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BRGM methodology for the geochemical monitoring of an active volcano in a dormant phase Lamongan (East Java) J.-C. Baubron Expert to the Volcanologicai Survey of Indonesia (Bandung) with the collaboration of J.-C. Sabroux Advisory Volcartologist B. Bourdon assistant and MM. Yustinus Sulisto and Suryono Merapi Volcanologicai Laboratory - Yogyakarta September 1988 88 DT 035 ANA BUREAU DE RECHERCHES GEOLOGIQUES ET MINIERES DIRECTION DE LA TECHNOLOGIE Département Analyse B.P. 6009 - 45060 ORLÉANS CEDEX 2 - France - Tél.: (33) 38.64.34.34 DIRECTORATE GENERAL OF GEOLOGY AND MINERAL RESOURCES GEOLOGICAL AND MINERAL SURVEY PROJECT - ADB.LN 641 INO (Agreement n° 813/76/DDX 188} Volcanologicai Survey of Indonesia Jalan Diponegoro 57 - BANDUNG 40122 INDONESIA
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of an active volcano in a dormant phase
Lamongan (East Java)
J.-C. Baubron Expert to the Volcanologicai Survey of Indonesia (Bandung)
with the collaboration of J.-C. Sabroux
Advisory Volcartologist B. Bourdon
Merapi Volcanologicai Laboratory - Yogyakarta
BUREAU DE RECHERCHES GEOLOGIQUES ET MINIERES DIRECTION DE LA TECHNOLOGIE
Département Analyse B.P. 6009 - 45060 ORLÉANS CEDEX 2 - France - Tél.: (33) 38.64.34.34
DIRECTORATE GENERAL OF GEOLOGY AND MINERAL RESOURCES GEOLOGICAL AND MINERAL SURVEY PROJECT - ADB.LN 641 INO
(Agreement n° 813/76/DDX 188} Volcanologicai Survey of Indonesia
Jalan Diponegoro 57 - BANDUNG 40122 INDONESIA
SUMMARY
Abstract
appeared in 02-1988
- Tjurah Buntu
- Gunung Kendeng
64
64
68
68
72
ABSTRACT
soil atmospheres (CO2, Rn, and He) carried out on the
western foot of the Lamongan volcano (E-Java) shows that the
thermal leak discovered six months before was still active.
The amount of gas escaping, now about 5 50 T per day of
CO2 from an area of 35 km^ , has increased strongly since
September 1987 and appears to be closely linked to the
seismicity. The February 1988 swarm is probably the cause of
the large increase observed.
that the process involved is a hot water system expanding
upward into the upper crust. The remaining low levels of He.
suggest that the cause of this is more likely to be a deep
seismic event, rather than a dyke intrusion.
- 1 -
maars and eruptive vents located on its lower slopes,
presently in a dormant phase with only some low temperature
furaaroles in the summit crater, has been investigated for
its potential hazards in 1987 and 1988 (M. Aubert,
J.C. Baubron and D. Westercamp, 1988, Report in progress).
These results, obtained in September 1987, show that in
a large area located at the western foot of the main cone,
some of the anomalies observed give gas concentrations large
enough to indicate a regional thermal leak.
Most of the fissures of the 1978 and 1985 seismic
crisis were located in this zone. This target was also the
epicentre of the February 1988 swarm, which has focussed
attention on its present behaviour. For this purpose, the
surveillance of the cold outgassing through the ground
should be an effective method for estimating the volcanic
activity, locating the most active area, and possibly for
distinguishing the surficial effects (seasonal variations,
hot water movements near the surface) of the deep activity,
i.e. a magmatic intrusion into the upper crust.
2. METHOD OF SURVEILLANCE
Analyses have been made in four areas in the
surroundings of the site of the February 1988 crisis and at
the presumed limits of the affected area.
_ 1 _
The analyses were run once a month for 3 months on :
- Two areas previously investigated
was revealed in September 1987. This is the
Kenek-1 traverse in the southern part of the
seismically active area where many cracks appeared
during the 1985 seismic swarm.
. In the northern part, across a recent roughly
NNW-SSE fault system of the volcano. This is the
W. RANÜ PANDAN traverse. No anomaly was observed
there in September 1987.
of last February. These are namely the TJURAH-BUNTU
traverse, across a crack field, and the GUNüNG KENDENG
traverse, near the blow hole of the 10th April, 1988.
This type of surveillance is done using two kinds of
measurement :
In addition, analyses of mofettes or gases from thermal
springs are compared with the previous work.
3. MONITORING INSTRUMENTS
carbon dioxide, radon and helium.
Why are these gases useful?
Carbon dioxide: Every thermal event in the crust will
induce degassing of the heated rocks, creating CO^ . For
instance, 0.5% of carbonates in the rocks over the area
affected by the heating, gives about 10® tonnes of CO2 for a
thickness of 10 km. Magmatic CO2 is not taken into account.
Radon: Radon is the direct daughter of radium and has
no intrinsic velocity. It is carried away by the underground
water, and there is a direct relationship between radon
activity and water temperature. As radon has a short
half-life (3.8 days), the investigations give information in
the range of about 100 metres below the surface.
Helium: This is the magmatic tracer. Helium is a
product of the uranium decay chain: every alpha particle
will give a helium atom. As it has the smallest atomic
radius of the rare gases, it has the highest intrinsic velocity
and it forms no compounds with other elements.
In practise, all the data published, and my own results
show that on an active volcanic field, the anomalous concen¬
trations of He fall in the range of 7 to 10 ppm or more (the
atmospheric concentration of helium is 5.2 ppm).
Thus, a gas survey based on these three components will
give information on three possible levels in the crust where
any magmatic intrusion would create distinctive anomalies:
- High activities of Rn will indicate a shallow thermal
anomaly, such as a hydrothermal convection cell.
- 4 -
biological contribution and related to a low Rn anomaly
will indicate a medium to deep thermal anomaly.
High concentrations of He (more than 7 ppm) will
indicate a deep thermal event.
4. METHOD OF ANALYSIS
- Sampling is done through a steel probe, 1 cm in diame¬
ter, with an internal teflon tube, driven into the soil
at a depth of 0.7 metres. The sampling interval is
usually 10 metres.
meter, so results are obtained on the spot, which can
be useful for choosing the next sampling point. In the
range 1% to 100%, the accuracy is typically 1%.
- Radon is analysed by alpha counting of ZnS-coated bulbs
filled with soil gas after elimination of the aerosols:
. First, on the spot in order to give an approximate
idea of the activity.
the radon activity.
mass-spectrometer after inflating a teflon bag of
0.5 litre of gas in the field.
_ =;
- Gas flow measurements are made in the following way:
. Half a container of about 150 1 and 0.6 m' is laid
down on a scraped area of ground, the open side
toward the soil .
. Gas samples are taken every half hour for 6 hours.
The graph of concentration against time usually gives a
straight line between T^, + 2 h and T^ + 6 h, which can be
taken as the mean increase per time unit. Flows are then
easily calculated.
5. RESULTS
1968
This N-S traverse is located across the southern branch
of the track from Ranu Lamongan to Gunung An jar. The
negative sites are on the northern side, zero being at the
side of the track. It probably crosses a N 80 tectonic
direction active in 1985.
The trace of a fault found on this traverse in
September 1987 (Table 1 - Figure 1) was characterised by a
maximum CO^ concentration of 5% against a local background
of about 1%.
die away with distance from this point.
- 6 -
- 7 -
1987 (M. Aubert - Figure 2.1) show a slight decrease of
potential from north to south, with a small positive anomaly
(+ 50 mv ) near the 150-200 m sites.
This can be interpreted as a small convection cell in a
general groundwater transfer toward the north from the zero
point, in agreement with the topography.
We point out that the SP anomaly ends on its north side
where the sharp CO2 anomaly begins. This illustrates the
fact that the SP anomaly is located on the wetter soil where
the gas cannot exude because of lower permeability.
In March 1988 (Figure 2.2) the same SP traverse showed
a contrasting shape: the general elefctric signal is one
order of magnitude lower, with reduced noise; the trend to
the north is similar but there is a sharp negative anomaly
near the zero point, combined with a good positive anomaly
between the 50 and 100 sites.
The heavy rains explain the decrease of the electric
signal and the lower background, but the negative anomaly
associated with the positive anomaly is most probably
related to a convective cell in a fissure. This is probably
the 1987 system rejuvenated by the seismic swarm of February
1988.
In March 1988 (Figures 1 and 3) the same traverse shows
a large increase of CO2 - traverse 88-1. In this figure, it
can be seen that the new profile is an homothetic
translation towards higher levels of CO2. The low values in
the first profile correspond to the lowest values in the new
one, in particular the negative anomalies near the "30" and
"-30" sites.
It should be noted that the highest increase is located
between the "50" and "200" sites, where the positive SP
anomaly was located in September 1987. Moreover, the highest
1988 concentrations of CO2 are linked with the positive
1988 SP anomaly.
It can be assumed that the convective cell found in
September 1987 is still operating and its intensity has
increased .
Figure 3) shows that there has been a further increase.
While the highest values show a slight increase, the lowest
values display a large one.
The last measurements, made in May (88-3, Figure 4)
show the same increase again, but now there are almost no
samples with low levels. The highest CO2 value is strictly
connected with the positive SP anomaly.
It can be concluded that the surficial effect of the
deep thermal leak thus revealed continued for the three
months of survey. Soil CO2 measurements appear to be a good
instrument for appreciating the changes in the thermal
discharge from the ground, even when the absolute levels are
low (i.e. no apparent manifestation).
This phenomenon can also be observed with frequency
analysis, the mode increases regularly (Figures 5, 6, 7, 8).
- 9 -
The evolution of the histograms from 87 to 88-3 indi¬
cates that whereas the mode (45% of the data are between 1
and 2% cf CO2 ) was connected with the biogenic output in
September 1987, the distribution became bimcdal for the
first recording of 1988 : 22% of the samples remain between
1 and 2% of CO2 but 24% went up to 5 to 6%. The latter peak
is linked with the thermal leak.
The last two charts show the continuing increase: the
mode increases from the 5 to 7% CO2 level (60% of the
samples) to the 6 to 8% level in only a month.
This explains why the biogenic concentrations are in
the range of 1 to 2% and the surface effect of the thermal
leak about 4 to 5%. At the end of our experiments, most cf
the sites were within the thermal anomaly.
The close relationship between the successive
concentrations of CO2 is confirmed by the diagram where CO2
concentrations for the first month are plotted versus the
CO2 concentrations for the succeeding months (Figures 9
10). The correlation coefficients are respectively 0.82 and
0.74 for 88-1, 2 and 3.
As the result is a straight line, it can be concluded
that the increase of CO2 is proportional to the CO2
concentration. In this example, we have:
CO2 (t + 1) = CO2 (t) X 1.1
t = expressed in months.
CO2 = percentage of CO2
gas emanation increases exponentially with time. In this
case, dangerous levels (100% CO2 ) will be reached in less
than 2 years of seismic activity, from an initial background
level of about 3 to 4%.
- 10 -
up, the sites of lowest concentration increase at the rate
of 1.6 a month, which is much higher than the rate measured
subsequently. But this was during the climax of the seismic
crisis .
The radon spectrum shows the same relationship, the
main difference is that Rn maxima are on the edges of the
CO2 anomalies (Figures 11, 12). This was discernible in
September 1987 with the low activities but is much clearer
with the data obtained in March 1988.
As for CO2, Rn activities rose to high levels: during
the investigations of 1988 I 600 to 800 pCi/litre. These
increases can be seen in the variation of the histograms,
most cf the samples have activities higher than
150 pCi/litre which is the common anomaly threshold (Figures
13, 14, 15).
which is in the form of:
Rn (T^ 1) = Rn ( T^ ) x 1.3
This is the same function as the relationship found
with the rises in CO2, i.e. an exponential increase cf Rn
activities with time.
against those of March 1988 (Figure 17) cannot be explained,
the seasonal changes in the climatic parameters probably
gave too many individual variations.
11 -
The He line chart (Figure 18) illustrates the point
that the levels are always in a low range, 0.2 ppm higher
than the results obtained in September 1987, but still
lacking any high concentration which would indicate a
magmatic component.
activities show a two fold relationship (Figures 19, 20):
- The upper area (high CO2 relative to the medium Rn
values) corresponds to the highest He concentration of
the traverse.
normal He.
With this kind of diagram, the places where the soil
atmosphere anomaly is produced by deep gases can be
distinguished from the samples where the gases are connected
with shallow ground water: this can be either originally
deep water from which the deep gases have previously been
extracted or rain water with a relative high horizontal
velocity (high radon) and/or an input of surficial
(biogenic) and deep CO2 after lateral transfer.
It can also reflect simply soil moisture: CO2 and He
escape more easily where the soil pores are not saturated
with water. In these sites, Rn activity is high because of
the direct relationship between Rn and water content. This
can explain the approximately inverse relationship between
He and Rn as shown on Figure 20.1.
The flow measurements show the same increase as seen in
the concentrations. The mean flow is about 0.4 l.m~^.h~^ of
CO2 (0.3 in April 22nd, 0.45 in May 6th).
12
Sanóle
D(aï2(«)-e8-1
Fig. t
Sample
100
1
ng. 5
Fig.4
- 17 -
Kenek
22 5,
[) 1 2 3 4t 5 6 7 8 9 IC
C02 («)-88-1
14
12
10.
8-
6.
4.
2
0.
4 5 6 7
4 5 6 7 8 ç
C02 (?.)-88-1
DRn (pCi/l)-88-1
Fig.11
- 22
200
100.
0.
-100. ' I 1 I I I r- -I 1 1 1- -I 1 1-
-200 -150 -103 -50 0 50 100 150 200 250 300
Sample
10.
8.
6.
4.
2.
0 1 1 1 1 1 1 1 1 1 1 p r r'
0 100 200 300 400 500 600 700 800 900 1000
Rn (pCi/1)-87
100 200 300 400 500 600 700 800 90> 1000
Rn (pCi/l)-88-1
200 300 400 500 600 70)
Rn (pCi/l)88-2
O 100 200 300 400 500 600 700 800 900
Rn (pCi/l)88-2
Fig.l 6
Rn (pCi/l)-88-1
Fig.l 7
I I r- -I 1 1 1 1 1 r-
100 200 300 400 5CN3 600
Rn (pC1/i:e8-2
This East-West traverse is located along the track,
west of Ranu Pandan; the last site on the western side, near
a little bridge where the track turns left.
This 1200 metre long traverse (Table 2, Figures 21-1,
-2, -3) gives the same figure as the KENEK-1 profile: the
increases of CO2 are proportional.
From September 1987 to the beginning of April the
increase is very high; the CO2 concentrations increasing by
a factor of 4 in 6 months. During the last two months the
scatter becomes concentrated around the modal value: 73% of
the samples give CO2 concentrations between 2 and 4% in
April (88-1), then 80% in May (88-2) (Figure 22-1, -2, -3).
This can also be illustrated with graphs ( CO2
87 versus 88-1 and CO2 88-1 versus 88-2), (Figures 23-1,
-2) .
It can be assumed that this mean value (3%), which is
moderate, is the increase since the February seismic crisis.
- 31 -
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Sample
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
370
380
390
400
410
C02(%)87
8.0E-1
5.0E-1
7.0E-1
1.0
6.0E-1
6.0E-1
1.8
9.0E-1
3.0E-1
4.0E-1
2.ÜE-1
2.0E-1
4.0E-1
3.0E-1
3.0E-1
1.0
9.0E-1
1.5
1.2
7.0E-1
1.2
8.0E-1
4.0E-1
8.0E-1
1.4
4.0E-1
1.3
5.0E-1
5.0E-1
4.0E-1
1.8
7.ÜE-1
4.0E-1
2.0E-1
6.0E-1
4.DE-1
5.0E-1
5.0E-1
2.0E-1
4.0E-1
6.0E-1
5.0E-1
C02(%)-88-l
4.4
3.2
5.4
3.4
4.9
4.8
4.1
3.8
2.6
3.3
2.8
3.5
6.6
3.9
1.9
3.7
3.6
4.6
3.8
2.1
3.2
3.7
2.0
3.7
2.9
1.8
3.2
2.4
2.4
4.8
2.9
3.2
2.4
1.3
2.2
2.0
1.4
2.0
7.ÜE-1
1.1
2.2
2.4
C02(%)-88-2
3.1
2.2
3.4
6.6
4.6
5.9
3.9
2.9
3.5
3.5
2.9
4.0
4.1
3.4
3.2
3.8
3.4
3.8
4.0
2.7
3.8
3.9
2.8
3.8
3.6
1.6
3.1
2.3
2.7
4.2
4.9
2.7
2.5
2.2
3.4
1.4
2.4
2.2
1.6
1.0
2.3
3.8
¡Tablez
- 32
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
Sample
420
430
440
450
460
470
480
490
500
510
520
530
540
550
560
570
580
590
600
610
620
630
640
650
660
670
680
690
700
710
720
730
740
750
760
770
780
790
800
810
820
830
C02(%)87
4.0E-1
4.0E-1
6.0E-1
8.0E-1
6.0E-1
1.2
8.0E-1
5.0E-1
1.5
9.0E-1
5.0E-1
5.0E-1
7.0E-1
5.0E-1
1.2
1.3
8.0E-1
1.3
8.0E-1
5.0E-1
8.0E-1
5.0E-1
1.0
9.0E-1
1.2
1.1
8.0E-1
9.0E-1
4.0E-1
6.0E-1
8.0E-1
5.0E-1
6.QE-1
4.0E-1
3.0E-1
7.0E-1
3.0E-1
5.0E-1
3.0E-1
4.QE-1
7.0E-1
1.0
C02(%)-88-l
2.1
Sample
- 35 -
50
40.
30
20.
10
C02CSÇ)-88-2
1 (
7
5.2.1. TJURAH BUNTU
Figures 24, 25). Once again the relationships are the same,
but here, although the level was high in March (88-1), a
slight decrease occured in April (88-2), and a small
increase at the beginning of May (88-3). This can also be
seen in the frequency analysis (bar graphs.
Figures 26, 27, 28), where 49% of the samples were in the 3
to 4% space the first month, 37% in the 2 to 3% space the
second month and 35% in the 3 to 4% space again in the last
month .
sites) shows a permanent increase. It should be noted that
the part of the traverse where the cracks occurred is the
part with the mean lowest CO2 concentrations and where the
instability is most apparent. Reaction with the open cracks
is possible.
trations. Rn frequency diagrams show the same decrease
between 88-1 and 88-2 as that displayed by CO2 (Figures 29,
30, 31, 32, 33).
sites where the low concentrations of CO2 are low. Moreover,
near site -100, the CO2 concentration increases as the Rn
activity decreases.
He is always quite low: 5.4 to 5.5 ppm only.
- 40 -
COa flows vary from 0.5 in the first month tc 0.3 then
to 0.4 in the last month (expressed as litres per hour per
square metre). The place where the measurements were made is
located near the -100 m site.
In this traverse, the depressed zones correspond to the
areas where the cracks are located. This has also been
observed at a smaller scale. The cracks are open systems
connected with the free atmosphere where the atmospheric
component is the most important in the gas mixture.
To verify this possibility, some CO2 mapping was done
along the perpendicular track which crosses the traverse
near the 90 site.
These measurements allow us to define some trends of
high and low CO2 levels, which are roughly N 80, i.e. the
direction identified on the ground where the fissures were
still evident (Tables 4-1, -2, -3) (Figure 34).
- 41 -
(se) 88-2 DC02 (95) 88-3 .
-1 1 1 <- -I 1 I 1 r -1 1 1 I r- 1
-150 -100 -50 0 50 1(K) 150 200 250 300 350
Sample
Fîg.25
A4 -
C02 (S?)-88-1
> 7 8
1 1
C02 m 88-2
*8 0 0 ° 0 9 0
0 0 0 ^ o 0 "
2 3-1567
C02 (%)-80-l
4 5 6
3 4 5
CM ^-^
2.5
2.
1.5.
1 I 1 1 1 1 1 1 I 1 1 1 1 1 1 1
-20 0 20 40 60 80 100 120 140
Sample
Fig.34
5.2.2. GUNUNG KENDENG
This N-S traverse is located 50 metres east cf the blow
hole of the 10th April . The first analysis was done the 12th
April (Table 5, Figures 35, 36).
We see a medium-level profile with high concentrations
in the southern part.. Once again, the whole profile
decreases (- 28%) before increasing (+ 7%) at the beginning
of May. This can also be observed on the bar-graph diagrams
(Figures 37, 38, 39).
The profile is characterised by a depressed zone near
sites - 40 and 50 (northern part of the profile), which is
everywhere below the mean value of the traverse. This could
indicate the crossing of the traverse by an open fault.
Rn activities are always connected with COa concen¬
trations and here. He gives the highest ccncentration in
this area, namely 6 ppm. Nevertheless, this value is quite
lew for a volcanic field.
Ten metres west of the blow hole a short traverse was
measured to check the influence of the open pipe on surface
concentrations (Table 6, Figure 40). It shows that the open
system acts as a drain for the deep flow, the concentrations
in soil atmospheres decreasing as the proximity to the blow¬
hole increases.
It can be assumed that the remaining concentration
(about 0.5%) is the mean value of the biogenic CO2 concen¬
tration .
= 6 -
Direction of the fault. If the depressed zone observed
on the main traverse is joined on a map to the blow hole and
then to the depressed zone on the short traverse, the three
points fall on a straight line trending roughly N 70 to
N 80, which is the direction measured for most of the
February cracks, and that inferred from the gas anomalies on
the TJURAH BUNTU traverse.
The measured gas flow changes from 0.22 at the begin¬
ning cf April tc 0.18 at the end of April, and 0.45 in the
middle of May, which is a considerable increase.
- 57 -
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Sample
-193
-180
-169
-158
-146
-135
-122
-111
-96
-85
-75
-63
-52
-41
-26
-13
-1
0
10
20
33
43
53
65
76
88
96
110
121
133
144
158
168
179
192
202
214
225
235
247
260
272
COZ(%)-í
5.5
2.9
3.5
3.8
2.4
2.9
3.8
3.4
4.2
4.2
3.1
2.6
3.1
4.5
3.8
4.7
5.8
6.2
4.7
8.4
5.5
5.2
6.9
5.9
4.6
5.6
4.3
5.1
6.2
4.5
3.9
6.8
4.8
4.8
4.2
4.3
4.4
6.8
5.6
5.6
5.8
4.1
Fig.35
- 59 -
C02 (S5)-1
C02 (95)-2
0 1 2 3 4 5 6 "3
C02(«)-3
Sample
Fig.40
- 64 -
atmospheres at the same sampling site at 0.7 metres depth
show a direct relationship (Table 7, Figure 41) in the form:
P = 1.17 E-03 SQR (COz %. )
P = (litres per hour per square metre)
This is the "normal" outgassing trend, established with
data from the Dieng plateau. Where there is seismic
activity, flows are higher than the common v^alues. This can
be interpreted as higher permeability due to the deep
multiple cracking caused by the earthquakes (all else being
equal ) .
In this way, the value of the flow versus the concen¬
tration cf the soil atmosphere can be used tc characterise
the area as active or not. For instance, it is clear from
this graph that, even though the measurements have been made
on anomalous sites, the flows recorded in Duren and Ranu
Lading are not in the active zone.
The mean value in the whole active area investigated is
about 0.35 litres per hour per square metre. So, the CO2
emanating from this 35 km- zone is in the range cf 550 to
600 tons per day. This is quite high, but 3 times less than
the CO2 escaping from the slopes cf Mt Etna (corrected for
the areas involved). The analogy should be moderate because
the techniques used are not similar: the measurements here
- 6;
are made in a static mode, whereas on Etna it was made in a
dynamic mode. The latter should give higher values. For
comparison, the mean value calculated here is half that of
the escape from the cold area into the active crater of La
Sulfatara (Flegrean fields - Italy). Measurements made in
the same way give flows of about 0.03 1 m~^.h~^ to 0.06 1
m~^.h~^ for the biogenic activity in a forest located on
volcanic soil (Vesuvius volcano, Italy).
- 66 -
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Sample
should be done (on anomalies).
- Traverses give the locations of faults (fractures areas
with high permeabilities).
even if they vanish due to the heavy rain showers.
- Traverses give a quick appreciation of the deep gas
flow variation within time, because concentrations and
flows are closely linked for a particular site. A 1500
to 2000 m traverse can easily be done in a day whereas
a single flux measurement needs 6 hours or more at the
same place.
6.3. Concentrations
CO2 : in this area there has been a large increase of
deep gas exhaust since last year. This could be connected
with the February crisis.
CO2 concentrations reach the limits of dangerous levels
according tc the data obtained on Dieng plateau. 9% CO2 in
soil atmospheres means that high concentrations exist
withing a few hundred metres of the surface. These large
volumes of CO2 could be rapidly released during a seismic
crisis with the related risks: small to large blows with
mechanical effects, toxicity of CO^ , etc.
CO2 concentrations (and flows) are linked with seismic
activity: the minor crisis observed at the beginning cf May
corresponds with an increase in gas. Moreover, a short
period of monitoring of CO2 in the blow hole has shown that
variations in CO2 were linked with seismic noises (or
tremors): see figure 42 with explanations.
Ranu Pakis is a maar lake located on the very active
KENEK-1 fault system. As it is about 150 metres deep and the
fault system delivers large quantities of gas, this lake
could soon be very dangerous, acting as an enormous CO2
reservoir .
involved here is a hot water system.
He: there are no high concentrations of He (about
10 ppm or more) in the areas investigated. We can
therefore infer that at the localities where the
analyses were run there is no evidence of an underlying
magmatic intrusion.
But, it should be remembered that whereas CO2 and Rn
give large or very large haloes. He will give only a narrow
anomaly if the source is a dyke. In such a case, a traverse
must be made across the system.
- 70 -
! ' I
(2 ranges). T = tremor; Q = shallow quake.
Note that the CO2 level on the day of the analysis was
about 2%. The level decreased slowly from more than 3% two
days after the blow to less than 1% two weeks after.
No interpretation is given for the increase seen every
day between 8 and 10 a.m. and the decrease after 3 p.m.
Every pulse seen during the increase was connected with
a ground vibration which could have been a real tremor.
Unfortunately, no record of the seismicity exists for
the period 4 p.m. to 4 a.m. because of a failure in the
instrument. No explanation can therefore be given for the
marked anomaly recorded between 0.30 and 2.00 a.m. However a
regional quake occured that night.
The last two sharp peaks recorded at 5.30 and 7.35 a.m.
were synchronous with two very shallow quakes close to the
site (some kilometres to the north).
- 72 -
6.4.1. Tiris
New data (Table 8) obtained on gases from the Tiris
springs agree with the analysed soil atmospheres (analyses
from the Merapi - Volcanological laboratory - YOGYAKARTA) .
1 !
2
3
4
Sample
ijen
Lamongan
There has been no change in the spring temperature
since September 1987, but there is an increase in the CO2
concentration and a decrease in the He concentration (down
to 2.5 ppm ) .
(no residual oxygen) are as follows (Table 9) .
1
2
3
4
SdmpEb
Ijen
Lamongan
Tiris
Kolbah
coz (%)
97.53
87.54
98.05
.24
N2 (%)
2.47
11.31
1.95
45.70
Sample
For the gases from Tiris, there is still a small
atmospheric contamination. The corrected data give a nitro¬
gen residue. This is an atmospheric component from which
oxygen has been used in the FeO/FezOa conversion. This
atmospheric component probably originates from the super¬
ficial water percolated through the upper layers and mixed
with the deep flow.
noticed last September (78.4 %). Radon activities, stan¬
dardised, give 527 pCi/1 and 482 pCi/1 respectively for
april and September. The September result recalculated from
the april result, would give 430 pCi/1 which agree with the
field measurement. These data indicate that Rn and CO2 have
the same source .
- 74 -
This can be explained by an input of shallow CO2 into
the area where the water table is heated. Either the new
input of energy is of shallow origin, or the thermal water
has a lateral source, possibly a connection with the active
area. On the latter hypothesis. He will not be carried along
with water.
6.4.2. Kolbah
The Kolbah spring, located ESE of the volcano (Tanggul
sheet, half way between Kaliboto 3 and Kotokan 1) rises at a
normal temperature in a small pool without any ferrous oxide
deposit .
The gas analysis, recalculated, gives a very surprising
result: the gas is half N2 and half CH* with a high He
concentration (173 ppm). CO2 is a minor constituent: only
0.29%. The theoretical composition of the gas (0.24% CO2 )
agrees with the field CO2 measurement. Such a mixture of
gases has sometimes been found on faults that are probably
very deep, which could be the case here.
6.4.3. Lamongan
Fumaroles from the Lamongan summit will contain only
10 ppm of He in the dry gas (87.5% of CO2 ) . This value is
very common in a quiescent volcanic period. It is the only
sample analysed which contains hydrogen. The 6^^C%o gives an
intermediate value: -2.4 and -2.9 ± 0.1. This can be
explained by a mixture between deep carbon and carbon from
carbonates of the crust. Values in the same range have been
found before by P. ALLARD (1986) in gases of Ijen, Merapi
and Central American Volcanoes.
The area investigated as a monitoring example shows
a large input of heat, carried up by water. There is no
magmatic evidence for the system in this particular area.
However, the four traverses where the analyses have
been conducted do not allow a general verdict for the whole
area. Ivestigations should be made soon on traverses close
to the seismic epicentres to confirm this conclusion.
8. RECOMMENDATIONS
understand clearly the phenomena.
- A geochemical map should be drawn to avoid working on
irrelevant areas. A 20 to 25 m interval is suggested.
- Frequent He measurements should be made to check the
magmatic input, which can change very quickly: He will
change with magma ascent, but CO2 and Rn will probably
change more slowly because they are connected with
water. This needs modern instruments to obtain accurate
data .
- Permanent monitoring over medium to long periods should
be done for Rn and CO2. If H2 can be linked with He
escape, it will be a good tracer.
- As an important input of heat exists somewhere,
attested by gas escape, shallow waters must.be heated
(in the range of some tens of degrees); Since the area
is confined, this cannot be done without an uplift. The
three lakes (Ranu Pakis, Lamongan and Bedali) should be
used as large integrating tiltmeters. Light automatic
equipment is commercially available for this purpose
(3 sensors per lake).
The depth of Ranu Pakis should be checked and the
gas/water ratio measured in the deepest water layers.
A general survey of the depths of all the lakes should
be made to obtain a general idea of the risk of
outgassing of water.