11:00 AM - 11:15 AM
[SEM12-07] Challenges of UAV-mounted Electromagnetic Survey to monitor resistivity structure of potential source of hydrothermal eruptions
Keywords:UAV, steaming area, hydrothermal eruption, hydrothermal system, TDEM, CSAMT
In the Owakudani steaming area of Hakone volcano, CSAMT surveys have detected resistivity structure changes associated with the 2015 hydrothermal eruption (Mannen et al., 2019). More frequent survey is thus expected to detect precursor changes before hydrothermal eruptions. To this end, we have been testing UAV-mounted electromagnetic surveys (hereafter UAV survey) in Owakudani since 2021 as a five-year project. Here is its interim report.
Our UAV survey uses Time Domain Electromagnetic (TDEM) method which is composed of a transmission source installed on the ground and a receiver suspended from a UAV. The transmitter can be a loop or a bipole, which is composed of electrodes installed at two distant locations. At the transmitter source, the electric current is applied repeatedly with opposite polarity rectangular pulses with 50 % duty cycle.
The current at the source creates a magnetic field that penetrates into the ground. This is called the primary magnetic field. When the current at the transmitter stops, the primary magnetic field also disappears and causes electromagnetic induction underground. The electromagnetic induction due to the temporal change of the primary magnetic field generates an induced current in the subsurface, which in turn generates a so-called secondary magnetic field. The TDEM method uses an induction coil suspended from a UAV to capture the time variation (transient response curve) of the secondary magnetic field.
In 2021, a bipole source was employed, which was expected to achieve deeper exploration depth than loop sources. A UAV electromagnetic survey and a CSAMT survey were conducted on almost the same survey line using the same transmission source in order to compare the two. In this campaign, the UAV electromagnetic survey unexpectedly failed to extract transient response curves in a significant portion of the survey line, while the CSAMT was successfully obtained resistivity structure up to 400 m deep. The reasons for the failure were considered to be (1) small signal strength and (2) electromagnetic noise from unknown source synchronized with the transient response curve.
In 2022, the signal strength was improved by reducing earthing resistance of the bipole source by increasing the number of electrodes to reduce grounding resistance (from 130-150Ω in 2021 to 126Ω) and increased the current from 5.9 A in 2021 to 6.3 A. In addition, the duration of transmission cycle was changed from 16.0 ms in the first year to 8.0 ms to avoid synchronous noise. However, measurements of secondary magnetic field at the UAV and on the ground surface revealed that, there existed abnormally strong secondary magnetic field or stray current that made the measurement of the secondary magnetic field significantly difficult. In particular, anomalous transient response curves were obtained on and near the road in the survey area, and artificial underground conductors was suspected to create such complex electromagnetic fields.
In 2023, we replaced the bipole source by a loop-type transmission source, to create more homogeneous primary magnetic field. While we obtained significantly improved response curve than that obtained before, the survey depth was too shallow (50 to 100 m) to detect the target structure of this project (~200m). We therefore applied one-dimensional inverse analysis on a numerically integrated transient response curves, which weight the low-frequency component. This technique greatly improved the survey depth up to 300 m deep. However, the obtained resistivity structures show significant differences from the two-dimensional structure revealed by the CSAMT survey and further discussion and analysis are required.
Our efforts revealed that the most serious obstacle to apply UAV electromagnetic survey were artificial noise and conductors in the survey area. This indicates that sufficient time for preliminary survey is essential before implementing UAV electromagnetic monitoring in touristy steaming areas.
Our UAV survey uses Time Domain Electromagnetic (TDEM) method which is composed of a transmission source installed on the ground and a receiver suspended from a UAV. The transmitter can be a loop or a bipole, which is composed of electrodes installed at two distant locations. At the transmitter source, the electric current is applied repeatedly with opposite polarity rectangular pulses with 50 % duty cycle.
The current at the source creates a magnetic field that penetrates into the ground. This is called the primary magnetic field. When the current at the transmitter stops, the primary magnetic field also disappears and causes electromagnetic induction underground. The electromagnetic induction due to the temporal change of the primary magnetic field generates an induced current in the subsurface, which in turn generates a so-called secondary magnetic field. The TDEM method uses an induction coil suspended from a UAV to capture the time variation (transient response curve) of the secondary magnetic field.
In 2021, a bipole source was employed, which was expected to achieve deeper exploration depth than loop sources. A UAV electromagnetic survey and a CSAMT survey were conducted on almost the same survey line using the same transmission source in order to compare the two. In this campaign, the UAV electromagnetic survey unexpectedly failed to extract transient response curves in a significant portion of the survey line, while the CSAMT was successfully obtained resistivity structure up to 400 m deep. The reasons for the failure were considered to be (1) small signal strength and (2) electromagnetic noise from unknown source synchronized with the transient response curve.
In 2022, the signal strength was improved by reducing earthing resistance of the bipole source by increasing the number of electrodes to reduce grounding resistance (from 130-150Ω in 2021 to 126Ω) and increased the current from 5.9 A in 2021 to 6.3 A. In addition, the duration of transmission cycle was changed from 16.0 ms in the first year to 8.0 ms to avoid synchronous noise. However, measurements of secondary magnetic field at the UAV and on the ground surface revealed that, there existed abnormally strong secondary magnetic field or stray current that made the measurement of the secondary magnetic field significantly difficult. In particular, anomalous transient response curves were obtained on and near the road in the survey area, and artificial underground conductors was suspected to create such complex electromagnetic fields.
In 2023, we replaced the bipole source by a loop-type transmission source, to create more homogeneous primary magnetic field. While we obtained significantly improved response curve than that obtained before, the survey depth was too shallow (50 to 100 m) to detect the target structure of this project (~200m). We therefore applied one-dimensional inverse analysis on a numerically integrated transient response curves, which weight the low-frequency component. This technique greatly improved the survey depth up to 300 m deep. However, the obtained resistivity structures show significant differences from the two-dimensional structure revealed by the CSAMT survey and further discussion and analysis are required.
Our efforts revealed that the most serious obstacle to apply UAV electromagnetic survey were artificial noise and conductors in the survey area. This indicates that sufficient time for preliminary survey is essential before implementing UAV electromagnetic monitoring in touristy steaming areas.