eruptive history of the dieng mountains region, central java, and potential hazards from future...

8/11/2019 ERUPTIVE HISTORY OF THE DIENG MOUNTAINS REGION, CENTRAL JAVA, AND POTENTIAL HAZARDS FROM FUTURE

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EVALUATION OF INITIAL INVESTIGATIONS

DIENG GEOTHERMAL

AREA,

CENTRAL

JAVA, INDONESIA

by

L . J . P .

Muffler

U . S .

Geological

Survey

U * S . Geological

S u r v e

OPEN TILE REPORT

T h i s r e p o r t i s preliminary and

has

n o t been e d i t e d o r reviewed f o r .

o o a f

onaity vith G e o l o g i c a l Survey

s t a n d a r d s o r n o m e n c l a t u r e *


2/24

CONTENTS

Page

Summary and recommendations 1

Introduction

1

Evaluation procedure

2

Summary

a n d

analysis

of

initial investigations

3

Electrical geophysical survey 3

Dipole survey 4

Modified

Schlumberger profile 6

Schlumberger sound ings

6

Audio-magneto-telluric measurements

7

Geochemical survey

7

Geologic

interpretation

10

Recommendations

12

Objectives and

requirements

o f

drilling

14

V olc a n i c, earthquake, a n d l a n d s l i d e

h azar d s 1 7

Organization

o f t h e

exploratory program

18

Supporting geological and geophysical investigations 19

References cited 20

Illustration

Figure 1 . To p o g ra p h i c m a p

at

1:25,000

o f

Dieng-Batur area (20-meter

contour interval. Shows dipole surveys made

from

source

A

a n d

source

B

o f Jacobson, Pritchard, a n d Keller ( 1 9 7 0 ) . Also

s h o w s

drill sites recommended

by

Muffler

a n d t h e

other

members

o f t h e

evaluation

team

f o r

200-meter

exploratory

h o l e s . Sites are numbered in order

o f

decreasing

priority. j n

pock

Table

Table

1

Hist o ri c

phreatic

eruptions a n d hydrothennal explosions

i n t h e Dieng'Mountains.

1 7 a


3/24

SUMMARY AND

RECOMMENDATIONS

Evaluation o f

t h e geophysical,

geochemical,

a n d

geological

studies

carried

o u t i n

1970 i n

t h e

Dieng

Mountains has

been completed.

On the

basis of

t h e e v a l u a t i o n ,

i t

i s

recommended

that

exploratory drilling

of t h e Dieng

geothermal field be

undertaken following

t h e

general

program

outlined

by

Muffler ( 1 9 7 0 ) .

I t

i s recommended that

s i x

explorato ry holes be

drilled,

each

to a dep th

o f approximately

200

m . Proposed sites

a r e :

1 .

Pagerkandang Crater

2 .

Just west o f

Telaga Terus

3 . Just

north

o f

Pawuhan

4 . 3 / 4 km east o f Sekunang

5 . Just

southeast o f

Sidolok

6 .

J u s t west o f

Dieng

Wetan

After completion

o f t h e

2 0 0

m holes,

t w o

holes should

be

drilled t o

a

dep th o f approximately 650

m .

INTRODUCTION

Interest in possible utilization

o f

the geothermal resources o f the

Dieng

Mountains

dates back t o 1 9 2 8 , w h e n the Dienst

Van Den

Mijbouw drilled

several exploratory h o l e s , t h e

deepest

t o 80 m (Purbo-Hadiwidjojo, 1968;

Hoesni, Arismunandar, a n d

R a d j a ,

1 9 7 1 ) .

Although

temperatures up

t o

145C

were f o u n d ,

t h e r e was n o

follow

up t o this limited

exploratory drilling.

Renewed interest

i n

t h e area

was

stimulated by

t h e

UNESCO Volcanological

Mission ( T a z i e f f , Marinelli, and Gorshkov,

1 9 6 6 ) ,

and was furthered by a

geothermal

mission

sponsored

by

t h e

French

government

i n

1 9 6 8 .

Both

missions recommended further investigations

a n d

development

o f t h e

geother

resources

o f t h e

Dieng

a r e a .

At the request o f

t h e

Indonesian

government

and under t h e auspices of t h e

United States Agency

f o r

International Development ( U S A I D ) ,

a

report

recommending

s t e p s t o evaluate an d

( i f

warranted) t o

develop

t h e

geotherma

potential o f t h e Dieng

Mountains was

prepared by t h e United States

Geological

S u r v e y ( M u f f l e r ,

1 9 7 0 ) . This

report

was based on

a

study of

t h

available literature a n d a brief

visit

to t h e Dieng Mountai ns. The report

concluded

t h a t t h e

Dieng probably contained significant reserves of

geothermal

e n e r g y , a n d

recommended

a three s t e p

program

with

initiation

o f

each

successive s t e p t o be

conditional upon

favorable indications

from

t h e

preceeding s t e p . Recommended steps were:


4/24

The alms

o f

s t e p s

1

a n d 2 were t o determine:

a )

whether the fumaroles are

Indeed t h e surface

manifestations

o f a

l a r g e , deep geothermal

system, b )

the optimum siting o f exploration a n d development wells, c )

the

size of

t h e geothermal

s y s t e m , a n d

d )

t h e

characteristics

of t h e

reservoir ( d r y

steam v s hot

w a t e r ;

acid v s

neutral; temperature;

transmissibility;

e t c . ) .

Geophysical, geological,

and

geochemical investigations recommended

as

s t e p 1

i n

t h e

USGS repor t

were carried o u t

during

t h e dry season

of 1970

as f o l l o w s :

a .

An electrical

geophysical

survey was conducted

by Group

Seven

I n c . ( J a c o b s o n a n d

Pritchard,

1 9 7 0 ; Jacobson,

Pritchard,

and Keller, 1970)

under

contract with USAID with strong counterpart support from t h e

Geological Survey o f Indonesia

( G S I )

a n d

t h e

Power Research Institute

( P R I ) .

b . Fumaroles, hot springs, a n d rivers o f t h e Dieng Mountains a n d

surrounding areas were sampled

a n d

chemically analysed

by

t h e

USGS and

t h e

G S I .

The

data are

summarized

and interpreted in

a preliminary geochemistry

report ( T r u e s d e l l ,

1 9 7 0 )

and in

a

final geochemistry report ( T r u e s d e l l , 1971)

c . Excellent

aerial photographs

of t h e

Dieng

Mountains an d

surrounding

terrain were provided by Geotronics

( p r o c e s s e d

by

P . N .

Aerial

Surveys)

un der

contract t o U S A I D .

d .

Using these photographs, a photogeologic map o f the Dieng area

was prepared by t h e GSI ( P a r d y a n t o ,

1 9 7 0 ) .

Additionally, four topographic maps at 1:5,000

scale

of about

2 1

km^

of the

Dieng

a n d Batur areas were

prepared from

surveying

data by

t h e

Indonesian

Power Research Institute ( P R I ) . Contour

interval

i s

5

meters.

EVALUATION PROCEDURE

The results o f

t h e

initial investigations have been reviewed and evaluated

by

a

team

composed o f

the following

persons:

?

L . J . P . Muffler, Geologist, USGS, Chairman

Djajadi

Hadikusumo, C h i e f ,

Volcanological

S u r v e y , GSI

H . L . O n g , Lecturer, Institute

Technology

of

Bandung

( I T B )

Vincent

R a d j a ,

Chief

o f

Operation, Electric Power Survey

Project, PRI

M . T . Z e n , Senior Lecturer, ITB


5/24

The evaluation t eam had

t h e

following

g o a l s :

1 . T o review and evaluate t h e initial investigations completed in

1 9 7 0 .

2 . T o

recommend

whether

o r

n ot t o proceed w i t h

exploratory

d r i l l i n g .

3 .

I f exploratory drilling was recommended, t o indicate

t h e n u m b e r ,

approximate

location, and

depth

o f holes

required

to

adequately

assess

t h e

potential

o f t h e

geothermal

s y s t e m .

The

evaluation

team

( e x c l u d i n g

O n g )

visited

t h e

Dieng

Mountains on January

1 4 an d 1 5 , 1 9 7 1 , with M . R . KLepper ( U S G S ) , D . D .

Fowler

( U S A I D ) ,

Sutardono

( P R I ) , an d

I

Sjamsu ( P R I ) . Blessed by

relativel y dry

weather,

the group evaluated

t h e

thermal features

and

their geologic

s e t t i n g ,

inspected possible drilling

sites and

considered

problems o f

road

access,

site

preparation, and

sources of

drilling

water.

Evaluation

o f t h e initial investigations

was carried

o u t a t t h e GSI in

Bandung

o n

January

18-21 a n d a t

USAID

in

Djakarta January

2 2 . A

preliminary

evaluation

r e p o r t ,

dated January 2 2 ,

1 9 7 1 ,

was written by Muffler , with

the

counsel

and

assistance

o f the

other members of

t h e

evaluation team

and

w i t h

the invaluable assistance o f M .

R . K l e p p e r .

The final evaluation

report was prepared by Muffler

a t t h e

USGS

i n Menlo Park, California,

February

1 - 1 0 , 1 9 7 1 , and supersedes t h e

preliminary

report.

SUMMARY AND

ANALYSIS OF INITIAL

INVESTIGATIONS

E lec tri c a l

Geop hys ical

Surv ey

An electrical

geophysical survey

using

several techniques was conducted

i n

t h e Dieng

Mountains

in

July a n d August, 1970

by

Group

S e v e n ,

I n c . under

contr act AID/ea-123. Assistance and location surveying

were

provided

b y

G S I .

Results of

t h e

electrical

survey

are contained

in

a preliminary

report dated August 2 7 ,

1970 ( J a c o b s o n

and Pritchard,

1 9 7 0 )

and

in a

final

report dated November 1 , 1970 ( J a c o b s o n , Pritchard, an d

Keller,

1 9 7 0 ) . At

the

request

of USAID, Group Seven

subsequently prepa red

a revised Plate I

showing metric coordinates.

These reports contain

useful

data

a n d

interpretative

information, and

are

valuable aids

in

evaluating

t h e

geothermal potential

o f t h e

Dieng Mountain s

a n d

i n

recommending

sites for exploration drill h o l e s . Efficient use of

t h e reports, however,

i s

impaired by several

deficiencies

i n

coverage

and

in

format

of

data

presentation:

1 .

The dipole survey did not extend sufficiently far

o u t

from

t h e

low resistivity

anomalies t o give

a

reliable indication

o f

background

resistivity values i n non-thermal

g r o u n d .

A dipole source near B a t u r ,

f o r

example,

would have been very

useful.


6/24

2 . Both dipole sources

were

sited in thermal

ground,

whereas at

least one s h o u l d

have been

sited in

non-thermal

ground, as

outlined by

prelimi nary Schlumberger

s u r v e y s .

>

r i i %

i

^ * v

I . 7

3 .

Additional Schlumberger profiling would have been useful,

particularly along

the roads t h a t connect

K awah

Sikidang w i t h

the Dieng-

Batur r o a d . 1 /

4 . The

maps

accompanying

t h e final

report are very difficult t o

use,

primarily because

o f

inadequate geographical

control.

The three

plates

accompanying

the final report show no coordinates, bench marks,

or

triangulation

points, and t h e DIeng-Batur road i s s o generalized that i t

i s difficult t o use

as a r e f e r e n c e .

These deficiencies occurred despite

the availability

o f

published topographic

maps.

Also, Plate

I I

for

some

inexplicable reason was

drafted with north t o t h e bottom.

5 .

There

are several discrepancies between illustrations. For

example,

figure

2

of

t h e final

report

i s

misleading,

i n t h a t there

should

be

a

gap

of

2-1/2

km

between locations

3 and 4 ( c f .

figure

1 of the

final

report

a n d

figure

5 o f

the preliminary

report).

6 . The tables accompanying

t h e

final report are very difficult t o

use

because there

i s n o

explanation

of the

tabular

format, a n d

because

the

column

headings

o f

table 2 are incompletely

l a b e l e d .

Three electrical techniques were used by

the

Group Seven field

p a r t y :

dipole

s u r v e y ,

modifie d Schlumberger profiling,

a n d

Schlumberger

s o u n d i n g s .

In

addition, t w o audio-magneto-telluric traverses

were made, as

well

a s

o n e electromagnetic

s o u n d i n g .

Dipole Survey

The dipole

results

are

presented by Jacobson, Fritchard, and

Keller

( 1 9 7 0 )

a s t w o

apparent resistivity

maps ( P l a t e s

I

and I I )

contoured

in ohm-meters.

One map

gives results

from

a

source dipole

located south

o f Dieng

Rulon

(Source A ) , a n d t h e other

gives results from

a

source northwest o f Karangengah

( S o u r c e B ) . These maps have been redrafted at

1:25,000

and

are

shown on figure

1 o f

this

r e p o r t .

Resistivity

varies

inversely w i t h temperature, salinity,

* /

There are significant discrepancies between the topographic map

( b a s e d on the published 1:50,000 Batur

s h e e t )

and the sketchy base m aps

used for Plates I and

I I o f

Jacobson, Fritchard

a n d

Keller

( 1 9 7 0 ) .

The

discrepancies are particularly

bad

in the vicinity

of

dipole source B

a n d

could not

be

satisfactorily resolved

on figure

1 o f this

r e p o r t .


7/24


8/24

The

boundary

of the

geothermal

system c a n n o t be

defined precisely

from t h e

resistivity

data,

for data points outside

t h e

anomalous area are insufficient

t o reliably determine t h e apparent resistivity of

t h e

non-thermal volcanic

r o c k s .

T h e

5 ohm-meter contour chosen

b y Jacobson,

Pritchard, and Keller

( 1 9 7 0 )

i s

a reasonable g u e s s ,

b u t

there

i s

n o a

priori

reason

why

t h e

boundary

o f t h e

system

should

correspond

t o t h i s

particular

contour.

The

absolute

value

o f

resistivity within

a geothermal system i s a

complex

function

o f

temperature,

salinity, porosity, a n d

clay and zeolite mineral

c o n t e n t , a n d t h e

aggregate

effect these factors

have

on

resistivity varies

from one geothermal system t o

a n o t h e r ,

depending

on

t h e

relative importance

o f t h e f a c t o r s .

T h e s e c o n d

area

o f

low resistivity,

a t

Tjandradimuka, i s incompletely

defined,

b u t

appears t o have

an

extent

o f at

least 1 k m ^ . The dipole data

indicate t h a t

there

i s

no

apparent connection

between

t h e

Tjandradimuka

geothermal

system

and t h e much larger Dieng geothermal

s y s t e m ,

a t least at

depths shallower than several k i l o m e t e r s .

Modified Schlumberger profile

A resistivity profile along

t h e road

from

Dieng

Kulon

t o

Batur

was

constructed from

modified

Schlumberger arrays about five source

array

l o c a t i o n s .

The resistivity

contrast on t h e

profile

i s good, and t w o

areas

o f low resistivity are defined ( j u s t

west of

Dieng Kulon

and t o t h e

northwest o f Karantengah). These zones o f low resistivity were emphasized

in t h e

prelimin ary Group Seven

r e p o r t , b u t

deprecated

in t h e

final

r e p o r t .

There

seems t o

be n o good reason

for

t h i s downgrading

o f t h e

data, and

the

anomalies shown on

figure

5

o f

Jacobson

a n d

Pritchard ( 1 9 7 0 ) and which

can

easily be contoured

o n

figure

2

o f

Jacobson, Pritchard, and Keller

( 1 9 7 0 )

appear t o be valid. I t should

be

noted t h a t dipole survey A shows

a

lobe

o f resistivity less than 10 ohm-meters covering

t h e

area northwest of

Karantengah.

No resistivity low

i s shown

on Plate

I I ,

but

t h e

data near

s o u r c e

dipole

B i s

suspect

f o r t h e s a m e reasons t h a t

applied

t o source

dipole

A (discussed a b o v e ) .

Schlumberger sound ings

F o u r

Schlumberger soundings

t o spacings

( A B / 2 )

o f 2 0 0

meters were made over

areas t h a t showed

very

low

resistivity

i n

the

dipole

s u r v e y . These

show

t h a t

ground

o f

very

low

resistivity

a t

Kawah Sikidang

a n d

Kawah Sileri

fumaroles extends downward

t o

at least a depth

o f

approximately 200

m .

A

sounding

within Pagerkandang

crater s h o w e d that

resistive rock overlies

material o f

low resistivity

t h a t c a n readily be interpreted

a s

h o t .

The

fourth sounding, on the south

rim

o f Pagerkandang c r a t e r , probably

i s n o t

meaningful owing t o steep

t o p o g r a p h y .

At

m o s t ,

i t reflects

t h e

highly

resistive rock expected

i n a ridge 1 0 0 m above t h e

bottom

o f

Pagerkandang

c r a t e r .


9/24

m easurem en t s ( A M I )

travers es

were

made: o n e along

the

Dieng-B atur road,

the other

for

1

km NNE

o f

Kaw ah

Sikidang.

Data

are given

only for the

although Jacobson, Pritchard, and Keller

( 1 9 7 0 ,

p . 2 7 )

both surveys yielded resistivity

values

w h i c h w e r e similar in

t o those

measured by

the m o r e cumbersome profiling and soundi ng methods.

Dieng-B atur trav erse showed

low

resistiv ity at Pawuhan, near

Sidolok,

Timbang.

The

trav erse

NNE of Sikidang

showed a near-surface layer

6 0 thick w i t h a resistiv ity near 100 ohm-meters

un derlain

by

o f

somewhat lower resistivity. Resistivities near Sikidang

itself

quite low, as expected

from

the extensiv e fumarolic activity.

Geochem ical

Survey

geochemical

survey

o f the hot springs,

fumaroles,

and

streams

of

the

and n eighborin g areas

w as

carried out

in

August,

1970

by

. H . Truesdell of the U.S.

Geological

Survey.

A

prelimi nary report o n

w o rk

w a s

sub mitted t o

USAID on August 2 4 ,

1 9 7 0 . The

final

geochemical

(Truesdell, 1971) w as

delayed pendi ng

receipt o f geochemical analyses

USGS laboratorie s i n

Menlo Park.

systems

may

p roduce either

a

m ix ture

of hot-water and

s team ( a

s y s t e m ) ,

or

s team

alone ( a v a p o r - d o m i n a t e d or dry steam system).

importa nt

in geothermal

ex p lorat ion t o

determi ne

w h i c h

type o f

system

in

a giv en

area*

Up w ard transfer of 1^0 and heat o ccurs by m o v e m e n t

f

liquid

w at er

in

a

hot w a t e r

system, but

by movem en t

of

steam

in a

v a p o r -

system. Since most chlorides are not appreciably soluble in

team

below

300C, a

hot-w ater

system can be p r e d i c t e d w i t h a

high degree

f

c o n f i d e n c e

i f

the surface

springs have

appreciable chloride, generally

than 50 mg/1.

In the Dieng

Moun t ain s

the abunda nce o f fumaroles, the scarcity

of

flowi ng

springs,

and

the

low chloride contents

o f

m o s t flowi ng springs w ould suggest

a

vap or-dom in ated

system

w ere

i t no t for the high-chloride water at

Kaw ah

Sileri (173

mg/ 1 ) an d

at P ulosari

(426

mg/ 1 ) springs.

Inasmuch

as high-

chloride springs canno t be associated w i t h a dry-steam system

(White,

an d Truesdell, 1971),

the

Dieng geothermal system i s almost certainly

a hot-water system,with a probable chloride

content

o f 700

m g / 1

(Truesdell,

1971,

p .

1 1 ) .

In this

respect,

the

Dieng system

i s

likely

t o

be

similar

t o

Wairakei, New Zealand or Hengil, Iceland, but to differ

from

the two k n o w n

commercial

vap or-dom in ated

systems of

Lardarello,

Italy, and The Geysers,

California.


10/24

8

Samples from

rivers

draining

the

Dieng Mounta ins were taken

t o

determine

J

whether any high-chloride thermal water diluted by ground water i s 1

discharging directly

into r i v e r s . Many rivers draining t h e Dieng

Mountains

I

contained chloride

i n

excess o f

background

( 1 0 - 2 5

mg/1), and in t w o rivers

( K a l i Tulis

a t

160-219 mg/1 a n d Kali

Dolok

at 61 mg/1) the chloride content

J

was quite h i g h . These

t w o

rivers drain

the

Sikidang

and Sileri

a r e a s , S

respectively, and t h e high

chloride values are

interpreted

t o

reflect

mixture o f chloride-bearing thermal water w i t h dilute meteoric

w a t e r .

On

t h e

basis o f

t h e

distribution o f thermal

activity,

Truesdell ( 1 9 7 0 )

concluded

t h a t i n

t h e

Dieng Mountains

at

depths

l e s s than 200 m there

are

three

geothermal

s y s t e m s ,

which may be interconnected

a t greater d e p t h s *

O f t h e s e , t h e Pagerkandang

system

(including

S i l e r i ,

S i g l a g a h , a n d Bitingan)

t h e l a r g e s t ,

with an area o f

2 . 5

k m ^ . Smaller systems

are

a t

Sikidang and

Tjandradimuka. The chloride-rich water

a t

Pulosari may

represent

outflow

from

either

t h e

Sikidang

o r

Pagerkandang s y s t e m .

Truesdell

( 1 9 7 0 ,

p .

1 3 )

notes t h a t the elevation

o f

K awah Sileri

i s

1875

m ,

and

he

assumes that this level

marks

the top of chloride

water

in t h e

Pagerkandang a r e a . There i s

a

4%

gradient between

Sileri a n d P u l o s a r i . I f

a similar gradient i s assumed from Pulosari

t o Sikidang, t h e

estimated

elevation

o f

chloride water under Ka wah Sikidang

(elevation 2035

m )

i s

1850

m .

The floor

o f

Pagerkandang crater

i s

at about

2 0 3 5 m . Therefore,

both

at Pagerkandang and

Sikidang a

hole

o f a t least

200 m would be

required t o

intercept chloride-bearing

w a t e r .

Hot-spring

fluid compositions,

particularly

t h e

content

o f S i O

and the

alkali

a n d

alkali-earth

r a t i o s ,

can be used

t o predict t h e subsurface

temperatures

o f

hot-water systems ( M a h o n ,

1970;

Fournier

a n d

Truesdell, 1970)

Use

o f

these chemical indicators, however,

i s

based on a number

o f a s s u m p t i o n .

( W h i t e , 1 9 7 0 ) :

1 . Temperature-dependent reactions, with adequate

supply of

constituents in t h e local reservoir

r o c k s ,

2 .

Water-rock equilibration

w i t h specific mineral assem blages

a t

high reservoir temperatures,

3 . Rapid flow o f water

from

reservoir t o surface s p r i n g s ,

4 .

Negligible reacti on in

transit at

lower

temperatures,

5 .

Absence o f dilution or

mixing with

other waters

a t

intermediate

l e v e l s .

White ( 1 9 7 0 )

emphasizes

t h a t

depending on discrepancies

t h a t

may

exist

between

these assumptions

a n d the actual conditions within a

s y s t e m , t h e

indicated temperatures may be only slightly

in error ( g e n e r a l l y

indicating

a

minimum) or grossly incorrect, either high o r low.


11/24


12/24

1 0

Geologic Interpretation

Aerial

photographs w ere m ade in August, 1970 by Geotronics Surveys under

contract

AID-47

7-25-1 from USAID. Approximately 125 km^

over

the Dieng

Moun t ain s w as

flown

at

1:20,000,

an d

a n

area

of

3,500

km

2

extendi ng

from

Gun un g

Slamet east to the

Dien g Moun t ain s

and thence southeast to

Gunun g

Sumbing w as flown

at 1:35,000.

The photography

i s of

superb quality,

and

the contractor

should

be commended for doi ng an excellent

j o b .

Based on these photographs, a

photogeologic

m a p a t 1:25,000 w a s

prepared

by

GSI

(Pardyanto, 1970).

Geologic

features of particular significance

s h o w n o n the

m a p

are ( a ) the

n u m erous

young flows that radiate

from

the

volcan oes around Gunung Pakuwadja,

( b )

the^ major phreatic explosion

craters, ( c ) a zone o f

volcan ic lin eam e n t s

extendi ng northwest

from

Gun un g

B utak

t o

Pagerkanda ng

crater,(d)

conspicuous east-west structural

lineaments south o f Batur,

an d

( e ) N.10 W . structural lineaments

in the

v i c i n i t y

o f

the

Dieng-B atur road between Batur

and

Gunung

Nagasari.

The

1:50,000

geologic m a p

accompa nying the

thesis o f

Gunawa n ( 1 9 6 8 , Plate

I I )

i s

the most detailed geologic m ap

seen t o datei'. Although

there

appears

t o

be n o m a p legend, the units on the m a p can be correlated

w i t h

the thesis t e x t , w h i c h gives a detailed

sequence

o f v olcanic rocks, based

o n

geomorphic criteria. The m a p shows

in

a

general w ay the same volcan ic

and structural lineaments

that

Pardyanto ( 1 9 7 0 )

presents,

and

in

addition,

the text, Plate

X ,

and figure

2

giv e m u c h d e t a i l o n the N.10

W . and

N.10

E . lin eam e n t s defined by the explosion craters at Timbang and Gua Djimat.

Petrologic data

are contained in

Gunawa n

f

s thesis ( 1 9 6 8 ) an d

in a

pa per

by Neum an n v a n Pa da ng ( 1 9 3 6 ) . B oth writers

suggest

a sequence of

m a g m a t i c

d ifferen t iat ion

t h at

correlates well

w i t h

the

age sequence deduced

from

geomorphology. There are

no analy tical

ages, and no fossil

control.

All

o f the

volcan ic

rocks appear t o be o f Quaternary age ( l e s s than 3 m i l l i o n

years).

Although there

hav e

been numerous historic phreatic eruptions,

only

one

truly

volcan ic

( i . e . , mag ma tic) eruption

from the

Dieng

Moun t ain s

has

been reported

in historic time, that

o f

Gunung Pakuwadja

in

1 8 2 6

(Gunawan,

1 9 6 8 ,

p . 8 9 ) .

* /

A v olcanic lineament as

used

in

this

report i s a linear array of

d iscrete

v olcanic eruption points.

A structural lineament

i s an alinement

o f to pographic o r v egetativ e features that reflect a fracture o r fault in

the

earth.

B oth

volcan ic

lineaments

and

structural lineaments are commo nly

interpreted t o

be

surface expressions o f

deep zones

o f fracture

or weakness

in the earth.

- f c

Regretably,

Gunawa n's m a p

an d

thesis w ere not ma de av ailable either

t o Pardyanto

for

use

i n his photogeologic evaluation, t o Truesdell for use

in locati ng fumaroles an d

hot

springs, or t o

Muffler in

1 9 7 0 . Early access

t o

Gunawan* s

w ork could have greatly simplified and expedited the

w o rk

o f

all three scientists.


13/24

1 1

summary of these three sources of geologic

data,

the

Dieng

Moun t ain s

a complex of separate extrusions (lavas and p yroclastic cones)

in composition through the

Quaternary

from

early

basalt

(Gunung

through oliv ine-bearing

andesite, pyroxen e andesite, hornblen de

(Gunung Butak) t o the young

biotite

andesites of the m o u n t a i n s

Gunung Pakuwadja. Laharic breccias,

ash,

and some lake sediments

complexly

intercalated w i t h

the

pyroclas t ic

cones

and

lav a

flows*

The

itself

i s a

tuff-

and

sedi ment-filled

to pographic d e p ression

extrusions.

canic lineaments are obvious: ( a ) a line of craters and flows

the m o u n t a i n crest east o f Gunung Butak

( f i g *

l )

b ) a zone of craters extendi ng southeast from Pagerkanda ng

crater

to

Pakuwadja

and

thence

m any miles

southeast to Gunung Sundoro and

Sumbing ( s e e figure 2 of Muffler, 1970).

geothermal systems

o f the Dieng

Moun t ain s

are clearly related to the

structural

lineaments. The resistiv ity

d a t a ( f i g . 1 ) show

the thermal

areas

of Pagerkanda ng and Sikidang are probably

a t d e p t h t o

form

a

single geothermal system, here named the Dieng

system. This geothermal system

i s

coincident w it h

the

zone

of

lineaments that extends from Pagerkanda ng

crater

t o Gunung

The area of most intense surface thermal activ ity ( i n and

the

Pagerkanda ng crater)

i s

at the intersection of this

n orth w es t-

and the east-west volcanic lineament along the crest of

t he

smaller

and probably separate Tjandradi muka geotherm al

falls

o n this

same east-west v o l c a n i c lineament

at

its in t ersect ion

the

N .

10W.

structural

lineaments o f Timbang, Gua Djimat, e t c .

associat ion

o f the two geothermal systems w i t h tectonic zones of

(characterized

by fracturing and

volcan ic

activity) suggests that

permeability at

d e p t h

in the g e o t h e r m a l

systems

w i l l be

d u e

t o

fractures,

as at

The

Geysers

in

California. There is n o

in the Dieng Moun t ain s for an extensiv e cap-rock overlying a

d efin ed

reservoir,

as seems to

be

the case

a t

Larderello,

Italy,

in many oil fields.

^Tazieff,

Marinelli , and Gorshkov ( 1 9 6 6 )

ap p ear

to

placed excessiv e

emphasis

on the possibility that the

basin -fillin g

of the Dieng

Plateau could form a

cap rock. Data

acquired d urin g

present inv estigations

show that

these sediments overlie

only

a

small

part

the

Dieng geothermal system

as

d e f i n e d by the resistivity

l o w .

There

well be

layers o f

restricted permeability at d e p t h in

the

Dieng

and indeed the existence o f such

layers m ay

be

importa nt

in

field characteristics under exploitation. But one can n ot

predict'

existence or location of such layers

from

available

surface data,

and

the cap

rock concept

in

the

Dieng

Moun t ain s

i s

of little p ract ical

in geothermal exploration.


14/24

1 2

RECOMMENDATIONS

I n view o f t h e favorable f i n d i n g s f r o m t h e i n i t i a l i n v e s t i g a t i o ns , i t i s

r e c o m m e n d e d t h a t

e x p l o r a t o r y d r i l l i n g i n

t h e

D i e n g

g e o t h e r m a l

s y s t e m be

undertaken f o l l o w i n g

t h e

g e n e r a l p r o c e d u r e

o f

s t e p

2

o f

t h e

program

o u t l i n e d b y

Muffler ( 1 9 7 0 , p . 1 1 ) .

S e v e r a l s l i m c o r e

d r i l l h o l e s s h o u l d

b e d r i l l e d t o

a p p r o x i m a t e l y

2 0 0 m e t e r s

t o d e t e r m i n e

t e m p e r a t u r e g r a d i e n t s ,

c a l i b r a t e

t h e

g e o p h y s i c a l d a t a ,

a n d

acquire

f l u i d a n d

r o c k

s a m p l e s .

A f t e r c o m p l e t i o n

o f t h e s e 2 0 0 - m e t e r h o l e s , t w o h o l e s s h o u l d be d r i l l e d

t o

a p p r o x i m a t e l y

6 5 0 m e t e r s . T h e p u r p o s e s

o f

t h e s e 6 5 0 - m e t e r

h o l e s

a r e t o

d e t e r m i n e t h e

b a s e t e m p e r a t u r e

o f t h e g e o t h e r m a l s y s t e m , t o

allow c o l l e c t i o n

o f f l u i d s a m p l e s , a n d

t o

a l l o w t e s t i n g o f t h e p r o d u c t i o n c h a r a c t e r i s t i c s

o f

t h e

g e o t h e r m a l s y s t e m .

I t

is m y

r e c o m m e n d a t i o n ,

an d t h e r e c o m m e n d a t i o n of

the o ther four m e mber s

o f the e val u at ion team, that

the

e x p l or at ory

d r i l l i n g

p r o g r a m i n c l u d e

si x

h o l e s

e ach

2 0 0

m e t e r s

d e e p r a t h e r t h a n

t h e f o u r

or igin al ly

s u g g e s t e d by

Muffl e r

(1970).

The

a d d i t i o n a l cos t for the t w o

ex tra

ho les sho uld no t

exceed $50,000 an d m i g h t be s ign if ican t ly

less.

The

two

a d d i t i o n a l h o l e s

a r e

p r op os e d

to

e v a l u a t e

p a r t s

of the D i e n g ge ot h e r m al sys t e m tha t cou l d

n o t b e

ad e q u at e ly as s e s s e d by

i n f e r e n c e

f r o m only four holes.

I ndi v i dua l

si tes a r e s h ow n o n

f i g u r e

1 . A l t h o u g h the si tes

a re

s h ow n a s

points,

they

a re no t

to

be i n t e r p r e t e d

as

exa c t lo c a ti o n s tha t c a n n o t be

c h a n g e d

by

a

f e w ten s or ev en

1 0 0

m eters.

T he superv i si n g geo lo g i st

or

e n g i n e e r s h o u l d h a v e l a t i t u d e

to

shi ft p r e c i s e l o c a t i o n s t o m i n i m i z e

l o g i s t i c

or e n g i n e e r i n g problems.

I ndi v i dua l

sites,

li sted in

o r d e r

of d e c r e a s i n g p r i o r i t y a r e as follows,

w i t h m e t r ic coor d in at e s re f e r r in g

t o

f igu r e

1 :

1

Within

t h e P a g e r k a n d a n g

C r a t e r ( 8 9 , 0 5 0 N ; 3 7 , 8 5 0

E ) ; e x a c t

l o c a t i o n within

t h e

c r a t e r

i s n o t

i m p o r t a n t . T h e s i t e

i s

s u g g e s t e d

a s a p r i m e

t a r g e t b y

t h e

g e o p h y s i c s ,

t h e g e o c h e m i s t r y ,

a n d t h e g e o l o g y . A c c e s s

t o

t h e

s i t e will

r e q u i r e r o a d

w o r k ,

p o s s i b l y i n c l u d i n g c o n s t r u c t i o n o f a b r i d g e o v e r K a l i

D o l o k .

T h e c o n s e n s u s o f

t h e e v a l u a t i o n

t e a m a n d

M e s s r s .

F o w l e r , S u t a r d o n o ,

a n d

K l e p p e r

was t h a t

upgrading

o f

an e x i s t i n g b r o a d t r a i l up

t h e

s o u t h e a s t

s i d e o f

t h e c r a t e r

f o r

a

d i s t a n c e

o f a b o u t 1 km w o u l d

p r e s e n t f e w e s t d i f f i c u l t i e s . A g r o u p o f s p r i n g s

a n d s e e p s

a t t h e

c r a t e r

bottom w o u l d

p r o v i d e a s o u r c e

o f

d r i l l

water

d u r i n g t h e

r a i n y s e a s o n .

T h e s e s p r i n g s

a r e r e p o r t e d

t o

d r y

u p d u r i n g

t h e

d r y

s e a s o n ;

i f

t h e y d o ,

w a t e r will

b e

a p r o b l e m .

2 A s i t e s o u t h w e s t o f T e l a g a T e r u s

( 8 6 , 5 0 0

N ; 3 9 , 6 0 0 E ) on

t h e

margin

o f

t h e l o w r e s i s t i v i t y

a r e a a t K a w a h

S i k i d a n g . T h e s i t e

i n d i c a t e d i s a t a s u f f i c i e n t d i s t a n c e

f r o m o b v i o u s ,

vigorous

t h e r m a l a c t i v i t y

t o

p r o v i d e g o o d

g r o u n d f o r

t h e s h a l l o w p a r t s o f

t h e h o l e a n d t o o t h e r w i s e minimize l i k e l i h o o d o f o p e r a t i o n a l

p r o b l e m s a n d h a z a r d s . N o r o a d c o n s t r u c t i o n

s h o u l d

be n e c e s s a r y ,

a l t h o u g h s o m e s t r e n g t h e n i n g

o f

t h e

b r i d g e a c r o s s K a l i

l u l i s

may

b e r e q u i r e d .

W a t e r

c a n b e o b t a i n e d

from

K a l i T u l i s , 1 0 0

m

t o

t h e

s o u t h e a s t .


15/24

1 3

3 .

This site

( 8 8 , 4 2 5

N ; 38,600 E ) on

t h e

bench

j u s t

north

of the

village o f Pawuhan i s chosen

t o

evaluate

t h e

southeastern part

o f t h e Pagerkandang 2 . 5 ohm-meter resistivity

low

a t

the lowest

feasible

drill

collar

elevation

( 2 , 0 2 5

m )

and t o see

i f

t h e

Pagerkandang center o f thermal activity extends

t o

t h e southeast

a t d e p t h . Access could

be

v ia the road constructed to

Pagerkandang

c r a t e r . Drill

water

i s available from Kali D o l o k .

4 . A

s i t e

( 8 5 , 3 0 0 N ; 39,675 E ) north

o f t h e

road junction 750 m east

o f

t h e

village

o f Sekunang, i s chosen t o evaluate

t h e

southeast

ward extension

o f the

resistivit y anomaly

a n d t o s e e

whether the

geothermal reservoir i s continuous a t depth between Sikidang an d

t h e fumaroles

a t

Telaga Tjebong. Access problems are minimal;

t h e nearest water appears

t o

be

a t S e k u n a n g , a

distance

o f

about

800 meters.

5 .

A site ( 8 8 , 2 2 5 N ;

37,000

E ) southeast o f t h e

village

of

Sidolok

on the Dieng-Batur road

i s

chosen

t o

t e s t whether or not t h e

l o w

resistivity

zone shown on

the

modified Schlumberger survey

represents a deep thermal

anomaly or merely

near-surface outflow

from Kawah

S i l e r i .

Access

i s n o

p r o b l e m ;

water

probably

can

be

had

from Kali Dolok

o r

related irrigation

c a n a l s within 2 0 0

meters

o f

the

s i t e .

6 . A site

( 8 7 , 6 2 5

N ;

39,875

E ) about 1 / 4 km west o f

Dieng

Wetan and

about 1/2

km

south o f

Dieng Kulon

i s chosen t o evaluate

whether

o r

n o t

the

geothermal anomaly extends

t o t h e northeast

side

o f

t h e

Dieng

P l a t e a u .

Access

i s e a s y , a n d

water

can

be

obtained

from t h e immediately

adjacent Kali

T u l i s .

T h e order o f

priority given

i s not

necessarily t h e

order i n

which t h e holes

should

be

d r i l l e d .

The Pagerkanda ng

a n d

Telaga

Terus holes

are

likely

t o

be in the hottest ground and t o

present

greatest drilling problems.

Accordingly, perhaps a hole such

a s

Pawuhan

o r Sidolok s h o u l d

be

drilled

f i r s t , i n

order t o

allow

t h e

contractor

t o

gain

experience i n local

conditions

and

I n t h e problems o f geothermal d r i l l i n g .

There should be sufficient latitude in t h e drilling contract for t h e

supervising geologist or engineer

t o

terminate

a

hole

a t

depths shallower

than

2 0 0

m o r

t o

drill onward

f o r

several tens o f meters beyond 200 m

s h o u l d

conditions

warrant.

There i s nothing magic a b o u t t h e

2 0 0 m number;

i t

was

chosen

a s a

figure

which

on t h e

average

should be deep enough t o

allow acquisition o f information needed

t o

more

fully evaluate t h e

geothermal

s y s t e m .

Drilling at Tjandradimuka i s n ot

recommended

a t present f o r four reasons:

1 . The

small

size of

t h e

resistivity

a n o m a l y .

2 . Difficult a c c e s s .

3 . History

o f

violent phreatic eruptions

a t

Timbang

t o t h e

southwest

a n d

Gua Djimat t o

t h e southeast.


16/24

A .

Presence o f

toxic gases at

Timbang

and

Gua

Djimat.

Sites for the

650-m holes

should not

be

selected

until

most o f t h e

data

from t h e 200-m holes are a v a i l a b l e .

( T h i s

was Muffler's 1970

i n t e n t ,

although his report

i s n o t explicit i n t h i s respect.) The

650-m holes

should

be

sited

principally on t h e basis

o f thermal gradients

determined

from

t h e 200-m

h o l e s ,

keeping i n

m i n d

that

t h e

primary object o f

t h e

650-m holes i s t o determine

t h e

base

temperature o f

t h e

geothermal s y s t e m .

I t

i s

obviously inappropriate t o

s i t e a n y

development holes until both

t h e

200-meter and 650-meter exploratory holes are drilled

a n d

the results

(including geochemical sampling and analysis of f l u i d s ) a r e a v a i l a b l e .

OBJECTIVES AND REQUIREMENTS OF DRILLING

Drilling

should

be conducted s o

as t o

achieve

t h e

objectives outlined on

p . 1 1 o f

Muffler's

report ( 1 9 7 0 ) .

The 200-meter holes

should

be

designed

t o :

a .

Determine

temperatures

a n d

temperature gradients,

b .

Allow collection o f water

a n d gases

from

the

holes

after

completion o f drilling

c .

Sample

t h e rocks

drilled

d . Test

t h e

geophysical, geochemical, and geological i n d i c a t o r s .

An accurate

determination

o f t h e temperature gradient from 100 t o 2 0 0

meters

i s

i m p o r t a n t . Recent USGS research drilling in Yellowstone National

P a r k ,

Wyoming, U S A , has

shown t h a t t h e

only accurate method

o f

determining

p r e -

drilling

ground temperatures i n geothermal areas i s t o drill f o r no longer

than

eight h o u r s , . l e t the

hole

stand f o r 1 5 hours, and t h e n take a bottom-

hole tem perature^

.

Accordingly,

a t

depths

of

100 t o

2 0 0

meters, drilling

should

progress

a t n o

more

than 3 3

meters per

d a y ,

s o

t h a t four o r

more

In

the Yellowstone

d r i l l i n g , i t

was feasible

t o leave t h e rods i n t h e

drill hole overnight

a n d take the

bottomhole temperature through

the rods

a n d t h e drill b i t .

I n

caving

g r o u n d ,

i t

may

n o t

be

possible

t o

leave t h e

rods in the

hole

overnight

without danger

of them

becoming

s t u c k .

I f

drill

mud

i s

u s e d ,

i t

may

prove impossible t o lower

a

temperature probe

t o

t h e

hole bottom through

t h e

mud

after i t

has

settled

for 1 5 h o u r s . I f

s o ,

i t

may be necessary t o

l o w e r t h e

probe

j u s t

after

t h e e n d o f t h e

drilling s h i f t ,

a n d

leave the probe a t hole bottom overnight.


17/24

1 5

temperatures

can be

determi ned from

1 0 0

t o 200 m e t e r s

d e p t h * - .

m ad e t o determi ne bottomhole temperatures at depths

than

100 m e ters

whenev er feasible.

of w a t e r a n d gases should

be

collected from

the

holes after

o f

d rillin g and

recovery

of the

hole

f r o m

drill-water loss.

the hole shows a tendency to cave, slotted liner should

be

hung

in

the

s o that

there will be conti nuing access t o

all

d e p t h s of the

hole

fluid sampling.

The min im u m

internal diameter of

the

liner should

be

least 2 - 1/4 inches ( 5 . 7 2 c m ) t o

allow

2-inch O.D. sampling dev ices t o

Accordingly, holes

should

be drilled at a dia meter sufficient

allow slotted

liner o f at least 2-1/4 inches I . D .

to be

inserted.

w o u l d

be ideal t o hav e

the

holes cored throughout their total d e p t h

20

m e ters

or s o , but this may no t be

fin an cially

feasible. Some core,

least

every 30 meters,

i s

essential, however, to

permit laboratory

o f

physical properties such as porosity, den sity,

e t c . This obviously

w ill

require a dia mo nd drill rig

w i t h

capability.

Assum in g

t h at

the rig i s equipped for wire-line

o f core,

i t

may not cost appreciably m o r e t o

core

conti nuously

t o

core only

intermittantly.

These alternativ es should be explored

possible contractors.

are

t w o

p ossible programs

for interrelati ng the

20 0 -meter

a n d

the

holes:

1 . Drill t he 20 0 -meter holes at a dia meter sufficiently

great

t o

allow

a

string

o f

casing to be cemented

in a t

20 0 -250 m eters,

retaini ng sufficient diameter

w i t h i n

this

casing

t o

allow

d rillin g

t o

proceed

safely and successfully t o

650

meters.

2 . Drill the 20 0 -meter

holes

at

small

diameter s, and drill completely

n e w

650 -meter holes

after thorough

e v a l u a t i o n o f best diameters

and casing program.

There are

a number

of

ways of

m easurin g

temperatures at depth.

ters, thermocouples, resistance t hermo meters and other electrical

accurate

and

efficient

w h e n they

work,

but in

a

number of

d rillin g

projects ( e . g . ,

Yellow s t on e

and Nicaragua)

they

have

a

d is tressin g

tendency

t o fail,

owi ng

t o

shorti ng

of the

cable

or

t o

of

the po tti ng

compound

under the

high-tem p erature

condi tio ns

and

the presence o f

a n

electrically conducti ng fluid.

Maxim um -record in g

(mercury in glass) do no t have

this

p ot e n t ial

for

electrical

but care must

be

taken t o insulate

the

thermo meter w hen temperature

are

suspected. I f temperatures at

d e p t h

in the Dieng Moun t ain s

determ in ed usin g electric al methods, great care

m u s t

be taken

t o

insure

gear

i s reliable and will

n ot fail;

the Dieng Mountains are a long,

way from the

nearest point where

electronic gear can

be repaired or

At the

very

least,

several m ax im u m -read in g thermo meters

(with

tight restrictions s o

that

the mercury column i s not

shaken

d o w n

during

withdraw al) should be

o n h a n d

t o

back up the

ge ar i n

case of failure.


18/24

1 6

A choice between these alternatives requires

c o s t and

drilling equipment

data

not

available

to m e , and s h o u l d

be

made after

preliminary consultation

with

prospective drilling contractors

a n d

after evaluation o f t h e USAID-

assisted 2,000-foot drilling program in

Nicaragua.

The drilling a n d casing

program

should be designed t o preclude any

uncontrolled eruptions

o f s f c e a m and w a t e r , particularly any

eruptions

around ( o u t s i d e ) the cas i n g Based

on t h e

nature o f the surface activity

i n t h e Dieng a r e a , i t i s anticipated t h a t t h e geothermal

water

level

will

be a t considerable

d e p t h , perhaps a s much a s 2 0 0 meters.

I f

this i s the

c a s e ,

pressures

are

unlikely

t o be

high until

t h a t

d e p t h ,

and

even

a t

considerably greater depths

pressures should be largely

counter-balanced

a n d controlled

by

the weight o f

water

standing i n t h e h o l e . However, a t

most depths there i s

some

potential f o r a surprise

e r u p t i o n , a n d

accordingly

casing

s h o u l d

be cemented

i n

t h e

hole

s o t h a t any eruption will occur through

the casing^

.

I f

there

i s

any indication

o f

pressure, an appropriate valve

a n d stuffing

box

should

be

mounted on

t h e

c a s i n g ,

a n d

drilling should be

done through

the

valve

a n d

stuffing b o x . I f wireline i s

used,

provision

s h o u l d

b e

made t o withdraw t h e core under positive pressure

within

the drill

s t r i n g .

With

regard

t o appropriate drilling operations

a n d

casing

p r o g r a m ,

i t i s

important that t h e level o f water standing i n t h e

hole

b e noted and

recorded

each d a y . Gradual rise o f water

l e v e l

on successive days can give warning

o f increasing

pressure

a n d potential drilling o r casing p r o b l e m s .

Water

level

measurements are

also

very

important scientific data essential t o

t h e

interpretation and evaluation o f t h e geothermal s y s t e m .

The wild

bores

a t T h e G e y s e r s ,

California

a n d

a t

Wairakei, New

Zealand are apparent ly both due

t o

insufficient depth

o f c a s i n g ,

with high-

pressure

steam

coming up

t h e open hole t o t h e f o o t o f the casing and then

o u t into t h e

formation

t o the s u r f a c e .

It

i s

not

possible

to g i v e

positive

guidelines

f o r casing depths a n d

number o f s t r i n g s .

I n t h e

USGS Yellowstone

d r i l l i n g ,

where

t h e

thermal

water

t a b l e

was a t the ground s u r f a c e ,

a n d

appreciable pressures

over

hydrostatic developed

a s

drilling p r o g r e s s e d , one string o f 4-1/2

x

5-1/2

inch casing

was

generally

s e t

a n d cemented a t 2 0 f e e t , and

a

s e c o n d string

o f s m a l l e r

diameter near 100 f e e t . A

t h i r d s t r i n g

was s e t

a n d

cemented

a t

about 500 feet i f much greater depth a n d higher temperatures were anticipated

I f

t h e

geothermal

water

table

i n t h e Dieng

Mountains

i s

near

the

predicted

elevation ( 1 8 7 5

m . ,

Truesdell, 1 9 7 1 ) , t h e

casing

program need

n o t

be

s o

thorough

a s a t

Yellowstone. Casing programs

f o r

developmental geothermal

wells

a r e

discussed by Giovannoni ( 1 9 7 0 ) a n d by Dench ( 1 9 7 0 ) , b u t their

discussions related

t o

deep

( 1 , 0 0 0 - 2 , 3 0 0

meter) production holes t h a t

require f a r

more

casing

o f

f a r

larger diameter t h a n t h a t

needed

f o r t h e

200-m exploratory

holes

i n t h e Dieng Mountains.

__________


19/24

1 7

VOLCANIC, EARTHQUAKE, AND

LANDSLIDE

HAZARDS

g e o t h e r m a l

system r e q u i r e s a p o t e n t

h e a t s o u r c e

a t

d e p t h .

Although s u c h

h e a t s o u r c e h a s n e v e r been d r i l l e d

o r

s a m p l e d , various l i n e s

o f

g e o l o g i c

t h e

presence

o f

a

h o t b o d y o f i n t r u s i v e rock a t d e p t h s

f 3 km o r g r e a t e r . Meteoric w a t e r c i r c u l a t e s t o t h e s e d e p t h s , a c q u i r e s

f r o m

t h e h o t body o f r o c k , a n d

b u o y a n t l y r i s e s

t o t h e

s u r f a c e i n t h e

c o r e o f

t h e g e o t h e r m a l s y s t e m .

n m o s t

e x p l o r e d

geothermal

a r e a s ,

i g n e o u s a c t i v i t y i s

n o t

o n l y

i n f e r r e d

t h e e x i s t e n c e o f t h e

g e o t h e r m a l

s y s t e m , b u t i s directly e x p r e s s e d a t

e a r t h ' s s u r f a c e by Q u a t e r n a r y v o l c a n i c r o c k s . T h e s e r o c k s

r e p r e s e n t

o f molten rock ( m a g m a ) t h a t made

t h e i r

way

t o

t h e

s u r f a c e r a t h e r

s o l i d i f y i n g

a t d e p t h .

A

n u m b e r

o f

g e o t h e r m a l a r e a s a r e

i n

regions where

t h e r e h a s

been

h i s t o r i c

volcanic a c t i v i t y .

F o r

t h e Wairakei

g e o t h e r m a l

f i e l d i n New Z e a l a n d i s l o c a t e d

only

1 0

m

n o r t h o f Lake T a u p o ,

t h e

l o c u s

o f t h e 1 3 1

A . D .

T a u p o

Pumice

e r u p t i o n ,

world's l a r g e s t volcanic e r u p t i o n o f t h e p a s t 2 , 5 0 0 y e a r s .

geothermal f i e l d i n New Z e a l a n d i s l o c a t e d 2 5

km

n o r t h e a s t o f

T a r a w e r a ,

which

l a s t e r u p t e d i n 1 8 8 6 .

Die n g

g e o t h e r m a l s y s t e m i s a lso lo c a ted

in

a n a r e a

of

v e r y y o u n g a n d

c o n t i n u i n g

vol can is m . Al t h ou gh there

a r e

n o r a d i o m e t r i c d a t e s

n

t h e reg i o n, the p hys iogr ap hy

of

m o u n t a i n s suc h

as

G u n u n g Fakuwa dj a,

K e n d i l , a n d

G u n u n g Sun do ro

a n d

of

c ra ters

s u c h

as

P age rkan d an g,

Mer d ad a, a n d Fa n g o n e n i n d i c a t e s

that

v o l c a n i c a c t i v i t y i n the Die n g

h a s

p e r s i s t e d u n t i l v e r y

rec en t times. Indeed,

G u n u n g P a k u w a d j a

s

r e p o r t e d

to

h ave er u p t e d in

1826,

a l t h o u g h

the p r e c i s e n a t u r e o f t h e

is

n o t clea r f r o m

the

sc a n ty repo rts (Gunawan,

1 9 6 8 ).

h a v e

bee n n o t r u e v o l c a n i c

e r u p t i o n s

(i.e.,

tho se t h at

e r u p t i o n

of

m o l t e n rock) in the

Dieng Mo u n t a i n s s i n c e

1826,

there

p h r e at i c er u p t ion s o r hyd r ot h e r m al explosions^- ; t h e s e

b e e n d e s c r ib e d

in d e t ail

by

Gu n aw an (1968,

p . 9 0 -1 0 3 ) ,

an d

a r e

i n

t able 1

of

t h is rep o rt.

a p p a r e n t f r o m

tab le

1 t h at viol e n t p h r e at ic er u p t ion s or hyd r ot h e r m al

i o n s h a v e o c c u r r e d i n rec en t y ears

at

K a w a h Sileri, a n d t ha t si m i la r

c a n

be

e x p e c t e d i n the

future. Accor d in gly,

d r i l l i n g a t K a w a h

s h ou l d be

a v o i ded, an d a ny buildi n g s or

g e n e r a t i n g

f a c i l i t i e s

P h r e a t i c

e r u p t i o n s

a n d h y d r o t h e r m a l e x p l o s i o n s are

c o m m o n l y

c o n f u s e d ,

p h r e a t i c eruption s e n s u s t r i c t u o c c u r s when magma c o n t a c t s g r o u n d w a t e r ,

i t t o steam

with c o n s e q u e n t e x p l o s i v e

e x p a n s i o n producing a

A hydrothermal

e x p l o s i o n

i s

p r o d u c e d

when w a t e r c o n t a i n e d i n n e a r -

t e m p e r a t u r e s

u p

t o

p e r h a p s 2 5 0 C f l a s h e s i n t o steam a n d

disrupts

t h e

c o n f i n i n g r o c k s ; n o

m a g m a

i s d i r e c t l y i n v o l v e d . I t

s

n o t

a l w a y s p o s s i b l e t o d e t e r m i n e

w h e t h e r a

g i v e n eruption i s

a p h r e a t i c

o r a

h y d r o t h e r m a l

e x p l o s i o n , p a r t i c u l a r l y when

observational

d a t a

B o t h

t y p e s

o f a c t i v i t y s e e m t o h a v e

t a k e n

p l a c e

n

t h e D i e n g M o u n t a i n s .


20/24

T

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21/24

1 8

should be located away from Kawah S i l e r i , probably near Dieng

W e t a n .

Any

eruptions from Kawah

Sileri

are likely

t o

be accompanied by some earthquake

a c t i v i t y , o f

uncertain but probably rather low

intensity.

Planning

f o r

any

plant construction should

t a k e

i n t o account

t h i s

potential local

s e i s m i c i t y ,

as

well

as

any regional

s e i s m i c i t y .

The l a s t truly

volcanic

eruption in the Dieng

Mountains took

place

a t

Gunung Pakuwadja in 1 8 2 6 ( 7 ) . One cannot assume

t h a t

volcanic

activity has

ceased, yet there i s no

way

t o

predict

i f ,

when,and where, another volcanic

eruption

will

o c c u r . I f a

volcanic

eruption should occur,

one might

expect

i t s locus t o be

near Gunung

Pakuwadja.

Landslides have been very common through historic

time in the

Dieng Mountains,

and

t h e

village

o f Legetang was buried i n 1955 by

a

landslide from

Gunung

Pengamun-amun ( G u n a w a n ,

1 9 6 8 , p .

6 3 ) . Telaga

Sewiwi (northwest o f

Karantengah)

apparent ly was

formed

by

a

landslide

in 1786 t h a t may

have been

triggered by the

phreatic

eruptions a t

Tjandradimuka

and Timbang. Landslid es

clearly present

a

hazard

t h a t

must be considered

i n

any generating

plant

l o c a t i o n .

ORGANIZATION OF EXPLORATORY PROGRAM

T o date there has

been

no specific organizational structure for t h e initial

investigations

o f

t h e Dieng Mountains, and no single operational and

coordinating focus

such a s a project m a n a g e r . This pattern, though

generally satisfactory, has resulted i n intermittent

confusion

a n d

misunderstanding

among

t h e

various institutions, contractors, and

individuals involved a s

t o t h e i r

specific responsibilities

and t i m e s c h e d u l e s .

There

have been

n o major p r o b l e m s , h o w e v e r , and t h e Initial Investigations

have been completed satisfactorily,

i n great

part due

t o g o o d

will and sense

o f

cooperation exhibited

by

a l l .

In particular, t h e

expediting

efforts o f

D r . Johannas

o f t h e G S I , D r .

Arismunandar

o f t h e P R I ,

D r . Klepper o f

t h e

U S G S , a n d M r . Kent a n d M r . Fowler o f USAID have

been

invaluable.

T h e evaluation t e a m , h o w e v e r , unanimously feels t h a t

a f o r m a l ,

recognized

projec t organization, p robably headed

b y a

project m anager

with clearly

defined

responsibility

a n d a u t h o r i t y , i s essential f o r t h e exploratory

drilling

a n d

any subsequent

s t e p s .

The anticipated increase

i n

c o s t ,

personnel

and

complexity greatly

increases

the potential f o r serious

misunderstandings, lack

o f coordination, and omissions. I n particular,

l o n g a n d

short-range planning

a n d

timing

o f

contracts, advisory

personnel,

a n d

supporting services i s essential.

I t i s beyond t h e scope of this report t o suggest any specific organizational

s t r u c t u r e . However,

attention

should b e

drawn t o t h e organization

o f

t h e

state-run

geothermal

enterprises i n

New

Z e a l a n d , I t a l y , a n d J a p a n .


22/24

1 9

SUPPORTING

GEOLOGICAL AND GEOPHYSICAL INVESTIGATIONS

Although n o t required f o r assessment

o f

the geothermal resources

o f

the

Dieng Mounta ins, several

l i n e s

o f scientific investigation could

well

provide

d at a

useful

i n

planning

t h e development o f the f i e l d .

I t

i s t h e

hope o f

t h e

evaluation

team

t h a t t h e s e investigations might be investigated

by t h e

cooperating

institutions during t h e coming year s o t h a t s o m e results

would be available

in t h e e v e n t a

decision i s made t o proceed with

development

o f t h e

f i e l d .

I t i s recommended

t h a t

regional photogeologic analysis o f the 3,500 km^

covered by t h e

1:35,000

aerial photographs be completed.

Preliminary

assessment i s currently being made by W .

H . C o n d o n , U S G S .

Gravity

studies i n t h e

Dieng Mountains

may prove

useful

i n

determining

t h e thickness

o f

the inter-volcano sediments i n t h e Dieng-Sikidang area

a n d

i n

t h e flat lands surrounding B a t u r . An initial gravity survey i s

currently being conducted along t h e Dieng-Batur road

by t h e

G S I . I f

t h e

results

o f

t h i s survey

are

f a v o r a b l e , i f

may

be

appropriate to

extend t h e

survey throughout the Dieng M o u n t a i n s .

I t i s

a l s o

recommended t h a t a microseismic

monitoring

system be installed

i n t h e

Dieng

Mountains. Such a

monitoring

system

would

serve t h r e e

p u r p o s e s :

1 . T o locate

earthquake

epicenter concentrations t h a t may indicate

deep geothermal targets

( c f .

Ward

a n d

Bjornsson,

1971; W a r d ,

Palmason, a n d D r a k e , 1 9 6 9 ; Lange a n d

Westphal, 1 9 6 9 ) *

2 . T o detect

any

increase i n frequency o f

microearthquakes

o r

change

in

location

o f e p i c e n t e r s , i n

order

t o

anticipate

possible

volcanic

(including p h r e a t i c ) e r u p t i o n .

3 . I f development o f

t h e

geothermal field proceeds, to

monitor

any increase

i n number o f

microseismic

events

related

t o large-

s c a l e fluid

withdrawal

( H e a l y ,

R u b e y ,

Griggs,

a n d

Raleigh, 1 9 6 8 ) .


23/24

2 0

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eruptive history of the dieng mountains region, central java, and potential hazards from future...

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