chapter 3 - the geology and geophysics of the geysers geothermal area figure 3.1 diagram showing the...

Chapter 3 - The geology and geophysics of The Geysers geothermal area

Figure 3.1 Diagram showing the northward progression of Tertiary and Quaternary volcanism with time, the San Andreas fault and the extrapolated positions of the Mendocino Triple Junction between 3 and 5 Ma (from McLaughlin, 1981).

37


Figure 3.2 Map of California showing major fault zones. Faults are shown as blacklines (dotted where concealed). Arrows and numbers indicate direction and amount ofmotion of the Pacific and Gorda plates with respect to the North American plate (from Hill et al., 1990).

Key: B = Banning; BP = Big Pine; BS = Bartlett Springs; BZ = Brawley seismic zone; C = Calaveras; CN = Concord; CU = Cucamonga; DV = Death Valley; E = Elsinore; FC = Furnace Creek; G = Garlock; GV = Green Valley; H = Hayward; HG = Hosgri; HRC = Healdsburg-Rodgers Creek; IM = Imperial; LVC = Long Valley Caldera; M = Maacama; M = Mission Creek; NI = Newport-Ingelwood; OT = Ortigalita; PM = Pinto Mountain; PV = Palos Verdes; PVA Panamint Valley; R = Rinconada; RC = Rose Canyon; SA = San Andreas; SC = San Clemente Island; SG = San Gregorio; SJ = San Janinto; SN = Sierra Nevada; SNA = Sur-Nacimiento; W = Whittier; WM = White Mountains; WW = White Wolf.

38

Figure 3.3 (A) Seismicity adjacent to the Mendocino Triple Junction. (B) Cross-section of seismicity from (A). The hypocentral pattern shows the subduction of the Gorda plate beneath the North American plate (from Walter, 1986).


40

Figure 3.4 Locations of earthquakes with M in California and western Nevada 1980 - 86, and mapped Holocene faults (from Hill et al., 1990).


41

Fig

ure

3.5

(a)

Nor

th-w

est t

o so

uth-

east

cro

ss-s

ecti

on s

how

ing

eart

hqua

kes

wit

h M

1

.5 a

djac

ent t

o T

he G

eyse

rs 1

980

- 86

.(b

) A

s (a

) bu

t for

a s

outh

-wes

t to

nort

h-ea

st c

ross

-sec

tion

(fr

om H

ill e

t al.,

199

0).

Key

: M

= M

aaca

ma,

BS

= B

artl

ett S

prin

gs, S

A =

San

And

reas


42

Figure 3.7 Structural model of The Geysers geothermal system (from McLaughlin, 1981).


45

Figure 3.8 Major crustal features of northern California and their relation to the emplacement of magma beneath The Geysers (from McLaughlin, 1981).


46

Fig

ure

3.9

Sch

emat

ic c

once

ptua

l mod

el o

f T

he G

eyse

rs g

eoth

erm

al s

yste

m. H

TR

= h

igh

tem

pera

ture

res

ervo

ir. O

pen

arro

ws

repr

esen

t the

flo

w o

f st

eam

. Clo

sed

arro

ws

repr

esen

t the

flo

w o

f w

ater

(co

nden

sate

and

wat

er)

(fro

m T

rusd

ale

et a

l., 1

993)

.


50


Fig

ure

3.10

(a)

Map

of

The

Gey

sers

, sho

win

g re

sidu

al g

ravi

ty b

ased

on

redu

ctio

n de

nsit

ies

of 2

.67

g/cm

3 . C

onto

ur in

terv

al =

2

mG

al, (

from

Ish

erw

ood,

197

5). (

b) M

ap o

f T

he G

eyse

rs s

how

ing

mag

neti

c fi

eld

cont

inue

d up

war

d to

3 k

m.

Con

tour

inte

rval

, 20

gam

ma

(fro

m I

sher

woo

d, 1

975)

. T

he s

outh

-wes

t to

nort

h-ea

st li

ne r

efer

s to

the

cros

s-se

ctio

n us

ed f

or g

ravi

ty a

nd m

agne

tic

fiel

d m

odel

ling

(F

igur

e 3.

11).

53

Figure 3.11 Gravity and magnetic field modelling for the cross-section shown on Figure 3.10. (a) Calculated and observed gravity fields, along with the gravity fieldcalculated for a spherical body centred at 13.5 km depth. (b) Calculated and observedmagnetic fields along the same profile. (c) Crustal model from gravity and magneticmodelling (from Blakely & Stanley, 1993; Isherwood, 1975).

Figure 3.12 Geoelectrical modelling for the cross-section shown on Figure 3.10. The model was compiled from direct current soundings, bipole-dipole mapping data, time domain electromagnetic soundings and a two-dimensional magnetotelluric (MT) model. Numbers indicate resistivities (in ohm-m) of the units in the model. (A) and (B) represent alternative 2 ohm-m magma bodies added to test the sensitivity of the data (from Blakely & Stanley, 1993).


54

Fig

ure

3.15

Map

of

The

Gey

sers

pro

duct

ion

area

sho

win

g th

e ar

ea o

pera

ted

by e

ach

deve

lope

r in

199

2 (f

rom

Bar

ker

1992

). T

he lo

cati

on o

f po

wer

gen

erat

ing

unit

s is

indi

cate

d by

the

sym

bol w

ith

asso

ciat

ed n

umbe

r. I

nfor

mat

ion

on th

e ge

nera

ting

uni

ts is

giv

en in

Tab

le 3

.3.


65

Figure 3.17 As 3.14, but with the locations of power generating units operating duringthe year represented superimposed on the seismicity (from Ross, 1996).


71

Fig

ure

3.19

(a)

Epi

cent

res

(dot

s) lo

cate

d by

the

UN

T n

etw

ork

(Sec

tion

4.1

.1)

betw

een

11/1

988

and

8/19

89 w

ith

Md

> 0

.7.

Ope

n ci

rcle

s re

pres

ent

UN

OC

AL

inje

ctio

n w

ells

, wit

h ci

rcle

are

a pr

opor

tion

al to

the

volu

me

inje

cted

ove

r th

e af

orem

enti

oned

pe

riod

. N

umbe

red

squa

res

are

sele

cted

PG

&E

pow

er p

lant

s. (

b) C

ross

-sec

tion

sho

win

g ea

rthq

uake

s lo

cate

d in

the

vici

nity

of

inje

ctio

n w

ell D

X-6

1 S

epte

mbe

r to

Dec

embe

r 19

86 (

from

Sta

rk, 1

992)

.


74

Figure 3.20 Plot of monthly average delivery rates for the south-east Geysers effluent pipe project (http://www.geysers-pipeline.org/).

7

64

211

255

363375

334331332

383378

0

50

100

150

200

250

300

350

400

Sep-

97

Oct

-97

Nov

-97

Dec

-97

Jan-

98

Feb-

98

Mar

-98

Apr

-98

May

-98

Jun-

98

Jul-

98

Mon

thly

Ave

rage

Del

iver

y R

ate,

in K

ilogr

ams

per S

econ

d


75

chapter 3 - the geology and geophysics of the geysers geothermal area figure 3.1 diagram showing the...

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