Abstract
Audio-magnetotelluric (AMT) method is a kind of frequency-domain sounding technique, which can be applied to gas hydrate prospecting and assessments in the permafrost region due to its high frequency band. Based on the geological conditions of gas hydrate reservoir in the Qilian Mountain permafrost, by establishing high-resistance abnormal model for gas hydrate and carrying out numerical simulation using finite element method (FEM) and nonlinear conjugate gradient (NLCG) method, this paper analyzed the application range of AMT method and the best acquisition parameters setting scheme. When porosity of gas hydrate reservoir is less than 5%, gas hydrate saturation is greater than 70%, occurrence scale is less than 50 m, or bury depth is greater than 500 m, AMT technique cannot identify and delineate the favorable gas hydrate reservoir. Survey line should be more than twice the length of probable occurrence scale, while tripling the length will make the best result. The number of stations should be no less than 6, and 11 stations are optimal. At the high frequency section (10∼1000 Hz), there should be no less than 3 frequency points, 4 being the best number.
Highlights
Gas hydrate is a solid crystal with cage structure consisting of water molecules and natural gas, which mainly exists in the terrestrial permafrost regions and beneath the sea along the outer continental margins of the world’s oceans (Sloan 1998)
As gas hydrate in the Qilian Mountain permafrost occurs abnormally in the local scope, the gas hydrate reservoir can be modeled as high-resistance abnormal body
To determine gas hydrate reservoirs favorable to AMT prospecting in the study area, the response characteristics of AMT forward modeling for high-resistance abnormal body of gas hydrate are analyzed with variation of resistivity, occurrence scale, and bury depth
Summary
Gas hydrate is a solid crystal with cage structure consisting of water molecules and natural gas (mainly CH4), which mainly exists in the terrestrial permafrost regions and beneath the sea along the outer continental margins of the world’s oceans (Sloan 1998). Determining appropriate survey line length, station spacing and the number of frequency points which must be considered in the field data acquisition, will be convenient to indoor AMT data processing and interpretation. Due to the different occurrences of gas hydrate in the Qilian Mountain permafrost region, resistivity, occurrence scale, and bury depth of gas hydrate vary a lot, and the corresponding AMT response modes are unclear, making data processing and interpretation difficult. Based on the geological conditions of gas hydrate in the Qilian Mountain permafrost, by establishing high-resistivity anomaly model for gas hydrate and carrying out forward modeling with finite element method, this paper obtained the AMT response characteristics of the corresponding models. Nonlinear conjugate gradient method (NLCG) was used to simulate AMT response characteristics under the conditions of different acquisition parameters, including survey line length, station spacing, and number of frequency points. Through comparing and analyzing the differences of response characteristics resulted from different acquisition parameters, an acquisition parameter setting scheme in favor of prospecting gas hydrate reservoir in the study area was obtained
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