGURE 3 STATION LOCATION MAP Mount Princeton area, Chaffee County,Colorado T - 1 8 4 2 16
Table 1
Station Location * Station # X Y Z Line Distance ______Geometry From Climax
1 + 11.8 + 6.7 + 0.2 A 61.4 2 +11.2 +4.6 + 0.2 A 63.5 3 + 14.3 +0.8 + 0.0 Base 67.2 4 +12.2 + 2.6 + 0.1 A 65.4 5 +12.7 -0.2 + 0.1 A 68 .2 6 + 6.7 -6.3 + 0.2 A 74.7 7 + 9.2 -3.3 -0.2 A 71.5 8 +16.4 -2.8 -0.1 B 70.8 9 + 15.4 +14.6 +0.1 A 53.4 10 +13.7 + 12.2 + 0.1 A 55.8 11 +12.2 -0.2 +0.1 A 68.2 12 +10.1 +3.1 + 0.2 A 65.0 13 +15.6 -7.3 -0.3 B 75.3 14 +16.6 -4.1 +0.0 B 72.1 15 +15.5 -1.3 -0.1 B 69.3 16 +18.5 + 6.6 + 0.0 B 61.6 17 +18.9 +9.1 -0.1 B 59.1 18 +19.3 +17.8 -0.2 B 50.5 19 + 18.1 +0.9 -0.5 B 67.2 20 +17.3 -4.6 -0.1 B 72.7
Origin at 38°45''N 106°001'W Elevation datum at 8570' (Station #3) * +X is west and +Y is north + Z is down Coordinates in kilometers T-1842 17
EQUIPMENT
Ten independent microearthquake recording systems were em
ployed 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 4 is a schematic diagram of the microearthquake recording system. A detailed explanation of the individual components is given below.
MARK _PRODUCTS_ SPRENGNETHER MEQ-800 TRUE TIME I HZ VERTICAL AMPLIFIERS INTERNAL <— WWVB RECIEVER SEISMOMETER ------. TIMING FILTERS 1 _ _
VISUAL DRUM RECORDER
FIGURE _4 SCHEMATIC DIAGRAM of EQUIPMENT USED IN SURVEY
Seismometer— The Mark Products L-4C seismometer is a one- hertz natural-frequency vertical seismometer. The open-circuit damp ing 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 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 { 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# although 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
5_. From this figure, the essentially flat system response to velocity over a selected frequency range can be seen. T - 184 2 19
CO LO 00 o o o o o o o
r—I
o c
IT)
rH H
ro CN O rH I—I AiLIAIlLISNSS nrt
F ig u re! re! u ig F 40
20 ■REFERENCE TIME IUE h*— HOURS- MINUTES et n h US t oa eya 7.4 n 5 5 E) 54 approximately .52 N and (73.04 Zemyla USSR Nova at the in test he dfeet itne. Figure distances. different three the order of 30 ms per day. per ms 30 of order the rneo sre ilsrtn h simc intrs rm ore at from sources signatures seismic the illustrating survey Princeton Significant deviations with standard time were noted and appropriate appropriate and noted were time standard with deviations Significant internal the of Adjustment pulse. second generated internal-clock the orcin md oarvl ie. omn rf cretos ee on were corrections Common drift times. arrival to made corrections limits. acceptable within to standard the with agreement kept clock ing (on an oscilloscope) the beginning of the WWVB second pulse with with pulse WWVB the of second beginning the oscilloscope) (on an ing record. each record day as an absolute time and date identification for the the for identification date and time absolute an as day record each TheWWVB systems. recording microearthquake the of clocks timing code (as shown in figure _6) was recorded at the beginning and end of end and beginning the at figure _6) in recorded shown (as was code Standards 60.kilohertz time-standard broadcast from Fort Collins, Collins, Fortfrom broadcast time-standard 60.kilohertz Standards Colorado. This time standard was used to synchronize the internal internal the synchronize to used was standard time This Colorado. T—1842 10 Data-- Figures Figures Data-- WWVB with compar daily by synchronized were MEQ systems The W B WV i te ai cl cd frte ainl Bureauof National the for code call radio WWVB the WWVB— is 15 v! * DAY YEAR OF 25 co! O OjO Ol v ^ — 9 r eape o eod fo h Mount from the records of examples are n 30 7_ is a sample recording of a nuclear nuclear a of recording sample a is 35 CORRECTION (MSEC. TO UT2 ) n 40 co o o o OlO o o cDj^r o 45 i o OiO CM O 50 00 ^CM 55 20 , m 60 T-1842 21 7500 km to the north-northeast. A local event, within the prospect area, is shown in figure _8. Figure j) 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. 9491 FIGURE 7 TELESEISM - NOVA ZEMYLA, USSE, Dist 69° date 294 , time 1232 f station #5 , gain 84 db , filters 5 -10 hz nuclear test ,approx. Mag 6.4M, 73.04 N 54.52 E (alluvium) W f l — —maammmsmm 2100 ASSK»aSAS?S gmiw.«^iig >iii^ '»iBiw ^.w>n!!»mwinMBnMWwa»iinjaijaiiiifcMSMln MWWlii s a ^ — ws sss ^S ssssssssa sS!l£iwSBSwMs5roK8BS mbbbS msm o t s s s s BBBBfiMltemBBasa s a m a i a mm FIGURE 8 LOCAL EVENT and CLIMAX SHOT date 294 f station #11 gain 90 db f filters q "10 hz (alluvium) local event f time 2051 epicenter +12.8, 0.0 f near sta #11 Climax event ^ time 2100 f dist 70 km f mining blast T- 1&*2 23 £gg| I a 11 ■ tiS B P •rH > 3 rH i—1 I iff 1 i ! 0 N cn x: P -P o -p S -P m m m i—i cn 0 i 0 03 0 ■■m e-*® -p i—i 0 rH Klilif P X! 0 X 0 X! m wm Cn rX Cn 0 P 1-1 P •rH M ■H u0 Einh -P S3 B H X > cp ■" w B o g V X ro rX pi X! CN '« te -5!satl C 03 o o S3 -P r - O O 0 m 11 H cn -P ■H -P ii 11 0 0 03 0 H P ■rH •rH « ■rH 03 ** 0 CN 03 Cn *> O 0 ro CN CN 0 *• i—t CN in Kill .H i—1 CN x ■—i CN 0 CN S g 0 ■H 0 H £ B -p g P 0 •rH •rH u •rH -P *• •P II » I -P ■p I f II: 0 £ -P -P 0 -P 0^1 0 P > P 0 w 0 w *■ > > « IT) 0 i—i 0 P> cn 0 i r i Kill!I s iE | OBSERVATIONS During the four days of field work, 17 seismic events were re corded. 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 near- regional 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 JJ3 thru \7 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. T-1842 25 TABLE 2 RECORDED EVENTS date time UTC # of sta location dist, from (Julian Day) in fit 293 2109 2 Climax 70 km 293 2110 1 (unknown) 294 0453 5 local (near sta #2) local 294 1232 5 Nova Zemyla 6 9° 294 1723 4 local (Arkansas Hills) local 294 2051 3 local (near sta #5) local 294 2100 7 Climax 70 km 295 0301 5 Elkhead Mtns 230 km 295 0521 7 Kuril Is. 21° 295 1324 1 local (near sta #8) local 295 2113 5 Climax 70 km 295 2202 7 Elkhead Mtns 230 km 295 2252 7 Climax 70 km o 00 296 0125 5 Southern Mexico CM 296 2045 7 Monarch Pass 25 km 296 2355 9 Climax 70 km 297 0100 9 Regional WNW 100 km TELESEISMS date location dist*from array from NEIS*, Golden, Co o 294 11 59 55.5 73.04 N 54.52 E USSR CD CD 295 05 09 51.6 47.53 N 148.74 E Kuril Is 21° 296 01 19 58.1 15.83 N 94.63 W Mexico 28° *NEIS - National Earthquake Information Service REFRACTION SHOTS date time Climax, Colorado 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 36 .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 g e o l o g y ,After making the assumption of plane waves, knowledge of the coordinates and arrival times at three stations u- niquely 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 accu racy, 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, 19 72) 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 de termining 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 _1£ thru 1J2 are time-distance curves of the tele seisms recorded during the survey. Figure 10 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 6.4M. 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. NOVA ZEMYLA, USSR 294- 1210 dist 69° az NlO °E MP o 57 HT H 21 km/se 17.7 km/sec RELATIVE DISTANCE in Km 20 FIGURE 10 REFRACTION PROFILE TELESEISM USSR KURIL IS. 295- 0521 dist 21° az N85°W 18 km/sec HT 11 u 0 w 11 c •H 12 w s M E* > 10 H 05 10 20 RELATIVE DISTANCE in Km FIGURE 11 REFRACTION PROFILE TELESEISM KURIL IS. ARRIVAL TIME in Sec 18 19 20 21 22 FIGURE FIGURE 14 15 EAIEDSAC inKm DISTANCE RELATIVE 11 2 ERCINPOIETLSIM S. MEXICO TELESEISM PROFILE REFRACTION 12 10 19 12km/sec 9km/se MP 16 20 it 28° dist az due S due az S. MEXICO S. MEXICO 296- 0125296- HT 30 T-1842 30 The apparent velocity from an event from southern Mexico, (figure 12) shows good agreement with a phase velocity of approxi mately 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 K3, 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 interpre tation 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 three- station 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 ARRIVAL TIME in Sec 2 3 4 18*4-2 - T 18 +; dist z N60°W az ELKHEAD MTNSELKHEAD 9- 2202295- 0301295- FIGURE 13 REFRACTION PROFILE ELKHEAD MTNS ELKHEAD PROFILE REFRACTION 13 FIGURE 5.6km/sec 17 16 EAIEDSAC inKm DISTANCE RELATIVE 19 10 1 + 15 14 20 HT 31 ARRIVAL TIME in Sec 2 4 43 44 45 46 T- T- 1842 * / 18 km/sec / 5.6 + + / FIGURE 14 REFRACTION PROFILE REGIONAL WNW REGIONAL PROFILE REFRACTION 14 FIGURE / / EAIEDSAC inKm DISTANCE RELATIVE '17 / / /l6 / 19 / +■ / / it 130 km dist MTN RUTH maybe z a wnw REGIONAL 297- 0100297- ^ + 3 / N45 t* +8 15 u / °W 12 / ' , 7 32 T-1842 33 shows late arrivals at stations #15, 18, and 19 and e a rly a r r iv a ls a t stations #3, and 8. Time residuals as great as the one at station #3 (almost one second) probably result from misidentification of the re fracted arrival phase, p , in the background noise. The scatter on y the plot is a result of the uncertainty in location. Contradictory so lutions 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 f it 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 ,16 shows the true geographic distances and the expected 5.6 km/sec phase velocity. This profile shows similar behavior to the ARRIVAL TIME in Sec 18*4-2 - T 0 it 70 km dist I CO , X A LIM C z a 9- 2113295- 2100 294- 2109293- 9- 2355296- 2252295- . / 4.8 IUE 5 ERCINPOIE lmx Co Climax, PROFILE REFRACTION 15 FIGURE 4 / km/sec/ N25°E T 18 EAIEDSAC inKm DISTANCE RELATIVE +1 / 10 /+17 A 2 ll A / 12 / 5.6km/sec 4 / / / / m / / + 194 / 4L5 e W / < 0 2 ---- CHALK f 14 / P / MP / > CREEK r i j ARRIVAL TIME in Sec 4 0 3 2 1 18 IUE 6 RFATO RFL nra LMX CO CLIMAX, normal 16.PROFILE REFRACTION FIGURE it 70km dist LMX CO CLIMAX, az due N due az shots5 5.6km/se EAIEDSAC inKm DISTANCE RELATIVE 10 6 1 1 12 19 11 20 CHALK CREEK HT + 14 13 adjusted profile, figure 1_5, but valuable information concerning size and lateral extent of the anorhalous zone is masked by the lateral re fraction. The final profile, figure 17_, 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 S3 0W, but once again the incoming wave front seems to be focused in a more east-west direction, as the apparent azimuth solution was S6 0W. Figure 18 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. ARRIVAL TIME in Sec 16 15 13 14 2 4 8 1 - T IUE 7 ERCINPOIE OAC AS CO.PASS, MONARCH PROFILE REFRACTION 17 FIGURE HL CREEK CHALK 4.8km/sec EAIEDSAC inKm DISTANCE RELATIVE 16 15 17 10 MONARCH PASS,CO MONARCH it 30 km dist z S60°W az 9- 2045296- 18 MP 20 37 T-1842 38 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 the-wave train. The areas of relative amplitude amplification (shown in figure 18) appear to correlate with arrivals with a predomi nance 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 neces sary 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 con stant 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 pro gram used, four arrivals or three arrivals and an origin time are needed. TRACE AMPLITUDE in mm NORMALIZED TO 10 mm at STATION #18 10 20 30 T- FIGURE 18 AMPLITUDE VERSUS DISTANCE CURVE DISTANCE VERSUS AMPLITUDE 18 FIGURE EAIEDSAC inKm DISTANCE RELATIVE 18 10 CO1 17 16 9 L5 8 19 20 CHI .LK 14 EE R C 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 estim ate an origin time by assuming a Poissons Ratio of 0.25 and using the Wadati Curve (plot of arrival time versus s-p time for a given velocity ratio). Table _3 Local Events date time UTC time local # of sta TQ X Y Mag 294 0453 1053PM 5 12.81 +11.0 +4.9 -0.5M 294 1723 1123AM 4 12.35 -8. 7 -7.5 +1.25M 294 2051 0251PM 3 24.05 +12.8 0.0 0.0M 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 magni tude corrected for distance and frequency response. The upper Arkansas Valley and the San Luis Valley are areas of very little historic seismic activity (Hadsell, 1968). A microearthquake sur vey of the San Luis Valley (Keller and Adams, 1976) recorded 6 local events in a three week survey, a very low level of seismicity consider ing the sensitivity of the recording equipment. Although the level re corded 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 3 0 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 T—1842 44 solution was modified by introducing a timing error of -25 ms and + + -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 4* Q the expected timing error in the system, led to solutions -5 from the observed apparent azimuth (see figure _19 for description of the tech nique 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 geo logic 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 N3 0W. Data from the Monarch Pass and Elkhead Mountains events sug gests a second boundary striking north-south corresponding to the mountain front on the west side of the valley. An interpretative illus tration of the lateral refraction is shown in figure 20. It must be stressed that this interpretation is not unique. Assump tion of a velocity contrast and orientation of the refracting horizon were T- 18^2 45 STA#1 Normal to Apparent Apparent Wave Front Wave Front ST A# 2 ST A# 3 FIGURE 19 THREE STATION APPARENT VELOCITY VECTOR Method D arrange stations in increasing arrival time £) find distance a by T 2 - Tl = a T T b 3-1 ^ draw apparent wave front between STA#2 and a draw normal to apparent wave front -Aapparent velocity equals m x 2-1 IU ^LIM/AA' V2 BOUNDARY v V1=0.75 V2 A: APPARENT AZIMUTH T: TRUE AZIMUTH TO MONARCH 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 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 be tween the array and the Monarch Pass quarry. In contrast, the bound ary 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 lbs., 18 delays) and October 22nd (44,000 lbs., 16 delays), were of sufficient size to warrant further consideration. A crustal refraction survey (Jackson and Pakiser, 1965) of the south ern Rocky Mountains used blasts from Climax to investigate the crustal structure from North Park, Colorado to Casper, Wyoming. In this sur vey, a blast of 25,500 lbs. was observed at distances out to 200 km using recording equipment with less sensitivity than in the present sur vey. 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 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 Moun tains. 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 sub surface velocity contrast under Chalk Creek. Both models are con sistent with the amplitude and dominant frequency measurements as they 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 ex plained 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 T - 1 8 ^ 2 U< D 1 3 -P ! u 3 1 3 P 1 W I tj cn CN 3 rH 1 > 03 CD 1 >i O P 2 i •H U 1 o co ft r H 1 > El < X Q w ^ > < 15 p W . . Q vh C/3 H > S3 C/3 O W D-i H X Eh W U s 2 O H H kP Q W o < Eh H > Eh M O < X (3 Eh § Pn cn w a 8 H X T-1842 50 of the zone. An approximate velocity contrast of 40% would be nec essary 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 move ment 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 specifi cally 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 1_8), 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 c a n n o t 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 g^oUnd niotion 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 explalnation 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 obser vations. 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 com pared to the conventional method when the following conditions exist. Xhe importance of lateral refractions in interpretations drawn from a large scale refraction survey in these cases cannot be discounted with out 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 w d uld add only minor deviations to arrivals from deep refractions or reflections, and 2) the existence of regional geo logic features such as, boundaries of readily discernable geologic pro vinces, 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 optimi zation 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, there fore, 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 con ditions 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 dis tances 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: 1) Several stations should be placed outside the area of in terest for the duration of the survey. These stations will act as con trol for identification and independent confirmation of regional seismic structure. 2) Several stations inside the project area should remain sta tionary 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 geo logic 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 place ment of stations, special attention should be given to specific targets of interest. 4) Roving stations should be left in one position until a suffi cient sampling of available sources, considering both distance and azi muth, is obtained. 5) The density of stations must be high to insure statistical sig nificance 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. T-1842 55 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. V T-1842 56 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, equip ment 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 . T-1842 57 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 de fine 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 pro vide data to help interpret the significance of the structure. Detailed elec trical 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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