10:45 AM - 12:15 PM
[STT40-P02] Geological Survey using Drone Airborne Electromagnetic Survey with Ground Loop Source Method
Keywords:Drone airborne electromagnetic survey, Drone airborne electromagnetic survey with Ground Loop Source Method, Airborne Elector Magnetics
The drone airborne electromagnetic survey with ground loop method was carried out for a tunnel construction project in Singapore to obtain data such as groundwater distribution and fault fracture zones up to 100m deep for evaluating the subsurface risks. The topography of the survey site is hilly, and the geology is granitic with very weathered sedimentary rock in top layers.
The loop source method for wave transmissions was selected for this survey. Although the bi-pole source method is usually used for the drone airborne electromagnetic survey, which requires electrode rods with straight transmission cables near the survey site, the ground loop method, which requires installing transmission cable around the survey site, was adopted for this project. The merits of the ground loop method are that heavy electrode rods for carrying currents into the ground from cables are not required and the transmitter can be much smaller than the bi-pole source method. However, the possible survey depth for the ground loop method is approximately 100m, which is shallower than 200m of survey depth measured by the bi-pole source method.
The vertical value of magnetic field was measured with a reception coil suspended from a drone. The transmitter and receiver were synchronized by GPS's PPS (Pulse Per Second) and a high precision clock, and the receiver recorded the transient responses of electromagnetic waves at intervals of 1 μsec. A waveform of 16.0 ms per cycle was used for current waveform and reception wave, and the amount of data in one cycle was 16,000.
The analysis used the transient response when the induced magnetic field was cut off. The reception waves of 16 ms per cycle for 2 seconds were stacked to make one file per batch. The resistivities of each layer were analyzed by the nonlinear least-squares method with assumed layered structure models. The analyzed resistivity data was represented by cross-section, plan and 3D model views, and the geological structure was examined from the resistivity distribution up to 100m deep from the ground.
The bi-pole source method of the drone airborne electromagnetic survey generates unanalyzable areas near transmission sources. For example, at a point 50m from the transmission source, the data deeper than 50m, called as data dead zone, cannot be obtained. However, with the ground loop source method, the data dead zone is limited, and the data near the ground loop cables could be obtained.
The drone airborne electromagnetic survey can measure the underground resistivity structure either by the bi-pole or the ground loop source method. The bi-pole source method has a deeper detectable depth, but a large transmitter and heavy electrode rods are required. On the other hand, the ground loop source method requires installing loop source cables around the survey site and detectable depth is shallower, but it can survey even in the areas where only light vehicles can access due to its lightweight equipment.
The methods of the drone airborne electromagnetic survey can be selected depending on required specifications and various survey site conditions, and the drone airborne electromagnetic survey is expected to be used in more projects in the future.
The loop source method for wave transmissions was selected for this survey. Although the bi-pole source method is usually used for the drone airborne electromagnetic survey, which requires electrode rods with straight transmission cables near the survey site, the ground loop method, which requires installing transmission cable around the survey site, was adopted for this project. The merits of the ground loop method are that heavy electrode rods for carrying currents into the ground from cables are not required and the transmitter can be much smaller than the bi-pole source method. However, the possible survey depth for the ground loop method is approximately 100m, which is shallower than 200m of survey depth measured by the bi-pole source method.
The vertical value of magnetic field was measured with a reception coil suspended from a drone. The transmitter and receiver were synchronized by GPS's PPS (Pulse Per Second) and a high precision clock, and the receiver recorded the transient responses of electromagnetic waves at intervals of 1 μsec. A waveform of 16.0 ms per cycle was used for current waveform and reception wave, and the amount of data in one cycle was 16,000.
The analysis used the transient response when the induced magnetic field was cut off. The reception waves of 16 ms per cycle for 2 seconds were stacked to make one file per batch. The resistivities of each layer were analyzed by the nonlinear least-squares method with assumed layered structure models. The analyzed resistivity data was represented by cross-section, plan and 3D model views, and the geological structure was examined from the resistivity distribution up to 100m deep from the ground.
The bi-pole source method of the drone airborne electromagnetic survey generates unanalyzable areas near transmission sources. For example, at a point 50m from the transmission source, the data deeper than 50m, called as data dead zone, cannot be obtained. However, with the ground loop source method, the data dead zone is limited, and the data near the ground loop cables could be obtained.
The drone airborne electromagnetic survey can measure the underground resistivity structure either by the bi-pole or the ground loop source method. The bi-pole source method has a deeper detectable depth, but a large transmitter and heavy electrode rods are required. On the other hand, the ground loop source method requires installing loop source cables around the survey site and detectable depth is shallower, but it can survey even in the areas where only light vehicles can access due to its lightweight equipment.
The methods of the drone airborne electromagnetic survey can be selected depending on required specifications and various survey site conditions, and the drone airborne electromagnetic survey is expected to be used in more projects in the future.