2.3 Geophysical Investigations - JAEA · 2.3 Geophysical Investigations 2.3.1 Aims ... Geophysical...
Transcript of 2.3 Geophysical Investigations - JAEA · 2.3 Geophysical Investigations 2.3.1 Aims ... Geophysical...
2525
2.3 Geophysical Investigations
2.3.1 Aims
- To identify potential water-conducting features and impermeable argillaceous layers.
- To acquire information on the orientation and geometry of geological discontinuities.
- To acquire geophysical properties by continuous wire line logging to support the
geological and hydrochemical modelling.
- To characterise in situ neutron flux production for hydrochemical interpretations.
2.3.2 Work performed
Geophysical logging: A suite of borehole geophysical logging methods, consisting of
temperature, spontaneous potential, resistivity, micro-resistivity, natural gamma, gamma
spectrum, neutron, density, acoustic, X-Y calliper and deviation logging methods were
employed from the surface to the bottom of each borehole. In MSB-1, because the
groundwater level in the borehole was below 60 mabh, geophysical logging was conducted in
an injection state above the static water level.
BTV: BTV investigations were performed from the surface to the bottom of each borehole.
Below 92 mabh in MSB-3, the BTV investigation was performed through PQ rods installed to
the depth as casing pipe, because borehole collapse had occurred during drilling in the
interval from 91 to 102 mabh near the NNW fault.
2.3.3 Results
Geophysical logging:
- Continuous profiles of the geophysical properties were acquired from the surface to the
bottom of each borehole. Geophysical logs are presented in Figures 21, 22, 23 and 24.
- Conglomerate layers and coarse sandstone layers intercalated in the Akeyo Formation and
the Toki Lignite-bearing Formation and the unconformity between the Mizunami Group
and the Toki Granite are distinct anomalies on the spontaneous potential, resistivity, micro-
resistivity, neutron, acoustic and density logs. Although these coarse clastic layers in the
sedimentary formations are generally inferred to be permeable, the depths at which distinct
anomalies on the geophysical logs occurred are not always coincident with fluid loss zones
and anomalies identified by fluid logging (Figures 21, 22, 23 and 24, see section 2.4.3 for
relevant matters).
- Relatively high gamma anomalies (500 to 2,000 API above background) were identified in
the arkosic sandstone immediately above the basal conglomerate of the Toki-Lignite
bearing Formation in MSB-1, 2 and 3, and immediately above the unconformity between
the Mizunami Group and the Toki Granite in MSB-4.
- The tuffaceous sandstone and tuff layers above the top of the basal conglomerate and
below the mudstone or muddy sandstone in the Akeyo Formation were defined as low
resistivity sections in each borehole. According to the study in the Tono Mine area [6], the
sections are possibly impermeable layers in the sedimentary rocks.
3030
BTV:
- Distributions of fracture frequency and orientation in each borehole, presented in Figures
25 and 26, were observed in BTV images and also coincided with the data from the core
description.
- Subhorizontal fractures are dominant in the all sections of the sedimentary rocks and in the
granite, except for the granite in MSB-4, in which steep WNW striking fractures are
dominant. NS and NNW striking fractures dipping at steep angles towards the east are
dominant in the weathered granite in MSB-3.
- Two fracture zones, defined by an abrupt increase in cumulative fracture frequency, were
identified in the Akeyo Formation in MSB-1. In these zones, EW and NE striking fractures
dipping at steep to moderate angles to the south are predominant. Also, one fracture zone
was identified along the NNW fault in MSB-3, in which NW striking fractures dipping at
steep to moderate angles towards the east are predominant.
- Four water-conducting features identified in MSB-1, based on fluid loss during drilling, are
EW and EWE striking and dipping at steep to moderate angles towards the south or north.
2.3.4 Evaluations
- Some coarse clastic layers were identified as anomalies by the geophysical logging.
However the anomaly depths did not always coincide with the depths of fluid loss during
drilling nor did with the anomalies on the fluid logging.
- Although argillaceous layers were not identified, tuffaceous sandstone layers of low
resistivity were defined in part of the Akeyo Formation. They are possible to be
impermeable layers in the sedimentary rocks.
- Information was acquired on the orientation and geometry of many of the geological
discontinuities. But we could not acquire this data for the NNW fault, and some of the
fractures in the conglomerate and the weathered granite because of sludge accumulation on
the borehole wall and significant enlargement of borehole diameter, making it difficult to
get reliable BTV images.
- Continuous profiles of geophysical properties were acquired in each borehole to support
the geological and hydrochemical modelling.
- Relevant data on in situ neutron flux production was acquired in each borehole.
2.3.5 Lessons learned
- A unidirectional scan line on core should be drawn to accurately infer the orientation of
discontinuities, preparing for loss of BTV images because of sludge accumulation,
borehole enlargement and turbidity of drilling fluid.
- Logging methods applicable to extremely low groundwater level (unsaturated or partially
saturated conditions) should be included in working programme.
- Employment of other geophysical logging method should be considered for identification
of potential water-conducting features in sedimentary rocks, because depths of anomalies
from the borehole geophysics did not always coincide with the depths of fluid loss during
drilling and of the fluid logging anomalies.
Figure25FracturefrequencyandorientationinMSB-1and2
Frac
ture
zone
:abr
upti
ncre
ase
incu
mul
ativ
efra
ctur
efre
quen
cyco
inci
dew
ithhi
gher
fract
ure
occu
rrenc
eslo
gged
inco
re.
31
MSB
-1
Miz
unam
iGro
up
E-W
/05S
N84
E/58
S
WC
F2
WC
F1
N=5
5
WC
F3
WC
F4
Frac
ture
zone
1
Orie
ntat
ion
Toki
Gra
nite N79
W/2
7S
N40
E/10
E
N=1
0
Frac
ture
zone
2
0-3
0°30
-60°
60-9
0°Di
pAn
gle
BTV
Frac
ture
s
75.6
2
87.6
0
32.3
2
34.7
2
Frac
ture
zone
1
Frac
ture
zone
2
Frac
ture
Dens
ity(N
m-1
)Cu
mula
tive
Num
ber(
N)2
46
0 050
100
201.
00
Geo
logi
calO
bser
vatio
ns
DrillingDepth(mabh)
20 40 80 100
120
140
160
180
200
60
78.9
0
195.
3319
5.60
131.
32
179.
48
117.
10
69.7
8
2.00
8.52
LithostratigraphicalColumn
Tuffa
ceou
ssa
ndst
one
Tuffa
ceou
ssa
ndst
one
Arko
sic
sand
ston
e,M
udst
one,
Gran
ulecg
l.,Li
gnite
Basa
lco
nglom
erat
e
Basa
lco
nglom
erat
e
TokiLignite-bearingFormationAkeyoFormation
LithostratigraphicalDescriptions
Mud
ston
e
Mudd
ysan
dston
e
Silts
tone
No
core
Tuffa
ceou
ssa
ndst
one,
Tuff,
Gran
ulecg
l.
TokiGranite
Oidawara
F.
wea
ther
edzo
neme
dium-g
rained
biotite
granite
30.0
036
.00
0-3
0°30
-60°
60-9
0°Di
pAn
gle
Frac
ture
Dens
ity(N
m-1
)Cu
mula
tive
Num
ber(
N)2
46
0 050
100
Miz
unam
iGro
up
N=5
5
Orie
ntat
ion
Toki
Gra
nite
BTV
Frac
ture
s
00/0
0
N=1
9
N45
E/03
E
N=9
MSB
-2 DrillingDepth(mabh)
20 40 80 100
120
140
160
180
60
77.5
4
170.
9517
3.35
132.
10
69.3
0
49.0
0
38.5
0
180.
00
LithostratigraphicalColumn
Geo
logi
calO
bser
vatio
ns
Tuffa
ceou
ssa
ndst
one
Arko
sic
sand
sotn
e,M
udst
one,
Gran
ulecg
l.,Li
gnite
Basal
conglo
merate
Basa
lco
nglom
erat
e
TokiLignite-bearingFormationAkeyoFormation
LithostratigraphicalDescriptions
Mud
ston
e
Tuffa
ceou
ssa
ndst
one,
Tuff
weath
ered
zone
medium
-grained
biotiteg
ranite
TokiGraniteN
oco
re7.
80
Figure26FracturefrequencyandorientationinMSB-3and4
32
Frac
ture
zone
:abr
upti
ncre
ase
incu
mul
ativ
efra
ctur
efre
quen
cyco
inci
dew
ithhi
gher
fract
ure
occu
rrenc
eslo
gged
inco
re.
MSB
-3 DrillingDepth(mabh)
20 40 80 100
120
140
160
180
60
72.5
0
177.
70
190.
60
140.
90
88.7
0
54.1
0
LithostratigraphicalColumn
Geo
logi
calO
bser
vatio
ns
3.80
42.2
2
199.
00
LithostratigraphicalDescriptions
Tuffa
ceou
ssa
ndst
one
Arko
sic
sand
ston
e,M
udst
one,
Gran
ulecg
l.,Li
gnite
Basa
lco
nglom
erate
Basa
lco
nglom
erate
TokiLignite-bearingFormationAkeyoFormation
Mud
dysa
ndst
one
No
core
Tuffa
ceou
ssa
ndst
one,
Mud
ston
e
weath
eredz
one
med
ium
-gra
ined
biot
itegr
anite
TokiGranite
faultzone
Miz
unam
iGro
up
N=3
8
Orie
ntat
ion
Toki
Gra
nite
N=3
2
N33
W/1
0W
N30
W/8
8EN33
W/5
8E
Frac
ture
zone
N12
W/0
5W
N87
W/6
4S
N17
W/6
6EN
31E/
81E
NN
Wfa
ult
Wea
ther
edzo
ne
0-3
0°30
-60°
60-9
0°Di
pAn
gle Frac
ture
zone
Frac
ture
Dens
ity(N
m-1
)Cu
mula
tive
Num
ber(
N)2
46
0 050
100
88.1
6
103.
50
BTV
Frac
ture
s
MSB
-4 DrillingDepth(mabh)
20 40 8060
76.6
0
93.9
0
63.4
0
Geo
logi
calO
bser
vatio
ns
99.0
0
LithostratigraphicalColumn
Tuffa
ceou
ssa
ndst
one
Ark
osi
csa
nd
sto
ne
,M
ud
sto
ne
,G
ranu
lecg
l.,L
ign
ite
Basa
lco
nglom
erate
TokiLignite-bearing
FormationAkeyoFormation
LithostratigraphicalDescriptions
Mud
ston
e
No
core
Tuffa
ceou
ssa
ndst
one,
Tuff,
Gran
ulecg
l.
medium
-grained
biotiteg
ranite
TokiGranite
34.4
530
.50
13.5
0
0-3
0°30
-60°
60-9
0°Di
pAn
gle
Frac
ture
Dens
ity(N
m-1
)Cu
mula
tive
Num
ber(
N)2
46
0 050
100
BTV
Frac
ture
s
N70
W/8
4S
N74
W/6
4N
Miz
unam
iGro
up
N=2
4
Orie
ntat
ion
Toki
Gra
nite
N=2
N43
W/1
6W