Marine Geophysical Researches 21: 1–21, 2000.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
1
Seafloor electromagnetic induction studies in the Bay of Bengal
E. John Joseph1,∗ , H. Toh1 , H. Fujimoto1, R.V. Iyengar2 , B.P. Singh2 , H. Utada3 & J. Segawa4
1 Ocean
Research Institute, The University of Tokyo, Japan; 2 Indian Institute of Geomagnetism, Mumbai-5, India;
The University of Tokyo, Japan; 4 Institute of Oceanic Research and Development,
Tokai University, Japan
3 Earthquake Reaearch Institute,
Received 8 December 1998; accepted 27 October 1999
Key words: Electromagnetic induction, ocean bottom magnetometer, 85◦ E ridge, Ninety East ridge, geomagnetic
depth sounding, vertical gradient sounding, thin-sheet modelling, 3-D forward modelling
Abstract
Seafloor magnetometer array experiments were conducted in the Bay of Bengal to delineate the subsurface conductivity structure in the close vicinity of the 85◦ E Ridge and Ninety East Ridge (NER), and also to study the upper mantle conductivity
structure of the Bay of Bengal. The seafloor experiments were conducted in three phases. Array 1991 consisted of five seafloor
stations across the 85◦ E Ridge along 14◦ N latitude with a land reference station at Selam (SLM). Array 1992 also consisted of
five seafloor stations across 85◦ E Ridge along 12◦ N latitude. Here we used the data from Annamalainagar Magnetic Obervatory
(ANN) as land reference data. Array 1995 consisted of four seafloor stations across the NER along 9◦ N latitude with land
reference station at Tirunelveli (TIR). OBM-S4 magnetometers were used for seafloor measurements. The geomagnetic Depth
Sounding (GDS) method was used to investigate the subsurface lateral conductivity contrasts. The vertical gradient sounding
(VGS) method was used to deliniate the depth-resistivity structure of the oceanic crust and upper mantle. 1-D inversion of the
VGS responses were conducted and obtained a 3-layer depth-resistivity model. The top layer has a resistivity of 150–500 m
and a thickness of about 15–50 km. The second layer is highly resistive (2000–9000 m) followed by a very low resistive
(0.1–50 m) layer at a depth of about 250–450 km. The 3-component magnetic field variations and the observed induction
arrows indicated that the electromagnetic induction process in the Bay of Bengal is complex. We made an attempt to solve
this problem numerically and followed two approaches, namely (1) thin-sheet modelling and (2) 3-D forward modelling. These
model calculations jointly show that the observed induction arrows could be explained in terms of shallow subsurface features
such as deep-sea fans of Bay of Bengal, the resistive 85◦ E Ridge and the sea water column above the seafloor stations. VGS
and 3-D forward model responses agree fairly well and provided depth-resistivity profile as a resistive oceanic crust and upper
mantle underlained by a very low resistive zone at a depth of about 250–400 km. This depth-range to the low resistive zone
coincide with the seismic low velocity zone of the northeastern Indian Ocean derived from the seismic tomography. Thus we
propose an electrical conductivity structure for the oceanic crust and upper mantle of the Bay of Bengal.
Introduction
Electromagnetic (EM) induction produces current
flow inside the earth on a global scale. The source
field for this is the current outside the earth, in the
ionosphere and beyond in the magnetosphere. EM
waves penetrate inwards (according to the skin-depth
rule) as deep as the lower mantle. Attenuation of the
diffusing wave and the current induced by it is de∗ Present address: Geological Survey of Japan, 1-1-3 Higashi,
Tsukuba, Japan 305. E-mail: john@gsj.go.jp
pendent on the electrical conductivity of the earth.
It is possible to investigate conductivity variations as
a function of depth by measuring magnetic field at
the earth’s surface. The magnetic field measured at
the earth’s surface includes contributions from external (inducing) and internal (induced) currents. Exact
separation into parts of external and internal origin is
difficult. Schmucker (1970) suggested that instead of
separating the field into external and internal parts,
one can study the subsurface conductivity structure
by separating the field into normal and anomalous
2
parts. The normal part by definition is the vector sum
of the external field and the field from currents in a
laterally uniform earth. However, the exercise needs
data from an array of magnetometers recording three
components of the magnetic field.
Recent development in theory, methodology, instrumentation and marine geology have spawned increasing interest in the area of EM methods for
seafloor exploration. Many of the seafloor EM techniques are the adaptation of terrestrial (land) methods
such as magnetovatiational (MV) and magnetotelluric
(MT) soundings. The land areas are in direct contact with a near insulator, the atmosphere, which
allows the nearly instantaneous propagation of EM
signals with limited attenuation. On the contrary, at
seafloor the overlying highly conducting seawater column acts as a low pass filter, i.e., the high frequency
EM signals get attenuated depending on the thickness
of sea-water column. Two competing sources of EM
field are present in the oceans: the ionospheric source
which propagate through the ocean and the underlying
lithosphere, as if the ocean were static; and secondly,
the motion of sea-water through earth’s static magnetic field produces an electric field of dynamo action.
At periods longer than a day, the energy spectrum of
ocean sources dominate the observed EM field. As a
result, seafloor MV and MT response functions are
band limited and restricted to 2-3 decades of period.
When analysing the seafloor EM data, the tidal periods
should also be filtered out.
Major seafloor EM experiments were conducted in
Pacific Ocean jointly by various research groups from
many nations. EMSLAB (EMSLAB group, 1988),
EMRIDGE (Heinson et al., 1993) and MELT (Chave
et al., 1996) are famous among them. Other seafloor
EM experiments include Tasman Project of Seafloor
Magnetotelluric Exploration by Australian researchers
(Lilley et al., 1989) and seafloor EM experiments
conducted every year from 1986 through 1992 in the
vicinity of of Japanese Island-Arc System by Japanese
scientists (Segawa and Toh, 1992; Toh and Segawa,
1995). In Atlantic Ocean the seafloor EM studies
were conducted by various groups (e.g., Poehls and
von Herzen, 1976; Shneyer et al., 1991). In Indian Ocean region, seafloor EM experiments were
conducted around the Indian peninsula by Indian researchers (e.g., Iyengar et al., 1992a,b; Joseph et al.,
1995). In the present study we report the seafloor
EM experiments conducted in the Bay of Bengal,
northeastern Indian Ocean.
The Bay of Bengal in the northeastern Indian
Ocean is a geologically complex region with deep-sea
fans, submarine canyons, subduction zones and the
aseismic ridges namely the 85◦ E Ridge and Ninety
East Ridge (NER). The 85◦ E Ridge is fully buried under the thick sedimentary deposits whereas the NER
topography could be traced up to 10◦ N latitude. Towards the North the NER plunges beneath the thick
sedimentary deposits. Though both these ridges are
believed to be of hot spot origin (Morgan, 1978;
Curray and Munasinge, 1991) they show distinctly different free-air gravity anomaly; the 85◦ E Ridge shows
negative anomaly while NER shows positive anomaly
(Liu et al., 1982; Chaubey et al., 1991; Sandwell and
Smith, 1997). Though the hot spot origin and tectonics of the NER is fairly established (e.g., Curray
et al., 1982), controversies still exist regarding the
origin and extension of the 85◦ E Ridge (e.g., Muller
et al., 1993, Storey, 1995; Ramana et al., 1997). The
simplest explanation for the gravity being low over
the 85◦ E Ridge is that the ridge is less dense than
the adjacent region. However strong magnetic signals
over the northern parts of the ridge is observed (e.g.,
Chaubey et al., 1991; Ramana et al., 1997). The unusual geophysical properties observed over the 85◦ E
Ridge may point to the geothermal state of its subsurface. The geothermal state has a profound effect on the
geoelectrical conductivity. The EM method has been
proven to be a powerful tool in mapping subsurface
electrical conductivity structures. Generally oceanic
crustal materials are considered to be less conductive. If a subsurface conductivity anomaly associated
with the 85◦ E Ridge is present, the induced current
in the Bay of Bengal crust by the transient geomagnetic field variations may get concentrated through the
ridge. On the other hand, if the ridge is resistive, the
induced currents will avoid flowing through it. These
two situations will produce distinctly different signatures in the records of geomagnetic field variations at
the seafloor across the ridge. In the study reported
here, arrayed seafloor magnetometer measurements
were conducted to deliniate the subsurface electrical
conductivity structure in the close vicinity of the 85◦ E
Ridge and NER, and also to understand the geoelectrical structure of the oceanic crust and upper mantle of
the Bay of Bengal.
3
Figure 1. Station locations with detailed bathymetery of the Bay of Bengal. Sea water depths are labelled in meters (created using ETOPO5
elevation and depth data set).
4
Experiments and instrumentation
The deployment of ocean bottom magnetometers
(OBMs) were conducted in three phases, and the station locations are shown in Figure 1. These sites were
selected with respect to a seismo-geological map of
the Bay of Bengal prepared by the Oil and Natural
Gas Corporation of India (personnel communication).
Array 1991 consisted of five seafloor stations across
the 85◦ E Ridge along 14◦ N latitude with a land reference station at Selam (SLM). The seafloor sites were
selected such that BYB 5 and BYB 8 were on the western and eastern flanks of the 85◦ E Ridge, while BYB
6 and BYB 7 were over the ridge. BYB 9 was selected
to be remote from both the 85◦ E Ridge and Ninety
East Ridge (NER). Simultaneous data were collected
for a period of 12 days in March 1991.
Array 1992 also had five seafloor stations across
the 85◦ E Ridge along 12◦ N latitude. Here the station
BYB 10 was near the eastern continental margin of
India, BYB 12 and BYB 14 were over the western
and eastern flanks of the 85◦ E Ridge, and BYB 13
right over the ridge. BYB 11 was between the eastern
continental margin of India and the 85◦ E Ridge and
remote from both the places. Simultaneous data were
collected for a period of 16 days in February–March
1992, and data from the permanent magnetic observatory Annamalinagar (ANN) was used as the land
reference.
Array 1995 consisted of four seafloor stations
across the NER along 9◦ N latitude. Here the station
BYB 15 was very close to the northern extension of
Sunda Arc and the Andaman and Nicobar island chain.
BYB 16 and BYB 17 were right over the NER, while
BYB 18 was on the western side of NER with a land
reference station at Tirunelveli (TIR). Simultaneous
data were collected for a period of 12 days in March
1995. Details of station locations and recorded data are
given in Table 1. Seafloor stations of the above three
arrays together covered an east-west profile across the
entire Bay of Bengal.
OBM-S4 magnetometers (Segawa et al., 1986)
were used for seafloor measurements. It is basically
a fluxgate magnetometer with low temperature drift
and sensitivity of 0.1 nT. Data were collected at a
sampling interval of one minute (two minutes in the
case of array 1995) using a Z-80 microprocessor-based
system and stored in EPROM cards. Each EPROM
card has a capacity of 128 K bytes, and can store
up to one month of three-component magnetic data
with one minute sampling intervals. Lowering of the
OBMs were carried out by free falling from the sea
surface, after the sites of deployment were accurately
located using the Global Positioning System (GPS).
Recovery of the instruments from the seafloor were
conducted by an onboard acoustic release system. In
array 1991, deployment and retrieval of OBMs were
carried out from aboard R.V. Samudra Manthan of
Geological Survey of India. For array 1992 and array
1995, the same were conducted from aboard O.R.V.
Sagar Kanya of Department of Ocean Development,
Government of India.
Data analysis and results
The OBM assembly settles at the seafloor in a random orientation, but upright because of the weight
attached with it. Since the sensor assembly is mounted
on a gimbal, the horizontal sensors are oriented in a
horizontal plane while the vertical sensor is oriented
in a vertical plane. The vertical sensor thus measures
the field directly, while the two horizontal sensors are
in a direction orthogonal to each other but at a random orientation with respect to geographic north and
east. Since the system measures the values of ambient
field components, the values recorded during the quiet
periods in nighttime can be used in conjunction with
model values from International Geomagnetic Reference Field (IGRF) to estimate the orientation of the
two horizontal sensors (Joseph et al., 1995).
After applying the geographical corrections, selected data were first plotted for visual examination.
The area under consideration is an equatorial region and hence for the day-time events, the presence
of equatorial electrojet leads to non-uniform source
fields. This puts restriction on the use of day-time
data in the analysis, as all the conventional approaches
presuppose the source field to be uniform. Hence we
selected local night-time data and got 6–8 moderate
events for each array. Figure 2 shows the stack plots
of one of the local nighttime events recorded simultaneously for each array (i.e., for array 1991, array 1992
and array 1995). Data from BYB 8 (array 1991), BYB
12 (array 1992), BYB 16 and TIR (array 1995) were
found noisy and could not be used for analysis.
Visual examination
Visual examination of the time series and the enlarged
portions of them (Figure 2) reveal improtant features
of the data. The magnetic field components shown
5
Table 1. Details of the station locations and the data sets
Stations
SLM
BYB 5
BYB 6
BYB 7
BYB 8
BYB 9
ANN
BYB 10
BYB 11
BYB 12
BYB 13
BYB 14
TIR
BYB 15
BYB 16
BYB 17
BYB 18
Array
1991
1991
1991
1991
1991
1991
1992
1992
1992
1992
1992
1992
1995
1995
1995
1995
1995
Position
North
East
Depth
(m)
Sampling
(min)
Period
(days)
11◦ 550
14◦ 000
14◦ 000
14◦ 000
14◦ 000
14◦ 000
11◦ 220
12◦ 000
12◦ 000
12◦ 000
12◦ 000
12◦ 000
08◦ 400
09◦ 000
09◦ 000
09◦ 000
09◦ 000
78◦ 100
84◦ 500
85◦ 100
85◦ 300
86◦ 000
88◦ 000
79◦ 410
81◦ 000
83◦ 000
85◦ 000
85◦ 300
86◦ 000
77◦ 490
91◦ 300
90◦ 300
90◦ 000
88◦ 300
Land
3138
3125
3070
3072
2980
Land
3310
3430
3325
3317
3285
Land
3387
2756
2963
3435
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
20
12
12
12
12
12
PMO∗
16
16
16
16
16
15
12
12
12
12
∗ Permanent Magnetic Obsevatory.
in the diagrams are Hx – (geographical north), Hy
– (geographical east) and Hz - (vertical down ward).
The Hz - variations at the seafloor stations in the close
vicinity of the 85◦ E ridge (i.e. at BYB 5, BYB 6,
BYB 7, BYB 13 and BYB 14) is very small. But at
seafloor station BYB 10 (near the eastern continental
margin of India) and the land stations SLM and ANN,
Hz - variations are quite prominent. Further, the Hz and Hx - are in phase at all these stations. Corresponding to positive Hx - variations, the external current
in the equatorial region is eastward and the related
induced current in the sea water and subsurface region would be in the westward directions. At land-sea
boundary (continental margin) this westward current
gets deflected in the north-south direction (parallel to
the coast).
The external current during night hours being uniform and oriented in the east-west direction will produce only north-south (Hx ) field and no east-west (Hy )
field. Further, a uniform inducing field would also
have no vertical (Hz ) field of external origin. The field
of internal origin will then be more pronounced in the
Hy and Hz variations. We do find significant Hy and
Hz variations at BYB 10 and ANN. Analysis of the
Indian magnetic observatory data (Nityanada et al.,
1977; Rajaram et al., 1979) and South Indian magnetometer array data (Thakur et al., 1981 and 1986)
proposed the presence of a regional effect arising from
deflection in the direction of induced current flow in
the Bay of Bengal. This north-south flowing induced
current will produce anomalous Hy and Hz variations
which are clearly seen at BYB 10 and ANN. Agarwal
and Weaver (1989; 1990), by their numerical calculations, showed the deflection of current flow at the
coast lines and concentration of currents at the southeast Indian coast. A similar type of current pattern was
also noticed by Takeda and Maeda (1979). Though the
coast effect may dominate at ANN which is close to
the coast line, we do not expect such a large effect at
land station SLM which was about 175 km away from
the coast line. But the Hz variations observed at SLM
seems to be anomalous. This anomalous Hz at SLM
may be due to some other local or regional structure.
Detailed study on the presence of such a structure is
beyond the scope of this paper.
Even though Hz - variations are small at BYB 11,
BYB 13 and BYB 14, Hy - variations are anomalous
at all these stations. Similarly at BYB 5, BYB 6, BYB
7 and BYB 9 also showed small Hz -variations, while
Hy -variations are significant at these stations. For array 1995, as the land records were found noisy, we
were left with only three seafloor station data. BYB
15, BYB 17 and BYB 18 showed almost similar behavior on all three components except that the BYB
6
Figure 2. Geomagnetic field variations recorded simultaneously for (a) Array-1991 stations corresponding to 13: 31–00:30 h (UT) of 19–20
March 1991; (b) Array-1992 stations corresponding to 12: 31–00.30 h (UT) of 2–3 March 1992 and (c) Array-1995 stations corresponding to
12: 31–00.30 h (UT) of 28–29 March 1995.
15 showed slightly larger Hz variations. This station is
very close to the Andaman and Nicobar island chains
(i.e., close to a lateral conductivity contrast). Another
interesting feature is that at seafloor stations, the Hx
and Hy variations are in phase. Knowing that Hy
is totally of internal origin, it can remain in phase
with Hx at seafloor, provided its major source is current induced in sea water. The Hx - variations at all
seafloor stations are attenuated relative to that at corresponding land stations. This is natural to expect
since in the Hx -field, both external and internal parts
add up at land while at seafloor the current induced
in the sea water becomes an external source for the
point of observation; i.e. at seafloor, contributions
of currents in the sea water would produce Hx -field
in opposite sense of Hx produced by ionospheremagnetosphere part. This cause attenuation of the Hx
-field at the seafloor, which is called as the ‘shielding
effect’ of the electrically conducting sea water column
(e.g., Schmucker, 1970). Apart from the above explained visual examination, we adopted two different
approaches namely: geomagnetic depth sounding and
vertical gradient sounding.
7
Geomagnetic depth sounding
The anomalous Hz field at a site may be related to horizontal Hx and Hy fields by complex transfer functions
A and B in the frequency domain as
Hz = AHx + BHy .
(1)
This method, known as geomagnetic depth sounding
(GDS) is routinely used to map the subsurface lateral
electrical conductivity contrasts. The transfer functions A and B are generally represented as real and
quadrature induction arrows of magnitude R and I
respectively, with orientation φR and φI relative to
geomagnetic coordinates. These quantities are defined
by
q
−1 −Ar
2
2
, (2)
R = (Ar + Br ), φR = tan
−Br
I =
q
(A2i
+ Bi2 ),
−1
φI = tan
Ai
Bi
.
(3)
Here the Parkinson convention (Parkinson, 1959) is
adopted so that the arrows point towards the conductors. The Induction arrows were calculated for all
magnetometer stations using robust remote-reference
algorithm (Chave et al., 1987; Chave and Thomson,
1989) which is proved to be the best among the EM
data processing techniques (Jones, 1989). For array
1991, array 1992 and array 1995, we used BYB 9,
BYB 11 and BYB 18 as the remote-reference stations
respectively. Induction arrows were computed for five
selected periods, such as 10.67, 16.00, 32.00, 64.00
and 128.00 min. Figure 3 shows the induction arrows
observed for two periods at all the stations. The observed induction arrows for seafloor stations are small
in magnitude with a general southerly trend (i.e., point
towards deeper waters). Quadrature arrows are much
smaller than the real arrows. This southerly trend of
the induction arrows as well as the larger real part of
the arrows with respect to the quadrature part may
be caused by the magnetic field polarisation in this
equatorial region. Arrows at BYB 5, which is over the
western flank of the 85◦ E ridge, showed a prominent
southwesterly trend, especially at shorter periods (i.e.,
pointing away from the ridge). This may be indicating that the induced currents in the Bay of Bengal
crust avoid flowing through the 85◦ E ridge and getting
concentrated in the sedimentary column between the
ridge and the eastern continental margin of India (i.e.
the ridge is resistive and sediments are conductive).
The Isopach map of the Bay of Bengal (Curray, 1994)
clearly shows that the western and eastern sides of the
N-S trending 85◦ E ridge have sedimentary thickness
of 8–10 km and 6–8 km respectively. Right over the
ridge the sediment thickness reduces to about 2–3 km.
These sediment thicknesses mentioned are along the
lines of magnetometer arrays of year 1991 and 1992.
But corresponding to longer periods, as seen in Figure 3b, the direction of the induction arrows at BYB 5
become more southerly. This may be a clear indication
that the lateral conductivity contrast exists only within
the shallow region. The southerly trend of induction
arrows at longer periods may be attributed partially
to the persistance of coast effect at station BYB 5
and also to the source field polarisation. Induction arrows for BYB 17 and BYB 18 are very small. This
is the case with all periodicities selected, which may
indicate the absence of any lateral conductivity in the
close vicinity. Moreover, sediment thickness is much
less across NER along the array 1995 station locations
(Curray, 1994). At seafloor stations close to the landsea boundary (BYB 10 and BYB 15), induction arrows
showed comparatively larger magnitude. This may
also be explained in terms of coast effect as well as
the source field polarisation. But at the same time we
must keep in mind the close vicinity of Sunda Subduction zone to BYB 15 and pondicherry rift to BYB 10
(Burke et al., 1978). We may require a denser array of
magnetometers in this region to clear the ambiguities
and deliniate these subsurface structures.
Induction arrows observed at land stations are quite
large. At SLM induction, arrows point in the SE direction at the shorter periods (T = 10.67–32.00 min) and
in the SSE direction at longer periods (T = 64.00–
128.00 min). Since this station is away from the east
coast of India, one does not expect the coast effect
to prevail at shorter periods. At SLM induction, arrows are large at shorter periods. Larger magnitude
induction arrows are also observed corresponding to
longer periods. As mentioned in the previous section,
this anomalous behavior may be due to some local or
regional subsurface structures. Land station ANN also
showed larger magnitude induction arrows pointing
southward at all periods. Generally at a coastal station
one would expect the induction arrows to be pointing towards sea and perpendicular to the coast. The
unusual behavior of induction arrows at ANN was noticed by many earlier workers (e.g., Singh et al., 1977,
Nityananda et al., 1977, Srivastava et al., 1984, Thakur
et al., 1986) and suggested that it may be either an
indication of induced current channeling through Palk
Strait (between India and Srilanka) and currents in the
8
Figure 3. Induction arrows observed for (a) T = 16.00 min and (b) T = 64.00 min. The red and yellow arrows represent the real and the
quadrature arrows respectively. (For land stations SLM and ANN, the scale of induction arrows is unit = 0.25.) Induction arrow magnitudes
are plotted with two standard deviation error bars.
9
Indian ocean or a manifestation of the source field
bias, because at the periods selected, the arrows tend to
orientate themselves with the direction of source field
polarisation. A very narrow continental shelf and the
presence of Pondicherry rift (Burke et al., 1978) may
have some influence on the anomalous field variations
observed.
Vertical gradient sounding
Attenuation of the horizontal magnetic field through
the ocean layer can be used to derive seafloor EM
impedance estimate, a technique known as vertical
gradient sounding (VGS) (Poehls and von Herzen,
1976; Law and Greenhouse, 1981). Defining a transfer
function P as the ratio of horizontal magnetic field at
the seafloor to that at the sea surface, the seafloor VGS
impedance tensor ζ is given by
ζ=
(iωµ − P iωµ cos h(αh))
,
(P α sin h(αh))
where
α = ±(1 + i)
r
ωσµ
,
2
(4)
(5)
ω is the angular frequency, µ is the magnetic permeability, σ is the electrical conductivity of the sea
water and h is the depth of ocean. Then the apparent
resistivity(ρa ) and phase (φ) are computed as
2
ζ ,
(6)
ρa =
ωµ
Im(ζ)
φ = arctan
.
(7)
Re(ζ)
We made an attempt to compute the VGS responses at
the seafloor stations. Due to the difficulty of making
vector magnetic measurements at the sea surface, we
followed the suggestion of Weaver (1963) and used
land-based data collected to represent the sea surface
measurements. There are some limitations for the application of VGS method by considering horizontal
components measured at land stations to represent the
sea-surface values. This is because the land station
horizontal components may be affected by the subsurface structures below the land stations and obtained
VGS responses may be biased. However since the land
stations are approximately in the same latitude and
not very far from the region of seafloor stations, we
followed the same approximation adopted in previous
studies by various groups around the world (e.g., Law
and Greenhouse, 1981; Ferguson et al., 1990; and
White and Heinson, 1994). Then VGS impedances
were calculated for the seafloor stations of both array 1991 and array 1992 using the robust analysis
algorithm of Chave et al. (1987).
The impedance ζ in Equation (4) were used to
compute the apparent resistivities and phases by Equations (6) and (7) for periods ranging from 8 min to
128 min Then the depth-resistivity profiles were obtained for the seafloor stations using the 1-dimensional
(1-D) inversion scheme (Utada, 1987) based on the
Monte Carlo Technique, which searches for minimum
variance. Figure 4 show the VGS responses, apparent
resistivities, phases, and the 3-layer depth-resistivity
model obtained by 1-D inversion corresponding to the
seafloor stations of array 1991 and array 1992. VGS
responses at all these seafloor stations showed approximately similar characteristics, whether the stations
are close to the 85◦ E ridge or away from it. The 3layer depth-resistivity profiles obtained showed a top
resistive layer (150–500 m) with a thickness of about
15–50 km. This layer is followed by highly resistive
layer (2000–9000 m) with a thickness of about 235400 km. The third layer showed very low resistivity
(0.1–50.0 m). Thus the 1-D inversion models show
that the oceanic crust and upper mantle are resistive
followed by a highly conducting region at a depth of
about 250–450 km. Since the VGS responses corresponding to the periods used in the present study (i.e.,
8.00–128.00 min) do not have the resolving power in
terms of depth-resistivity for shallow structures, the
responses corresponding to the sedimentary column
and the 85◦ E ridge could not be seen. As described
earlier, the Hy -variations at SLM is very small and
that at ANN is anomalous. So we restricted the calculation of VGS responses to the attenuation of the
Hx -field only. For array 1995, we could not compute
VGS impedances since the simultaneous land-based
data at TIR were found noisy.
Model calculations
Seafloor EM observations are influenced by the conducting sea water column as well as the subsurface
geological structures. To evaluate such effects, it may
be advisable to do model calculations. In the present
study, visual examination of the time series and induction arrows calculated indicated that the EM induction process in the Bay of Bengal is complex.
We made an attempt to solve this problem numerically and adopted two approaches, namely: thin-sheet
Figure 4. The apparent resistivities (ρa ), phases (φ) and the 3-layer depth-resistivity profile obtained from the 1-D inversion for the seafloor stations (a) BYB 5, (b) BYB 6, (c) BYB 7, (d) BYB
9, (f) BYB 10, (g) BYB 11, (h) BYB 13 and (i) BYB 14. The dots with error bars and thick lines are the observed VGS responses and 1-D inversion responses respectively. (e) and (j) are the
3-layer depth-resistivity profiles obtained from the 1-D inversion for these seafloor stations.
10
11
approximation (McKirdy et al., 1985) and three dimensional forward modelling (Mackie et al., 1994).
Any geophysical data interpretation or model calculations without considering the available geological and
geophysical constraints may lead to incorrect results.
Here in these model calculations we incorporated as
many a priori constraints as possible.
Thin-sheet modelling
Thin-sheet electromagnetic approximation has been
proven to be good in resolving 3-D induction problems where the conductivity anomalies are associated
with shallow sub-surface features (i.e., conductivity
variations occur in a thin-sheet above a layered earth).
Thus this method is well suited to study the effect of
sea water column on the seafloor EM measurements.
We followed the method of McKirdy et al. (1985) in
which calculation of the total field is required to have
a Neumann boundary condition of vanishing outward
gradient at infinity. The study area and the surrounding region were divided into 50 × 50 grids with a
grid-node spacing of 55 km. The edge effect has been
minimized by extending the grids a sufficiently larger
distance away from the observational domain. The
sea-water column conductance values have been averaged at every 100 m up to a water depth of 4500 m
(assuming the average sea water conductivity σ as
3.3 S/m) to provide 45 different conductance values.
The surrounding land region (thickness = 4.5 km) was
given a conductance value of 22.5 S. The thin-sheet
conductance map is shown in Figure 5.
Since the present seafloor EM experiment is the
first of its type for this region, information about the
subsurface resistivity structure of the Bay of Bengal
was not available. So, the average 1-D depth – resistivity profile obtained from the present study (i.e.
from VGS method) was incorporated as the initial
layered structure beneath the thin-sheet. Here in this
model calculations, all thin-sheet conditions (Weaver,
1982) were fully satisfied. Thin-sheet calculations
were made with two orthogonal polarisations of the
inducing field. These polarisations are denoted as
TM-polarisation for the field in the X-direction and
TE-polarisation for field in the Y-direction. X- and
Y- directions refer to the geomagnetic north and geomagnetic east directions respectively, although the
thin-sheet grid may not necessarly be aligned with
geomagnetic co-ordinates. The computed EM components for TM- and TE- polarisations of the induc-
ing field contains all parameters required to calculate
thin-sheet 3-D responses.
The GDS induction arrows may be computed using the corresponding horizontal magnetic fields for
on and below the thin-sheet. Verical component is assumed to be continuous across the sheet. Since our
objective was to obtain seafloor model responses, we
used the magnetic fields for the lower side of the thinsheet. The total field (i.e., both polarisation) is used
for computing the thin-sheet model seafloor induction arrows. In the final model, the layers beneath
the thin-sheet were such that, the first and second
layers have resistivities of 1000 m and 5000 m
with thicknesses of 50 km and 350 km, respectively.
These layers were followed by a third layer which
has a resistivity of 0.1 m and thickness extending to
infinity. The thin-sheet model induction arrows were
then calculated. Figure 6 shows the thin-sheet induction arrows and the observed induction arrows for two
periods (T = 16.00 and 64.00 min). Model induction
arrows clearly show that at all periodicities coast effect
persist even at the seafloor stations in the close vicinity
of the 85◦ E ridge which is about 500 km away from
the coast line. This is on the contrary to the land and
sea surface where the coast effect diminishes within
a few tens of kilometers corresponding to the periodicities chosen in the present study. As noted from
the observed induction arrows, the quadrature arrows
are very small compared to the real arrows (see Figure 3). In general, the real part is much larger than the
quadrature part and hence the real part is used more
often (Rikitake and Honkura, 1985). So we restricted
our model comparisons to real induction arrows only.
Thin-sheet induction arrows corresponding to the positions of seafloor magnetometer stations in the close
vicinity of the 85◦ E ridge and the NER agree well
with the observed induction arrows (i.e., point towards
deeper waters) with an exception at BYB 5, where
the direction of observed arrows point southwesterly
at shorter periods.
For the stations close to the land-sea contrast, the
oberved and model induction arrows could not be
matched well due to the limitations of the modelling
technique and also due to not incorporating the subsurface conductivity structure at land-sea boundaries
in the thin-sheet model.
One must also remember that the seafloor measurements were carried out on a thick sedimenatry column.
A more exact model calculation would need inclusion
of this sedimentary column. The sediments are considered to be conductive mainly due to its porocity.
12
Figure 5. Thin-sheet conductance map of the Bay of Bengal and the surrounding region. (Colour coding of the conductance values are also
given.)
Figure 6. Thin-sheet induction arrows and the observed induction arrows for (a) T = 16.00 min, and (b) T = 64.00 min. Red and green arrows represent observed and thin-sheet arrows. For
clarity thin-sheet arrows are plotted only for the profiles corresponding to array 1991, array 1992, and array 1995.
13
14
Within a thick sedimentary column, as the depth increases, the porocity decreases. Water-filled porocity
φ of sediments can be related to a depth of burial z by
a simple exponential function
φz = φ0 .e−az
(8)
where φ0 is the porocity of sediments at seafloor and
a is a constant (a −1 ranges from 1.5–2.0) (Le Pichon et al., 1990; Heinson and Segawa, 1997). Archie
(1942) showed the relationship between porocity and
conductivity. Brace et al. (1965) have shown that an
empirically derived Archie’s law formulation is applicable to a wide range of rock types and porocities.
Hermance (1979) modified and rewrote this relation as
σ∗ = σs + (σf − σs )φt ,
(9)
where z is depth, exponent term t varies in between
1 and 2 (laboratory studies), σs and σf are the conductivities of solid phase and fluid phases and σ∗ is
the conductivity of the rock sample to be determined.
Using this method we have computed the sediment
conductivity at various depths.
The isopach map of the Bay of Bengal (Curray,
1994) shows that the sediment thickness varies from
22 km at about 20◦ N to 2 km at the equator. In the
region of seafloor EM measurements, both sides of the
85◦ E ridge has about 8–10 km sediments whereas the
ridge crest has about 2–3 km thick sediments. These
sediments are too thick to be incorporated into the
thin-sheet and too thin to be added as a separate layer
just beneath the thin-sheet (i.e., thin-sheet conditions
could not be satisfied). To alleviate this problem we
made an attempt to do 3-D forward modelling.
3-D forward modelling
We adopted the 3-dimensional finite difference algorithm of Mackie et al. (1994) which is based on
the integral forms of the Maxwell’s equations. Here
in the model calculation we assign the tangential Hfields on the boundaries of the model for appropriate
polarisation. These boundary values come from a 2dimensional (2-D) TM-mode calculation where each
vertical plane of the 3-D model is treated as the inner
part of a large scale 2-D model. The values obtained
at the positions corresponding to the boundaries of
the 3-D model are then used as boundary values for
the 3-D problem. The TM-mode values are used since
these are appropriate for a current that crosses resistive
boundaries. The study area and the surrounding region
were divided into 50×50×16 blocks (not counting the
air layers required in the forward modelling) totalling
40,000 model parameters. Here the horizontal spacing
of blocks at the region of interest was taken as 55 km,
similar to the thin-sheet grid-node spacing. The source
field for magnetotelluric problem is a uniform current
sheet that is put at the earth’s surface. At the top of
the air layers, a 1-D plane wave impedance for the
outgoing fields is used. Likewise, a 1-D plane wave
impedance for a layered media is used at the bottom of
the earth model. Depth-resistivity values were adopted
as mentioned in the case of thin-sheet calculations.
The advantage of this method is that it could incorporate the thick sedimentary column as well as the
known subsurface geological constraints from various geological and geophysical studies (e.g., Curray,
1994; Gopalarao et al., 1994; Ramana et al., 1997;
Sandwell and Smith, 1997). Our initial model incorporated only the sea water column and the sedimenatry
layers of varying thickness and resistivities followed
by 1-D structure obtained from VGS method. Though
the seafloor model MT responses were comparable
with 1-D inversion responses, the model failed to reproduce the GDS responses. Later we incorporated
other subsurface geological features such as 85◦ E
ridge and NER (both with 500 m resistivity) as
seen on the tectonic map of Bay of Bengal which is
given as Figure 7. Then the 3-D model responses were
computed.
The 3-D model induction arrows show a general
trend towards deeper waters as seen in the case of thinsheet arrows. At a short period (T = 16.00 min) for
the point corresponding to the seafloor station BYB
5, 3-D model induction arrows show the change in
direction (points southwesterly), which agrees well
with the observed induction arrow. As the period increases, the direction of induction arrows change is as
shown in Figure 8. This behavior fairly agrees with
that of the observed induction arrows at all periods
for this station, indicating the possible presence of a
lateral conductivity contrast at shallow depths, (i.e.
the lateral conductivity contrast between the 85◦ E
ridge and the surrounding sedimentary column). The
induced current avoids flowing through the resistive
85◦ E ridge and gets concentrated towards less resistive sediments in the western side of the 85◦ E ridge.
Observed 1-D responses (VGS) were compared with
3-D model responses and found that they are in good
agreement especially for deeper layers (see Figures 9)
for which the resolution is better corresponding to the
range of periods used in the present study (T = 8.00–
128.00 min). Our final 3-D model for the study region
15
Figure 7. Tectonic map of the Bay of Bengal.Sediment isopachs (km) are given by dotted line (after Curray, 1994).
16
Figure 8. Comparison of induction arrows observed, computed from thin-sheet and 3-D model calculations for selected periods at seafloor
station BYB 5. Red, green and yellow arrows represent observed, thin-sheet and 3-D arrows, respectively.
is shown in Figure 10. This model has a resistive
oceanic crust (layers of 1000 m) and an upper mantle
(layers of 3000 m, 1000 m, 500 m and 200 m),
followed by a very low resistive region (0.1 m) at a
depth of 400 km.
As explained, in the final model, complex 3-D
structures were restricted to the shallow region and
the periodicities used in the present study are between
8.00–128.00 min. The prominent lateral conductivity
contrast between the conducting sediments and the
resistive 85◦ E ridge is seen on the behavior of 3-D
model induction arrows (Figure 8). At the same time
the 3-D model resitivity and phase responses show that
the shallow 3-D structures like the 85◦ E ridge and
17
NER have less influence on the depth-resistivity profile obtained. This can be seen from Figure 9, where
the observed responses and the model responses agree
fairly well for seafloor stations BYB 5 and BYB 6 of
array 1991 and BYB 13 and BYB 14 of array 1992
(stations over the 85◦ E ) as well as BYB 9 of array
1991 and BYB 10 and BYB 11 of array 1992 (stations away from the 85◦ E). Model comparisons were
carried out between 3-D model seafloor MT responses
and conversions of the observed VGS responses. So
the final 3-D model still has some limitations in the
sense that the horizontal magnetic field at the land reference stations may be different from the sea-surface
magnetic field above the seafloor stations. Despite
these limitations, the final 3-D model proposed here
has the following features: (1) This model can handle
the very thick sedimentary layers of the Bay of Bengal;
(2) This model gives better fits to observed GDS and
synthetic MT responses as shown in Figure 9 than any
1-D or thin-sheet models. The authors think this 3-D
model may be useful as first hand model for the future
EM induction studies in this region.
Conclusions
Seafloor magnetometer station of arrays 1991, 1992
and 1995 together covered an east-west profile from
the eastern continental margin of India to the northern
extension of the Sunda Arc (i.e., an east-west profile
of the entire Bay of Bengal). Visual examination of
the time series and the selected data clearly indicated
the complexity of the EM induction process in the
Bay of Bengal. Though the observed induction arrows
were small in magnitude, they revealed the direction
of induced currents in the Bay of Bengal and subsurface. Induction arrows have a general trend towards
the south (i.e., to the region of deeper waters). Arrows
corresponding to the seafloor station BYB 5, which
was situated at the western flank of the 85◦ E Ridge,
showed a southwesterly trend at shorter periods and
as the period increases the trend has changed more towards the south. This oscillation characteristic of the
induction arrows at BYB 5 may be attributed to the
presence of a shallow lateral conductivity contrast in
the close vicinity of this seafloor station.
Thin-sheet and 3-D forward model calculations
jointly showed that the observed induction arrows at
seafloor sites could be explained in terms of nearsurface features such as the thick sedimentary deposits
(i.e., deep-sea fans of the Bay of Bengal), the resis-
tive 85◦ E Ridge and the thick seawater column above
the seafloor sites. The induced currents in the Bay of
Bengal crust avoid flowing through the resistive 85◦ E
Ridge and getting concentrated towards less resistive
sediments in the western side of this ridge. As per the
seismic reflection data (Gopalarao et al., 1994), the
eastern flank of the 85◦ E Ridge is gently sloping while
the western flank is steep. So the lateral contrast at
the western flank is sharper and is clearly reflected on
the induction arrows at seafloor station BYB 5, which
was situated right over the western flank of the 85◦ E
Ridge (i.e., at the lateral conductivity boundary). Thus
the present study favours the propostition of Liu et al.
(1982), that the gravity low over the 85◦ E ridge is due
to the thickening of oceanic crustal material forming
the ridge with its underlying root in the lithosphere.
Induction arrows observed at seafloor stations
close to the land-sea boundary (BYB 10 and BYB
15) could not be fully reproduced by the model calculations. This may be due to not incorporating the
complex structures at the land-sea boundary, e.g., proposed conductor beneath Palk Strait (Rajaram et al.,
1979) and Pondicherry rift (Thakur et al., 1986) in
the case of BYB 10 and Sunda subduction zone in
the case of BYB 15. A dense network of seafloor EM
stations may throw light on these complex structures
(e.g., Wannamaker et al., 1989; Toh, 1993; White and
Heinson 1994). Of course we must keep in mind that
any model calculation has its own limitations. Coverage of larger area and limited computer memory
allocation restricted us from incorporating every local
subsurface feature into the 3-D model.
To investigate the depth-resistivity profile, we
adopted the VGS method. Though there were some
limitations with data sets, a reasonable 1-D resistivity structure was obtained for the subsurface of the
Bay of Bengal. The observed depth-resistivity profiles
showed a resistive oceanic crust and upper mantle followed by a low resistive region at a depth of about
250–450 km. Since the depth-resistivity information
of the land region included in the model calculation
was not clearly known, model comparisons were carried out between 1-D VGS responses and 3-D model
seafloor MT responses. As mentioned earlier, there
are some limitations for the application of the VGS
method by using the horizontal components measured
at the land station to represent the sea surface values.
This is because the land horizontal components may
be affected by the subsurface structures below the land
station and the obtained VGS responses may be biased. To alleviate this one should observe variations in
Figure 9. Comparison of the 3-D model responses with the observed VGS and 1-D responses corresponding to the seafloor stations (a) BYB 5, (b) BYB 6, (c) BYB 7 (d) BYB9 (f) BYB 10,
(g) BYB 11, (h) BYB 13 and (i) BYB 14 respectively. Dots with error bars, thick line and dotted line represent observed VGS responses, 1-D inversion responses and best fitting 3-D forward
model responses respectively. (e), and (j) are the depth-resistivity profiles obtained from 3-D model calculations (dotted lines) compared with the 1-D inversion (solid lines) corresponding to
the seafloor stations. The thin and thick dotted lines represent model values corresponding to stations right over the 85◦ E ridge and away from the ridge respectively.
18
19
Figure 10. Best fitting 3-D model for the Bay of Bengal. Subsurface has been divided in to 16 layers. Layer numbers and thicknesses are clearly
marked. The resistivities (m) are shaded according to the scale at the bottom.
20
the horizontal electric and magnetic field components
at seafloor, namely, the direct MT method. However,
in the present study we had only seafloor magnetic
field measurements and VGS was the only alternative
approach to detect the subsurface electrical conductivity structure. Computed 3-D model responses fairly
agree with the observed VGS responses.
Very few techniques like seismic tomography,
electromagnetic induction and gravity measurements
can provide clues to the structure and physical properties of earth’s mantle. Seismic tomography has been
proven to be the best among them. So there is a general
tendency to correlate any geological or geophysical
finding corresponding to deeper structures with that of
the results from seismic studies. Tarits (1994), in his
review on EM studies of global geodynamic process,
showed that the electrically resistive subsurface layers may be correlated to fast seismic velocity zones,
and conducting layers to slow velocity zones. Here in
the present study, we compared the depth-resistivity
profile obtained for the Bay of Bengal with P-wave
seismic tomography model (Inoue et al., 1990) and Swave tomography model (Li and Romanowicz, 1996).
The depth to the highly conducting region observed
(250–450 km) agrees well with the seismic low velocity zone of the northeastern Indian ocean derived
from the seismic tomography. Thus we propose an
electrical conductivity structure for oceanic crust and
upper mantle of the Bay of Bengal. Need remains for a
detailed survey with dense network of seafloor EM instruments for a better understanding of the subsurface
electrical conductivity structures.
Acknowledgements
We thank L.A. D’Cruz, A. Dhar, M.R. Kulkarni, A.
Hanchanal and P. Gawali for their assistance during
the data collection. Thanks are also due to G. Heinson,
and R.L. Mackie, who kindly provided the thin-sheet
modelling and three-dimensional forward modelling
codes and clarified doubts in the course of model calculations. J.R. Curray and Dick Batchman are thanked
for providing the isopach maps of the Indian Ocean
and the Bay of Bengal. One of the authors (E.J.J.)
thanks the Ministry of Education, Science, Sports and
Culture of Japan for the research fellowship. Three
anonymous referees are thanked for their critical comments which improved the manuscript. Figures are
generated using Generic Mapping Tool (GMT).
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