Yuji Mitsuhata

2-D electromagnetic modeling by finite‐element method with a dipole source and topography

I present a method for calculating frequency‐domain electromagnetic responses caused by a dipole source over a 2-D structure. In modeling controlled‐source electromagnetic data, it is usual to separate the electromagnetic field into a primary (background) and a secondary (scattered) field to avoid a source singularity, and only the secondary field caused by anomalous bodies is computed numerically. However, this conventional scheme is not effective for complex structures lacking a simple background structure. The present modeling method uses a pseudo‐delta function to distribute the dipole source current, and does not need the separation of the primary and the secondary field. In addition, the method employs an isoparametric finite‐element technique to represent realistic topography. Numerical experiments are used to validate the code. Finally, a simulation of a source overprint effect and the response of topography for the long‐offset transient electromagnetic and the controlled‐source magnetotelluric me...

2.5‐D inversion of frequency‐domain electromagnetic data generated by a grounded‐wire source

Interpretation of controlled‐source electromagnetic (CSEM) data is usually based on 1‐D inversions, whereas data of direct current (dc) resistivity and magnetotelluric (MT) measurements are commonly interpreted by 2‐D inversions. We have developed an algorithm to invert frequency‐Domain vertical magnetic data generated by a grounded‐wire source for a 2‐D model of the earth—a so‐called 2.5‐D inversion. To stabilize the inversion, we adopt a smoothness constraint for the model parameters and adjust the regularization parameter objectively using a statistical criterion. A test using synthetic data from a realistic model reveals the insufficiency of only one source to recover an acceptable result. In contrast, the joint use of data generated by a left‐side source and a right‐side source dramatically improves the inversion result. We applied our inversion algorithm to a field data set, which was transformed from long‐offset transient electromagnetic (LOTEM) data acquired in a Japanese oil and gas field. As dem...

Various-scale electromagnetic investigations of high-salinity zones in a coastal plain

Three different-scale electromagnetic (EM) measurements have been performed in the Kujukuri coastal plain, southeast Japan, to investigate the distribution of saline groundwater. The three techniques were audio-frequency magnetotelluric (AMT), transient electromagnetic (TEM), and small loop-loop EM measurements. The resistivity sections estimated from these data sets reveal three independent resistivity distributions extending to different depths. The AMT method reveals a regional-scale resistivity distribution across the plain to a maximum depth of approximately 500 m and the existence of deep conductive zones, which are inferred to be associated with fossil seawater trapped in a Pleistocene formation. The TEM results show a medium-scale resistivity distribution to depths of approximately 100 m, in which two shallow conductive zones are recognized. It is concluded that these features are caused by present seawater intrusion and high-salinity salt-marsh deposits formed during sporadic marine regressions. The small loop-loop EM method provided a shallow resistivity profile that highlights the conductive salt-marsh deposits and resistive sandy ridges. Although these resistivity sections correspond to different depth ranges, the overlapping portions of the sections are very consistent with one another. These EM methods are useful in detecting and interpreting important resistivity features. Taking the geologic evolution of the coastal plains into consideration is crucial when interpreting resistivity profiles such as these, and our results suggest that the presence of fossil seawater is an important factor controlling resistivity at a variety of depths.

Non-destructive Analysis of Oil-Contaminated Soil Core Samples by X-ray Computed Tomography and Low-Field Nuclear Magnetic Resonance Relaxometry: a Case Study

Non-destructive measurements of contaminated soil core samples are desirable prior to destructive measurements because they allow obtaining gross information from the core samples without touching harmful chemical species. Medical X-ray computed tomography (CT) and time-domain low-field nuclear magnetic resonance (NMR) relaxometry were applied to non-destructive measurements of sandy soil core samples from a real site contaminated with heavy oil. The medical CT visualized the spatial distribution of the bulk density averaged over the voxel of 0.31u2009×u20090.31u2009×u20092xa0mm3. The obtained CT images clearly showed an increase in the bulk density with increasing depth. Coupled analysis with in situ time-domain reflectometry logging suggests that this increase is derived from an increase in the water volume fraction of soils with depth (i.e., unsaturated to saturated transition). This was confirmed by supplementary analysis using high-resolution micro-focus X-ray CT at a resolution of ∼10xa0μm, which directly imaged the increase in pore water with depth. NMR transverse relaxation waveforms of protons were acquired non-destructively at 2.7xa0MHz by the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence. The nature of viscous petroleum molecules having short transverse relaxation times (T2) compared to water molecules enabled us to distinguish the water-saturated portion from the oil-contaminated portion in the core sample using an M0–T2 plot, where M0 is the initial amplitude of the CPMG signal. The present study demonstrates that non-destructive core measurements by medical X-ray CT and low-field NMR provide information on the groundwater saturation level and oil-contaminated intervals, which is useful for constructing an adequate plan for subsequent destructive laboratory measurements of cores.

On-Site Bias Noise Correction in Multi-Frequency Slingram-type Electromagnetic Induction Measurements

Slingram-type horizontal-loop electromagnetic induction systems are popular for environmental and engineering investigations of relatively shallow subsurfaces. The Slingram method is effective for the detection of anomalous zones by mapping and profiling large areas. However, analogous to airborne electromagnetic and induction borehole logging methods, zero level adjustment is crucial for further inverse analysis to estimate subsurface resistivity structures. To address this problem, we propose a simple and practical procedure for on-site measurement and correction of bias noise. To attenuate the response generated by electromagnetic induction in the ground, we raise the Slingram-type sensor vertically off the ground and measure background noise as the bias noise. By subtracting the bias noise from the original raw data, bias correction is implemented. Two applications, a high-salinity groundwater investigation and a levee assessment survey, demonstrate that this procedure is effective for quadrature data...

Chapter 9 2-D inversion of frequency-domain EM data caused by a 3-D source

Abstract We present a 2.5-D inversion algorithm for frequency-domain controlled-source electromagnetic data. This algorithm employs 2.5-D forward modeling with a finite-element method, calculation of sensitivities matrix based on the adjoint-equation approach and a linearized least-square method with a smoothness constraint. To find a reasonable balance between the data fitting and the smoothness constraint, we introduce the quasi-linearized Akaikes Bayesian information criterion. The tests for synthetic data show our inversion algorithm improves model resolution as iteration proceeds and can obtain final convergence automatically. Moreover, the algorithm provides a smoother and lower-contrast image of structure from noisy data, and shows dependence of resolution on the location of the source.

Characterization of organic-contaminated ground by a combination of electromagnetic mapping and direct-push in situ measurements

Contamination of soil and groundwater by synthetic volatile organic compounds (VOCs) and hydrocarbons nhas recently raised public concern. Geophysical techniques are frequently used to characterize contaminated sites and to specify subsurface contaminant plumes in Europe and America, but there have been very few such surveys in Japan. Electromagnetic (EM) induction mapping was applied to investigate a contaminated site on reclaimed land near a harbour in central Japan. The use of EM mapping enabled efficient coverage of a study area in the site and imaging of the subsurface resistivity distribution down to approximately 10 m. In situ direct-push membrane interface probe (MIP) and electrical conductivity (EC) in situ measurements were also performed as more direct sensing techniques, and the results were compared with soil core samples. The results suggest that the first and second conductive zones mapped by this investigation correspond to clayey soil zones that act as barriers to prevent the infiltration of contaminants. In addition, the in situ MIP measurements and laboratory analyses indicate multiple occurrences of contamination by VOCs and oil. Although EM mapping was not able to clearly specify a contaminant plume, it was demonstrated as a useful technique to delineate the infiltration pathways of contaminants by illustrating the subsurface distributions of clayey zones. In addition, the combination of direct-push in situ measurements and EM mapping is demonstrated as an essential characterization strategy to verify the interpreted resistivity structure and to determine the relationship between the heterogeneous resistivity and contaminant distribution.

Case Study on a Low-enthalpy Geothermal Exploration in Pohang Area, Korea

ABSTRACT Korea Institute of Geoscience and Mineral Resources (KIGAM) launched a new project to develop the low-enthalpy geothermal water in the area showing high geothermal anomaly, north of Pohang city, for large-scale space heating from Korea Research Council of Public Science & Technology(KORP) funding. Surface geologic and geophysical surveys including Landsat TM image analysis, gravity, magnetic, magnetotelluric (MT) and controlled-source audio-frequency MT (CSAMT) and self-potential (SP) methods have been conducted and the possible fracture zone was found that would serve as deeply connected geothermal water conduit. By the end of 2003, two test wells of 1 km depth were drilled, various kinds of borehole survey along with additional MT measurements and sample analysis will follow and then the detailed subsurface condition is to be characterized. Next step would be drilling the production well of 2 km depth and all further steps remain to be determined depending upon the results of the test well studies.

Sub-seafloor resistivity sensing using a vertical electrode configuration

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.

Application of MT to hydrocarbon exploration in northeast Japan

The Yurihara area is located at the southern edge of Akita Prefecture, an important hydrocarbon‐producing province in Japan (Figure 1). This area, about 20 km north of Chokai volcano, consists mainly of the Yurihara Plateau. The geologic structure has been analyzed by drilling, surface geology, and seismic surveys (Figure 2). Deep and commercial oil and gas reservoirs have been found in the Miocene age Nishikurosawa Formation (mainly basalt lava and pyroclastic rocks).