In the field of exploration geophysics, as in many other fields, there has been a growing interest in unmanned (autonomous) or manned remotely piloted aircraft, ground vehicles, and underwater vehicles (so-called drones). This manuscript, we focus on small unmanned aerial vehicles (UAVs) and introduce the history and classification of UAVs and their use in the research and development of geophysical exploration methods, including on seismic, radioactive, ground-penetrating radar (GPR) surveys and especially magnetic and electromagnetic methods. In the fields of magnetic and electromagnetic surveys, research and development of "new" exploration methods such as "UAV (drone) magnetic" and "UAV electromagnetic (EM)" methods are rapidly progressing, exploiting the mobility and portability of small UAVs. The magnetic field sensors (magnetometers) used in magnetic exploration, which is a passive exploration method, are classified into two types: vector type and scalar type—and examples of some applications of UAV magnetic exploration are introduced. Since there are both active and passive measurements in UAV EM exploration, the research, development, and applications of UAV EM methods are explained by dividing them into four categories according to the types of transmitter and receiver. Through these reviews, we can find that in geophysical surveys using small UAVs, the weight limit of the onboard measurement device affects the accuracy of the observed data and the depth of investigation. In addition, battery capacity is a constraint on flight time, distance, and exploration efficiency. When using the multicopter type UAVs, we need to consider the electromagnetic noise generated by the rotor drive and the oscillating noise due to flights with loading and suspending the survey instruments. Skilled pilots and ground engineers are also essential for safe and efficient exploration. UAVs can fly more flexibly than manned flights, thereby increasing the possibility of high horizontal resolution and data acquisition along the terrain. UAVs are also extremely advantageous in terms of the safety of workers against hazards such as natural disasters and exposure to radioactive and toxic substances. The possibility of exploration in steep terrains, wetlands, and environmental conservation areas, where manned exploration is difficult, will be investigated. For this purpose, it is important to make the geophysical survey technology not only smaller and more accurate but also more power-efficient. With the acquisition of large amounts of data, it is necessary to develop software for rapid processing, visualization, and inverse analysis of data including depth information. In this context, UAV geophysical survey technology is expected to become the new standard geophysical exploration method in the future.

　The use of unmanned autonomous vehicles (UAVs) for volcanic observations has received attention since the beginning of this century, as they enable various kinds of measurements to be carried out without a risk of human casualties. The UAVs can fly along pre-programmed routes with high positional accuracy as much as tens cm, and thus a setup of ideal measurement lines can be realized, for example, to keep constant altitudes above ground and spatial intervals of measurements, and to make repeated observations along the same measurement lines at certain time intervals to detect the temporal change of the observables. In this study, an autonomous driven unmanned helicopter was used to conduct aeromagnetic surveys in volcanoes to keep a constant spatial resolution of geomagnetic field measurements over a complex terrain, and to detect changes in the geomagnetic field caused by volcanic activities. At Mt. Mihara, Izu-Oshima island, high magnetization zones were identified around the central cone, which is considered to have been solidified underground and not erupted in the past. At Mt. Shinmoe-dake, Kirishima, we could clearly detect the magnetization of the lava in the crater over time due to cooling through repeated aeromagnetic surveys since the 2011 eruption. The development and use of drones have been remarkable in the last few years, and volcanic observations by UAVs are expected to mature in the future.

　Industrial drone market is growing rapidly, and domestic and foreign drone manufacturers are in the middle of the development race. The evolution of flight performance and auxiliary functions is advancing its technical level to a level of practical use. Successive research development and demonstration experiments are crucial for solving technical problems dependent on aircraft specifications. This paper describes the development of technology and aircraft mounting in demonstration experiments in the field of geophysical exploration. An electromagnetic exploration sensor suspended from a drone is used to search for vehicles buried in the ground by automatic flight. The sensor is suspended, and the aircraft is automatically flown over the exploration route. Those experiments found that the stable positioning and precise measurements require the following technical points: 1) the done should have enough payload to allow the additional load due to the oscillation, and 2) a range finder should be properly set up on the done. In order to achieve a goal to instrument flight of drones without an ancillary over occupied areas by 2022, the technical element issues of drones will progress from hardware system development to software system development to flight control technology development. In particular, autonomous flight control is an important elemental technology, because it will enable the self-learning of the present status, the self-making of the flight route, and the optimal decision for the flight. The autonomous flight control provides the precise information of the drone position, navigation speed, and traveling direction, by applying the Extended Kalman Filter (EKF) to the sensor data obtained from GPS, gyroscopic instrument, accelerometer, compass, barometer, and ultrasonic sensor. The technical evolution of drones for industrial field use will continue with a combination of the above elemental technologies, environmental arrangement and consolidation of the legal system for the drone flight, and data management system.

　An AUV is an unmanned underwater robot that can dive under autonomous control with pre-programmed information, and remarkably has developed in recent years. It is benefit that an AUV without the towed cable is not affected by the motions of the mother vessel unlike ROVs and does not need to move with the mother vessel. Its freedom of vehicle movement in water allows it to obtain data over a wide area near the sea bottom. A cruising AUV typically equipped some acoustic instruments such as a multi-beam echo sounder to obtain the detailed bathymetric data. In recent, she has been tried to use for the geophysical exploration survey. For example, the case studies about some anomaly detection associated with the hydrothermal deposit have been reported using the magnetometer and electrometer. As a new geophysical survey using multiple AUVs, the direct current survey and self-potential survey has been conducted using two AUVs for electrical current transmission and receiver, and have successfully detected negative SP anomaly areas and low resistivity zones around the known hydrothermal deposit areas. In addition, an ASV, which is also autonomous robot on the water surface, has been developed. It is expected that AUVs and ASVs will allow us unmanned and labor-saving geophysical exploration in the sea. In this paper, we will overview these underwater robots and introduce their application both in Japan and abroad.

　Airborne electromagnetic (AEM) survey methods can be divided into two types: 1) AEM survey methods, in which a transmitter is installed on the ground and measurements are made in the air, and 2) methods in which electromagnetic waves are transmitted from the air and received in the air. The drone exploration method has been developed and put into practical use. Ground-source type airborne electromagnetic survey method (D-GREATEM) installs a transmitter on the ground during the exploration. D-TEM is also in practical use, and has both transmitter and receiver in air and measures by flying two drones simultaneously. One drone towing transmitter and other a receiver. D-GREATEM and D-TEM were conducted to understand the geological structure and hydraulic properties in the Seino area of the Nobi Plain in order to develop a groundwater utilization system for use during the event of major earthquake or other disaster.

　D-GREATEM survey was carried out in two flight lines continuous from the mountainous area to the alluvial fan and D-TEM survey on one flight line in the alluvial area during the high and the low rainfall seasons. The same positions of flight lines were reproduced by using programmed flight. The geological structure, faults, and groundwater were estimated by calculating the difference in rate of change from the results of the two surveys and comparing them with previous data. Although each survey method has its own challenges and limitations, it was confirmed that the geological model, faults and groundwater level changes can be understood from the exploration results.

　The drone AEM method has a larger amount of data than other survey methods that can easily obtain detailed three-dimensional information of the subsurface. Since the same measurement conditions can be reproduced by the automatic flight of the drone, four-dimensional exploration with time variation is possible. It is expected that the drone airborne electromagnetic survey technology will contribute to geological surveys in various fields in the future.

　In recent years, several types of drones such as unmanned aerial vehicles (UAV) and unmanned ground vehicles (UGV) have been developed, researched, and applied in various fields. This technology may be especially useful for the investigation of shallow subsurface problems at depths of meters to dozens of meters below the surface, which are of concern in fields such as infrastructure maintenance, environmental geology, and civil engineering. In particular, the demand for shallow subsurface exploration over a wide area and along a lengthy survey line without the need to dig up the ground is rapidly increasing. In response to these needs, we have begun to develop a new UGV electromagnetic (EM) exploration system (UGVEM system), which consists of a single-frequency multi-coil electromagnetic probe and UGV. In this study, we first conducted basic experiments at a test site to verify whether it would be feasible to use the new system designed from both a UGV and EM instruments. Then, to further evaluate the feasibility and effectiveness of our system, field experiments were conducted along part of the Miyato River-bank in Kuno, Koga City, Ibaraki Prefecture, Japan. According to the data acquired from field experiments with our new system and apparent resistivity cross-sections generated by the observed data, we were able to conclude that the developed system can be applied for efficient field explorations of shallow subsurface geo-electrical structures.

　Concentrated metals such as seafloor hydrothermal deposits and rare earth rich mud are expected to be used as marine mineral resources. Understanding subseafloor structures by the seismic reflection survey is necessary to understand the spatial distribution of marine mineral resources. The imaging accuracy of the subseafloor structure depends on the source frequency and data density. Therefore, densely acquiring data near the seafloor, it is possible to acquire high-resolution data that matches the scale of the target.

　In this study, we investigated a survey method for establishing a seismic reflection survey system using an autonomous underwater vehicle (AUV) that enables efficient data acquisition in the deep sea. As a result of the survey experiment of the portable autonomous hydrophone cable developed for mounting on the AUV, it was shown that it is possible to obtain a high resolution cross section by correcting the cable attitude under water. In addition, as a result of the AUV mounting test of the portable autonomous hydrophone cable, we observed the system noise caused by the AUV thruster and the hydrophone cable required for the selection of the seismic source. In the future, it is expected that the seismic reflection survey system using the AUV will be developed by the offshore survey experiment.

　In Japan, magnetic surveys using unmanned aerial vehicles (UAV) began with a balloon magnetic survey for crustal characterization of the Tohoku arc in the early 1970’s. As it is difficult to control the horizontal position of a balloon, blimp-borne magnetic surveys were then developed with a successful test flight in the early 1990’s. In the early 2000’s, magnetic survey systems using model planes (e.g., Ant-Plane) were developed by the Polar Research Institute of Japan to conduct magnetic surveys over relatively wide areas in Antarctica. The systems were constructed using inexpensive model planes and instruments such as MR and fluxgate magnetometers because of a high risk of losing planes in critical conditions of the Polar region. Also in the middle 2000’s, a new magnetic survey system using an unmanned autonomous helicopter was developed by the Earthquake Research Institute, Univ. of Tokyo, to conduct magnetic surveys over rugged volcanoes more economically than with manned aircraft. This system is still used frequently for magnetic surveys to map the subsurface structure of active volcanoes for volcanic hazard mitigation.

　In 2010’s, the emergence of multi-copter systems has dramatically changed drone magnetic surveying. Recently new, lighter, optical pumped magnetometers have been developed for use in drone surveys and have been employed especially for surveys over active volcanoes with great success. Repeated drone magnetic surveys are a promising way to detect temporal magnetic changes and estimate volcanic activity of active volcanoes.

　According to a review about ongoing overseas activities, we realize that drone magnetic surveys are being applied to various purposes such as searching for UXO and abandoned oil and gas wells, mineral exploration, archeological surveys, and environmental surveys over landfills.

　The Geological Survey of Japan, AIST, has recently opened the aeromagnetic processing and interpretation software, AMSS3, to the public. This software was originally developed for the aeromagnetic data observed by manned helicopters but can be used for data collected by drone. This software includes the reduction of data to an arbitrary surface, which is crucial for drone magnetic data observed at different altitudes.

　Although there are still problems to be solved, peripheral devices and control techniques for drone magnetic surveys are developing rapidly. The future of drone magnetic surveys as a useful technique for near surface applications is bright.

　The Hokusetsu area located in the northern part of the Osaka Basin, Japan, has the complicated geological structure including graben, called the Imogawa lowland, between the Hokusetsu Mountains and the Senri hill. We estimated the S-wave velocity structures in both the graben site and the hill site by carrying out microtremors array surveys with the maximum radius of around 100 m (S-array) and 500 m (L-array), respectively. The observed phase velocity up to 2000 m/s was obtained by these microtremor array surveys. On the other hand, the estimated depth of basement rock (Vs > around 3000 m/s) is not accurate enough in the microtremor array surveys. To estimate the deep structures, therefore, we reanalyzed data of two different seismic reflection surveys conducted at the Hokusetsu area and integrated the results of the microtremors array surveys and the seismic reflection surveys. To explain well both the observed phase velocity dispersion curves from the microtremor array survey and the depth of basement rock from the reflection seismic survey, we combined the structures shallower than about 500 m estimated by the microtremor array survey and the ones deeper than about 500 m estimated by the reflection seismic survey. As the result, the depth of basement rock (Vs > around 3000 m/s) of the integrated structural models is about 1000 m at the Imogawa lowland and about 600 m at the Senri hill, respectively, indicating the sharply inclined bedrock spread in the Hokusetsu area. Around the temporary seismic observation stations set up by the Railway Technical Research Institute (RTRI) at the Hokusetsu area, microtremor array surveys with the maximum radius of around 30 m (XS-array) were also carried out to estimate a shallow S-wave velocity structures. These surveys revealed that the sedimental layer (Vs < 500 m/s) has a thickness of about 200 m at the Imogawa lowland and about 100 m at the Senri hill, respectively.

　Currently, water on the Moon surface is considered a potential resource. Furthermore, we need to know geological information to build base camps on the Moon. Therefore, estimating the S-wave velocity of the underground of the Moon has been crucial. In the Moon exploration, it is difficult to bring heavy and large-sized equipment because of the spacecraft’s limited capacity. Therefore, we investigated the applicability of a small extraterrestrial exploration system based on the microtremor survey without a seismic source. This study evaluates the investigation depth for each arrangement of small seismometer arrays on the Moon when analyzing microtremor data by the Centerless Circular Array (CCA) method. The result of the CCA method depends on a given NS ratio (the power spectral ratio of the incoherent noise to the coherent signal), the number of seismometers, and an array radius. Therefore, we estimated the NS ratio on the Moon using seismometer data of Apollo 14, 15, 16, and 17. Using the estimated NS ratio, the number of seismometers and the array radius, we calculated the minimum detectable wavenumber k_min at each frequency by the approach of Cho et al. (2006). The minimum wavenumber k' – its lunar data-based wavenumber k_obs exceeds the minimum detectable wavenumber k_min – estimated the maximum detectable wavelength. We then calculated one-third of the maximum wavelength to estimate the investigation depth for each array arrangement. First, we analyzed the earth data using the CCA method and confirmed that the investigation depth could be evaluated from the three parameters and the observed dispersion curve. Second, we analyzed Apollo data and evaluated the investigation depth on the Moon. Finally, we summarized applicability of the CCA method for the Moon explorations, considering appropriate array arrangement and noise level to analyze the desired depth. For example, our evaluation demonstrates that we can investigate from a surface to 3 m depth by the pentagon-type array with 0.3 m radius or the triangle-type array with 0.5 m radius. Thus, our evaluation approach helps design seismic arrays in lunar exploration projects in the future.

　We proposed a method for estimating porosity using the Bouguer anomaly by replacing the density structure with a porosity–density contrast combination between the pore fluid and host rock. We applied this method to Bouguer anomalies obtained in Jigoku-dani, which is located in Midagahara volcano, and it was predicted that low-density areas filled primarily with volcanic gas and/or water would have a porosity contrast 0.06 – 0.35 higher than the surrounding host rock. Comparisons with effective porosities based on existing well data in geothermal areas and analyses of the relationship between porosity and density revealed that the estimated porosities in this study would be distributed across a range of realistic values.

　We had conducted a temporary strong ground motion observation around the Fujikawa-kako fault zone in the eastern part of Shizuoka prefecture, Japan, for understanding of earthquake ground motion features in the area. In this paper we explain the outline of the strong motion observation and characteristics of observed ground motions. We calculated ratios of horizontal spectra of ground motion records at the stations to that of a rock reference site, and found that the large ratios are concentrated in the river mouth area of the Fujikawa river. The ratios at frequencies less than 0.8 Hz are particularly large in the coastal part of the mouth area. The spatial variation of the ratios at frequencies from 0.8 to 3 Hz is significant suggesting complex site amplification effects. On the other hand, the ratios are small at the stations in the southern part of Yamanashi prefecture. Small ratios are also found at the stations with firm soil conditions in the Izu peninsula and the western coastal part of the Suruga bay. We conducted microtremor array explorations at the stations to know shallow S-wave profiles. Average S-wave velocities in the top 30 meters were calculated from the profiles. We found that the spectral ratios in a frequency range from 0.8 to 3 Hz are well correlated with the average velocities.

　We have developed an electrical exploration system with a vertical electrode array to easily detect the shallow resistivity structure on rough topographic areas, such as submarine hydrothermal deposit areas. The new system with a vertical electrode array enables self-potential surveys by measuring the vertical electric field. The vertical electric field can better detect areas with self-potential anomalies because both the vertical electric field and the self-potential have a peak above a current source. We carried out an experimental observation using the new system on a known submarine hydrothermal deposit field in the Izena Hole, the mid-Okinawa Trough. Several distinct anomalies were observed in the vertical electric field at the same locations where self-potential anomalies were detected by previous studies. Estimated six vertical electric fields data using seven electrodes located at different distances from the seafloor show a clear tendency. Both the amplitude and the contribution of small spatial-scale components decrease with distance. Direct measurement of the vertical electric field can easily detect anomalies associated with ore bodies without complicated data processing or numerical calculations.

　Yuda Hot Spring in Yamaguchi City, Yamaguchi Prefecture, is an alkaline simple hot spring that flows up into the plains. This hot spring yields an abundant amount of water (2,000 tons/day), but its flow path has not yet been clarified. In this study, we analyzed the water chemistry of Yuda Hot Spring and performed resistivity measurements of geological samples around the spring. In particular, the resistivity structure was examined by electromagnetic survey, core observations were made by spring source boring, and a numerical simulation was carried out to elucidate the flow path of the hot spring water. The electromagnetic survey revealed a difference in resistivity between the bedrock and the overlying sedimentary layer, and the upper surface of the bedrock in the region was found to be depressed, sandwiched between multiple faults crossing the Yuda Hot Spring area. However, a resistivity structure could not be obtained that indicated the flow path of the hot spring. The measured electrical conductivity (EC) of the hot spring water was about 100 mS/m, an order of magnitude higher than that of groundwater in surrounding rock. The resistivity of a rock sample with groundwater present as pore water was half that of a rock sample with hot spring water present as the pore water. The obtained core sample was primarily composed of hard sandy schist and pelitic schist, and the frequency of fractures was 1.25 fractures/m. On the other hand, cracks were present in a part where a quartz porphyry with a width of about 0.6 m intruded at high angles, so it is possible that the cracks in intrusive rock such as quartz porphyry serve as the flow path of the hot spring water. Based on these results, we constructed a geological model of Yuda Hot Spring and conducted a numerical simulation. The results showed that resistivity would be difficult to detect when the intrusive rock where cracks are predominant, which we regarded as the flow path, has a width of 100 m or less. These results suggest that the flow path of Yuda Hot Spring is less than 100 m wide.

　Various research and development of technology that realizes optimization and stabilization of steam production are being conducted for geothermal fields in Japan. One of the means is water recharge by injection, which is a kind of EGS (Enhanced / Engineered Geothermal System). This technology attempts to recover the amount of steam by artificial injection of river water into geothermal reservoirs using a well specially prepared for the injection in addition to the production and reinjection wells that are used for regular plant operations. Recharge injection tests were conducted in Okuaizu Geothermal Field, (Yanaizu-town, Fukushima, Japan) from 2015, as part of Geothermal Reservoir Evaluation and Management Technology Project conducted by of Japan Oil, Gas and Metals National Corporation (JOGMEC). In this project, National Institute of Advanced Industrial Science and Technology (AIST) conducts the observation of microseismic events. For the purpose of accurately illuminating the effect of water injection in real time. In this study, we conducted a semi-real time analysis of the number of micro seismic events and hypocenter distribution regarding the water injection tests based on the data observed at seismographs installed in boreholes as well as on the surface. It leaded to monitoring the behavior of the injected water. Recharge water injection test s were conducted once in 2018 and twice over from 2019 to 2020. In the water injection test in 2018, the number of micro seismic events during the water injection period did not increase compared to the pre-injection term. Particularly, the number near the well did not increase as well. In the former water injection test over 2019 to 2020, an increase in the number of microseismic events was observed during the water injection period compared to the pre-injection term. At that time, microseismic events evolved toward deeper depths than those during the water injection test in 2018. In the letter water injection test over 2019 to 2020, although the number of microseismic events did not increase compared to the pre-injection term unlike with the previous water injection test, the evolution of microseismic events toward deeper depths was confirmed again. From those microseismic monitoring results, we concluded that the relationship between water injection and micro seismic events was not clear in the 2018 water injection test. On the other hand, the injected water well spread over deeper depths in the water injection tests over 2019 to 2020.