Assessment of Gas Hydrate Resources in Ross Sea Area, Antarctica Based on Inversion of Gravity and Magnetic Data

： The Ross Sea is located between Victoria Land and Mary Bird Land in West Antarctica. In this paper, the published gravity and magnetic data in the Ross Sea area are fused with the high-precision gravity and magnetic data measured by the ship. Then ， The gravity anomaly data is used to invert the Moho depth by the Parker-Oldenburg method; the magnetic anomaly data is used to invert the Curie depth of the Ross Sea area by the power spectrum method. Finally, according to the inversion results of the Moho depth and Curie depth, the high-precision heat flow distribution in the Ross Sea area is calculated. And compared with the actual measured heat flow value and other inversion results, it shows that this inversion result has obtained a higher resolution. At the same time, the geothermal gradient is calculated by heat flow and thermal conductivity. According to the temperature-pressure equation for formation and storage of gas hydrate, the thickness of the gas hydrate stability zone in the study area was quantitatively calculated.

conditions (Lifeng W, et al., 2013). Therefore, temperature and pressure conditions are very important for the analysis of gas hydrate accumulation.
In this paper, based on the magnetic data inversion of the heat flow distribution in the Ross Sea, the accumulation conditions of this area are analyzed with(by using) the temperature, pressure and thermal factor(data). The thickness of the gas hydrate stability zone is calculated, and the resource prospect of gas hydrate in this area is preliminarily estimated by volume integral method.    Figure 2, the abscissa represents the temperature, and the ordinate represents the water depth and the corresponding pressure. The gray part of the picture can form natural gas hydrate. But for the marine environment, the part below the freezing point is not analyzed. It can be seen from Figure 2 that the lower the seabed temperature, the lower the pressure required to form hydrates; the deeper the water, the higher the temperature at which the corresponding hydrates can exist stably.Antarctica is the coldest continent on earth. Its average temperature in January in summer is minus a few degrees Celsius, and in July in winter it is minus 20 degrees Celsius (Barker P F et al.,1999), while the seabed temperature of Antarctica is lower than that in other regions. The measured seabed temperature of ODP178 voyage 1095 hole (Lee M W et al.,1993) and ODP188 voyage 1165 hole ( Cooper A K and PE O'Brien ,2004 ) are both lower than 0 ℃. The latitude of the Ross Sea is between 72°S and 85°S, which is higher than the latitude of other continental seas except the Weddell Sea. The Ross Sea shelf area is affected by the erosion of the continental ice sheet, and the shelf water depth is much deeper than that outside the Antarctic continent. This causes the seabed pressure of the Ross Sea shelf to be much higher than other marginal sea areas, corresponding to the seabed temperature much lower than other sea areas. , The Ross Sea area is easier to meet the stable occurrence conditions of hydrates.  In addition to the above data, through the accumulation of many Chinese Antarctic scientific expeditions, a considerable amount of geophysical data has also been accumulated in the Ross Sea area, which has been processed and calculated and combined with public data for use. Figure 3, the previous Ross Sea scientific expedition survey lines in Antarctica, each survey line has carried out water depth, gravity and magnetic measurement, most of which have carried out seismic survey and heat flow measurement.

Gravity and magnetic inversion curie, Moho depth
Heat flow is one of the most elusive geophysical observables, it is difficult to measure directly, and its measurement results are of poor regularity and have great variability (Davies, 2013). Li and Wang (2016) pointed out that the current understanding of the changes in the heat flow on the earth's surface is not sufficient to infer the deep thermal structure, and the Curie-point depth (CPD) derived from magnetic anomalies can be better constraint. Therefore, when there are difficulties in measuring the heat flow around the Antarctic surface, the amount of data is small, and it is difficult to truly reflect the internal thermal structure, the calculation of CPD becomes the best method for inversion of the thermal structure of the lithosphere.
CPD refers to the depth at which a magnetized material above a certain temperature becomes paramagnetic. In the crust, iron-titanium oxide (titanium magnetite) is the main source of magnetic anomalies (O'Reilly, 1976), and the Curie point temperature of magnetite is generally 580°C. This boundary provides a key basis for calculating the distribution of heat flow in the Ross Sea. The calculated CPD change also needs to be compared with the obtained change in crustal thickness (CT).

Magnetic anomaly inversion curie
The wavelet multi-scale decomposition method is used to separate the magnetic anomaly information of the upper and lower interfaces of the magnetic layer, and the logarithmic power spectrum analysis is performed. Using the power spectrum method to invert the depth of the inner burial, the depth of the bottom surface of the magnetic layer, as the inner depth of the place, represents the average depth of the place, the inversion result is shown in Figure 4.
In the formula, Tz Tcpd  is the temperature of the bottom magnetic layer.
When CPD is higher than Moho, it is calculated as a single-layer problem: But when CDP is lower than Moho, the calculation is a two-layer problem ( Figure 6): Zm is the depth of the Moho, kc and km are the thermal conductivity of the crust and mantle, respectively.

Estimation of gas hydrate reserves in the Ross Sea
Combined with the actual environmental conditions of the Ross Sea, and modified the pressure calculation equation and related assumptions of Milkov (2001) , combined with the boundary curve equation of the methane hydrate stability zone in seawater proposed by Miles (1995) for calculation. 。P is the pressure value, the unit is MPa, T is the temperature, the unit is °C. This assumption of static pressure is valid at shallow seabed depths, so when calculating the relationship between pressure and water depth, the hydrostatic pressure is used to approximate the actual seabed pressure: 12 P gh gz  