AUV-CSEM: An Improvement in the Efficiency ofMulti-sensor Mapping of Seafloor Massive Sulfide(SMS) Deposits with an AUV
Steve Bloomer
Ocean Floor Geophysics
Burnaby, BC, Canada
steve.bloomer@oceanfloorgeophysics.com

Matthew Kowalczyk
Ocean Floor Geophysics
Burnaby, BC, Canada
matthew.kowalczyk@oceanfloor
geophysics.com

Peter Kowalczyk
Ocean Floor Geophysics
Burnaby, BC, Canada
peter.kowalczyk@oceanfloorgeo
physics.com

Eldad Haber
Department of Earth, Ocean andAtmospheric Sciences
University of British Columbia
Vancouver, BC, Canada
eldad@compgeoinc.com

Toru Kasuga
Fukada Salvage and Marine
Works Co. Ltd.
Tokyo, Japankasuga.toru@fukasal.co.jp

Abstract— Seafloor massive sulfide (SMS) deposits are ofinterest because of their potential as copper and gold orebodies.
This mineralization is often highly conductive, and consequently,
electromagnetic mapping of these deposits for ranking of drilling
targets often follows autonomous underwater vehicle (AUV)
surveys that include multibeam bathymetry, side scan sonar, andchemical sensor data collection. This paper describes the resultsof two engineering tests to enable the simultaneous collection ofstandard payload AUV data with electromagnetic data for rapid
determination of drilling targets. The resulting system is shown
to be an efficient and cost-effective method for concurrent highresolution multisensor and electromagnetic mapping in an SMS
environment.
Keywords—autonomous underwater vehicle, submarine
massive sulfides, controlled source electromagnetic, self-potential,
multibeam, bathymetry, 3-D inversion, 3-D visualizationI.

INTRODUCTION

Seafloor massive sulfide (SMS) deposits found nearseafloor hydrothermal venting sites are of economic interest
because they contain metals such as gold and copper at highenough grades to be potential orebodies. These deposits werefirst discovered in 1980 [1]. Serious exploration for these
deposits has been ongoing in the last decade because of theirpotential to be highly profitable in the future and as a strategic
requirement of many nations to secure stable supplies ofmetals for their industrial base.
Typically, the search for new vent fields starts with
regional mapping consisting of shipborne bathymetry to
identify appropriate seafloor morphological features such as
calderas. Towed ocean chemistry surveys with side-scan sonar
are then done to detect plumes that identify locations forfollow up high-resolution surveys. These detailed surveys of

Steven Constable
Scripps Institution of
Oceanography
University of California, San
Diego
La Jolla, CA, USA
sconstable@ucsd.edu

SMS fields are typically done with AUVs using multibeam
sonar (MBES), side-scan sonar (SSS), sub-bottom profilers(SBP), water chemistry sensors, and magnetometers.
In addition to the sensors listed above, electromagnetic(EM) methods are also useful for mapping SMS deposits. EM
methods can map the subsurface distribution of these deposits
in detail because SMS mineralization containing copper orgold is often highly conductive. This is a strong argument for
the use of EM in an SMS exploration program because EM
can identify the SMS deposits most likely to contain economic
mineralization and, consequently, allows a rapid ranking oftargets for drilling. However, EM surveying has been typicallydone separately from the AUV component of the follow up
high-resolution surveys.
This paper presents the results of two engineering tests
mounting electrodes to an AUV to enable simultaneouscollection of data from a standard AUV payload and EM
systems. These tests were conducted by OFG, Fukada
Salvage and Marine Works (Fukada), and the Scripps
Institution of Oceanography (Scripps). The resulting AUVmounted controlled source electromagnetic system (AUVCSEM) has successfully demonstrated that it can provide an
efficient and cost-effective method to do subsea EM in the
SMS exploration environment. AUV-CSEM systematicallyapplied in conjunction with data collected with the other AUV
sensors provides opportunities for very efficient exploration,
leading rapidly to the discovery of buried and inactive SMS
deposits, and the ranking of drill targets.
II.

MARINE EM SURVEYS

EM surveying is useful for ranking potential drilling
targets in SMS exploration. Consequently, there has been anongoing development of marine EM systems for this purpose.
In 2008, Ocean Floor Geophysics Inc. (OFG) developed andpatented the first EM system to successfully map the limits ofconductive near-surface mineralization in a survey of an SMS
deposit at Solwera1 [2]. Since then, towed coincident loop
time domain systems have been developed. The Waseda
University system [3] in offshore tests have detectedmineralization buried at 30 m. The Golden Eye system [4] hasdetected both conductivity and chargeability anomalies over
SMS fields along the Central Indian Ridge. Clear anomalies
have been detected over sulfide mineralization at the TAG
field on the mid-Atlantic ridge using a deep-tow controlled
source electromagnetic system (CSEM)[5].
In addition to EM methods using an active source, selfpotential measurements can be used in the marine
environment for the study of SMS deposits. In early surveys
on land, self-potential measurements over metal ore bodies
were observed to produce negative potential anomalies, with
[6] presenting a classic model that explains this phenomena as
the result of oxidation-reduction gradients across the ore body.
In the marine environment, SP measurements were first
documented in [7]. Likely the first successful application ofmarine SP in studies of SMS deposits was with an electrodearray connected to the submersible Cyana in a survey of aseamount near the axis of the East Pacific rise [8]. A similar
procedure was used in [9] in a submersible Alvin survey of the
TAG hydrothermal mound. More recent SP studies using a
towed array showed that SP was critical in the detection ofpotential SMS targets in the northern equatorial zone of the
Mid-Atlantic Ridge [10] and that SP methods could be used todetect buried SMS deposits in a survey of Izena hole in the
Okinawa trough [11].
Over the last four years, using a controlled electricalsource and an array of CSEM receivers described in [12],OFG has partnered with Fukada and Scripps to successfully
map methane hydrate deposits to several hundreds of meters
below the seafloor. These methane hydrate deposits areresistive targets and the extension of this CSEM technology tomap conductive SMS deposits to a comparable range of
depths below the seafloor would be valuable.
However, towed CSEM systems consist of a long array ofelectrical transmitters and receivers. Therefore, a large ship isrequired to tow the system. For surveys with high spatial
resolution with tight line spacing, slow towing speeds andtime-consuming turns (up to 4 hours) are required.
Additionally, it would be difficult to tow the system at aconstant low altitude that was used in mapping hydrate
deposits in the rugged terrain associated with hydrothermal
vents.
To mitigate these problems, reduce costs on ship and crew
to deploy the system, and to enable the simultaneouscollection of high-resolution complementary data (MBESbathymetry, SSS, SBP, ocean chemistry, magnetometry), an
AUV-CSEM concept was developed to (1) use battery
powered transmitters placed on the seafloor to generate
electrical fields that interact with the subsurface geology; (2)integrate the EM receiver electrodes into the Fukada AUV“Deep1” to measure the resulting electric fields; and (3)eventually use these E-field measurements to generate 3-D
subsurface conductivity distributions over the survey area.
This paper will summarize the highlights of two AUV-CSEM
engineering tests performed in 2015 and 2016 by OFG,
Fukada, and Scripps to resolve technical uncertainties andadvance the use of CSEM technology with a survey AUV as aplatform.
III.

RESULTS OF THE FIRST ENGINEERING TESTThe first engineering tests involved measuring the ambient
electric field on the CSEM electrodes mounted to an AUV
without any transmitted electrical (E-) field during the test.
EM modelling prior to the AUV tests demonstrated thatsignificant EM anomalies above background could be detectedusing a standard towed CSEM system. The E-field noiselevels generated by an AUV were not known prior to the test,
but it was thought that the noise levels would be sufficiently
low to enable useful CSEM data, while causing only minor
loss of AUV vehicle performance.
This test was performed in 2015 in Nago Bay, Okinawa,
Japan in approximately 300 m of water. CSEM electrodes andan underwater data logger were contributed by Scripps.
Fukada provided ship time and the AUV and modified thevehicle for mounting the electrodes. OFG designed andfabricated the mounting apparatus for the electrodes. OFG and
Fukada assembled the apparatus and electrodes on the AUV.
Two pairs of electrodes were mounted perpendicular to themain axis of the AUV, with one pair mid AUV and one paircloser to the vehicle propeller (Fig. 1). The subsea data logger
was secured in the main flooded section of the AUV to record
the ambient electric fields.
An experiment was designed to study how the ambient
noise levels on the E-field measuring electrodes changed
while varying operational parameters. A series of short survey
loops were run, while turning off the payload devices (sonars,
magnetometers, and chemical sensors) one by one and varying
the heading of the vehicle. Later, the speed of the vehicle wasvaried by changing the propeller rotation rate while running aseries of short lines. Finally, a short, but typical, sonar
multibeam survey was run to determine the effect of the dragof the electrodes on the AUV speed over ground and on
vehicle energy consumption.

Fig. 1: The first (2015) test set up with a pair of electrodes mounted on
top of Fukada’s Deep1 AUV.

After the test, the data were divided into subsets for each
set of operational parameters. Spectra were then generated onthese subsets, and the time series data and the spectra wereanalyzed. Three major observations were made:1) The principal source of the noise was related to thethruster system and the payload devices had little effecton the noise level on the electrodes (Fig. 2).
2) The noise level varied with the heading of the vehicle, theorientation of the electrodes, and the position of theelectrodes with respect to the thruster.
3) The speed tests showed that the noise on the electrodes
had a red spectrum, with noise power decreasing withfrequency with a series of narrow spectral peaks
superimposed on that spectrum. These narrow peaks
correspond to the rotation rate of the propeller of the AUV
and its harmonics (Fig. 3), consistent with the hypothesisthat the thruster system is the principal source of noise.
These observations were very encouraging. Since the noise
added by the payload devices was minor, CSEM and standard
payload surveying can be run simultaneously during a survey,
increasing survey productivity and providing more data to aid
geological interpretation for determining drill targets. As well,
despite the presence of narrow, propeller related, peaks in thenoise spectrum, there are bands of reduced noise between thepeaks that can be matched with peaks in the transmitted
electric field spectra to produce bands in the electric field
spectra with high signal to noise ratios. This can be easily
done by adjusting the rotation rate of the AUV propeller
relative to the known source waveform spectral peaks.
Finally, the AUV speed for the small multibeam survey
was reduced from an expected 1.5 m/s to 1.1 m/s due to theFig. 2: The noise levels on the AC channels as the payload equipment
was successively turned off, with the actions indicated on the bottom
axis. The noise level with all the equipment on is only slightly higher
than most of the equipment turned off. Turning the vehicle motor off was
the only action that substantially decreased the noise level, indicating the
primary noise source was related to the thruster.

increased drag caused by the electrodes. Power consumption
was approximately 0.8 kWh/hr of dive time, which is slightly
more than a typical 0.7 kWh/hr for a Deep 1 survey. At these
rates of AUV speed and battery consumption, 50 lines
kilometers would be achievable in a 24-hour dive plus battery
charge cycle, which would be better than the 40-line kms/day
achieved with a ship towed system. Despite of the negative
effects of drag caused by the electrodes, the logistics of the
AUV system are still more efficient than a towed system,
because of the absence of large turns. Additionally, payload
data can be collected without affecting the electric field data,and navigation of the AUV over the seafloor is improved.
IV.

SEAFLOOR TRANSMITTER TESTThe results of the first set of tests indicated a strong
possibility that a AUV-CSEM survey system would be
successful. The last remaining uncertainty was the range fromthe AUV to a Scripps ocean bottom EM transmitter in whichhigh quality E-field data could be collected. Forward
modelling results suggested that to a range of 350 m from thetransmitters, operating with a standard transmitted current,
useful E-field data could be collected in a conductive region.
It was also thought that the bands of low spectral noise were
sufficiently wide that two or more transmitters, transmitting
signals with different fundamental frequencies, could be
operated simultaneously and that their signals could be
separated.
For the transmitter test in 2016, an AUV electrodeconfiguration like the 2015 test was used, with a pair ofaddition vertical electrodes added mid-AUV. The Scripps
seafloor transmitters were battery powered and transmit a
programmable binary waveform into an orthogonal pair of 10
m antennas (Fig. 4). The battery life of each transmitter with astandard transmit current was approximately 12 hours. The
transmitter assemblies, which included flotation and a

Fig. 3: Noise spectra run at two speeds showing shift in spectral peaks.
The spectral peaks appear to be related to the frequency of the vehicle
propeller. A 1.1 m/s AUV speed was achieved with a propeller rate of
201 RPM (~3.35 Hz) while the 1.5 m/s AUV speed was achieved with
264 RPM (~4.4 Hz).

Fig. 4: AUV CSEM survey, Sea of Japan Nov 2016. Left, AUV with electric dipole mounts on AUV, including a vertical pair of electrodes. Right, Scripps
DUESI seafloor transmitter being deployed.

weighted platform, were equipped with an USBL positioning
beacon to monitor its descent to the seafloor and to determine
its final resting position and with an acoustic release to enable
recovery at the end of the survey
The survey took place offshore Japan in the Iheya Ridge
area of the Okinawa Trough at a water depth of approximately
1500m. The general area of this survey is known for activehydrothermal venting and SMS deposits [13] [14]. A previous
survey at the site indicated there were hydrothermal ventchimneys in the planned survey area. The planned survey
consisted of three roughly SW-NE 1 km lines space 100
metres apart, with one line along the axis of a line of chimneys,
and a perpendicular set of 1 km intersecting the three otherlines in the middle of the survey grid and running throughmore chimneys.
The planned locations of the two transmitters wereapproximately at opposite corners of the 200m square definedby the intersection of the survey lines. A third transmitter was
deployed prior to the survey to determine the drift offset of thetransmitter platform due to strong ocean currents. This drift
offset enabled the two transmitters for the test to be locatedwithin 100m of their planned locations, resulting in the central
region of the survey area to fall within the expected range ofthe transmitters. The two transmitters broadcasted 20 A at afrequency of 2 and 2.5 Hz and harmonics, with thetransmission polarities alternated every 30 s.
The survey grid was run three times at a nominal altitudeof 70m above the seafloor. The transmitter on times weresynchronized with the AUV mission to optimize transmitter
battery consumption and to attempt to create a survey test with
one circuit of the survey grid run only the first transmitter on,
once with both transmitters on, and once with only the second
transmitter on. This synchronization of current transmission
and repeated survey circuits was achieved, with electrical fielddata collected on all three runs along with the full suite of
AUV payload data: multibeam bathymetry, sidescan sonar
backscatter, subbottom profiler, OFG Self-Compensating
Magnetometer (SCM) system, optical backscatter (turbidity),
temperature, and ocean chemistry (pH and ORP - oxidation
reduction potential) data.
Final navigation for the survey was produced by shifting

the navigation from the inertial navigation system of the AUV,
which drifts as a function of distance travelled, to the USBL
navigation on a line by line basis using a least squares
minimization to calculate shifts in eastings and northings. By
picking the positions to the top of the numerous chimneys, a
5m accuracy between positions in the three circuits wasestablished. Although the positional accuracy could be
improved by matching the bathymetric features between
circuits, the navigational accuracy is sufficient to produce a
consistent navigational basis between the three circuits run
during the survey to evaluate the utility of an AUV platform
for CSEM surveys, and for the EM analysis described below.
For this assessment, the success of the test was gauged by: (1)how well the measured CSEM response of the massive sulfide
system at this site coincided with a field of SMS mounds andactive hydrothermal chimneys (Fig. 5) and (2) how consistent
the results of the CSEM response were from circuit to circuitas the transmitters were turned on and off.

Fig. 5: Bathymetry map of the test area derived from circuit 1 multibeam
bathymetry data. AUV tracks from circuit 1 (black), circuit 2 (brown),
and circuit 3 (red) are shown. Labels A-C show area of anomalous selfpotential.

V.

SEAFLOOR TRANSMITTER TEST RESULTS

The test results were very positive. The SP and CSEM
results presented here are summarized in [15]. We
successfully measured the CSEM response of a hydrothermal
chimney system with a strong anomaly over chimneys
presumed to contain massive sulfides. A comparison of the
DC electric field data from three sets of traverses showed astrong consistency (Fig. 6) and demonstrates that we canoperate at least two transmitters concurrently using separated
fundamental frequencies. The data over the entire survey area
was of high quality, with an estimated transmitter range
estimated to be at least 400m.
We also successfully operated all the AUV payload
systems while making these CSEM measurements. This
enabled the comparison of CSEM and groundtruth datasets byusing 3-D visualization tools (Fig. 7). Fig. 8 shows a single
frequency 1-D apparent conductivity map derived from theelectric field data recorded during one of the circuits of thetest. The two order of magnitude range of the conductivities iscapable of distinguishing conductive bodies (~10 S/m) fromresistive geology (~0.1 S/m). Comparison of this figure to thebathymetry (Fig. 5) shows that high apparent conductivities
for this frequency correspond to the bathymetric highs at B.
The apparent conductivity anomalies are interpreted torepresent massive sulfides. The southern conductivity highcorresponds to a series of mounds. The electric field data isalso more spatially consistent than the oxidation-reduction
potential data measured in the water column, suggesting thatstrong currents and/or episodic venting effect the distributionof the chemical plumes, and the consistent conductivity
anomalies are thus related to the local geology (Fig. 9).
Additionally, Fig. 8 is the result from a single frequency ofa multi-transmitter, multi-frequency CSEM survey. The
CSEM data were collected at multiple frequency bands with

Fig. 6: 30 second averages of the electric field data, after levelling, forthe eastern most lines of the southeast-northwest lines of the three
repeated circuits.

high signal to noise ratios between 2 to 20 Hz, providing a
multi-frequency data set amenable to 3D inversion to map out
the burial depth and limits of the conductive zones. A
preliminary inversion of the CSEM data was performed using
the total electric field data at multiple frequencies, producing a
subsurface 3-D conductivity model shown in Fig. 7. The
principal conductive body is centered beneath the large
mounds in the southern section in the survey area and extends
to depth. However, the sensitivities used in the inversion were
suboptimal, with higher sensitivities towards the transmitterand the seafloor. It is suspected that abnormal higher
conductivities have been concentrated close to the transmitter
locations as a result. This is observed in high conductivity
zones in the inversion model near the northwest transmitter.
The extension to depth of the main conductor in a region oflower sensitivity suggests this conductor is real.
In addition to the apparent conductivity maps generated
from this data, mounting the E-field sensors on the AUV
allows a highly sensitive self potential (SP) measurement to be
made. An Occam inversion was used to integrate DC electricfields with irregular data spacing to SP data on a uniform grid,using the approach of [16]. Here, the results (Fig. 10) show amuch stronger SP anomaly over the more northern set ofmounds (point A). The cause of the SP anomaly is uncertain:
it may due to mineralization, or hydrothermal venting, or both.
Given that the venting and SMS deposits are related, it still isa useful measurement in a suite of data for characterizing
potential drilling targets.
VI.

CONCLUSIONSThis paper has presented the results of two successful tests
that demonstrate the capability of mounting electrodes to an
AUV for EM prospecting of geological targets. The example
pilot study showed its capability to accurately map conductive
SMS targets, but would even be more effective mapping
resistive targets, such as gas hydrate deposits, wheretransmitter ranges would even be longer due to less signal
attenuation. The CSEM-AUV system presented here is more

Fig. 7: 3-D data model of the AUV-CSEM data. On the top layer are the
AUV tracks and the locations of the ORP minima (pink spheres). The SP
inversion result from Fig. 10 is painted on the multibeam bathymetry(vertical exaggeration 2x). Beneath the seafloor surface, a subbottom
profile is displayed. The inverted conductivity model derived from the
CSEM data is displayed as both a vertical slice through the model and an
isosurface. The volume is approximately 1100m x 1200m x 250m.

Fig. 8: Apparent conductivity (colored disks – log10 scale) overlain on bathymetry for two polarizations of CSEM transmission from a deployed transmitter(center black symbol). For this example, the transmission frequency is14 Hz, and the black lines show the directions of the polarization ellipse maxima
recorded by the AUV.

efficient than a towed system and has the added benefit of asuite of complementary instrumentation to aid interpretation.
These tests have also indicated the necessary
improvements required for future surveys. For the initial tests
presented here, the electrode mounting hardware was simple
with the purpose of extending the electrodes away fromelectrical interference from the vehicle. This resulted in dragon the vehicle, limiting the speed of the AUV and increasing
the battery consumption. For future measurements, the dipoles
will be streamlined to reduce drag, and increase the mapping

Fig. 9: Plan map view of a 3D model of the inverted CSEM conductivity
at a depth of 1632 mbsf. The 2 green triangles are the transmitter
positions, while the locations of the ORP minima for the entire survey areplotted as black circles for circuit 1, brown circles for circuit2, and red
circles for circuit 3.

productivity of the vehicle. Transmitter design will be
improved, especially increasing the output current to increasetransmitter range and acoustic control of the on/off switchingof the transmitter to conserve battery power and enable anefficient survey to be run by enabling the AUV to pass fromthe range of one transmitter to another.
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