Induction in electrically conductive seawater attenuates the magnetotelluric
(MT) fields and, coupled with the red nature of the natural magnetic fieldspectrum,
leads to a dramatic loss of electric and magnetic field power on the seafloor
at periods shorter than 1000 s. For this reason the marine MT method has
traditionally been used only at periods of 10^3 to 10^5 s to probe deep
mantle structure; rarely does a seafloor MT response extend to 100 s period.
To be useful for mapping continental shelf structure at depths relevant
to petroleum exploration, however, MT measurements need to be made at periods
between 1 and 1000 s. This may be accomplished using AC-coupled sensors,
induction coils for the magnetic field and an electric field amplifier developed
for marine controlled source applications. The electrically quiet seafloor
allows the attenuated electric field to be greatly amplified before recording,
and in deep (1 km) water, motional noise in magnetic field sensors appears
not to be a problem. In shallower water, motional noise does degrade the
magnetic measurement, but seafloor magnetic records can be replaced by land
recordings, producing an effectively sea-surface MT response. Field trials
of such equipment in 1 km deep water produced good quality MT responsesat
periods of 3 to 1000 s; in shallower water responses to a few hertzcan be
obtained. Using an autonomous seafloor data logger developedat Scripps Institution
of Oceanography, marine surveys of 50 to 100 sites are feasible.
The magnetotelluric (MT) method has been used map sedimentary structure
as an aid to petroleum exploration for several decades (e.g. Vozoff, 1972;
Orange, 1989). The reliability and usefulness of the MT method has improved
greatly over the past few years as a result of progress in several areas.
These include improvements in data acquisition technology, the introduction
of a remote reference to reduce bias associated with noise in the magnetic
field measurements,`robust' response function estimation methods, and improved
1D, 2D, and even 3D forward and inverse modeling codes. As a consequence,
the MT method represents an important non-seismic exploration tool, particularly
for reconnaisance surveys and in areas where the seismic reflection method
performs poorly. The latter include buried salt, carbonate, and volcanic
horizons that efficiently reflect and scatter acoustic energy. In the companion
paper (Marine Magnetotellurics
for Petroleum Exploration 2, Numerical Analysis of Subsalt Resolution
), 2D and 3D modeling is used to demonstrate the utility of the MT method
in delineating subsalt structure.
Many petroleum prospects are offshore, but because seawater attenuates the
magnetic source-field at frequencies below 0.01 Hz, the marine MT method
has traditionally been considered sensitive only to great depths. Indeed,
the focus of marine MT instrumentation has been drift-free measurement of
EM fields at essentially DC frequencies (e.g. Filloux, 1987). Deployed mainly
on the deep seafloor in persuit of mantle conductivity structure, these
instruments rarely provide responses at periods shorter than a few hundred
seconds. Investigations of shallow conductivity have been undertaken usinga
controlled source method, replacing the lost power at high frequencies using
a man-made transmitter positioned on or close to the seafloor (e.g. Cox
etal , 1986). However, the use of a marine controlled source method is technologically
challenging, and the method favours the more resistive, hard-rock seafloor
of the deep ocean over the conductive sediments of petroleum targets on
the continental shelf. Modeling a 3D source field also presents a greater
difficulty than modeling the MT plane-wave source.
The companion paper demonstrates that if instrumentation and processingare
optimized for the frequency band 0.001 to 1 Hz, then the marine MT method
is indeed viable for delineating subsalt structure. The work described here
resulted from field trials conducted for the express purpose of providing
the petroleum industry with a usable offshore electromagnetic explorationmethod.
Simple 1D theory demonstrates that for downward propagating energy, electric
and magnetic fields measured on the seafloor provide an MT impedance for
the subsea section that is independent of the overlying conductive layer.
It appears, then, that one may neglect the effect of the seawater layer
and approach seafloor MT just as one would for land MT. However, although
the field ratio is unchanged, the fields themselves will be attenuated by
induction in the seawater layer, so the instrumental and logistical impact
of smaller fields must be considered. Also, wave and current motion in shallow
water makes measurement of magnetic fields difficult, and it can be desirable
to use nearby land recordings of the magnetic field for the MT impedance
on the grounds that B is spatially coherent over large horizontal distances
(at least several hundred kilometers). The effect of taking seafloor E with
the equivalentof sea-surface B needs also to be considered. Attenuation
of the electric and magnetic fields through seawater depends on the electrical
structure of the seafloor, and so cannot be estimated in detail prior to
carrying out an electromagnetic survey. However, general predictions can
be made. The effect of the resistive seafloor is to slightly enhance the
magnitude of the electric fields over their half-space values. The effect
of the seafloor on the magnetic field, on the other hand, is much larger,
and increasing the resistivity of the seafloor greatly attenuates the fields
over their half-space values.
With the appropriate equipment the seafloor electric field is relatively
easy to measure, because although it is much smaller than normally encountered
on land, there is no cultural contamination and the isothermal, isosaline
environmentis ideal for non-polarizing electrodes. However, as intimated
above, motional noise in the magnetic sensors can degrade the quality of
the B field measurement, particularly in shallow water. The use of a land
magnetic sensor then becomes attractive, by taking the ratio of seafloor
E to land B to get a magnetotelluric impedance. Because the sea's effect
on the E field is small, the correction can be ignored or predicted quite
accurately using an estimated seafloor resistivity. One then has the equivalent
of a sea-surface MT response, and the first layer of the model will be the
seawater itself. Since sea depth and resistivity are well known, this layer
can be fixed during modeling or inversion to recover accurate estimates
of seafloor structure.
Marine MT studies have traditionally used fluxgate or torsion fiber magnetometer
sensors and DC coupled electric field sensors. On land, however, MT exploration
for shallower targets is conducted with induction coils which respond to
the time derivative ofthe magnetic field. This AC coupling removes the 50000
nT main componentof the field, but because the natural field spectrum is
very red at frequencies above 1 Hz coils can be used effectively to periods
in excess of 1000 s. We use the BF-4 magnetometer coil manufactured by Electromagnetic
Instruments Incorporated (EMI). It follows that AC coupling is also appropriate
for measuring electric fields, and the amplifier developed for seafloor
controlled source applications is well-suited to seafloor MT (Webb et al
., 1985). This amplifier is designed to take advantage of the low noise
and low impedance of the marine silver-silver chloride electrodes.
The MT method requires time series of electric and magnetic fields that
are many times longer that the longest period of interest; a typical recording
time of 10 hours would be used to obtain reliable MT responses at 1000 s
period or longer. It is impractical to moor a ship for this length of time,
and somewhat difficult to do so in deep water, and so we choose autonomous
data recorders rather than wireline instrumentation. Seafloor
data loggers have been developed for a variety of applications at Scripps
Institution ofOceanography (SIO). Remote reference processing requires that
data at the base and reference sites be acquired simultaneously with synchronized
acquisition systems. More importantly, processing seafloor data with land
magnetometer components requires precise timing; phases accurate to 5 degrees
at a frequency of 2 Hz requires timing to 7 ms or better. Since all the
seafloor instruments are autonomous, and beyond the reach of radio communication,
accurate timing must be accomplished by on-board quartz clocks. The on-board
clocks are started using a GPS time standard, with initial timing accurate
to 1-10 microseconds. After recovery, clocks are again checked against the
GPS standard to estimate drift or error. Drift rates are typically less
than 1 ms per day.
In order to evaluate the operation of the MT instrument system, we have
conducted testing off San Diego in various water depths. The first test
was carried out from the 7th to the 16th of April 1994 by fitting three
orthogonal BF-4 to coils one of the SIO controlled source electric field
receivers. This instrument was deployed in 1000 mwater in the San
Diego Trough, approximately 30 km offshore and due west of SIO. A second
seafloor data logger was `deployed' at Pinyon Flat Geophysical Observatory
about 150 km NE of the offshore site. This instrument was connected to 2
horizontal BF-4 sensors and to 75 m electric dipoles terminated by conventional
lead-lead chloride electrodes. An EMI electric field signal conditioner
was used with 40 dB of gain and 10 Hz low-pass filtering. Crystal oscillators
in both instruments were started from a precision clock at SIO prior to
deployment. Sampling rate was continuous at 8 Hz during the 10-day deployment.
The data were re-formatted from the SIO binary format used in the seafloor
instruments to EMI time series format,
and written to optical disk using the ISO 9660 standard. This allows considerable
volumesof data (up to 700 Mbytes at a time) to be read directly into MTR-93,
a commercial MT software package for computation of MT
response functions. A total of 9 days data were processed in this way.
Coherency is high at periods longer
than 10 s, showing not only that the magnetic fields are spatially homogeneous
over these distances, but also that the orientation of the seafloor instrument
has been correctly estimated and that the seafloor magnetic field is being
measured with fidelity. A dramatic drop in coherency at periods shorter
than 7 s comes from a combination of the red source-field spectrum and seawater
attenuation.
Also shown are response computed using seafloor E and remote B (rather than
seafloor B), to illustrate both the improved accuracy at higher frequencies
(i.e. smaller error bars) and the attenuation
associated with seawater. High frequency attenuation is associated with
diminished seafloor electric field; if one corrects for this the high frequency
is again asymptotic to seawater conductivity. At periods longer than about
100 s the response is depressed because the larger land magnetic field has
been used. The XY component agrees with the seafloor response to within
a factor of 2, because the lower apparent resistivity produces only minor
changes in the magnetic field, but the YX mode differs by an order of magnitude
because the higher seafloor apparent resistivity produces more attenuation
of seafloor B.
Shallow water deployments (around 100 m) have also been carried out. In
these deployments the quality of the electric field measurements is still
good, but motional noise associated with wave activity and water currents
degrades the quality ofthe magnetic field significantly. The use of land
magnetic recordings becomes essential, but the difference between the (effective)
sea-surface response and the seafloor response is less dramatic for the
thinner seawater layer. Excellent MT responses to frequencies of 2 Hz have
been obtained in this way.
Using a novel combination of induction coil sensors and AC-coupled electric
field amplifiers, good quality seafloor magnetotelluric responses at periods
of 3 s to 1000 s have been obtained in water 1 km deep. In the past, a short
period limit of 300 s would be typical for marine MT in water of this depth.
In shallower water, responses to several hertz can be obtained. This increased
bandwidth allows the MT method to be used on the continental shelf to map
sedimentary structure as an aid to petroleum exploration. While MT lacks
the resolution of the seismic reflection technique, it can be used in situations
where the seismic method performs poorly, such as imaging beneath salt,
volcanics or carbonates. Also, because electrical conductivity is a strong
function of porosity, it can be used alone or in conjunction with seismic
velocities to back out porosity and permeability (e.g., Evans, 1994).
In the companion paper (Hoversten et al., 1996), 10 to 20 stations of synthetic
data are inverted to recover accurate 2D structure for a variety of geometries.
In practice, several times this number of stations might be needed to characterize
a prospect, and a modern MT land survey can consists of up to 50 or 100
stations. The new SIO MT system has been designed with operations of this
size in mind. Oncea field project is mobilized, it takes about an hour to
prepare an instrument for deployment, and perhaps another hour to recover
a deployed instrument (depending on water depth and station spacing). Thus
about 5 instruments can be recovered and deployed in a 10-hour working day
and left overnight to record. At this rate a 100-station survey can be completed
in three weeks. Loss rates will vary between about 1 and 5% per deployment,
and so a fleet of 10 instrumentsat the start should ensure completion of
the marine operation. While the costs of the ship and instrument replacement
do not have to be borne by land surveys, the daily productivity is greater
for marine MT, and it is estimatedt hat the per station cost of a marine
survey would not greatly exceed the cost of an expensive land operation.
Indeed, both small and large proprietary surveys have already been conducted
during the development of the equipment and methodology.
Acknowledgements: An engineering team consisting of T. Deaton, C. Hollinshead,
D. Jacobs, J. Lemire, A. Nance, D.Willoughby, and P. Zimmer helped develop
the seafloor MT instrument. We thank captain L. Zimm and the crew of the
R.V. Sproul for assistance with the marine experiment, and Electromagnetic
Instruments Incorporated for the loan of equipment.
References:
Cox, C.S., S.C. Constable, A.D. Chave, and S.C. Webb, 1986: Controlled source
electromagnetic sounding of the oceanic lithosphere, Nature 320 (52-54)
Evans, R.L., 1994: Constraints on the large-scale porosity and permeability
structure of young oceanic crust from velocity and resistivity data, Geophysical
Journal International 119 (869--879)
Filloux, J.H., 1987: Instrumentation and experimental methods for oceanic
studies, in Geomagnetism, editied byt J.A. Jacobs. Academic Press (143-248)
Orange, A.S.,1989: Magnetotelluric exploration for hydrocarbons, Proceedings
of the IEEE, 77 (287--317)
Vozoff, K., 1972: The magnetotelluric method in the exploration of sedimentary
basins, Geophysics 37 (98--141)
Webb, S.C., Constable, S.C., Cox, C.S., and Deaton, T.K., 1985: A seafloor
electric field instrument, Geomagnetism and Geoelectricity 37 (1115-1129)