Structural exploration using longwave radio-clock time-signal

VLF methods have been used for one dimensional survey that ties records of the single point measurement with subsurface structure. Since VLF electromagnetic wave is not stable due to various effects in the propagation, subsurface structural exploration using VLF methods has limitations in resolution and in the applicability depending on the place of surveys. To overcome some of the limitations, we propose to use standard-time longwaveelectromagnetic transmissions (JJY in Japan), that could be more stable than VLF, for the exploration of underground structure. Once radio time-signal receivers have become popular, we may distribute many receivers in a wide area to record continuous time signal simultaneously for estimating subsurface resistivity distribution. Continuous measurements, moreover, might improve measurement efficiency and S/N ratio. In our study, we applied numerical experiments to confirm the method to work. First, we created a test data set composed of air and heterogeneous half space earth for which JJY signal propagates. Then, we estimate the distortion of time signal on the surface of the half space to evaluate the characteristics of underground response to JJY and to see the availability of JJY standard electromagnetic wave for structural exploration as well as for a VLF method. We used electromagnetic wave of 20 kHz as a VLF wave and 40 and 60 kHz as JJY standard electromagnetic waves and evaluated the resolution of the methods derived from the skin depth and the influence of the geometry for various combination of the orientation of anomalous structure, the propagation direction of radio wave, and the orientation of two- dimentionally aligned receivers. To estimate the influence of the geometry between the orientations of structural anomaly and the propagation direction, we evaluated the characteristic response of the survey as a function of difference angle of the orientations. Our results show the following confirmation: (i) there are little influence on the attenuation of the electromagnetic radiation if observation point is located above the resistivity anomaly, (ii) - higher the frequency becomes, shallower layer the influences come from, and (iii) the smaller difference angle becomes, better the sensitivity of survey becomes. Therefore, we conclude that the structural anomaly runs in the direction of radio wave propagation, the most ideal survey would be conducted as known well for electromagnetic surveys. Our study suggests that JJY signal or any other continuous time signal could be used for the estimation of subsurface resistivity distribution. In the future, we try to extend the method to VLF-MT for subsurface structure and to apply it for field data.