Mapping Gas Hydrate using Electromagnetic Methods

Steven Constable

Scripps Institution of Oceanography

Acknowledgements

Scripps Institution of Oceanography, Seafloor Electromagnetic Methods Consortium

SEG and SEG Foundation

Sponsored by Statoil

Sponsored by Paradigm

SEG Membership Benefits

SEG Digital Library – full text articles Technical Journals in Print and Online Networking Opportunities Membership Discounts on ! Continuing Education Training Courses ! Publications (35% off list price) ! Workshops and Meetings

Join Today!

Membership materials are available today!

Join Online at seg.org/About-SEG/Membership

Student Opportunities

Sponsored Membership Student Chapter Programs SEG/Chevron Student Leadership Symposium SEG/ExxonMobil Student Education Program Challenge Bowl Scholarships Field Camp Grants Geoscientists Without Borders® Student Expos & IGSCs Honorary and Distinguished Lecturers SEG Online

Learn more at seg.org/Education/Students-Early-Career

Section/Associated Society Opportunities

Host DL, HL, and DISC Programs Council Representation Annual Meeting Booth Discount Best Papers presented at SEG

Annual Meetings Joint Conferences, Workshops, and

Forums in partnership with SEG

For more information, including a list of benefits, please visit: www.seg.org/resources/sections-societies

Mapping Gas Hydrate using Electromagnetic Methods

Steven Constable

Scripps Institution of Oceanography

March 2013:

Hydrate: the What, the Where, and the Why

Laboratory studies of hydrate electrical conductivity

Marine EM methods

Hydrate Ridge experiment

The Vulcans

2015 San Diego Trough tests

Concluding remarks

0 10 20Temperature, °C

Dep

th, k

m

0

0.5

1.0

1.5

2.0

temperature

phase boundary

ocean

seafloor sediment

seismic BSR

gas hydrate stable

free gas stable

free gas stable

What:

Where:

US Geological Survey

Why:It is a hazard to drilling and infrastructure It is viewed by some as a potential energy source Methane release may play a role in climate change Is a significant part of the global carbon cycle Hydrate may play a role in marine CO2 sequestration It can confound interpretation of marine EM for exploration There is a lot of it

Photos courtesy Arnold Orange

indicated the possibility that gas hydrates could exist in

astounding volumes in the natural environment (Fig. 1). By the

mid-1990s, enough field data had been obtained to support

a general consensus that gas hydrate was a significant part of the

natural environment with profound implications for society.21

Recognition of the potentially immense scale of gas hydrate

occurrence has raised many questions. As a practical issue, gas

hydrates can play a role in a range of natural22 and industrial23

geohazards. The previously unappreciated pool of shallow

carbon represented by gas hydrates are only now being incor-

porated into conceptions of global environmental processes,

including carbon cycling24 and global climate.20 Furthermore, for

a planet where energy demands are steadily increasing and future

energy supplies are uncertain, the immense energy supply

potential of tens of thousands of trillions of cubic feet of methane

gas commands investigation. This paper will focus primarily on

these latter two issues, gas hydrates as a potential energy supply

and as a potential source of carbon input to the atmosphere, as it

is these issues that relate most directly to total resource volumes.

Until very recently, all published estimates of gas hydrate

resources, and by extension, judgments on gas hydrate energy

resource potential and role in climate processes, have relied on

estimates of total global volumes of methane housed in gas

hydrate.16 Such estimates are a necessary and important initial

step in understanding gas hydrate occurrence and its implica-

tions; however, as revealed by a series of recent field programs

(Fig. 2) gas hydrate in nature occurs in a wide range of condi-

tions. Gas hydrate is known to occur from the seafloor to depths

of more than 3000 m (10 000 feet) below sea-level, within and

below permafrost, at temperatures ranging from sub-zero to

more than 20 !C; at concentrations from 1–2% to more than 90%

of pore space; as disseminated grains and in massive sediment-

displacing forms.41 The profound differences in the nature of

these occurrences have significant implications for both the

potential producibility of gas hydrates and how gas hydrates may

respond to any given change in the natural environment.

Therefore, it is no longer necessary or appropriate to suggest that

the entire global gas hydrate resource volume has direct rele-

vance to either the energy supply or climate issues.

This report begins by reviewing the terminology used to

describe resources. We then briefly review the history and status

of gas hydrate assessment with respect to each resource category,

and close with some observations on the relevance of various

categories of gas hydrate resource volumes to the energy and

climate issues related to naturally occurring gas hydrate.

Global gas hydrate ‘‘resources’’

Descriptions of the size of the gas hydrate ‘‘resource’’ base are

common. However, these numbers can be easily misunderstood

if the precise intended meaning of the term ‘‘resource’’ is unclear.

In general usage, the term is applied to concepts that can range

from the very broad (every molecule that exists) to the very

narrow (what we could expect to be able to use), even to workers

within a common discipline. Therefore, discussion of gas hydrate

resources, like any hydrocarbon, is best conducted with careful

reference to one of the following ‘‘resource’’ subcategories

(Fig. 3).†

Gas-in-place (GIP) resources

Gas-in-place (GIP) is a term most commonly used to describe an

assessment that includes every methane molecule present in the

subject region or geologic formation without regard for resource

concentration, form, enclosing media, or potential recover-

ability. In traditional oil and gas industry usage, GIP refers to the

total hydrocarbon volume present within a given reservoir unit.

An estimate which intends to include all GIP within a broad

region, to include all forms and settings, is commonly called the

‘‘resource endowment’’. GIP is typically therefore entirely

a function of geologic condition as is determined using the

following formulation (in which the subscript ‘gh’ indicates gas

hydrate):

GIPgh ¼ area (m2) # thickness (m) # porosity (%) # satu-

rationgh (%) # volumetric conversion factor

The volumetric conversion factor in gas hydrate applications

relates to the cage occupancy of the hydrate lattice, and can vary

from about 160 to 180, with a value of 164 (equating to an 85%

occupation) being typically used. Unlike traditional hydrocar-

bons, this value is largely independent of the depth (and therefore

pressure and temperature) at which the gas hydrate occurs.

Therefore, gas hydrates that occur shallower in comparison to

sea-level than roughly 1200–2000 m ($4000 to 6000 ft: depending

on local conditions) will hold more gas (at standard temperatures

and pressures) than an equivalent reservoir volume of free

Fig. 1 Estimates of global gas-in-place in gas hydrate versus the publi-

cation date of the estimate. The observed trend in the more recent esti-

mates (indicated by letters) indicates that work over the past three

decades has not succeeded in constraining gas hydrate resource volumes,

which continue to range over nearly three orders of magnitude. (1)

Trofimuk et al.;2 (2) Trofimuk et al.;3 (3) Cherskiy and Tsarev;4 (4)

Trofimuk et al.;5 (A) McIver;6 (B) Kvenvolden;7 (C) Kvenvolden and

Claypool;8 (D) MacDonald;9 (E) Gornitz and Fung;10 (F) Harvey and

Huang;11 (G) Ginsburg and Soloviev;12 (H) Holbrook et al.;13 (I) Solo-

viev;14 (J) Milkov et al.;15 (K) Milkov;16 (L) Buffet and Archer;17 (M)

Klauda and Sandler;18 (N) Wood and Jung;19 (O) Archer et al.20 (figure

modified after Milkov21).

† All volume estimates correspond to the volume of gas at standardtemperatures and pressures included within the gas hydrate structure,and not to the volume of the solid phase gas hydrate itself.

This journal is ª The Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 1206–1215 | 1207

2010

Boswell and Collett, 2011, and Milkov 2004

A lot, but, global volume is highly uncertain:

Boswell and Collett, 2011

The hydrate resource pyramid.

1.000

1.100

1.200

1.300

1.400

1.500

1.600

1.700

1.800

1.900

2.000

2.100

2.200

2.300

2.400

230.012.0

230.0112.0

230.0212.0

229.0312.0

229.0412.0

229.0512.0

229.0612.0

229.0712.0

228.0812.0

228.0912.0

228.0955.0

Line:Trace:

1.000

1.100

1.200

1.300

1.400

1.500

1.600

1.700

1.800

1.900

2.000

2.100

2.200

2.300

2.400

seismics courtesy Anne Trehu, OSU

BSR shows free gas at edge of stability fieldbut provides no indication of hydrate above

Blanking zones, show hydrate orgas near the seafloor but little below

Quantification of hydrate volume using seismic methods is difficult.

Ken Sleeper

0 0.10.4

0.8

1.2

1.6

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Gas saturation

P-w

ave

velo

city

, km

/s

Seawater/gas in 50% porous sandstone

Velocity

Fizz-gas

Archie’s with fizz-gas

The BSR reflection is associated with small amounts of free gas - similar to the “fizz-gas” problem in hydrocarbon exploration.

1 Ωm 300

SonicVISION

- +

Deep RAB

low Resistivity high

N E S W NResistivity

Deep RAB avg.Ring

2.4

Logs from Walker Ridge 313-H, from Boswell et al., Fire In the Ice Summer 2009

Hydrate is electrically resistive, and so is a target for electromagnetic methods.

0 0.10.4

0.8

1.2

1.6

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1100

101

102

103

104

Gas saturation

Res

istiv

ity, Ω

m, l

og s

cale

P-w

ave

velo

city

, km

/s

Seawater/gas in 50% porous sandstone

HS lower bound

Arc

hie

’sVelo

city

Fizz-gas

Least resistivity contrast easily

detected with EM

Archie’s with fizz-gas

Hydrate/gas concentrations have to be high to generate an electrical signature - EM is a good tool to find the top of the pyramid.

Laboratory studies of hydrate conductivity

Du Frane et al., 2011

Apparatus to synthesize methane hydrate in a conductivity cell.

260

265

270

275

280

285

290

5 10 15 20 25

Tem

perat

ure,

K

Time , hours

Full reaction;no discontinuities

associated withfreezing

H 2O liq. ice

DC

BA

265

270

275

280

285

290

2 3 4 5 6

T,K

Time , hours

externalfluid bath

sample,center

B

C

A

sample, base

Pres

sure

, CH

4 (g),

MPa

Temperature, K

star

t/pr

essu

re ic

e +

gas

slow heating(ice + gas)

slow cooling(CH

4hydrate)

H2O (liq.)

+ CH4(g)

H2O (s) + CH

4(g)

CH4nH

2O

+ CH4 (g)

H2O (s) H

2O (l)

quenchto 77 kfor SEM

hold,12-15 h

0

5

10

15

20

25

30

35

240 250 260 270 280 290 300

AB C

D

Stern et al., Am. Min. (2004)

Synthesis of Methane Hydrate:

100 vol% CH4 hydrate

100 vol% CH4 Hydrate

50 vol% CH4 hydrate:

50 vol% Sand50 vol% ice:50 vol% sand

50 vol% CH4 hydrate:

50 vol% glass beads

10 vol% ice:90 vol% sand

Cryo-SEM is used to assess grain characteristics and phase distribution.

Du Frane et al., 2015

Impedance spectroscopy and equivalent circuit models allow removal of electrode effects:

Du Frane et al., 2011

0 vol% sediment

Du Frane et al., 2011

= o

eA/kT

Pure hydrate conductivity is 3-4 times lower than ice and well fit by Arrhenius model.

Mixed with silica sand, hydrate conductivity goes up until a percolation threshold is reached. We think that impurities from the sand, probably K+ and Cl-, increase the charge carriers available in the hydrate.

-5.0

-4.5

-4.0

-3.5

-3.0

3.4 3.5 3.6 3.7 3.8 3.9 4.0 103/T (K -1)

0°C -10°C -20°C 10°C 0°C -10°C -20°C 10°C

50:50 vol%

70:30 vol%

90:10 vol%

10:90 vol%

100:0 vol%

Ratio of hydrate/ice to sand

Log(σ

(S/m

))

Colors

10:90

90:10

70:30

50:50

100:0

Du Frane et al., 2015

Mixed with silica sand, hydrate conductivity goes up until a percolation threshold is reached. We think that impurities from the sand, probably K+ and Cl-, increase the charge carriers available in the hydrate.

-5.0

-4.5

-4.0

-3.5

-3.0

3.4 3.5 3.6 3.7 3.8 3.9 4.0 103/T (K -1)

0°C -10°C -20°C 10°C 0°C -10°C -20°C 10°C

50:50 vol%

70:30 vol%

90:10 vol%

10:90 vol%

100:0 vol%

Ratio of hydrate/ice to sand

Log(σ

(S/m

))

Colors

10:90

90:10

70:30

50:50

100:0

Du Frane et al., 2015

50/50 hydrate and sand is about 2,000 Ωm

Marine CSEM Methods

CRIP

PS I N

ST

ITUTION OF OCEANOGRA

PHY

UCSD

CRIP

PS I N

ST

ITUTION OF OCEANOGRA

PHY

UCSD

CRIP

PS I N

ST

ITUTION OF OCEANOGRA

PHY

UCSD

CSEM Transmitter

0 5 10 15 20 25 30 35 40 45 5010

-16

10-14

10-12

10-10

0.1 Hz Amplitude

Distance along line, km

Am

plitu

de

, V

/Am

2

0 5 10 15 20 25 30 35 40 45 50

-150

-100

-50

0

50

100

150

Distance along line, km

Ph

ase

, d

eg

ree

s

0.1 Hz Phasecrossline

inline

CSEM Data

Controlled-source electromagnetic (CSEM) sounding:

Field amplitude and phase is measured as a function of frequency and source/receiver position.

With frequency domain CSEM, the entire air-sea-seafloor system is illuminated continuously. Energy propagates preferentially in resistive rocks.

Air

Sea

Seafloor

With frequency domain CSEM, the entire air-sea-seafloor system is illuminated continuously. Energy propagates preferentially in resistive rocks.

Amplitude and phase of the magnetic/electric fields on the seafloor can be used to infer geological structure to depths of several km.

CRIP

PS IN

ST

ITUTION OF OCEANOGRAPHY

UCSD

Air

Sea

Seafloor

2E = µEt

+ µ2Et2

2E = 0

Radar

DC Resistivity

High frequency(megahertz)

Mid frequency(0.001 - 1000 Hz)

Zero frequency

The resolution of EM induction is between wave propagation and potential fields:

2E = µEt

r2U = 0

r2u = @u

@t+

1c2

@2u

@t2

Wave equation: Resolution ~ wavelength

Diffusion equation: Resolution ~ size/depth

Laplace equation: Resolution ~ bounds only

µ

= electrical conductivity ~

= magnetic permeability ~ = electric permittivity ~ 109 1011

104 106

3 106 S/mH/m

F/m

Inductive EM

Seismics

Gravity/Magnetism

Instrumentation:

Myer et al., Geophysics, 2015

Naif, PhD thesis, 2015

The many uses of marine CSEM:

MacGregor et al., GJI, 2001

Subduction zones

Mid-ocean ridges

Oil and gas exploration

Hydrate Ridge Experiment

130°W

130°W

125°W

125°W

120°W

120°W

40°N 40°N

44°N 44°N

48°N 48°N

52°N 52°N

0 100 200

km

Newport, Oregon

HydrateRidge

GordaPlate

Juan de Fuca Plate

PacificPlate

North American Plate

ExplorerPlate

Blanco Transform

MendocinoTransform

125.25°W 125.2°W 125.15°W 125.1°W 125.05°W 125°W 124.95°W44.55°N

44.6°N

44.65°N

44.7°N

44.75°N

-2200

-2000

-1800-1600

-1400

-1400

-1400-1200

-1200

-1200

-1000

-1000

-1000

-1000

-1000

-1000

-800

-800

-800

-3200 -2800 -2400 -2000 -1600 -1200 -800 -400 0meter

CSEM Tow 1

CSEM Tow 2

CSMT Tow

Receiver VE

Receiver MT

ODP Leg 204 drill sites

CSEM/MT Tow Lines

0 5

km

Northern

Hydrate Ridge

Southern

Hydrate Ridge

2004 pilot study at Hydrate Ridge

Weitemeyer et al., GJI, 2011

125.25°W 125.2°W 125.15°W 125.1°W 125.05°W 125°W 124.95°W44.55°N

44.6°N

44.65°N

44.7°N

44.75°N

-2200

-2000

-1800-1600

-1400

-1400

-1400-1200

-1200-1200

-1000

-1000

-1000

-1000

-1000

-1000

-800

-800

-800

-3200 -2800 -2400 -2000 -1600 -1200 -800 -400 0meter

CSEM Tow 1

CSEM Tow 2

CSMT Tow

Receiver VE

Receiver MT

ODP Leg 204 drill sites

CSEM/MT Tow Lines

0 5

km

Northern

Hydrate Ridge

Southern

Hydrate Ridge

-6000 -2000-4000 2000 40000 distance (m)

1600

1400

1200

1000

dept

h (m

)

3

2.5

2

1.5

1

0.5

Resi

stiv

ity

(Ohm

-m)

s02s03

s04

s05

s06

s07

s08 s09s10 s11

s12

s13s14

s15 s16

s19 s20s21

s17

s18

s22

1245 12461244

1252Seism

ic BSR

Hydrate above BSR

Free gas below BSR

seismics courtesy Anne Trehu

Weitemeyer, et al., GJI, 2010; 2011

2D inversion, using Schlumberger’s finite difference code

-6000 -4000 -2000 -0 2000 4000Distance (m)

-8000 -6000 -4000 -2000 0 2000 4000 6000

dept

h (m

)

distance (m)-8000 -6000 -4000 -2000 0 2000 4000 6000

dept

h (m

)

s01s02 s03

s04s05

s06s07

s08s10 s11 s12

s13 s14 s15 s16

s19 s20 s23s24 s25

s21

s17s18

s22

ODP 1245

ODP 1246

ODP 1244

BSR

transmitter tow

s09

1500

3000

2750

2500

2250

2000

1750

800

1000

1200

1400

1600

1800

2000

2200

velo

city

(m/s

)re

sistiv

ity (Ω

m)

3

2.5

2

1.5

1

0.5

800

1000

1200

1400

1600

1800

2000

2200

very low velocity

101 102 103104

105106

120(P)

121(P)

High resistivity below the BSR corresponds to low seismic velocities -> free gas, while high resistivity above the BSR suggests hydrate.

Weitemeyer, Constable and Tréhu, 2011

hydrate

free gas

resistivity

velocity

ODP 1245 ODP 1246ODP 1244

ODP 1252

0

75

150

225RelativeDe

pth(m)

A

B

B'

BSR

GHAnticline

i

Site 1245 Site 1246

Site

1252

B

B'

0

50

100

150

200

250

300

350

De

pth

(m

bsf)

B

B'

Horizon A

0.5 1 1.5 2 2.5 30.5 1 1.5 2 2.5 3

0.5 1 1.5 2 2.5 3

Resistivity

(Ω-m)

Resistivity

(Ω-m)

Resistivity

(Ω-m)

AC

AC

BGHSZ BGHSZ

BGHSZ

TopTop

Top

N E S W N E S W

N E S W

High

Site 1244

Shipboard Scientific Party Leg 204 (2003)

Comparison of inversion resistivities with well logs

Resistivities

Gas/hydrate saturations.

Weitemeyer, Constable and Tréhu, 2011

The Hydrate Ridge project was a success, but ...

There are a number of limitations with deployed seafloor receivers:

• Closely spaced receivers are costly in ship time and instruments

• Navigation errors increase with short source-receiver offsets

• There are still, inevitably, gaps in data coverage

This argues for a towed system.

The Vulcans

Bottom-dragged systems exist but

• Source-receiver offsets are limited

• Noise is high

• Equipment losses are frequent

• Only inline data are possible

Schwalenberg, et al., 2010

www.whoi.edu/cms/files/revans/2006/2/EM_System_7927.pdf

CRIP

PS IN

ST

ITUTION OF OCEANOGRAPHY

UCSD

EM transmitter

ElectrodesTransmitter antenna (100m) 200-500 m floating rope

Neutrally buoyant EM receiverTail float

5 15m0 90m 180m165 175

10−2

10−1

100

101

10−16

10−15

10−14

10−13

10−12

10−11

10−10

10−9

10−8

10−7

Frequency (Hz)

Po

we

r V2 /H

z

1.5 - 2 knots3 - 4 knots

The alternative is to fly an array above the seafloor.

But, noise induced by lateral motion of cable in Earth’s magnetic field

E = v B

Goto, et al., 2009

Our modeling also showed that it would be worth recording the vertical component of the electric field.

At lower frequencies, vertical field data can carry more information than horizontal.−2000 −1500 −1000 −500 0 500 1000 1500 2000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Norm

alized A

mplitu

de

−2000 −1500 −1000 −500 0 500 1000 1500 2000−30

−20

−10

0

10

20

Along track distance (m )

Phase D

iffe

rence (

degre

es)

Horizontal

Vertical0.5 Hz

−2000 −1500 −1000 −500 0 500 1000 1500 2000Along track distance (m)

B

C

200 m

1000 mtow direction

air

sea, 0.3 Ωm

sediment, 1 Ωm

fault, 10 Ωm

50 m

50 mTransmitters/receivers

D

0.5 Hz, 500 m offset, 50 m altitude

In 2007, we developed “Vulcan” for fixed offset frequency sounding.

CRIP

PS I N

ST

ITUTION OF OCEANOGRA

PHY

UCSD

CRIP

PS I N

ST

ITUTION OF OCEANOGRA

PHY

UCSD

CRIP

PS I N

ST

ITUTION OF OCEANOGRA

PHY

UCSD

CRIP

PS I N

ST

ITUTION OF OCEANOGRA

PHY

UCSD

CRIP

PS I N

ST

ITUTION OF OCEANOGRA

PHY

UCSD

EM TransmitterVulcan:

Towed Ex,y,z Receiver

Deployed

Ex,y,z Bx,y Receivers

33.5 Hz

23.5 Hz

13.5 Hz

6.5 Hz

3.5 Hz

1.5 Hz

0.5 Hz

0 0.5 1 1.5 2 2.5 3 3.5 4-400

-300

-200

-100

0

100

200

300

400

Time, seconds

Cur

rent

, am

ps

Waveform-D has a broad spectrum

96°W 94°W 92°W 90°W 88°W 86°W 84°W 82°W 80°W

24°N

25°N

26°N

27°N

28°N

29°N

30°N

31°N

0 100 200

km

-3500

-3000

-3000

-3000

-2500

-2500

-2500

-2000

-2000

-2000

-1500

-1500

-1500

-1000

-1000

-500

-500

0

0

0

0

0

00

0

0

00

0

GC 955

MC 118

AC 818

WR 313Fort

Lauderdale

Tampa

EM ReceiverGeophysical/Geochemical Sensors

Pipeline S-13591BP 24"

Fiber Optic Cable Boundary Marker

Popup_on_bottom/IDP_on_bottom

Fiber Optic CableCSEM Tow

Well #1 OCS-G-07925

24°N

25°N

26°N

27°N

28°N

29°N

30°N

31°N

96°W 94°W 92°W 90°W 88°W 86°W 84°W 82°W 80°W

-1100 -1000 -900 -800[m]

-1000

-950

-925

-900

-925

-925

-900

-900

-875

-850

88°30.6 88°30 88°29.4 88°28.8 88°28.2W

28°50.7N

28°51N

28°51.3N

28°51.6N

28°51.9N

28°52.2N

28°52.5N

28°52.8N

28°53.1N

s24 s23 s22 s21

s17 s18 s19 s20

s16 s15 s14 s13

s09 s10 s12

MMS Reserve Boundary

s08 s07 s06 s05

s01 s02s03

Tow 3

Tow 6

Tow 8

Tow 4

Tow 1

Tow 2

Tow 7

Tow 9

Tow 5

Tow 10

s04

s11

Bathymetry Data from CMRET provide by Leonardo Macelloni

0 0.5 1

km

From Sleeper et al. OTC 2006

SE crater

NW crater

SW crater

A

B

C

24 OBEM 500 m spacing 10 CSEM tows with towed receiver at a height of 65 m and 300 m offset.

MC 118, Gulf of Mexico using seafloor instruments and towed receiver:

inactiveclam

graveyard

active, out-cropping hydrate

active, in proximity to super-saline

waters

352.5

353

353.5

354354.5

355

355.5

356

356.5

3191

3192

3193

3194

3195

3196

1.0

1.2

1.4

1.6

UTM Eastin

g (km)

UTM

Northing (km

)

pseudo−depth

(

km

)

appare

nt re

sis

tivity

(Ω

-m)

1.5

2.5

2

3

0.5

1

MC118 Vulcan apparent resistivity frequency sections are in good

agreement with OBEM pseudosections

1.0

1.21.4

1.6

1.8

pseu

do−d

epth

(km

)

353353.5

354354.5

355355.5

356356.5

3191.53192

3192.5

31933193.5

3194

3194.53195

3195.5

UTM Easting (km)

UTM Northing (km)

appa

rent

resi

stiv

ity

(Ω

-m)

1.5

2.5

2

3

0.5

1

Mississippi Canyon 118 6.5 Hz OBEM

CRI P

PS I N

ST

I TU TION OF OCE ANOGRAPHY

UCS D

Transponder for navigationTransponder for navigation “Vulcan” towed 3-axis receivers

Seafloor EM receiver

SUESI - EM transmitter

Under Fugro funding in 2011 we developed Vulcan Mk II

•Real-time depth telemetry

•Real-time data samples

•3-axis accelerometer

•1000+ meter offsets

•Timing pulse from transmitter

seafloor instrument

seafloor instrument

10−2

10−1

100

101

102

10−19

10−18

10−17

10−16

10−15

10−14

10−13

10−12

10−11

10−10

10−9

Frequency (Hz)

crossline E

inline E

vertical E

10−2

10−1

100

101

102

10−19

10−18

10−17

10−16

10−15

10−14

10−13

10−12

10−11

10−10

10−9

Frequency (Hz)

1.6 knots 2.8 knots

crossline E

inline E

vertical E

Pow

er

V2 /H

z

Pow

er

V2 /H

z

Voltage noise is comparable to our seafloor instrument. (But, dipoles are 5-10 times shorter.)

2015 Southern California Tests A tale of two seeps

Work carried out by Peter Kannberg and supported by OFG and BOEM

LosAngeles

Scripps

Del MarseepSant Cruz

Basin

We have carried out two surveys, one targeting a known methane vent called the Del Mar seep, and one covering most of the Santa Cruz Basin.

−117.8 −117.6 −117.4 −117.2

32.8

33.0

33.2

−1400

−1200

−1000

−800

−600

−400

−200

0

methane vent

Depth (m)

La Jolla Fan

Thirtymile Bank

Coronado Bank

SanDiego

start

The Del Mar seep is a methane vent in the San Diego Trough, studied by Scripps students. It is in a pop-up structure bounded by two strands of the San Diego Trough Fault.

California Borderlands

Maloney et al., 2015

Ryan, et al., BSSA, 2012

Ryan et al. discovered this feature, in about 1,000 m water depth, and predicted fluid or methane venting, since confirmed by ROV dives and acoustics.

Maloney et al., 2015

Ryan, et al., BSSA, 2012

We also obtained an uncalibrated signal on a Contros methane sensor during an earlier CSEM test.

In March 2015 we towed across the vent with a 500 m Vulcan array, made a turn, and towed over it again.

Line 1

Line 2

Fault

0 5 10 15 20 25 30 35 40 45−1100

−1050

−1000

−950

−900

−850

−800

Along−track distance (km)

Dep

th (m

)

SUESISUESI seafloorATETVulcan 1Vulcan 2Vulcan 3Vulcan 4

0 5 10 15 20 25 30 35 40 45−20

−15

−10

−5

0

5

10

15

20Vulcan Pitch

Array Depths

Along−track distance (km)

Pitc

h (d

egre

es)

ATETVulcan 1Vulcan 2Vulcan 3Vulcan 4

0 5 10 15 20 25 30 35 40 45−10

−5

0

5

10Vulcan Roll

Along−track distance (km)

Rol

l (de

gree

s) ATETVulncan 1Vulcan 2Vulcan 3Vulcan 4

turn

methane vent

recovery

deployment

channel

seafloor

turn

channel

Navigation and stability of the receiver system is important.

Depth

Pitch

Roll

−117.8 −117.6 −117.4 −117.2

32.8

33.0

33.2

−1400

−1200

−1000

−800

−600

−400

−200

0

methane vent

Depth (m)

La Jolla Fan

Thirtymile Bank

Coronado Bank

SanDiego

start

~ Base of hydrate stability field

RMS 0.99

Line 1 inversion shows a uniform seafloor except in the seep area.

Frequencies of 1.5, 3.5, 6.5 Hz were fit for 3 Vulcans. Ey fits to 1% amplitude and 0.6°phase. As predicted, there is a strong low-frequency signal in Ez.

Distance along tow (km)

Inline data; RMS 0.99

Inline + vertical data; RMS 1.6

~ Base of hydrate stability field

~ Base of hydrate stability field

Addition of the vertical electric field data removes what appears to be a layering artifact and brings out a conductor that may be fluids feeding the vent.

Anisotropy (ratio of vertical to horizontal resistivities) is very high in the northern part of the region inferred to be gas hydrate.

Using Archie’s Law, resistivity can be converted to hydrate saturation. Integrating saturation provides an estimate of 2 billion cubic meters of methane, or 0.07 tcf.

Sh = 1

aRw

mRt

1n

where a=1, n=2, m=3, =0.5 Rw = 0.3Ωm, Rt = model resistivity

after Collet and Ladd, 2000

Santa Cruz Basin study: 21 seafloor receivers and 6 Vulcan tow lines. Water depths are over 2,000 m.

Highest resistivities appear to be on the flanks of the basin.

It looks as though we have discovered another seep.

~8 Ωm resistor lies entirely above the BSR, while a resistor to the east lies under (gas?)

Line 4 seep.

BSR Polarity reversal?Hydrate potential - 10 degree dipping beds crossing the BSR - seismic polarity reversal

March 2013

Over 1,000 line-km of Vulcan survey have been carried out off Japan as part of a national assessment of gas hydrate resources.

Research Consortium for Methane Hydrate Resources, Japan, 2015

Inversion of the CSEM data will provide a better estimate of resource potential than is possible with seismic/acoustic data alone.

Connecting the world of applied geophysics