Geophysical electromagnetic prospecting method and its discussion

1. Seafloor Massive Sulfide Exploration - A
New Field of Activity for Marine
Electromagnetics
Presented in: EAGE/DGG Workshop on Deep Mineral Exploration
Authors: Hendrik Müller et al
Published: March 2016
Seminar by: Samarth Pran

2. Table of contents
• Introduction
• Area of study
• Instrument used - Golden Eye Electromagnetic Profiler
• Principle
• Survey and results
• Conclusion
• References

3. Introduction
• Seafloor massive sulfides (SMS) are now in the focus of deep mineral exploration. Sulfidic ore complexes
are amongst natural rocks with the highest observed electrical conductivity.
• Hydrothermal systems at mid ocean ridges are the motor for high temperature mineralisations forming
high-grade polymetallic ore deposit. These systems contain heavy metals such as copper, gold, silver and
zinc - all materials of high demand in the growing high-tech industry.
• However, from a resource perspective, inactive systems are considered more prospective. These inactive
systems are usually difficult to identify because:-
1. Typical SMS deposits are small scale (100-500 m x 100-500 m) and occur in deep waters (2000-4000
m),
2. They are often sediment-covered with no or little seafloor expression, and
3. They usually show no or little water column indications which could be monitored with e.g. turbidity- or
water column sensors.
• The detectability of small-scale conductive targets in the topmost 5- 30 m below seafloor with EM methods is
hampered by the attenuation of EM fields in the conductive seawater, complexity of near-seafloor
operations in rough deep-water environments, and limited viability of EM instrumentation developed for the
hydrocarbon industry to detect SMS deposits.
• In this seminar, we will see the first results with a novel controlled source electromagnetic (CSEM) system
over active and inactive seafloor hydrothermal fields at the Central Indian Ridge.

4. Area of study
Junction of 3 mid oceanic ridges :-
1. Central Indian ridge
2. Southeast Indian ridge
3. Southwest Indian ridge
• Boundary of 3 plates – Indo-Australian,
African and Antarctic plate.
• Germany holds an exploration license for
polymetallic sulfides along the Central and
Southeast Indian Ridge SE of Madagascar.
• The Federal Institute for Geosciences and
Natural Resource (BGR) conducts annual
research and exploration cruises (INDEX) in
the license areas.
Rodriguez Triple Junction

5. Principle of hydrothermal activity along mid-ocean ridges Inactive chimney observed in the licence area.

6. Golden Eye Electromagnetic Profiler
• Instrument frame – Glass-fiber reinforced plastic
(GRP).
• Horizontal concentric coil system
o the transmitting coil
o bucking coil
o reference coil
o receiver coil
o calibration coil.
• The CSEM-Loop system is operated in the frequency
domain providing in-phase and quadrature values at
up to 10 frequencies in the range of 15 to 20,000 Hz
with a sampling rate of 30Hz.
• The transmitter has a maximum current output of 30 A,
providing a depth of investigation of 10-20 m below
seafloor.
Green components belonging to the CSEM Loop system
Red components belonging to the IP system
Black components used by both systems and for navigation

7. • The Golden Eye frame also hosts the BGR electrical current
transmitter with output to two orthogonal electrical source dipoles.
• DC electric field and Induced Polarisation (IP) measurements
are performed using three orthogonal electrical receiving dipoles
connected each to one of BGR’s data loggers originally developed
for gas hydrate investigations.
• Golden Eye is deployed from the vessel and lands on the seafloor
guided by video cameras, sonar and altimeter.
Golden Eye Electromagnetic Profiler
Equipped with
2 analog video cameras, LED lights, a CTD (conductivity,
temperature, depth) sensor, sonar, altimeter, and a three-axial
broadband magnetometer.

8. CSEM-Loop System
• The large horizontal transmitter coil induces eddy currents in the surrounding seawater and sub-
seafloor utilizing alternating magnetic fields of superimposed frequencies.
• The small bucking coil is tuned to compensate the primary field of the transmitter at the coaxial and
coplanar receiver coil.
• Highly sensitive in-phase and quadrature components of the secondary fields are measured in parts
per million (ppm) as calibrated ratios of the secondary field (measured by the receiver coil) to the
primary transmitted field.
• The sub-seafloor conductivity model is retrieved from one-dimensional inversion of the multi-
frequency, dual-phase subsurface signal.
• The seawater contribution to the signal is subtracted using the CTD-derived seawater conductivity.
Golden Eye’s Principle

9. Magnetometry
• Magnetic properties of subsurface hold important information about the nature and depth extend of
hydrothermally altered zones.
• While a lack of magnetization is often observed at basalt-hosted hydrothermal fields, a positive
magnetic anomaly can be found at ultramafic-hosted sites and allows evaluating the dimensions and
constitutions of the stockwork below the hydrothermal deposit.
• In addition, high resolution magnetic susceptibility measurements of the topmost 1.5 - 2 m below
seafloor are retrieved from the low-frequency in-phase component of the CSEM-Loop system.
IP System
• Ability of the ground to store charges (called chargeability) where the ground acts as a capacitor. When
the transmitter current is switched off, the voltage decay between potential electrodes to zero takes time.
• The highest chargeabilities have been reported for disseminated metallic ores, especially massive sulfide
deposits.
Golden Eye’s Principle

10. Survey and results
• A detailed investigation of
cluster 4, located north of the
Triple Junction was conducted.
3 SMS fields were known in
the 10 km x 10 km cluster from
previous investigations.
• 4 Golden Eye deployments were
completed during the three weeks
cruise covering all three fields
with 327 landings for CSEM-Loop
measurement, 119 for IP
measurements using about 100 h
of ship time.
Survey Details

11. Survey and results
• Apparent conductivities along
track derived from 2.7 kHz
quadrature data from the CSEM
Loop system corresponding to
an investigation depth of about
2 m below seafloor.
• High conductivities clearly
correlate with the known location
of the active and inactive fields.
But they also extend beyond for
the inactive sites.
• Blue + signs indicate Golden Eye
track lines of the other
deployments.
Results

12. Survey and results
• Seafloor conductivities derived from DC
electrical fields – measured along a
profile across the active field.
• Electrical fields measured 1 m above the
seafloor decrease in presence of an
anomalous conductive seafloor.
• Normalized with background electrical
fields seen outside hydrothermal field –
corresponds to a typical background
seafloor conductivity of 1 S/m.
• This figure shows the chargeability
derived from the switch-on transients.
• Both conductivity and chargeability show
distinct anomalies over the active field.
• Elevated values are also observed over the
inactive sites.
active
site

13. Survey and results
• Seafloor chargeability has been calculated
from the observed IP effect normalized by
the reference effect measured in seawater.
• Values also peak over the active site.
active
site
• DC electrical field data have been
normalized with the median background
values to derive average seafloor
conductivities which peak over an active
field for the shown profile.

14. Conclusion
• Golden Eye is a unique and novel marine multi-sensor EM system capable to identify even buried
SMS deposits based on anomalous electrical and magnetic subsurface properties.
• The system supports a comprehensive evaluation of the spatial extend, composition, and inner
structure of Seafloor Massive Sulfide deposits and helps to reduce the necessary amount of costly
deep-sea drilling.
• On its first deep-sea mission during the INDEX 2015 cruise, the Golden Eye system demonstrated
its great potential to investigate active and inactive hydrothermal sites in the German license areas.
• Layered inversions of the CSEM-Loop data develops the pseudo-3D models of the deposit, and
analysis of frequency domain IP data gives idea about the chargeability of the sea-surface.
• Measurements of the electrical and magnetic properties of rock samples collected from the same
areas will be used to calibrate the seafloor data, and to allow conclusions of the mineralization
and alteration status of the deposit.

15. References
• Goto, T., Takekawa, J., Mikada, H., Sayanagi, etal. [2011]. Marine Electromagnetic Sounding on
Submarine Massive Sulphides using Remotely Operated Vehicle (ROV) and Autonomous Underwater
Vehicle (AUV). Proceedings of the 10th SEGJ International Symposium, Kyoto, Japan, 20-22 November
2011: pp. 1-5. doi: 10.1190/segj102011-001.103
• Kowalczyk, P. [2008]. Geophysical prelude to first exploitation of submarine massive sulphides. First
Break, 26(11), 99-106.
• Müller, H., Schwalenberg, K, von Dobeneck, T. [2015]. Challenges in the deep sea: The GOLDEN EYE
electromagnetic profiler. Oral talk presented at the 75. Meeting of the German Geophysical Society (DGG),
Hannover, Germany, 23-26.03.2016
• Reynolds, J.M. [2011]. An Introduction to Applied and Environmental Geophysics, 2nd Edition
• Szitkar, F., Dyment, J., Fouquet, Y., Honsho, C., Horen, H. [2014]. The magnetic signature of ultramafic-
hosted hydrothermal sites. Geology, 42, 175-178. doi: 10.1130/G35729
• Wynn, J., Williamson, M. Fleming, J. [2012]). Induced Polarization for Subseafloor, Deep-Ocean
Mapping. Sea Technology, September 2012, p. 47-50.