Advancements in Electromagnetic Propagation Logging

Wharton, R.P.; Hazen, Gary; Rau, Rama N.; Best, David

doi:10.2118/9041-ms

Cited by 39 publications

(8 citation statements)

References 2 publications

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“…After bandpass filtering (frequency range between 3 and 9 MHz), the data were migrated to account for off-angle reflections from sloped interfaces (e.g., the lateral walls of the subglacial conduit/ice-shelf channels) using Kirchoff-Depth migration implemented in the open-source software Seismic Unix. The required radio-wave velocity model varies with depth using a depth–density parameterization 53 and a density–velocity relation 54 . The grounding-zone of Roi Baudouin Ice Shelf is characterized by an extensive blue-ice belt so that the radio-wave velocity is close to the pure-ice velocity (1.68·10 8 m s −1 ) everywhere.…”

Section: Methodsmentioning

confidence: 99%

Actively evolving subglacial conduits and eskers initiate ice shelf channels at an Antarctic grounding line

et al. 2017

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Ice-shelf channels are long curvilinear tracts of thin ice found on Antarctic ice shelves. Many of them originate near the grounding line, but their formation mechanisms remain poorly understood. Here we use ice-penetrating radar data from Roi Baudouin Ice Shelf, East Antarctica, to infer that the morphology of several ice-shelf channels is seeded upstream of the grounding line by large basal obstacles indenting the ice from below. We interpret each obstacle as an esker ridge formed from sediments deposited by subglacial water conduits, and calculate that the eskers' size grows towards the grounding line where deposition rates are maximum. Relict features on the shelf indicate that these linked systems of subglacial conduits and ice-shelf channels have been changing over the past few centuries. Because ice-shelf channels are loci where intense melting occurs to thin an ice shelf, these findings expose a novel link between subglacial drainage, sedimentation and ice-shelf stability.

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Section: Methodsmentioning

confidence: 99%

Actively evolving subglacial conduits and eskers initiate ice shelf channels at an Antarctic grounding line

et al. 2017

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“…Using this expression, v was expressed as a ε r(b) and used as input to the Complex Refractive Index Model (CRIM) to calculate gas content [e.g., Huisman et al ., ]. The CRIM is a volumetric three‐phase mixing model for the soil [ Wharton et al ., ],

{ε_{r (normalb)}}^{α} = θ {ε_{r (normalw)}}^{α} + ((), 1 - n) {ε_{r (normals)}}^{α} + ((), n - θ) {ε_{r (normala)}}^{α}

where ε r(a) , ε r(w) , and ε r(s) are the relative dielectric permittivity of gas (= 1), water (temperature dependent), and soil particles (= 2; after Comas et al . []), respectively, n is the porosity, θ is the volumetric soil water content, and α is a factor accounting for the orientation of the electrical field and the geometrical arrangement of minerals (typically 0.35 for peat soils [ Kellner et al ., ; Parsekian et al ., ]).…”

Section: Methodology and Experimental Setupmentioning

confidence: 99%

Investigating carbon flux variability in subtropical peat soils of the Everglades using hydrogeophysical methods

Comas

Wright

2014

J. Geophys. Res. Biogeosci.

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The spatial and temporal variability in accumulation and release of greenhouse gases (mainly methane and carbon dioxide) to the atmosphere from peat soils remains very uncertain. The use of near-surface geophysical methods such as ground penetrating radar (GPR) has proven useful during the last decade to expand scales of measurement as related to in situ gas distribution and dynamics beyond traditional methods (i.e., gas chambers). However, this approach has focused exclusively on boreal peatlands, while no studies in subtropical systems like the Everglades using these techniques exist. In this paper GPR is combined with gas traps, time-lapse cameras, gas chromatography, and surface deformation measurements to explore biogenic gas dynamics (mainly gas buildup and release) in two locations in the Everglades. Similar to previous studies in northern peatlands, our data in the Everglades show a statistically significant correlation between the following: (1) GPR-estimated gas content and gas fluxes, (2) GPR-estimated gas content and surface deformation, and (3) atmospheric pressure and both GPR-estimated gas content and gas flux. From these results several gas-releasing events ranging between 33.8 and 718.8 mg CH 4 m À2 d À1 were detected as identified by the following: (1) decreases in GPR-estimated gas content within the peat matrix, (2) increases in gas fluxes captured by gas traps and time-lapse cameras, and (3) decreases in surface deformation. Furthermore, gas-releasing events corresponded to periods of high atmospheric pressure. Changes in gas accumulation and release were attributed to differences in seasonality and peat soil type between sites. These results suggest that biogenic gas releases in the Everglades are spatially and temporarily variable. For example, flux events measured at hourly scales were up to threefold larger when compared to daily fluxes, therefore suggesting that flux measurements decline when averaged over longer time spans. This research therefore questions what the appropriate spatial and temporal scale of measurement is necessary to properly capture the dynamics of biogenic gas release in subtropical peat soils.

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“…Assuming peat soil is a low loss medium, the EM wave velocity ( v ) can be expressed as:

where ɛ r(b) is the relative dielectric permittivity of the peat soil and c = 3 × 10 8 m/s. In order to estimate gas content from v we applied the Complex Refractive Index Model (CRIM) [e.g., Huisman et al , 2003], which is a volumetric mixing model for a soil [ Wharton et al , 1980]:

where ɛ r(a) , ɛ r(w) , and ɛ r(s) are the relative dielectric permittivity of gas (1), water (80 at 21 °C) and the soil particles respectively, n is the porosity, θ is the volumetric soil water content and α is a factor accounting for the orientation of the electrical field and the geometrical arrangement of peat fibers (typically 0.35 for peat soils [ Kellner et al , 2005]). As later explained in the results section, gas content estimation using the CRIM accounted for: (1) changes in porosity as a function of time due to peat surface deformation; (2) changes in ɛ r(w) due to temperature variation of the peat column; and (3) changes in water table elevation concurrently monitored during GPR data acquisition.…”

Section: Experimental Field Design and Methodologymentioning

confidence: 99%

Seasonal geophysical monitoring of biogenic gases in a northern peatland: Implications for temporal and spatial variability in free phase gas production rates

2008

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[1] A set of high resolution surface ground penetrating radar (GPR) surveys, combined with elevation rod (to monitor surface deformation) and gas flux measurements, were used to investigate in situ biogenic gas dynamics within a northern peatland (Caribou Bog, Maine). Gas production rates were directly estimated from the time series of GPR measurements. Spatial variability in gas production was also investigated by comparing two sites with different geological and ecological attributes, showing differences and/or similarities depending on season. One site characterized by thick highly humified peat deposits (5-6 m), wooded heath vegetation and open pools showed large ebullition events during the summer season, with estimated emissions (based on an assumed range of CH 4 concentration) between 100 and 172 g CH 4 mÀ2 during a single event. The other site characterized by thinner less humified peat deposits (2-3 m) and shrub vegetation showed much smaller ebullition events during the same season (between 13 and 23 g CH 4 m À2 ). A consistent period of free-phase gas (FPG) accumulation during the fall and winter, enhanced by the frozen surficial peat acting as a confining layer, was followed by a decrease in FPG after the snow/ice melt that released estimated fluxes between 100 and 200 g CH 4 mÀ2 from both sites. Estimated FPG production rates during periods of biogenic gas accumulation ranged between 0.22 and 2.00 g CH 4 m 3 d À1 and reflected strong seasonal and spatial variability associated with differences in temperature, peat soil properties, and/or depositional attributes (e.g., stratigraphy). Periods of decreased atmospheric pressure coincided with short-period increases in biogenic gas flux, including a very rapid decrease in FPG content associated with an ebullition event that released an estimated 39 and 67 g CH 4 m À2 in less than 3.5 hours. These results provide insights into the spatial and seasonal variability in production and emission of biogenic gases from northern peatlands.Citation: Comas, X., L. Slater, and A. Reeve (2008), Seasonal geophysical monitoring of biogenic gases in a northern peatland: Implications for temporal and spatial variability in free phase gas production rates,

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Advancements in Electromagnetic Propagation Logging

Cited by 39 publications

References 2 publications

Actively evolving subglacial conduits and eskers initiate ice shelf channels at an Antarctic grounding line

Actively evolving subglacial conduits and eskers initiate ice shelf channels at an Antarctic grounding line

Investigating carbon flux variability in subtropical peat soils of the Everglades using hydrogeophysical methods

Seasonal geophysical monitoring of biogenic gases in a northern peatland: Implications for temporal and spatial variability in free phase gas production rates

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