results of a detailed gravity survey in the alamosa area, alamosa county, colorado 6979261

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RESULTS OF A DETAILED G R A V I T Y SURVEY I N THEALANOSA AREA, ALANOSA COUNTY, COLORADO

bY

C l a r o n E. Vacke lp rang

September, 1983

Work performed under Contract Number DE-AC07-801012079



DISCLAIMER

This report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency Thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressed hereindo not necessarily state or reflect those of the United StatesGovernment or any agency thereof.



DISCLAIMER

Portions of this document may be illegible inelectronic image products. Images are producedfrom the best available original document.



00E/ID/12079-1

ESL -12

RESULTS OF A DETAILED GRAVITY SURVEY I N THEALAMOSA AREA, ALAMOSA COUNTY, COLORADO

Cla ron E. backe lp r ang

September, 1983

Work performed under Contract Number DE-AC07-80ID12079

E a r t h S c i en c e L a b o r a t o r yU n i v e r s i t y o f Ut ah Res ear ch I n s t i t u t e

420 Chi pet a Way, Su i t e 120

P re p ar ed f o r U.S. Department o f Energy

D i v i s i o n o f Geo th er m al En er g y



T *

NOTI E

T h i s r epor t was prepared t o document work sponsored by the United States

Government.

Department of Energy nor any Federal employees nor any of their contractors

subcontractors or their employees makes any warranty express or imp1Sed o r

assumes any 1egal 1 ab i 1 t y or responsi b i 1i t y f o r the accuracy completeness .

Neither the U n i t e d States nor i t s agent t h e United S t a t e s

o r usefulness o f any information apparatus product o r process disclo sed o r

represents t h a t i t s use would not infringe privately owned r i g h t s .

NOTICE

Reference t o a company o r product name does not imply approval o r

recommendation of the product y the Univers i ty of Utah Research Institute o r

t he U.S. Department of Energy t o the excl usion of ot he rs t h a t may be s u i t ab l e .



TABLE OF CONTENTS

Page

ABSTRACT. ............................................................. .l

INTRODUCTION. ......................................................... .2

GENERAL GEOLOGY.. ..................................................... .2Gravi ty Data. .................................................... . 3

Survey Procedure.............................................5Survey Resul ts. ............................................. -5

Model Results................................................6

GEOLOGIC IMPLICATIONS..................................................8/

A P P E NDIX B.................................,

ILLUSTRATIO

F i g u r e 1. Location Map

P1 a te I A1 amosa Basin Geologic Cross-Seci

P l a t e I 1 Simple Bouguer Gravity Map o f thcA1 amosa Coun ty, Col or ad o

P l a t e 111 Computed three-d imensiona l g ra v i l

P l a t e I V 21/2- Dimensiona l Gravi ty Model Ba

P l a t e V 2 1/2- Dimensional Gravi ty Model D g

........................... 9

.......................... 11

.......................... 12

).........................13

ions

A 1 amosa Area,

1 Alamosa Area, Co lor ad o

I , Alamosa Area, Co lora do

B Alamosa Area, Co lor ad o



ABSTRACT

A t o t a l o f 322 s ta t i o n s , ce n te re d on t h e City o f Alamosa, were surve yed

These data have shown the Alamosai th a gravimeter dur ing September, 1981.h o r s t t o have an i r r e g u l a r t op . T h is i r r e g u l a r i t y i s th ou gh t t o be caused by

pa leova l ey s and /o r down-d ropped f a u l t b locks w i t h in t he Precambr ian ho rs t .

The City o f Alamosa l i e s d i r e c t l y o ve r a l o c a l g r a v i t y low. V o lc a ni c r o c k s

w i t h i n t h i s l o w may co n ta i n a r e s e rv o i r f o r ge oth erma l f l u i d s , as ye t

u n su b s ta n t i at e d by d r i l l i n g . Thermal f l u i d s a re t h o u gh t t o e n t e r t h e Alamosa

a r e a v i a a q u i f e r s from the west (San-Juan Mountains) and/or from t h e R i o

Grande R i f t zone with t h e f l u i d s r i s i n g al on g f r a c t u re s w i t h i n and b o r de r in g

t h e h o r s t. The m ost fa v o r a b l e d r i l l i n g t a r g e t s a pp ea r t o be e i t h e r n ea r t h e

c e n te r o f t h e l o c a l g r a v i t y lo w o r i n t h e f r a c t u r e zone a t t h e edges o f t h e

in fe r r e d down-dropped f a u l t b locks .

\

1



I NTRO DUCTI O N

A t t h e r e q u e s t o f DOE/DGE Idaho Operat ions Off ice th e E art h Science

Lab ora t ory /U n ive rs i ty o f U tah Research I n s t i t u t e (ESL/UURI) undertook a

d e t a i l e d g r a v i t y s ur ve y o f a 200 square m i l e a rea centered on th e City o f

Alamosa, Colorado.

bein g used by seve ral bus inesses i n o r nea r Alamosa.

he at in g o f motels , shopping ma l ls and pu b l i c swimming pools .

Low tempera ture (90-110°F) geothermal f l u id s are cu r r e n t l y

These i nc l ude d i rec t

The City o f A lamosa, t hr ou g h p a r t i c i p a t i o n i n t h e DOE/DGE User-Coupled

D r i l l i n g Program, had c o n t r a c t e d t o d r i l l f o r t h er m al f l u i d s w h ic h w ou ld be

used f o r d i s t r i c t h ea ti ng and i n d u s t r i a l processing. A d r i l l s i t e had been

se le c ted and d r i l l i n g scheduled b u t p r i o r t o i t s commencement, ESL rece ived

t h e r e qu e s t from DQE/DGE I da ho O pe ra ti on s O f f i c e t o p e rf o rm a g r a v i t y

survey.

t h e s e l e c ti o n o f a d r i l l s i t e w i t h i n t h e a re a most p r o m is in g f o r th er ma l

f l u i d s .

T h is g r a v i t y s u rv e y was t o p r o v i d e a d d i t i o n a l d a ta w hi ch w ou ld a i d i n

A meet ing was h e l d w i t h t h e d r i l l i n g c on tr ac to r, D r . Jay Kunze o f Energy

AServ ices Inc., Idaho Fa l l s , Idaho, immedia te ly fo l l o w in g th e f i e l d survey.

rough copy o f th e Bouguer g ra v i t y map was presented and i t s genera l

impl icat ions were d iscussed.

d r i l l e d i r r e s p e c t i v e o f t h e g r a v i t y r e s ul ts .

U nfo rt un ate ly , t h e i n i t i a l d r i l l s i t e was

GENERAL GEOLOGY

The gene ral ize d geology of th e Alamosa are a pre sen ted i n t h i s r e p o r t i s ,

f o r t h e most p a r t , a summary o f Colorado Geological Survey Sp ecia l Pu bl ic at io n

17 by D r . Ri ch ar d L. Burroughs (1981).

2



The City o f Alamosa i s l oca ted a t th e south-cent ra l end o f an

in te rmontane s t ru c t u r a l depression . Th is va l l e y i s app rox ima te ly 40 m i l e s

w ide and 70 m i le s lon g (F igure 1) and i s des ignate d t h e Alamosa Basin.

Trending nor th -south th rough th e center o f th e Alamosa Bas in i s a b u r i e d

bedrock ho rs t. The City o f A lamosa l i e s d i r e c t l y above t h i s h o r s t wh ic h

co ns is t s o f Precambrian rocks and has been penetrated a t a depth of about 5400

f ee t by a d r i l l t e s t l o c at ed ro ug hly 10 m i l e s n o r t h o f town i n t h e CSESW Sec.

16, T.29N., R.1OW.

A d d i t io n a l d r i l l i n g s t i l l f u r t h e r t o t h e n o r th o f town h as shown t h a t t h e

two grabens formed on e i t h e r s id e o f t h e Alamosa H ors t (P la te I ) have

developed a t d i f f e r e n t t i me s g e o l o g ic a l ly and t o s i g n i f i c a n t l y d i f f e r e n t

depths. The Baca Graben which formed t o t h e ea st o f Alamosa has an

a c cu m u la ti on o f v a l l e y f i l l e s ti m a te d t o be a t l e a s t 19,000 f e e t t h i c k .

g raben was w e l l formed p r i o r t o th e l a t e Eocene.

Th is

formed west o f t he ho rs t du r i ng t he Pre-Miocene and has accumulated about

10,000 f ee t o f ma te r i a l wh ich con ta i ns ash -f low t u f f s and vo l ca noc la s t i c s f rom

t h e San Juan v o l ca n i c f i e l d s t i l l f u r t h e r t o t h e west.

no t t hough t t o ex tend , i n any s i g n i f i c a n t t hi ckness , ac ross th e Alamosa Hors t

no r a re they p resen t i n t h e Baca Graben t o t h e east .

G r a v i t y Data

These volcanics are

'

/

The g r a v i t y d a ta i n t h e Alamosa B as in p r i o r t o t h e ESL survey consis ted,

i n p ar t , o f USGS data presented on Map GP-895 by Behrendt and Gajwa (1974); a

Masters Thesis, Un iv er s i ty o f Texas

approx imate ly 3500 s t a t i o n s from t h

Paso (Davis, 1979) which in cl ud ed

of the Department of Defense

g rav i t y l i b ra ry ; and recen t unpub l i shed da ta l from th e Co lorado Schoo l o f

Mines. Wi th th e except ion o f th e Co lorado Schoo l o f M ines data, these g ra v i ty



Figure 1 Pinysiographic subdivisions San Luis basin

modified after Upson 1939 .

4



a 3

d a t a a r e o f a r e g i o n a l n a t u r e w i t h s t a t i o n s l b e i n g o c cu p ie d g e n e r a ll y a t two

Avenues. An a l t e r n a t e base s ta t i o n was setI

a t benchmark H364 loc ate d on t h e

co ns is t i ng o f s ta t io ns numbered 20 0 1 to 2060 and 1 8 0 1 to 1822, was p rov ided by

1967 (Wool l a r d , 1979).

Survey Results

T h e re su l t s o f t h e su rve y a re p re se n te d

5

as the Simple Bouguer Gravity Map



*

O r . Ralph C. Holmer o f th e Colorado School o f M'ines. Th is pr o f i l e ind ica tes

the weste rn edge o f th e A lamosa h ors t as a s teep loc a l g rad ien t wh ich i s

These t roughs then fo rm an i nb ein g a t t h e i r j un c t i on .

Model Resu?ts

r t e dT (

superimposed upon a strong regional g r av i t y g rad ien t. The reg iona l t ren d

u n do u bte dl y co n ti n ue s wes tward i n t o t h e t h i c k vo l ca n i c se c t i o n co mp r i s in g t h e

San Juan vo l ca n i c f i e l d . The g r a v i t y p r o f i l e a c ro ss t h e h o rs t i n d i ca te s t h a t

th e c i t y o f Alamosa se ts on a gr a v i t y low. The Colorado School o f Mines data

d id no t de f ine the easte rn edge o f the A lamosa Hors t .

the re fo re ex tended an add i t i ona l f o u r m i l e s t o t h e e a s t b y ESL. The eastern

edge o f the h o rs t appears t o be loca te d approx imate ly one m i le east o f the

p o i n t where t h e School o f M ines p r o f i l e ended, ( a t s t a t i o n 1822). Whi le the

ESL exten s ion d i d no t comp le te l y c ross th e Baca g raben, s u f f i c ie n t da ta were

Th is p ro f i l e was

1 w i t h t h el o w

o n t h e h o rs t

a cq ui re d t o i n d i c a t e t h i s graben i s l i k e l y formed from severa l down-fau l ted

b locks

Three g r a v i t y h ighs, separa te and d i s t i n c t f rom each o the r , were

d e l i n e a t e d within t h e l i m i t s o f t h i s survey. These occur t o t he no r thwest ,

no r t hea st and sou th o f town and border the lo c a l g r av i t y l ow a t A lamosa.

low i s ce ntered roug hly midway between main s t r e e t (Highway 160) and the

This

a i r p o r t . The lo w i s l o c a l i z e d

ampl i tude. An east-west t r e n d

Monte Vista and Baca grabens.

and i t s t he s e t ro ug hs t h a t

The g rav i t y da ta p resen ted on P la te I were i n t e r p r e t e d i n te rm s o f

subsur face st r u c tu re by comput ing two and one-h a l f d imensiona l and thr ee -

6



d imens iona l models us ing a lgo r i th ms mo d i f i ed a t ESL. The resul tant models,

shown as Plate I11 t o P la te V, a r e e x c e ll e n t f i t s t o t h e Bouguer g r a v i t y

data. I n t e r p r e t a t i o n a l a m b ig u i t ie s i n c l u d e t h e d e n s it y , d e pt h o f b u r i a l and

th i ckn e ss o f t h e va r i o u s ro cks u n i t s , and t h ese r e s u l t i n a no n-un iq ue

i n t e r p r e t a t i o n o f t h e g r a v i t y data.

I n computing the th ree -d imens iona l model, rec ta ngu la r p r isms o f va ry ing

d ime ns io ns and d e n s i t y a r e co mbined t o a p p ro x ima te i r r e g u l a r i t i e s i n t h e

subsurface bedrock. The parameters f o r th e model determined t o be a best

f i t t o t h e Bouguer g r a v i t y map a r e l i s t e d i n Appendix A.

The model on Plate I11 suggests the two nor the rn g rav i t y h ighs a re caused

by Precambr ian rocks o cc ur r in g a t depths between 4,750 fee t and 5,600 f e e t .

The g ra v i t y l o w be twee n t h e n o r t h e rn h i g h s i s f ormed i n t h e model b y p l a c i n g a

down-dropped b lo ck a t a depth o f 5,600 f e e t . T h i s b l o ck h as an i n f e r r e d

s e c t i o n o f v o l c a n i c l a s t i c s 600 f e e t t h ic k above th e Precambr ian rocks. The

g r a v i t y l o w upon wh ich A lamosa i s l o ca t e d was modeled wi t h two down-dropped

b l o cks . The f i r s t , ce q te re d on t h e ow has i n f e r r e d v o l c a n i c l a s t i c s , 1400

feet t h i c k , i n the depth i n t e r v a l o f 5,600 f e e t t o 7,000 feet. The second,

wh ich f o r ce s a n e a s twa rd e l o n g a t i o n o f t h e lo w, h as i n f e r r e d vo l ca n i c l a s t i c s ,

900 e e t t h i c k , i n t h e d ep th i n t e r v a l o f 5,600 f e e t t o 6,500 f e e t . F i n a l l y ,

t h e g r a v i t y h i g h t o th e s ou th o f town was modeled w i th Precambr ian rock s a t a

d e p t h o f 5,000 f e e t .

P l a t e s I V and V show model r e s u l t s usi ng a - d imens iona l a lgo r i thm

o r i g i n a l l y d ev el oped by

d im e ns io na l i n t h a t i t a llo ws f o r p oly go ns o f f i n i t e r a th e r th an i n f i n i t e

USGS (Cady, 1977). T hi s al g or i t h m i s 2l/2-

7



4

Two p ro f i l e s , lab e le d B-B' and D - D , were computed as north-south and

east -west p r o f i l e s , respe c t i ve l y , across the su rvey a rea (P la te 11). Geologic

c o n t r o l f o r th es e p r o f i l e s i s t ak en fro m P l a t e I o f Burroughs, 1981. Th is

geo log i c c ross -sec t ion c rosses the A lamosa ho rs t t o t h e no r t h o f t he su rvey

' a r e a a nd u t i l i z e s deep d r i l l h o le d ata f o r c o n t r o l .

GEOLOGIC IMPL I C A T I O N S

O b vi ou sl y t h e t o p o f t h e Alamosa h o r s t i s v er y i r r e g u l a r , t h e most l i k e l y

cause b ei ng s t r u c t u r a l d e fo rm a ti on ( f a u l t i n g ) o f t h e h o r s t i t s e l f due t o

r i f t i n g . Based upon the picture presented by the ESL Bouguer gravi ty map and

the computer models, i t i s hypothes ized th a t t h e t roughs de f i ne down-faul ted

b locks w i th i n t h e ho rs t . The g r av i t y h i ghs a re more s ta b le po r t i o ns of t he

ho rs t and the l o ca l g r av i t y l ow cen te red on Alamosa i s p oss ib l y due t o an

i n t e r s e c t i o n o f e as t-w es t and n o r t h t r e n d i n g f a u l t s e t s wh ic h may have caused

a smal l b lo ck t o be down-dropped a few hundred feet.

C u t t i n g s from t h e d r i l l h o le r e c e n t ly c om pleted f o r t h e City o f Alamosa

show the presence of a s i g n i f i c a n t t hi ck n es s o f O l ig oc en e ( ? ) v o l c a n i c l a s t i c s

f i l l i n g i n t h e t ro ug hs . These v o l c a n i c l a s t i c s were e nc ou nte re d a t a depth

o f a p p r o xi m a te l y 5,300 fe e t and cont inued u n t i l t h e Precambr ian was

i n t e r s e c t e d a t 6,370 feet J . Z e i s l o f

l o c a t e d on t h e sou thwes t f l an k o f t h e l o c a l g r a v i t y l ow ; hence, a t h i c k e r

s e c t io n o f v o l c a n i c l a s t i c s p ro ba bly e x i s t s w i t h i n t h e c e n t er o f t h e g r a v i t y

1OW.

pers. comm.). T h is d r i l l h o l e was

It i s not known I f volcanic last ic rocks cap Precambrian basement beneath

th e g r av i t y h ighs. Concei vab ly t h e vo l c an i c l a s t i c s cou ld have fo rmed a

b la nk et over Precambrian h ighs and these vo lca n i c l as t i c s were subsequentl y

8\



J5

eroded during the Late Eocene.

in d ic a t i v e of Precambr ian rocks wi tho ut associa ted vo lcan ic cover as assumed

b y t h e 3-D model.

It i s a l s o p o s s ib l e t h a t t h e g r a v i t y h ig hs a r e

I n any event, th e recen t d r i l l - h o l e has shown th e presence

o f t h e v o l c a n i c l a s t i c s w i t h i n t h e l o c a l g r a v i t y l o w a nd t he s e ve ry p r o b ab l y

ex tend th roughout th e pos tu la te d t roughs .

EXPLORATION TARGET

Burroughs 1981) has mentioned th e presence o f p aleo val ey s and/or down-

dropped b locks a long the Alamosa ho rs t and has d iscussed t h e i r importance t o

th e d iscove ry o f geothe rma l f l u i ds . The de ta i l e d g r av i t y su rvey comple ted by

ESL ha s f u r t h e r i n d i c a t e d what appears t o be pa le ov al ley s and/or down-dropped

b lock s w i t h i n t h e Alamosa ho rs t . These appear t o t rave r se the c re s t o f t he

h or s t i n a no r t h and eas t-wes t d i re c t i o n and cou ld a l l ow f o r commun ication o f

water between the Monte Vista and Baca grabens.

Serv ices, Inc. has shown th e presence o f a t le a s t 1,060 f e e t o f

vo l can i c l as t i c s upon the A lamosa ho rs t nea r t he a i rpo r t .c om p ri se p a r t o f what i s c a l l e d t h e c o n fi n ed a q u i f e r t h e r e ap pea rs t o b e

a d e q u a t e r e s e r v o i r p o t e n t i a l f o r ponded g eo th erm al f l u i d s p a r t i c u l a r l y w i t h i n

the areas of the down-dropped b locks.

The ho le d r i l l e d by Ene rgy

Since these rocks

The source o f th e t te rs w i t h in th e A lamosa area i s no t known.

e R i f t zone sohe ar ea i s p a r t o f t h e R i

waters i s poss ib le .

f a u l t s o f t h e Alamosa h o r s t an d c o u l d co n c e iv a b ly f i l t e r t hr ou gh . t h e h o r s t

a long th e f ra c tu re s o f th e down-dropped b locks .

c o u l d be moving l a t e r a l l y away from a heat source i n th e San Juan vo lcan ic

f i e l d and i n t o t he A lamosa a rea as pos tu l a ted by Bu rroughs 1981). C e r t a i n l y

th e h ig h heat f low measurements 2.85 + 0.65 HFU) i n t he San Juan vo l ca n i c

ep c i r c u l a t i o n o f m e te or icI

These h e at ed f l u i d s w o uld l i k e l y r i s e al o ng t h e b o r d er

A l te rna t i ve l y , t he rma l wa te rs



f i e l d s u pp o rt t h e p re se nce o f a bur ied heat source.

vo lca nic rocks w i t h in t h e Monte V is ta graben would p la ce th e Alamosa area down

d ip and a l ong the hyd ro log i c g rad ien t .

The eas tward d ip o f the

Regard less o f t he therma l f l u i d source, i t i s s t i l l p os s ib le t o f orm ula te

an e xp l o ra t i on ta rg e t i n t he Alamosa a rea.

f o r t h e r m a l f l u id s, then an area such as t h e l o c a l g rav i t y l ow cen te red on

Alamosa wou ld l i k e l y have a th ic k e r v o lca n ic s ec t io n and hence a grea te r

I f t h e v o l ca n i cs f or m a r e s e r v o i r

chance fo r development o f a s iza b le res erv o i r .

th e same te x tu re or , more impo r tan t l y , th e same p or os i ty and permea b i l i t y .

d r i l l - h o l e c en te re d on t h e g r a v i t y low would l i k e l y i n t e r s e c t t h e g r e a te s t

Not a l l vo l can i c f l ows have

A

t h i ckness o f vo l c an i c l as t i c s and thus i nc rease th e odds f o r a p roduc t i ve we l l .

S i nce f l u i ds m igh t be f l ow ing a l ong f rac tu res as we l l as t h rough

a q u if e rs , an a l t e r n a t i v e d r i l l l o c a t i o n c o u l d t e s t a m a j o r f r a c t u r e zone.

Wh i l e t h e ma jo r bound ing f a u l t s t o t h e A lamosa h o r s t can no t be ove r l ooked as

p o t e n t i a l ta r g e t s , a b e t t e r a re a m i gh t be f ou nd on t h e h o r s t i t s e l f a l on g one

o f t he bo rde rs t o t he pos tu l a ted down-faul ted b l ocks. An a rea cou ld be

Id e n t i f i e d where two o r more o f these locks a nd /o r f a u l t s i n t e r s e c t .

a r e a m a y e x i s t a l o n g t h e n o r t h e r n s i d e o f t h e l o c a l g r a v i t y l o w .

necessa ry t o b e t t e r de f i ne t he edges o f any down-fau lt ed b l ocks be fo re

Such an

It would be

, *

i s area. Th is cannot be done w i t h a dd i t io n a l g r a v i t y d a ta b u t

d w i th a r e f l e c t i o n s e i sm i c s urve y. T h is c o u l d be q u i t e

expensive ($20,000 t o $40,000) b u t i t would s t i l l b e 1

a dry hole.

o s t l y t h a n d r i l l i n g

10



P

ACKNOWLEDGEMENTS

Spec ia l t hanks a re g i ven t o Dr. Ralph C. Holmer o f th e Colorado School o f

M ine s f o r h i s t i m e s p en t i n d i s cu s s in g t h e g e op h ys ic a l d a ta i n

Area and f o r pe rm iss i on t o use the g r av i t y da ta acqu i red by t h e

Mines along Highway 160. Thanks ar e a ls o gi ve n t o Bear Creek M

i n Tucson, A r iz o n a f o r t h e i r generous l o a n o f a grav imeter .

he Alamosa

School o f

ning Company

11



REFERENCES

Behrendt, John C., and Bajwa, LaCretia Y., 1974, Bouguer Gra v it y Map of

Bur roug hs, R ic hard L., 1981, A Summary o f t h e Geology o f the San Luis Basin,

Colorado Geolog ica l Survey Specia l Publ ica t ion 18.

Colorado, Geop hysical In v e s ti g a ti o n s Map GP-895.

Colorado-New Mexico w i t h emphasis on th e Geothermal Po te nt ia l f o r th eMonte Vista Graben.35 PP.

Da vis , Gene Ho we ll, 1979, A G ra vi t y Study o f th e San Lui s Basin, Colorado.

Wool lard, G. P. 1979, The New G ra vi ty System--Changes i n in te rn a ti o n a l

Thesis, Master of Science , Un ive rs i t y o f Texas a t E l Paso.

g r a v i t y base valu es and anomaly values. Geophysics, v. 44, no. 8, p.1352-1 366.



?

APPENDIX AProject name: A1amosa Area, Col orado

G r a v i t y S u r v e y

Model : 3-D G r a v i t y Model

U n i t s i n F eet

PRISM x1 x2 Y1 Y2 D1 D2 Dc

1

2

3

4

5

6

7

8

9

10

11

12

13

14

16

17

18

20

21

-85000.

-21120.

-12144.

15840.

8448.

21120.

18480.

-13728.

-26400.

-2640.

8448.

31680.

-26400.

-85000.

-26500.

-2640.

-85000.

75000.

52800.

-85000.

-26400.

8448.

100000. -85000. 265000. 0. 20000. -0.83

-2640. 10560. 27984. 5000. 5200. 0.83

-7392. 20000. 25000. 4900. 5200. 0.83

24816. 10000. 15840. 4500. 5200. 0.83

21120. 29568. 32736. 4750. 5200. 0.83

24816. 15840. 40000. 5000. 5200. 0.83

31680. -18480. -7392. 5400. 5600. 0.83

-2640. -18480. -5280. 5300. 5600. 0.83

52800. -85000. -18480. 5000. 20000. 0.83

8448. -10560. 6500. 5600. 7000. -0.45

2112. 5600. 6500. -0.45 #

18480. -10560.

52800. -18480. 265000. 7000. 20000. 0.83

31680. -18480. 47520. 5600. 20000. 0.83

-26400. -85000. 265000. 5600. 8000. 0.63

31680. 47520. 265000 5400. 20000. 0.83

8448. 6500. 47520. 5600. 6200. -0.45

-26400. -85000. 265000. 8000. 10000. 0.35

-85000. 265000. 12500. 20000. 0.83

75000. 5000. 265000. 10000. 20000. 0.83

-26400. -85000. 265000. 10000. 20000. 0.83

-2640. 6500. 40000. 5200. 5600. 0.83

31680. 6500. 40000. . 5200. 5600. 0.83

13



. v

Grid ParametersG r a v i t y ModelNumber o f Gr i d Po in ts i n X D i r e c t i o n : 19 Number o f Gr id Po in ts i n Y D i r e c t i o n : 15 G r i d Sp ac in g i n F e e t : 5280.00

X O f fs e t i n F eet : 0.00Y O f f s e t i n Feet : 0.00



A P P E N D I X B

P R I N C I P A L F A CT O RS FO R G R A V I T Y S T A T I O N S

15



.E a

2



--

Et .



EFFICIENCYLINE 22-210

L.



EFFICIENCY LINE 22-210

L

...



.

I



L



.... .- - . . ..- .

... -. .

. . . . . . . .- . .._ . - . .. - .

. . . . . . . . .

. . .

. . - -. .. - ....

. . . . . . . . ._

..._ . . . - -. . . ~ _._..

I



-*

J



b ,.......

@ * EFFICIENCYLlNEa 22-2IO



* EFFICIENCY LlNEe 22-2 1 0 Ld



@ * EFnCIENCY LlNEe 22-2 1 0

LJ I-,

ll p= WJ



.



MONTE VlSTA GRABEN ALAMOSA HORST

PLATE 1

ALAMOSA BASIN

CROSS-SECTIONS

ak er 8urroughr. 1981



X f :

h

n

W

ba

mb7;;a

,N.

aa

SSR=3 88

-

* *I II I I

II

0 10.56 21.82 31.68 42.24 52 80 63.36

K LOFEET

OBSERVED=CRLCULATED= X

I I I # '

1a l l u v i u m

OLYGON DENSITY

CONTRRST1 -0.43000

2 -0.331308

I I

1 WRFRCE

6

KILOFEET PLATE I V



\

10.56 21.12 31.68 42.24 52.80 6S36 73.92 84.48 55.04 115.60 116.16

OLYGON OENSUYILOFEET

OBSERVED=

CRLCULATED- X -o.ssoeo-0.s5oou-0.4soee-0.43000

I I I I I I I II I 1 .

W

1

a l l a v i a n

results of a detailed gravity survey in the alamosa area, alamosa county, colorado 6979261

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