17:15 〜 19:15
[SCG48-P02] On the feasibility of the Horizontal Spatial Gradient method for estimating the subsurface resistivity structure of Mars
キーワード：火星、比抵抗、水、磁場
The STEP1 concept of the Japanese Strategic Mars Exploration Program (JSMEP) plans to deploy three landers equipped with scientific instruments, including fluxgate magnetometers, on the Martian surface. Investigating the subsurface resistivity structure using magnetic field data from these landers is of significant scientific interest, as it can provide insights into the potential presence of water layer within the crust and constraints on the water content of the mantle.
The magnetotelluric (MT) method, which is commonly used for subsurface resistivity exploration, is not readily applicable to Martian exploration. This is because the MT method requires both the horizontal components of the electric and magnetic fields, but the landers of the STEP1 concept cannot deploy electrodes necessary for electric field measurements.
As an alternative, the Horizontal Spatial Gradient (HSG) method has gained attention. This method estimates resistivity based on the ratio of the vertical component of the magnetic field to its horizontal gradient. Jones (1980, 1983) applied the HSG method to an array of magnetic observation points in the Baltic Shield, determining the horizontal gradient by fitting a two-dimensional quadratic surface using least squares. Similarly, Schmucker (2003) used polynomial fitting on magnetic field data from 14 observation points in Europe to estimate resistivity. While, in theory, the horizontal gradient can be determined from a minimum of three simultaneous observations, additional observation points are typically required for higher accuracy. As a result, the HSG method, as applied on Earth, cannot be directly used in the STEP1 concept, which involves only three landers.
To address this issue, we focus on the method proposed by Pinçon et al. (2000), which estimates the horizontal gradient using data from only three observation points through a technique called the filter-bank approach (Pinçon and Motschmann, 1998). This method was originally developed in preparation for the NetLander mission on Mars, which was never realized. By assuming that the magnetic field propagation is dominated by a horizontally propagating plane wave with a single wavenumber vector, the wavenumber estimation directly provides the horizontal gradient. However, estimating the wavenumber vector from observational data requires spectral analysis in the wavenumber domain, and the limited number of observation points makes this process challenging. The filter-bank approach enables spectral analysis in the wavenumber domain even with only three observation points. When applied to synthetic data, this method successfully identified the wavenumber vector used to generate the data. As far as we know, however, this method has never been applied to real observational magnetic data.
In this study, we apply Pinçon’s method to three-point magnetometer arrays using INTERMAGNET observatory data in Europe. We compare the results with those obtained using the HSG method in Schmucker (2003), which used the same European array data. Through this comparison, we assess the feasibility of applying the HSG method with only three observation points.
The magnetotelluric (MT) method, which is commonly used for subsurface resistivity exploration, is not readily applicable to Martian exploration. This is because the MT method requires both the horizontal components of the electric and magnetic fields, but the landers of the STEP1 concept cannot deploy electrodes necessary for electric field measurements.
As an alternative, the Horizontal Spatial Gradient (HSG) method has gained attention. This method estimates resistivity based on the ratio of the vertical component of the magnetic field to its horizontal gradient. Jones (1980, 1983) applied the HSG method to an array of magnetic observation points in the Baltic Shield, determining the horizontal gradient by fitting a two-dimensional quadratic surface using least squares. Similarly, Schmucker (2003) used polynomial fitting on magnetic field data from 14 observation points in Europe to estimate resistivity. While, in theory, the horizontal gradient can be determined from a minimum of three simultaneous observations, additional observation points are typically required for higher accuracy. As a result, the HSG method, as applied on Earth, cannot be directly used in the STEP1 concept, which involves only three landers.
To address this issue, we focus on the method proposed by Pinçon et al. (2000), which estimates the horizontal gradient using data from only three observation points through a technique called the filter-bank approach (Pinçon and Motschmann, 1998). This method was originally developed in preparation for the NetLander mission on Mars, which was never realized. By assuming that the magnetic field propagation is dominated by a horizontally propagating plane wave with a single wavenumber vector, the wavenumber estimation directly provides the horizontal gradient. However, estimating the wavenumber vector from observational data requires spectral analysis in the wavenumber domain, and the limited number of observation points makes this process challenging. The filter-bank approach enables spectral analysis in the wavenumber domain even with only three observation points. When applied to synthetic data, this method successfully identified the wavenumber vector used to generate the data. As far as we know, however, this method has never been applied to real observational magnetic data.
In this study, we apply Pinçon’s method to three-point magnetometer arrays using INTERMAGNET observatory data in Europe. We compare the results with those obtained using the HSG method in Schmucker (2003), which used the same European array data. Through this comparison, we assess the feasibility of applying the HSG method with only three observation points.