mapping gas hydrates with marine controlled source ... · demonstrated the existence of hydrate...

MAPPING GAS HYDRATES WITH MARINE CONTROLLED SOURCE ELECTROMAGNETICS

Karen Weitemeyer1∗ and Steven Constable1 Scripps Institution of Oceanography1

University of California San Diego La Jolla, California, 92093-0225

UNITED STATES OF AMERICA

ABSTRACTEstimates of global methane hydrate volume spans four orders of magnitude, mainly because commonly used seismic methods are not easily sensitive to the bulk concentration of hydrate in the sea-floor section. Electromagnetic (EM) methods can be used to augment seismic data collected over hydrates, which are electrically resistive compared to surrounding sediment, and can provide additional information to constrain the volume of gas hydrate in the subsurface. We have been developing inductive marine EM techniques for this purpose and have collected controlled source electromagnetic (CSEM) data in two regions: Hydrate Ridge (Oregon) and the Gulf of Mexico. At Hydrate Ridge, we found a resistive region that coincides with the seismic bottom simulating reflector (BSR), a seismic horizon commonly associated with the phase transition of solid hydrate to free gas. Resistive regions above the seismic BSR are inferred to be hydrate, and resistive areas below are inferred to be free gas. At one of four locations surveyed in the Gulf of Mexico, Mississippi Canyon 118, we were able to differentiate chirp acoustic blanking due to carbonate and gas hydrate at two sea-floor vents. One vent is electrically conductive and is actively venting fluids and has sea-floor carbonates. The second vent is electrically resistive and is currently inactive, but the pavement of dead methanotropic clam shells suggests it once was active and any methane that was here froze into hydrate. We are able to distinguish between carbonate and hydrate, in contrast to the other mapping techniques, because the carbonate here is highly fractured, allowing saline pore waters to dominate the EM signal. These projects to date have provided a basis for the use of EM as an additional tool for carrying out global studies focused on directly quantifying the distribution, concentration, and total volume of hydrate accumulations.

Keywords: marine gas hydrates, electrical resistivity, CSEM, Hydrate Ridge, Gulf of Mexico, Mississippi Canyon 118

NOMENCLATUREBSR bottom simulating reflectorBx inline horizontal magnetic field (nT/Am)By crossline horizontal magnetic field (nT/Am)CSEM controlled source electromagnetic

DC direct currentEM electromagneticEx crossline horizontal electric field (V/Am2)Ey inline horizontal electric field (V/Am2)Ez vertical electric field (V/Am2)

∗ Corresponding author: Phone: +1 (858) 534 9430 Fax +1(858) 534 8090 E-mail: [email protected]

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

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GoM Gulf of MexicoHR Hydrate RidgeLBL long baseline acoustic navigationMC118 Mississippi Canyon 118MT magnetotelluricNW northwestOBEM ocean bottom electromagnetic receiverODP Ocean Drilling ProgramSW southwestSE southeastSUESI Scripps undersea electromagnetic source instrumentSBL short baseline acoustic navigationVE vertical electric1D one dimensional2D two dimensional3D three dimensionalρa Apparent resistivity (Ωm)ρ Resistivity (Ωm)

INTRODUCTIONThe estimated global volume of methane hydrate spans several orders of magnitude (see review by Boswell and Collett [1]). Two factors contribute to this uncertainty: (i) commonly used geophysical methods, such as seismic methods, are not easily sensitive to the bulk concentration of hydrate in the seafloor section; and (ii) direct sampling via drilling has a limited spatial extent. One seismic feature associated with gas hydrates is the seismic bottom simulating reflector (BSR), which typically marks the phase change from solid hydrate above and free gas below [2]. However, very little free gas is required to create the BSR [3] and so, the existence of a BSR does not infer the presence of significant hydrate above and the absence of a BSR does not imply that there is no hydrate within the sediment. Other seismic features are seismic amplitude blanking and seismic bright spots within the gas hydrate stability zone as documented at Blake Ridge [4]. Low concentrations of hydrate leads to amplitude blanking and high concentrations of hydrate may lead to amplitude enhancement within porous sediment [5]. However, these features are not unique, as other factors such as lithology, free gas, and sediment compaction can cause similar effects ([4] and references within). Further information is required before one can make claims about hydrate content. Complex 3D seismic inversion using rock physics principles and geologic models have made improvements to predicting gas hydrate saturations from seismic data, but there are ambiguities with inversion results and the rock

models maybe inadequate [6,7]. The most recent success for mapping and drilling gas hydrate deposits has come from using a petroleum systems approach [8].

A characteristic property of gas hydrates is that they are electrically resistive; well logs show an increase in resistivity when compared with water saturated zones [e.g. 9]. This property provides an EM target for controlled source electromagnetic (CSEM) methods. EM methods image the bulk resistivity structure of the subsurface and are sensitive to the concentration and geometric distribution of hydrate. The CSEM technique can thus be used to augment seismic data and provide additional information to constrain the volume of gas hydrate in the subsurface.

CSEMThe use of EM methods to estimate hydrate volume was first proposed by Edwards [10]. He and colleagues conducted several field studies using a transient electric dipole-dipole method on the Northern Cascadia margin off the west coast of British Columbia, Canada. These studies demonstrated the existence of hydrate when no BSR is present [11], and the existence of hydrate or free gas in seismic blanking zones thought to represent hydrate-bearing pipes [12]. A slightly different magnetic dipole-dipole CSEM survey which senses the top 30 m of sediment was used at two seafloor mounds in the northern Gulf of Mexico [13, 14]. The results showed that the mounds were electrically conductive (low resistivity), dominated by effects of raised temperature and pore fluid salinities rather then by the presence of massive hydrate expected within the mound. Both of these EM systems are dragged along the seafloor, and so their use is limited to

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places with little seafloor infrastructure or little bathymetry. The number of electromagnetic surveys dedicated to gas hydrate mapping is increasing [16,17,18,19,20] and the data quality is improving. The EM community is starting to move beyond 1D interpretations of the data, which is critical given the highly heterogenous distributions of hydrate documented by drilling [21].

The CSEM method used in this paper, shown in Figure 1, is a frequency-domain technique whereby a horizontal electric dipole transmitter is towed close to the seafloor and ocean bottom electromagnetic (OBEM) receivers record the transmitted electric and magnetic fields at various frequencies and ranges [22]. This is the same system that has been in use for the last ten years by the oil and gas industry to map deep offshore hydrocarbons [23]. The horizontal electric fields recorded by receivers are larger over resistive seafloor structures such as basalt, salt, carbonates, hydrocarbon reservoirs, or gas hydrates [24]. In this paper we will discuss the development of our CSEM equipment to map gas hydrates from our first pilot CSEM survey at Hydrate Ridge (2004) to our more recent CSEM surveys in the Gulf of Mexico (2008).

EXPERIMENTAL DESIGNUsing the 1D forward modeling codes of [25, 26] we made initial model studies of a horizontal hydrate layer. These studies indicated that an inline electric field geometry (where the transmitter is towed directly inline over OBEM receivers) is most sensitive to such a layer. 1D forward models at several frequencies (0.1 to 300 Hz) and ranges (transmitter-receiver offsets) also showed that a broad frequency spectrum was needed in order to be sensitive to a hydrate layer using different transmitter-receiver offsets (from 0 to 4 km) [see 15]. These 1D experimental design studies were specifically designed for our first pilot CSEM survey at Hydrate Ridge (2004).

The later development of a 2D finite element CSEM forward modeling code by Li and Key [27] allowed us to exam more realistic models of hydrate emplacement in hydrate filled faults or dikes of various thicknesses and resistivities, as suggested by Kleinberg [28, 29]. One example is shown in the top panel of Figure 2. Two different transmitter orientations are modeled: one with the transmitter inline (along the y-axis) and one with the transmitter crossline to the receivers (along the x-axis). The electric and magnetic field amplitudes due to a hydrate dike (solid lines) are

shown for a frequency of 0.5 Hz (middle panel) and for 16 Hz (bottom panel). Electric and magnetic fields are also shown for a background resistivity of 1 Ωm (dashed lines). The greater the deviation of the amplitude from the background model, the larger the signal due to the hydrate dike. In this case, with an inline transmitter, the Ez component has the largest signal (a decrease and increase in amplitude at low frequency; and a

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decrease in amplitude at high frequency), followed by the Ey and Bx components. For the crossline transmitter, only the By component has a signal due to the hydrate dike, and which is greater than the inline Bx. Sensitivity analysis shows a peak in the CSEM response at greater than 10 Hz, associated with induction at short source receiver offsets, and at less than 0.1 Hz, associated with a DC like galvanic effects. Consistently, the inline Ez component has the largest anomaly across frequencies varying from 0.1 to 300 Hz.

The 1D model studies showed that we were sensitive to a hydrate layer and that we needed to transmit a broad frequency spectrum. The 2D model studies of a slanted hydrate dike showed the importance of collecting the vertical electric field component in order to be sensitive to dipping hydrate filled fractures, such as those found at Hydrate Ridge by Weinberger and Brown [30] and in the Northern Gulf of Mexico by Cook et al. [31].

CSEM SURVEY AT HYDRATE RIDGEHydrate Ridge is situated on an accretionary ridge that is part of the Cascadia subduction zone. It is approximately 80 km off-shore from Newport, Oregon, and is in water depths of 820–1200 m (Figure 3). Hydrate Ridge was chosen as our pilot study area due to an opportunistic use of ship time, the existence of hydrate, and the extensive ground truth available from well logs of the Ocean Drilling Program (ODP) Leg 204 [32], a 3D seismic volume [33] and acoustic bathymetry mapping [34]. These other data sets allowed for a comprehensive comparison with our CSEM results.

Hydrate emplacement at Hydrate Ridge is lithologically controlled and due to a focused high-flux regime and a distributed low-flux regime [35]. Seismic horizon A is an example of the focused high-flux regime; a gas-charged conduit transporting thermogenic or altered biogenic gas from great depth within the accretionary prism to the summit of Hydrate Ridge [36]. Hydrate formation due to the presence of methane within the sediments from in situ microbial methane production leads to diffuse fluid flow and dispersed hydrate throughout the sediment, and produces a pervasive BSR over much of the Cascadia accretionary complex [37].

The CSEM survey consisted of 25 OBEM receivers and a transmitter tow line co-located

with seismic line 230 and four ODP Leg 204 drill locations. The EM transmitter, SUESI200, used a 90m antenna and transmitted a square wave of 5Hz at 100 Amperes. A commercial short baseline (SBL) acoustic navigation system was meant to provide transmitter navigation, but this system failed. Instead the transmitter was navigated through indirect means which are discussed in great detail in Chapter 4 of [38]. The OBEMs were configured as either a three component electric field sensor, or as a magnetotelluric (MT) instrument (recording horizontal electric and magnetic fields). These two types of configurations were alternated along the line, allowing us to collect both CSEM data and MT data. We were able to examine the fundamental frequency and the third harmonic (15 Hz). Initial results were presented as 1D apparent resistivity pseudosections [39]. The inline 5 Hz electric field data were later inverted using a 2D finite difference code through a collaboration with David Alumbaugh and Guozhong Gao of EMI-Schlumberger [40].

Figure 4 shows the 2D CSEM inversion with an overlay of seismic line 230. Two main stratigraphic layers, identified by Chevallier et al. [41] are imaged: the accreted abyssal plan sediments of the Astoria Fan which form the core of the accretionary complex; and the deposition of overlying slope basin sediment formed from the formation and evolution of the accretionary wedge fold-thrust belt. These main layers are distinguished by the gradual change from the deep resistive region in the 2D inversion to more conductive region at shallower depths. The deep resistive accretionary complex sediments agree with tomography seismic velocity inversion by Arsenault et al. [42] and MT data [43]. Higher velocities are associated with folded accretionary complex sediments. The folds in the velocity

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model correlate well with the high resistivity zone in the CSEM inversion [43]. Above the accretionary complex are slope basin sediments, which are within the gas hydrate stability zone. To the west of the profile the CSEM inversion contains a resistor at about the depth of the BSR. In this region of Hydrate Ridge the hydrates are biogenic and hence a concentration of gas at the BSR is expected [32]. The chaotic seismic region between sites 2 to 4 was interpreted as having high free gas or gas hydrate saturations in inversion by Zhang and McMechan [44]. Seismic horizon A is a gas-charged fluid conduit taking methane gas to the southern summit (out of the page), which also shows up as a resistor in the CSEM inversion. A low velocity zone was found by a seismic tomography inversion and is interpreted to indicate the presence of free gas beneath the BSR and in horizon A [43]. A conductive region below s06 exists within the hydrate stability field at the summit of this profile, suggesting lower hydrate concentrations and/or the presence of brines. The anticline under site 16 is present as a resistor and may be a result of a change in lithology.

EQUIPMENT DEVELOPMENTThe experience at Hydrate Ridge led to improvements in the collection of CSEM data, and to the seafloor instruments and transmitter. The unreliability of the commercial SBL led to the development of our own inverted long baseline (LBL) acoustic navigation system. We also developed a new instrument, Vulcan, which is a towed three axis electric field

receiver. Figure 5 shows how CSEM data was collected for the subsequent surveys in the Gulf of Mexico (GoM).

OBEM Receiver UpgradesAn external compass was developed to give orientation and tilt of the receiver, eliminating the task of determining how the instrument was oriented through indirect means [see Ch 5 of [38]]. Ship time was allocated for dedicated digital acoustic navigation of seafloor receivers, rather than collecting acoustic navigation simultaneously with CSEM transmission, giving locations of seafloor receivers accurate to 1-3m. Dedicated digital acoustic navigation also prevents the contamination of CSEM data with noise from the acoustic transponders mounted on the receivers. The 2D model studies discussed above pointed out

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The Barracuda system was perhaps the most disappointing aspect of instrument performance on the cruise. We hadvarious issues which included a loose cable in the SUESI-mounted Benthos acoustic ranging unit, a new transpondersystem which may be weaker than our older ones, intermittent failure of the GPS units in the paravanes, and collapseof the towing bridle in turns resulting in the paravanes failing to fly properly and even flying underwater. One unitflooded as a result of this, but we had a spare. However, this was a new instrument system and some startup problemsoften occur. We did get the system working for the majority of the MC 118 survey, and will have enough data totest its viability, and for a time we had real-time locations for the deeptow transmitter during the survey, which is ourobjective. Since we have ship’s position, wire out, SUESI depth, and antenna depth, the only critical parameters we donot have are the offline set and the antenna azimuth. The seafloor instruments are all well navigated using long baselineranging from the ship (standard errors in the positions are typically 1-2 m) and the recording compasses worked verywell (accurate to a few degrees), so we will recover these parameters using a method developed by Karen Weitemeyerduring her thesis work which uses the close-range geometry of the electromagnetic fields.

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Figure 5: Diagram of the inverted LBL system proposed for use during the surveys.



the importance of collecting five components of the electromagnetic field (Ex, Ey, Ez, Bx, By); which was done during the GoM experiment. We decreased the gain of the amplifier electric field to reduce saturation of the amplifier at short offsets (HR <750m; GoM <250m ), allowing us to record amplitude data at ranges more sensitive to shallow hydrate.

SUESI TransmitterThe original Hydrate Ridge project used the SUESI200, the first version of our new generation EM source instruments. Output current was kept between 100 and 150 A since it was the first use of the transmitter. Since then we have built a second version of this transmitter rated to 500 A (SUESI500) on a 200 m long antenna for deep crustal sounding. For the GoM project we use a much smaller, 50 m antenna, in order to better approximate a dipole field and to increase resolution at short source receiver offsets. This smaller antenna was only rated to 200 A, which is the current we used for GoM. We added a tail buoy which included an acoustic relay transponder and a recording depth meter to the end of the 50m transmission antenna. The dip of the antenna can then be computed by comparison of the tail depth with the depth gauge within SUESI.

Model studies led us to move away from transmitting a square wave (as was done for Hydrate Ridge), to transmitting a binary doubly symmetric waveform [45]. This allowed a dense and broad frequency coverage of several decades, in which the amplitudes of the third and seventh

harmonics are maximized. The waveform used in the Gulf of Mexico is shown in Figure 5 along with its amplitude periodogram. It has nearly two decades of frequency for which the harmonics are above 0.1 times the peak current, and the five largest harmonics are only a factor of three different in size, spanning more than one decade. For comparison, a square wave has little more than one decade of frequencies greater than one tenth peak current, and the two lowest harmonics differ by a factor of three, covering a factor of three in frequency.

VulcanElectromagnetic fields attenuate faster at a higher frequency and so in order to capture the full frequency content of the transmitted waveform we designed a three component electric field sensor towed at a fixed offset (300 m) behind the transmitting antenna. Vulcan’s development was motivated by model studies of dipping hydrate dikes, which produce signatures in the vertical electric field at short offsets, suggesting the need for more than the traditional horizontal receivers. In contrast to the seafloor instruments, for which navigation errors in the transmitter-receiver geometry become large at short ranges, the source-receiver offset for Vulcan is fixed and known. While towing at several knots the noise floor of Vulcan is comparable to the seafloor instruments when its shorter antennae are considered.

Inverted LBL Navigation SystemOur inverted LBL acoustic navigation system consists of a Benthos DS9000 installed on SUESI

and towed transponders behind the vessel on the surface using paravanes (Figure 5). The transponders have GPS receivers and radio modems, providing realtime estimates of position. The relay transponders on the tail buoy and Vulcan provide an LBL solution based on replies from either surface or seafloor instruments.

The GoM project gave us the chance to develop this system, but unfortunately, the inverted LBL system did not work well until the last survey at MC 118.

GULF OF MEXICO CSEM SURVEYSIn October, 2008 we collected CSEM data at four survey areas within the Gulf of Mexico (Alaminos Canyon block 818,Walker Ridge block 313, Green Canyon block 955, and Mississippi Canyon block 118). The areas studied during the cruise are in different water

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depths and have different geologic controls on the way hydrate is thought to be distributed. Two of these (WR313 and GC955) were drilled by the JIP and successfully encountered gas hydrate [8]. CSEM results at these two locations imaged geologic structures such as salt bodies and water saturated sand channels. AC818 is an intended JIP drill location and we are able image resistive anomalies associated with salt and with gas hydrate within the Oligiocene Frio sand. 1D interpretations can be made, but 2D or even 3D analysis will be necessary before more meaningful information can be gleaned from the CSEM data. We thus focus our attention on MC 118, which has relatively flat bathymetry and is suitable for 1D interpretation.

Data ProcessingThe data were processed by using a fast Fourier transform to convert the data from the time domain into the frequency domain. The data are stacked into 60 second stack frames and are merged with the transmitter navigational parameters. The major axis of the polarization ellipse was used to select the resistivity of a half-space forward model that matched the recorded data, to create an apparent resistivity for each transmitter receiver pair. Finally a pseudosection projection technique was used to make an image of the data. Figure 7 (left) shows a schematic of the data projection for mapping ranges into depths; the longer the transmitter-receiver offset the deeper that data point is projected.

A similar approach was taken with the Vulcan data except that the Vulcan apparent resistivity pseudosections were generated using the total field from all three components of the electric field (Ex, Ey, Ez) measured by the instrument. Forward modeling was again used to compute apparent resistivities as a function of frequency. The apparent resistivities were projected into a depth using skin depth attenuation for each frequency as shown in Figure 7 (right). Pseudosections provide a way to look at the data and observe lateral variations in resistivity, but do not provide reliable information about resistivity variations with depth. For this reason the depth scales in the following figures should not be taken literally. We estimate that the OBEM data are sensitive to the top few kilometers of sediment and the Vulcan data to the top few hundred meters.

Mississippi Canyon 118MC 118, a designated Minerals Management Services observatory, has large outcrops of hydrate on the seafloor but no direct evidence of hydrate at depth. The main area of interest is a hydrate/carbonate mound, a cold seep, consisting of three main craters venting methane gas into the ocean at various flux rates [47,48, 49](Figure 8C). Diapiric salt is present some 200-300 m below the seafloor

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Figure 7: Marine CSEM system with the towed 3-axis electric field receiver, called Vulcan. We can build apparent resistivity pseudosections two different ways: The left side shows apparent resistivities derived from seafloor instruments projected to a mid-point between the EM transmitter and deployed receiver, at a pseudo-depth proportional to the transmitter-receiver offset. For the fixed-offset towed receiver (right), apparent resistivities from different frequencies can be projected at the common mid-point based on skin depth, with lower frequencies having larger skin depths/deeper penetration. Figure from [46].



[51 and references within] . The movement of the buried salt body has created sedimentary faults allowing fluids to migrate from depth to the surface of the seafloor [52 and references within]. The migration of fluids through these conduits are dynamic and have evolved such that at present day the most active venting occurs within the SW and NW complexes [47,48,49]. Hemipelagic, sand and free clay constitutes the host sediment [52].

Twenty-four OBEM receivers were deployed in a 6 x 4 array and SUESI was towed over the 10 lines forming the survey grid. SUESI “flew” at an altitude of 65 m above the seafloor to avoid already installed equipment and pipelines (Figure 8B). In addition to the seafloor receivers, Vulcan was towed in tandem with and 300 m behind SUESI.

Pseudosections of the Vulcan data (Figure 9) show MC 118 to be rather conductive, with a background resistivity of 0.5-1 Ωm, and generally featureless except at the SE crater. No constraints

were placed on the intercepting tow lines and so the fact that three lines independently give a resistive body at the SE crater provides confidence that this is a geological feature (rather than an experimental artifact or navigation error). The E-W line that crosses through the SE crater is overlaid on chirp acoustic line 119 from [50] for comparison with electrical resistivity. The acoustic blanking or wipeout zones at MC 118 are attributed to authigenic carbonate as well as free gas and gas hydrate [53].

A pavement of dead methanotrophic clam shells exists on the seafoor at the SE crater [47,48]. There is no evidence for recent venting, suggesting that the conduit once supplying methane to these clams became blocked, perhaps due to hydrate formation [47,48, 29]. The SE crater resistor appears to have some depth extent and the acoustic blanking there is correlated with resistive seafloor. However, the acoustic blanking zone towards the SW crater is associated with the background resistivity of 1 Ωm. The acoustic signature here is

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attributed to shallow carbonates [54], suggesting that hydrate and carbonates, which we initially thought would be confounding electrical resistors, are in fact differentiable. It seems like a reasonable interpretation at this time that the resistor at the SE crater is hydrate, only drilling at the SE crater can confirm this.

Figure 10 shows the 6.5Hz OBEM pseudosections, which are consistent with those from Vulcan. Three CSEM tow lines show a resistor at the SE crater, again with a background resistivity of about 1 Ωm. The slightly elevated background resistivities from the OBEM data are probably a result of sampling deeper, more compacted, sediments. Inconsistencies between the Vulcan and OBEM pseudosections in the E-W tow line crossing site 9 maybe caused by navigational errors, although they could be due to a resistor, such as a salt body documented in [47,48] that is too deep to be visible by Vulcan.

CONCLUSIONWe have presented a brief history of Scripps Insititution of Oceanography’s development of marine CSEM to map gas hydrates. The first pilot survey at Hydrate Ridge has documented a resistive region at the seismic BSR. Further development of the technique led to the creation of a fixed offset towed sensor, which has imaged gas hydrates at MC118. At the three other sites in the Gulf of Mexico the CSEM technique has mapped geologic structures consistent with other available data sets.

ACKNOWLEDGEMENTSFunding for the Hydrate Ridge data set was provided by ExxonMobil and GERD, Japan. Anne Tréhu (OSU) is thanked for useful discussions and continuous support of this work. The 2D inversion of the Hydrate Ridge data was done in collaboration with David Alumbaugh and Guozhong Gao. The Gulf of Mexico hydrate experiment was funded by industrial sponsors ExxonMobil, WesternGeco, Chevron, EMGS, Fugro, CGG/Veritas, Shell, and Statoil, as well as the MMS and the NETL program of the US Department of Energy (contract DE-NT0005668). Ship time was provided by the University of

California Shipfunds Committee. We thank numerous colleagues in the JIP and Gulf of Mexico Hydrates Research Consortia for advice and support over the years this project took to mature. The seafloor electromagnetics consortium is thanked for supporting KW over the years. We thank present and past members of the marine EM Lab for ensuring the collection of high quality marine EM data: Brent Wheelock, David Myer, Kerry Key, Yugou Li, John Sounders, Cambria Berger, Jake Perez, Jacques Lemire, Chris Armerding, Arnold Orange, Patricia Chen, Garth Englehorn, Chris Winther, Jim Behrens.

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Figure 10 MC118 OBEM apparent resistivity pseudosections at 6.5Hz. From [46].



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