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.

Petrophysical data, including magnetic properties of rocks are very effective for interpreting results of magnetic surveys. However, details of rock magnetic properties have been examined rarely in parallel with magnetic survey data. Whereas, apparent magnetic lows are observed in the Musadake geothermal area, eastern Hokkaido by a recent high-resolution aeromagnetic survey of JOGMEC to estimate geothermal potential of the area. In this study, we have made laboratory measurements of density, magnetic susceptibility and natural remanent magnetization (NRM) of rock samples from outcrops in the area to evaluate its application to an interpretation of the magnetic anomalies.

Block samples had been collected from three outcrops: the right bank of the Ikeshomanai River (Site A), the Shura River (Site B), and the right bank of Kutekunbetsu River (Site C) and had been shaped into cylindrical specimens. All the specimens from the Site B and some specimens from the Site C indicate high NRM intensities (≥ 1.0 A/m) with high Qn ratios (≥ 1.0) and negative NRM inclinations. Whereas, all the specimens from the Site A and some specimens from the Site C show low NRM intensities (≤ 1.0 A/m) with low Qn ratios (≤ 1.0) and positive NRM inclinations.

AC demagnetization experiments were also conducted for selected specimens from the three outcrops and all the selected specimens were found to show negative NRM inclination, suggesting its reverse magnetization as the initial magnetization. This result is consistent with the obvious low magnetic anomalies observed over the rock sampling sites and our results imply that the low magnetic anomalies are not attributed to geothermal signs, but rather to the distribution of reversely magnetized volcanic rocks. According to photomicrograph and X-ray fluorescence analyses conducted additionally, the rock sample from the Site A is seen to lack opaque minerals by alteration, causing its low magnetic properties.

On a basis of good agreements between the petrophysical property and magnetic anomaly, it is concluded that examinations of petrophysical properties of rocks are useful when interpreting magnetic survey results in the study area.

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.

The purpose of this research was to establish a three-dimensional (3D) numerical modeling method for electromagnetic (EM) exploration problems, such as the magnetotelluric (MT) method, and to determine the iterative solver and preconditioner suitable for a system of linear equations of the 3D EM modeling. We developed a new numerical calculation code for solving Maxwell's equations based on EM potentials by a finite difference method incorporating the Lis (Library of iterative solvers for linear systems). The accuracy of the developed code was confirmed by comparing its results with those of the integral equation method using two synthetic geoelectrical models, which are typical for electromagnetic exploration. Thereafter, we used various Lis'iterative solvers and preconditioners to solve the system of linear equations. Calculation results indicate that a combination of FGMRES and ILU(0) is the best for the 3D finite difference modeling of the MT method.

In this paper, we describe an application of the Tikhonov regularization and electromagnetic (EM) migration to an inverse problem of a typical marine controlled-source EM (MCSEM) method used for offshore hydrocarbon exploration.<br>  The 3-D modeling and inversion of MCSEM data is a very challenging problem because of the number of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys.<br>  To overcome this difficulty, we apply Tikhonov regularized inversion along with the Jacobian matrix computed using the principles of EM migration. In order to obtain a stable solution, we use the conjugate gradient method to solve the iterative regularized inversion. The multigrid quasi-linear approximation developed by Ueda and Zhdanov (2006) is introduced for accelerating the computation, in the forward stage of the iterative inversion.<br>  The developed methods are tested on synthetic examples of 3-D MCSEM inversion for the model with the resistive anomaly embedded in the conductive sediment; we have found that the methods can be applied effectively to the interpretation of MCSEM data.<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.