We developed a drone-borne electromagnetic survey system using a commercial multi-frequency electromagnetic sensor equipped with a GPS receiver, a WiFi serial transceiver, and an ultrasonic distance sensor to measure the height of the electromagnetic sensor above the ground surface. The electromagnetic sensor was suspended from a drone with ropes. The distance between the drone and the electromagnetic sensor was adjusted to minimize the influence of electromagnetic noise generated by the drone, and to stabilize the electromagnetic sensor during flight. The system was tested at two experimental sites. The first site consisted of two buried vehicles to simulate a landslide. We assumed a scenario in which the search for the buried vehicles was urgent and accessibility to the area was limited. The second site consisted of wet and dry agricultural fields to test resistivity mapping. In the first test, we used the in-phase component of the measured data to locate the vehicles. The shallower vehicle was identified clearly, while the deeper vehicle was located successfully, albeit less easily. In the second test, the quadrature component was used for one-dimensional inversion after data processing, which included data smoothing, resampling and bias noise correction. The bias noise was measured while hovering the drone at a high altitude to negate the influence of ground conductivity. The results showed that the resistivity distributions could be mapped at some depths by using a five-frequency-processed quadrature component, and clearly showed the difference between the wet and dry fields. The crucial parameter in the evaluations of these targets was the height of the electromagnetic sensor above the ground surface, which was measured continuously during flight. The results demonstrated the potential of the survey system to search for buried metal objects and for shallow subsurface resistivity mapping over relatively large areas.

調査地域に分布する岩石の磁気的性質を含む物性情報は，磁気探査の結果を解釈する上で非常に有効である。しかし，これまで，磁気探査と並行して岩石磁気データの詳細が調べられる事は少なかった。一方，北海道東部の武佐岳地熱地域では，JOGMECにより地熱資源ポテンシャル調査を目的に高分解能空中磁気探査が実施され，顕著な低磁気異常域が観測されている。そこで，本研究では当該地域で採取された露頭岩石について，その密度，磁化率および自然残留磁化（NRM）を実験室内で測定することで，磁気探査データの解釈への活用可能性について検討を行った。

イケショマナイ川右岸（A地点），シュラ川流域（B地点）とクテクンベツ川右岸（C地点）の3箇所から採取した露頭岩石（ブロック試料）を円筒試料に整形して測定に用いた。B地点の全部とC地点の一部の試料は，高NRM強度（≥1.0 A/m）かつ高Qn比（≥1.0）で，また負の伏角を示す。一方，A地点の全試料とC地点の一部の試料は，低NRM強度（≤1.0 A/m）かつ低Qn比（≤1.0）で，また正の伏角を示す。

次に交流消磁実験を各地域から選定した試料について行ったところ，全ての試料はNRMの伏角が負となり，初生磁化が逆帯磁であることが分かった。この結果は，岩石採取地点付近に分布する顕著な低磁気異常と整合的であり，これらの低磁気異常が地熱兆候によるものではなく，逆帯磁の火山岩の分布によるものであることを示している。一方，A地点から採取した岩石の鏡下鑑定と蛍光X線分析による全岩分析を実施したところ，変質が進み有色鉱物が少ないことが判明し，結果として磁性を弱めていることが分かった。

以上より，北海道東部の武佐岳地熱地域では露頭岩石の物性測定結果と磁気探査との対応が非常に良い事がわかり，磁気探査の解釈を行う上で，表層部の岩石物性情報の詳細を調べることが有効であることが明らかとなった。

We consider an application of the ideas of electromagnetic (EM) holography/migration to the interpretation of a typical marine controlled-source EM (MCSEM) survey, which consists of a set of sea-bottom receivers and a moving electrical bipole transmitter. The three-dimensional interpretation of MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multitransmitter and multireceiver data acquisition systems used in these surveys. At the same time, we show that the MCSEM surveys with their dense system of transmitters and receivers are extremely well suited for application of the holography/migration method. The combined EM signal in the receivers forms a broadband EM “hologram” of the sea-bottom geological target. As in optical and radiowave holography, we can reconstruct the volume image of the geological target by “illuminating” this EM hologram with the reference signal. The principles of holography/migration imaging formulated in this chapter are tested on typical models of a sea-bottom petroleum reservoir. We also apply this new technique to the interpretation of an MCSEM survey conducted in the Troll West gas province, offshore from Norway.

In this paper we consider an application of the ideas of electromagnetic (EM) migration to the interpretation of a typical marine controlled-source (MCSEM) survey, which consists of a set of sea-bottom receivers and a moving electrical bipole transmitter. The 3-D interpretation of the MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. At the same time, we show that the MCSEM surveys with their dense system of transmitters and receivers happen to be extremely well suited for application of the migration method. In order to speed up the computation of the migration field, we apply a fast form of integral equation (IE) solution based on the multigrid quasi-linear (MGQL) approximation. The principles of the migration imaging formulated in this paper are tested on a typical model of a sea-bottom petroleum reservoir.

There is growing interest in marine direct current (DC) resistivity methods for sub-seafloor exploration of a broad range of geophysical and geological targets. To address this, we have developed a new marine DC method with a vertical electrode configuration (VEC). Compared to conventional marine DC methods that use a horizontal electrode configuration, the shape and position of our VEC cable can be controlled relatively easily. Therefore, the VEC is suitable for operations in regions of steep bathymetry and for expeditious sub-seafloor resistivity exploration. In this study, we introduce a water-resistant electrode array cable and an onshore multichannel DC measurement system for stable and rapid data acquisition. To evaluate the performance and efficiency of the new system, we conducted field experiments in the shallow water zone at Shimizu Port, Suruga Bay, Japan. In order to quantitatively analyze the VEC-DC data, we adopt a 1-D numerical modeling code that computes the electric potential and apparent resistivity generated by a point and dipole current source used in the VEC-DC measurement. These can be placed at any position with an arbitrary electrode configuration in a multilayered space, including seawater and sub-seafloor layers. We also develop an inversion code for the VEC-DC data based on a simulated annealing (SA) optimization and applied this to the field data. The observed data is of sufficiently good quality to be used for inversion, and the SA result demonstrates that the proposed VEC-DC system is able to estimate the sub-seafloor resistivity structure.

We have developed a new marine magnetotelluric (MT) measurement system that can reduce noises caused by sea wave motions and can be applied to measurements under very shallow seawater areas, such as a coastal region with a sea depth of 10 to 100 m. The difficulties of geophysical exploration in shallow water and coastal areas include (1) fishery activity, (2) limitations of survey vessel size, and (3) motion noise caused by sea waves. In order to overcome these difficulties, we selected a MT method that uses natural EM fields without transmitting an electric current in seawater, which enables the method to be used in areas with active inshore fisheries. In addition, the developed marine MT system is very short, which reduces motion generated by sea waves, and compact, which enables light-draft small survey boat operation. We conducted offshore data acquisition using a new MT measurement system and an onshore MT survey at the Horonobe coastal area, Hokkaido, Japan. High-quality data were successfully obtained in both onshore and offshore field surveys. Two-dimensional (2D) inversion for field data from onshore to sea bottom reveals that a quaternary sedimentary layer of a few hundred meters in thickness, which was determined by well logging to be a freshwater layer, extends horizontally offshore for several kilometers under the sea. The results obtained herein demonstrate that the newly developed marine MT measurement system can be used to clarify the geoelectrical structures of brackish/fresh groundwater distributions and in coastal areas. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.

This paper describes a three-dimensional ground penetrating radar (GPR) survey carried out around a levee of the Ara River in Saitama, Japan, where deformation of the ground was observed after heavy rainfall associated with the typhoon of September 2007. The high-density 3D GPR survey was conducted as a series of closely adjacent four directional sets of 2D surveys at an area surrounding vertical cracks on the paved road caused by deformations induced by heavy rain. The survey directions of the 2D surveys were 0, 90, 45, and 45 degrees with respect to the paved road and the intervals between lines were less than 0.5m. The 3D subsurface structure was accurately imaged by the result of data processing using Kirchhoff-type 3D migration. As a result, locations and vertical continuities of the heavy rainfall induced cracks in the paved road were clearly imaged. This will be a great help in considering the generation mechanisms of the cracks. Moreover, the current risk of a secondary disaster was found to be low, as no air-filled cavities were detected by the 3D GPR survey. © ASEG/SEGJ/KSEG 2009.

電磁探査法の一種であるMagnetotelluric(MT，地磁気地電流)法は，自然に存在する磁場(地磁気)とその大地への入射に伴う電流(地電流)を測定し，その比から地下の比抵抗構造を求める探査手法であり，資源や地殻等の地下比抵抗構造の調査で広く利用されている。MT法では赤道付近で発生した雷放電が電離層での反射を繰り返し全地球を伝播する現象や，太陽活動に伴って磁気圏や電離層に発生する電磁場を信号源としている。観測は入射する磁場とそれに伴って大地に発生する電場をそれぞれ磁場センサーと電極で時間領域において測定する。データ解析は取得した時系列データを周波数変換し，電場と磁場の比から算出した見掛比抵抗を利用した地下比抵抗構造の解釈に加え，より定量的な解析として，観測データから地下比抵抗構造を推定する逆解析も実施されている。MT法の測定データから地下構造を推定することは典型的な逆問題であり，この逆問題の中では通常，既知の地下構造から測定データを予測計算する順解析を複数回行う。順解析は大地が一次元構造の場合は解析解が得られるが，2次元以上の地下構造を想定する場合は，支配方程式であるMaxwell方程式を離散化して数値的に解く。陸域におけるMT法のデータ取得から数値解析は，2次元解析まではほぼ実用化されたと言ってよい段階にあるが，3次元解析については，計算コストと精度の問題により，一部商用探査での適用例もあるが，依然として研究段階である。さらに近年，陸域のみならず，海域におけるMT法の利用への注目が高まり，極めて低比抵抗の海水を数値計算の中で取り扱う必要が出てきた。そこで，これまで利用してきた計算手法の見直しや，あらたな計算手法の研究が世界的に進められている。MT法の順計算では，主として差分法，有限要素法，積分方程式法が用いられているが，特に差分法，有限要素法における連立方程式の求解(前処理を含めた反復法)について，地球物理分野ではこれまで主にCG法とその改良形であるBiCG法やBiCGSTAB法，あるいはQMR法やGMRES法などが用いられてきたが，それ以外の反復法の適用可能性についてはあまり検討されてこなかった。また前処理についても，基本的なJacobi前処理，不完全Cholesky分解，ILU(0)などが検討されてきたが，反復法，前処理を同時に比較する報告は，電磁探査の数値計算においてはほとんどなされていない。本研究では，Maxwell方程式を差分法により電磁ポテンシャルを用いて離散定式化した後，連立方程式を反復法プログラムLis (Library of Iterative Solvers for linear systems)を援用して種々の反復法，前処理により求解し，その特徴を調べるとと共に，Lisのような優れた計算工学分野の成果を地球物理・物理探査分野へ適用する可能性を検討した。その結果，通常用いられることの多いBiCGSTAB法やQMR法に比べて高速なFGMRES法が有効であることがわかった。また複数の前処理による計算結果からは，収束回数と計算時間を考慮した総合的な計算コストの点ではILU(0)が現時点では最も効果的であることを示した。電磁探査法の3次元応答計算における線形連立方程式は条件数が大きく，定式化や実装によって有効な前処理や反復法も異なってくると考えられる。そのような中で，Lisのように可搬性が高く，簡便に種々の反復法・前処理を利用できるライブラリを適用することで，今後重要になると考えられる沿岸域における陸海同時解析や，大規模構造解析において，より効率良く3次元解析手法の開発と適用が行えると考えられる。

本論文では，近年海洋石油ガス探査に利用されている海洋人工（制御）電流（信号）源電磁探査 (Marine CSEM, MCSEM) 法の逆解析へのTikhonov正則化 (regularization) と電磁 (EM) マイグレーションの適用について詳細に検討した。MCSEM法データの3次元解析は，取得データ量の増加や解析領域の複雑化による計算コスト上昇のために非常に困難な数値計算問題の一つとなっている。<br>  この問題に対処するために，本稿では電磁マイグレーションによってヤコビアン行列を計算するTikhonov正則化法の適用を試みた。また，より安定した解を得るために正則化逆解析の解法として共役勾配法を採用した。さらに反復計算における順解析部分には，高速化のために多重格子準線形 (Multigrid Quasi-Linear, MGQL) 近似 (Ueda and Zhdanov, 2006) を使用した。本稿で開発した手法を，海底下の3次元高比抵抗異常領域を想定した数値モデルを用いて検証した。その結果, MCSEM法データの解釈へ適用可能なことがわかった。<br>

In this paper we consider an application of the method of electromagnetic ( EM) migration to the interpretation of a typical marine controlled-source (MCSEM) survey consisting of a set of sea-bottom receivers and a moving electrical bipole transmitter. Three-dimensional interpretation of MCSEM data is a very challenging problem because of the enormous number of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. At the same time, we demonstrate that the MCSEM surveys with their dense system of transmitters and receivers are extremely well suited for application of the migration method. In order to speed up the computation of the migration field, we apply a fast form of integral equation (IE) solution based on the multigrid quasi-linear (MGQL) approximation which we have developed. The principles of migration imaging formulated in this paper are tested on a typical model of a sea-bottom petroleum reservoir.

Marine controlled-source electromagnetic (MCSEM) surveys have become widely used in off-shore petroleum exploration. However, the interpretation of the MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. In this paper we demonstrate that this problem can be solved using the method of electromagnetic migration. We extend the basic principles of electric field migration for the case of joint electric and magnetic field interpretation. A joint migration field is produced by back propagation within the conductive sea-bottom sediments of all electric and magnetic signals recorded in the receivers. We demonstrate that the joint migration of the EM field data provides a better quality image of the sea-bottom resistive structure (e.g., hydrocarbon reservoir) than the results of individual migration for the different (electric or magnetic) field components.

Marine controlled-source electromagnetic (MCSEM) surveys become widely used in off-shore petroleum exploration. However, the interpretation of the MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. In this paper we demonstrate that this problem can be solved using the method of electromagnetic migration. We extend the basic principles of electric field migration to the case of joint electric and magnetic field interpretation. The joint migration field is produced by back propagation within the conductive sea-bottom sediments of all electric and magnetic signals recorded in the receivers. We demonstrate that the joint migration of the EM field data provides a better quality image of a sea-bottom resistive structure (e.g., a hydrocarbon reservoir) than the results of individual migration for the different (electric or magnetic) field components.

The reconstruction of seismic images of the medium from crosswell travel-time data is a typical example of the ill-posed inverse problem. In order to obtain a stable solution and to replace an ill-posed problem by a well-posed one, a stabilizing functional ( stabilizer) has to be introduced. The role of this functional is to select the desired stable solution from a class of solutions with specific physical and/or geometrical properties. One of these properties is the existence of sharp boundaries separating rocks with different petrophysical parameters, e. g., oil- and water-saturated reservoirs. In this paper, we develop a new tomographic method based on application of a minimum support stabilizer to the crosswell travel-time inverse problem. This stabilizer makes it possible to produce clear and focused images of geological targets with sharp boundaries. We demonstrate that the minimum support stabilizer allows a correct recovery of not only the shape but also the velocity value of the target. We also point out that this stabilizer provides good results even with a low ray density, when the traditional minimum norm stabilizer fails.

Multitransmitter electromagnetic (EM) surveys are widely used in remote-sensing and geophysical exploration. The interpretation of the multitransmitter geophysical data requires numerous three-dimensional (3-D) modelings of the responses of the receivers for different geoelectrical models of complex geological formations. In this paper, we introduce a fast method for 3-D modeling of EM data, based on a modified version of quasilinear approximation, which uses a multigrid approach. This method significantly speeds up the modeling of multitransmitter-multireceiver surveys. The developed algorithm has been applied for the interpretation of marine controlled-source electromagnetic (MCSEM) data. We have tested our new method using synthetic problems and for the simulation of MCSEM data for a geoelectrical model of a Gemini salt body.

In this paper we consider an application of the ideas of electromagnetic (EM) migration to the interpretation of a typical marine controlled-source (MCSEM) survey, which consists of a set of sea-bottom receivers and a moving electrical bipole transmitter. The 3-D interpretation of the MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. At the same time, we show that the MCSEM surveys with their dense system of transmitters and receivers happen to be extremely well suited for application of the migration method. In order to speed up the computation of the migration field, we apply a fast form of integral equation (IE) solution based on the multigrid quasi-linear (MGQL) approximation. The principles of the migration imaging formulated in this paper are tested on a typical model of a sea-bottom petroleum reservoir. © 2005 Society of Exploration Geophysicists.

We developed a fast method for 3-D modeling of marine controlled-source electromagnetic (MCSEM) data in seabed logging (SBL) applications. This method is based on a modified version of quasi-linear (QL) approximation, which speeds up significantly modeling of multitransmitter-multireceiver surveys. The developed code has been tested using synthetic problems and for simulation of MCSEM data for a geoelectrical model of a Gemini salt body.