Quantitative imaging of water, ice and air in permafrost systems through petrophysical joint inversion of seismic refraction and electrical resistivity data

SUMMARY Quantitative estimation of pore fractions filled with liquid water, ice and air is crucial for a process-based understanding of permafrost and its hazard potential upon climate-induced degradation. Geophysical methods offer opportunities to image distributions of permafrost constituents in a non-invasive manner. We present a method to jointly estimate the volumetric fractions of liquid water, ice, air and the rock matrix from seismic refraction and electrical resistivity data. Existing approaches rely on conventional inversions of both data sets and a suitable a priori estimate of the porosity distribution to transform velocity and resistivity models into estimates for the four-phase system, often leading to non-physical results. Based on two synthetic experiments and a field data set from an Alpine permafrost site (Schilthorn, Bernese Alps and Switzerland), it is demonstrated that the developed petrophysical joint inversion provides physically plausible solutions, even in the absence of prior porosity estimates. An assessment of the model covariance matrix for the coupled inverse problem reveals remaining petrophysical ambiguities, in particular between ice and rock matrix. Incorporation of petrophysical a priori information is demonstrated by penalizing ice occurrence within the first two meters of the subsurface where the measured borehole temperatures are positive. Joint inversion of the field data set reveals a shallow air-rich layer with high porosity on top of a lower-porosity subsurface with laterally varying ice and liquid water contents. Non-physical values (e.g. negative saturations) do not occur and estimated ice saturations of 0–50 per cent as well as liquid water saturations of 15–75 per cent are in agreement with the relatively warm borehole temperatures between −0.5 and 3 ° C. The presented method helps to improve quantification of water, ice and air from geophysical observations.

Geothermal Regime and Deep Temperatures of the Siberian Platform

The geothermal regime of the southern segment of the East Siberian platform, where more than 200 heat-flow measurements have been carried out, is well-understood. The present work deals with the study of deep temperatures of the southern Siberian platform, based on results of geothermal measurements in more than 70 boreholes. In addition, measurements of thermal properties have been made mostly on core samples representing the Vendian terrigenous deposits and Riphean magmatic and metamorphic basement rocks. The basement rocks may be subdivided into two groups, with thermal conductivity coefficients varying in the range of 2 and 3 W/m/K. Higher coefficients indicate the presence of carbonate-halogen admixtures. Studies have also been made of the borehole thermograms and temperatures at the bottom and top of the Moti suite, of lower Cambrian age. These boreholes vary in depth from 1300 to 6000 m, and the borehole temperatures attain values as high as 70оC. In this region average heat flow is 38±4 mW/m2. Higher heat flow values (45±6 mW/m2) are observed in the anticlinal domes and salt-dome crests, while low heat flow seems to be typical of marginal uplifts. This peculiar geothermal condition is also closely related to hydrodynamic features of the area, where underground seepage flow penetrates to depths of 3-5 km while conductive diffusion of heat prevails in the deeper crust. It is argued that such anomalous conditions exert influence on the dynamics of hydrocarbon accumulation, which in turn is also predetermined by geothermal conditions.

Siple Dome shallow ice cores: a study in coastal dome microclimatology

Ice cores at Siple Dome, West Antarctica, receive the majority of their precipitation from Pacific Ocean moisture sources. Pacific climate patterns, particularly the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM), affect local temperature, atmospheric circulation, snow accumulation, and water isotope signals at Siple Dome. We examine borehole temperatures, accumulation, and water isotopes from a number of shallow ice cores recovered from a 60 km north–south transect of the dome. The data reveal spatial gradients partly explained by orographic uplift, as well as microclimate effects that are expressed differently on the Pacific and inland flanks. Our analyses suggest that while an ENSO and SAM signal are evident at Siple Dome, differences in microclimate and possible postdepositional movement of snow makes climate reconstruction problematic, a conclusion which should be considered at other West Antarctic coastal dome locations.

Impact of the Last Glacial Cycle on Late-Holocene temperature and energy reconstructions from terrestrial borehole temperatures in North America

Reconstructions of past climatic changes from borehole temperature profiles are important independent estimates of temperature histories over the last millennium. There remain, however, multiple uncertainties in the interpretation of these data as climatic indicators and as estimates of the changes in heat content of the continental subsurface due to long-term climatic change. One of these uncertainties is associated with the often ignored impact of the last glacial cycle on the subsurface energy content, and on the estimate of the background quasi steady-state signal associated with the diffusion of accretionary energy from the Earth's interior. Here we provide the first estimate of the impact of the development of the Laurentide ice sheet on the estimates of energy and temperature reconstructions from measurements of terrestrial borehole temperatures in North America. We use basal temperature values from the data-calibrated Memorial University of Newfoundland Glacial Systems Model to quantify the extent of the perturbation to estimated steady-state temperature profiles and to derive spatial maps of the expected impacts on measured profiles over North America. Furthermore, we present quantitative estimates of the potential effects of temperature changes during the last glacial cycle on the borehole reconstructions over the last millennium for North America. The range of these possible impacts are estimated using synthetic basal temperatures for a period covering 120 ka to the present day that include the basal temperature history uncertainties from an ensemble of results from the calibrated numerical model. For all the locations, we find that within the depth ranges that are typical for available boreholes (≈600 m), the induced perturbations to the steady-state temperature profile are on the order of 10 mW m−2, decreasing with greater depths. Results indicate that site-specific heat content estimates over North America can differ by as much as 50%, if the energy contribution of the last glacial cycle in those areas of North America that experienced glaciation is not taken into account when estimating recent subsurface energy change from borehole temperature data.