Exploration Geophysics - Volume 40, Issue 1, 2009

The holistic inversion approach for frequency domain airborne electromagnetic data has previously been employed to simultaneously calibrate, process and invert raw frequency-domain data where prior information was available. An alternative formulation has been developed, which is suitable in the case where explicit prior information is not available. It incorporates: a multi-layer vertically-smooth conductivity model; a simplified bias parameterisation; horizontal smoothing with respect to elevation; and cluster computer parallelisation. Without using any prior data, an inversion of 8.0 million data for 3.4 million parameters yields results that are consistent with independently derived calibration parameters, downhole logs and groundwater elevation data. We conclude that the success of the holistic inversion method is not dependent on a sophisticated conceptual model or the direct inclusion of survey-area specific prior information. In addition, acquisition costs could potentially be reduced by employing the holistic approach which largely eliminates the need for high altitude zero-level measurements.

On Aogashima Island, a volcanic island located in the southernmost part of the Izu Seven Islands Chain, vector magnetic anomalies were obtained in a helicopter-borne magnetic survey. The purpose of this study was to understand the volcanic structure of Aogashima Island in order to mitigate future disasters.

Commonly, to obtain the magnetic structure of a volcanic island, total intensity anomalies (TIA) have been used, even though they have intrinsic errors that have not been evaluated correctly.

Because the total intensity magnetic anomaly (TIA) is not a physical value, it does not satisfy Maxwell’s Equations, Laplace’s Equation, etc., and so TIA is not suitable for any physical analyses. In addition, it has been conventionally assumed that TIA is the same as the projected total intensity anomaly vector (PTA) for analyses of TIA. However, the effect of the intrinsic error (ॉT = TIA–PTA) on the analysis results has not been taken into account. To avoid such an effect, vector magnetic anomalies were measured so that a reliable analysis of Aogashima Island magnetization could be carried out.

In this study, we evaluated the error in TIA and used vector anomalies to avoid this erroneous effect, in the process obtaining reliable analysis results for 3D, vector magnetization distributions. An area of less than 1 A/m magnetization was found in the south-west part of Aogashima Island at the depth of 1–2 km. Taking the location of fumarolic activity into consideration, the lower-magnetization area was expected to be the source of that fumarolic activity of Aogashima Island.

In this study, we examined the applicability of a 3D electrical resistivity technique for the probing of underground cavities at a field test site in a karst area in Yongweol-ri, Muan-gun, in the south-western part of the Korean peninsula. At the test site, where the ground has subsided in the past, underground cavities are commonly found in the limestone bedrock, which is overlain with alluvial deposits. The limestone cavities at the test site are mostly filled with groundwater and clay; hence, they show levels of electrical resistivity that are significantly lower than those of the surrounding host bedrock. The results of this study demonstrate that the zones of low resistivity correspond to the zones of the cavities identified in the boreholes at the site, and that our 3D electrical resistivity survey is a very effective tool for detecting and mapping underground cavities in a karst area.

In field surveys using the dipole–dipole electrical resistivity method, we often encounter negative apparent resistivity. The term ‘negative apparent resistivity’ refers to apparent resistivity values with the opposite sign to surrounding data in a pseudosection. Because these negative apparent resistivity values have been regarded as measurement errors, we have discarded the negative apparent resistivity data. Some people have even used negative apparent resistivity data in an inversion process, by taking absolute values of the data. Our field experiments lead us to believe that the main cause for negative apparent resistivity is neither measurement errors nor the influence of self potentials. Furthermore, we also believe that it is not caused by the effects of induced polarization. One possible cause for negative apparent resistivity is the subsurface geological structure. In this study, we provide some numerical examples showing that negative apparent resistivity can arise from geological structures.

In numerical examples, we simulate field data using a 3D numerical modelling algorithm, and then extract 2D sections. Our numerical experiments demonstrate that the negative apparent resistivity can be caused by geological structures modelled by U-shaped and crescent-shaped conductive models. Negative apparent resistivity usually occurs when potentials increase with distance from the current electrodes. By plotting the voltage-electrode position curves, we could confirm that when the voltage curves intersect each other, negative apparent resistivity appears. These numerical examples suggest that when we observe negative apparent resistivity in field surveys, we should consider the possibility that the negative apparent resistivity has been caused by geological structure.

Natural magnetic fields are attenuated by electrically conductive water. For that reason, marine magnetotelluric surveys have collected data at long periods (1000–100 000 s). The mantle structure has been the main target of seafloor magnetotelluric measurements. To ascertain crustal structure, however, electromagnetic data at shorter periods are important, e.g. in investigations of megathrust earthquake zones, or in natural resource surveys. To investigate of the former, for example, electromagnetic data for periods of less than 1000 s are necessary. Because no suitable ocean bottom electromagnetometer (OBEM) has been available, we have developed a small OBEM and ocean bottom electrometer (OBE) system with a high sample rate, which has an arm-folding mechanism to facilitate assembly and recovering operations. For magnetic observation, we used a fluxgate sensor.

Field observations were undertaken to evaluate the field performance of our instruments. All instruments were recovered and their electromagnetic data were obtained. Results of the first experiment show that our system functioned well throughout operations and observations. Results of other field experiments off Tottori support the claim that the electromagnetic data obtained using the new OBEM and OBE system are of sufficient quality for the survey target. These results suggest that this device removes all instrumental obstacles to measurement of electromagnetic fields on the seafloor.

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 0.5 m. 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.

Ongoing and extensive urbanisation, which is frequently accompanied with careless construction works, may threaten important archaeological structures that are still buried in the urban areas. Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) methods are most promising alternatives for resolving buried archaeological structures in urban territories. In this work, three case studies are presented, each of which involves an integrated geophysical survey employing the surface three-dimensional (3D) ERT and GPR techniques, in order to archaeologically characterise the investigated areas.

The test field sites are located at the historical centres of two of the most populated cities of the island of Crete, in Greece. The ERT and GPR data were collected along a dense network of parallel profiles. The subsurface resistivity structure was reconstructed by processing the apparent resistivity data with a 3D inversion algorithm. The GPR sections were processed with a systematic way, applying specific filters to the data in order to enhance their information content. Finally, horizontal depth slices representing the 3D variation of the physical properties were created. The GPR and ERT images significantly contributed in reconstructing the complex subsurface properties in these urban areas. Strong GPR reflections and high-resistivity anomalies were correlated with possible archaeological structures. Subsequent excavations in specific places at both sites verified the geophysical results. The specific case studies demonstrated the applicability of ERT and GPR techniques during the design and construction stages of urban infrastructure works, indicating areas of archaeological significance and guiding archaeological excavations before construction work.

We report an implementation of the Ground Penetrating Radar (GPR) survey at a site that corresponds to a ruined castle. The objective of the survey was to characterise buried archaeological structures such as walls and tiles in Van Khan Tooril’s Ruin, Mongolia, by 2D and 3D GPR techniques. GPR datasets were acquired in an area 10 m by 9 m, with 10 cm line spacing. Two datasets were collected, using GPR with 500 MHz and 800 MHz frequency antennas. In this paper, we report the use of instantaneous parameters to detect archaeological targets such as tile, brick, and masonry by polarimetric GPR. Radar polarimetry is an advanced technology for extraction of target scattering characteristics. It gives us much more information about the size, shape, orientation, and surface condition of radar targets.

We focused our interpretation on the strongest reflections. The image is enhanced by the use of instantaneous parameters. Judging by the shape and the width of the reflections, it is clear that moderate to high intensity response in instantaneous amplitude corresponds to brick and tiles. The instantaneous phase map gave information about the location of the targets, which appeared as discontinuities in the signal. In order to increase our ability to interpret these archaeological targets, we compared the GPR datasets acquired in two orthogonal survey directions. A good correlation is observed for the alignments of reflections when we compare the two datasets. However, more reflections appear in the north-south survey direction than in the west-east direction. This is due to the electric field orientation, which is in the horizontal plane for north-south survey directions and the horizontally polarised component of the backscattered high energy is recorded.

Ground penetrating radar (GPR) is an effective tool for detecting shallow subsurface targets. In many GPR applications, these targets are veiled by the strong waves reflected from the ground surface, so that we need to apply a signal processing technique to separate the target signal from such strong signals. A pulse-compression technique is used in this research to compress the signal width so that it can be separated out from the strong contaminated clutter signals. This work introduces a filter algorithm to carry out pulse compression for GPR data, using a Wiener filtering technique. The filter is applied to synthetic and field GPR data acquired over a buried pipe.

The discrimination method uses both the reflected signal from the target and the strong ground surface reflection as a reference signal for pulse compression. For a pulse-compression filter, reference signal selection is an important issue, because as the signal width is compressed the noise level will blow up, especially if the signal-to-noise ratio of the reference signal is low. Analysis of the results obtained from simulated and field GPR data indicates a significant improvement in the GPR image, good discrimination between the target reflection and the ground surface reflection, and better performance with reliable separation between them. However, at the same time the noise level slightly increases in field data, due to the wide bandwidth of the reference signal, which includes the higher-frequency components of noise. Using the ground-surface reflection as a reference signal we found that the pulse width could be compressed and the subsurface target reflection could be enhanced.

The tau-p inversion algorithm is widely employed to generate starting models with most computer programs, which implement refraction tomography. This algorithm emphasises the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases which are indicative of shear zones. This study demonstrates the failure of the tau-p inversion algorithm to detect or define a major shear zone which is 50 m or 10 stations wide. Furthermore, the majority of refraction tomography programs parameterise the seismic velocities within each layer with vertical velocity gradients.

By contrast, the Generalized Reciprocal Method (GRM) inversion algorithms emphasise the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50 m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is confirmed by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, an analysis of the shot record amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers.

The major conclusion reached in this study is that while all seismic refraction operations should aim to provide as accurate depth estimates as is practical, those which emphasise the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the improved lateral resolution are obtained with 2D traverses in which the structural features can be recognised from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features such as faults which do not display any intrinsic variation or ‘signature’ in seismic velocities.

We propose a waveform inversion method for SH-wave data obtained in a shallow seismic refraction survey, to determine a 2D inhomogeneous S-wave profile of shallow soils. In this method, a 2.5D equation is used to simulate SH-wave propagation in 2D media. The equation is solved with the staggered grid finite-difference approximation to the 4th-order in space and 2nd-order in time, to compute a synthetic wave. The misfit, defined using differences between calculated and observed waveforms, is minimised with a hybrid heuristic search method. We parameterise a 2D subsurface structural model with blocks with different depth boundaries, and S-wave velocities in each block. Numerical experiments were conducted using synthetic SH-wave data with white noise for a model having a blind layer and irregular interfaces. We could reconstruct a structure including a blind layer with reasonable computation time from surface seismic refraction data.

The interpretation of observed waveform characteristics identified in refraction and wide-angle reflection data increases confidence in the crustal structure model obtained. When calculating traveltimes and raypaths, wavefront methods on a regular grid based on graph theory are robust even with complicated structures, but basically compute only first arrivals. In this paper, we develop new algorithms to compute traveltimes and raypaths not only for first arrivals, but also for fast and later reflection arrivals, later refraction arrivals, and converted waves between P and S, using the modified wavefront method based on slowness network nodes mapped on a multi-layer model. Using the new algorithm, we can interpret reflected arrivals, Pg-later arrivals, strong arrivals appearing behind Pn, triplicated Moho reflected arrivals (PmP) to obtain the shape of the Moho, and phases involving conversion between P and S. Using two models of an ocean-continent transition zone and an oceanic ridge or seamount, we show the usefulness of this algorithm, which is confirmed by synthetic seismograms using the 2D Finite Difference Method (2D-FDM). Characteristics of arrivals and raypaths of the two models differ from each other in that using only first-arrival traveltime data for crustal structure analysis involves risk of erroneous interpretation in the ocean-continent transition zone, or the region around a ridge or seamount.

Rupture directivity is the important parameter in estimating damage due to earthquakes. However, the traditional moment tensor inversion technique cannot resolve the real fault plane or the rupture directivity. To overcome these limitations, we have developed a new inversion algorithm to determine the moment tensor solution and the rupture directivity for moderate earthquakes, using the waveform inversion technique in the frequency domain. Numerical experiments for unilateral and bilateral rupture models with various rupture velocities confirm that the method can resolve the ambiguity of the fault planes and the rupture directivity successfully. To verify the feasibility of the technique, we tested the sensitivity to velocity models, which must be the most critical factor in practice. The results of the sensitivity tests show that the method can be applied even though the velocity model is not perfect. If this method is applied in regions where the velocity model is well verified, we can estimate the rupture directivity of a moderate earthquake. This method makes a significant contribution to understanding the characteristics of earthquakes in those regions.

In recent years, microtremor array observations have been used for estimation of shear-wave velocity structures. One of the methods is the conventional spatial autocorrelation (SPAC) method, which requires simultaneous recording at least with three or four sensors. Modified SPAC methods such as 2sSPAC, and linear array methods, allow estimating shear-wave structures by using only two sensors, but suffer from instability of the spatial autocorrelation coefficient for frequency ranges higher than 1.0 Hz.

Based on microtremor measurements from four different size triangular arrays and four same-size triangular and linear arrays, we have demonstrated the stability of SPAC coefficient for the frequency range from 2 to 4 or 5 Hz. The phase velocities, obtained by fitting the SPAC coefficients to the Bessel function, are also consistent up to the frequency 5 Hz. All data were processed by the SPAC method, with the exception of the spatial averaging for the linear array cases. The arrays were deployed sequentially at different times, near a site having existing Parallel Seismic (PS) borehole logging data.

We also used the imaginary part of the SPAC coefficients as a data-quality indicator. Based on perturbations of the autocorrelation spectrum (and in some cases on visual examination of the record waveforms) we divided data into so-called ‘reliable’ and ‘unreliable’ categories. We then calculated the imaginary part of the SPAC spectrum for ‘reliable’, ‘unreliable’, and complete (i.e. ‘reliable’ and ‘unreliable’ datasets combined) datasets for each array, and compared the results. In the case of insufficient azimuthal distribution of the stations (the linear array) the imaginary curve shows some instability and can therefore be regarded as an indicator of insufficient spatial averaging. However, in the case of low coherency of the wavefield the imaginary curve does not show any significant instability.

The Tamar rift valley runs through the City of Launceston, Tasmania. Damage has occurred to city buildings due to earthquake activity in Bass Strait. The presence of the ancient valley, the Tamar valley, in-filled with soft sediments that vary rapidly in thickness from 0 to 250 m over a few hundreds metres, is thought to induce a 2D resonance pattern, amplifying the surface motions over the valley and in Launceston. Spatially averaged coherency (SPAC), frequency-wavenumber (FK) and horizontal to vertical spectrum ratio (HVSR) microtremor survey methods are combined to identify and characterise site effects over the Tamar valley.

Passive seismic array measurements acquired at seven selected sites were analysed with SPAC to estimate shear wave velocity (slowness) depth profiles. SPAC was then combined with HVSR to improve the resolution of these profiles in the sediments to an approximate depth of 125 m. Results show that sediments thicknesses vary significantly throughout Launceston. The top layer is composed of as much as 20 m of very soft Quaternary alluvial sediments with a velocity from 50 m/s to 125 m/s. Shear-wave velocities in the deeper Tertiary sediment fill of the Tamar valley, with thicknesses from 0 to 250 m vary from 400 m/s to 750 m/s.

Results obtained using SPAC are presented at two selected sites (GUN and KPK) that agree well with dispersion curves interpreted with FK analysis. FK interpretation is, however, limited to a narrower range of frequencies than SPAC and seems to overestimate the shear wave velocity at lower frequencies. Observed HVSR are also compared with the results obtained by SPAC, assuming a layered earth model, and provide additional constraints on the shear wave slowness profiles at these sites. The combined SPAC and HVSR analysis confirms the hypothesis of a layered geology at the GUN site and indicates the presence of a 2D resonance pattern across the Tamar valley at the KPK site.

Using a discrete wavenumber method, we examine the effects on Logging-While-Drilling (LWD) logs when a mismatch exists between the amplitudes or generating times of the signals from individual monopoles in a LWD multipole source. An amplitude-mismatched LWD dipole/quadrupole source produces non-dipole/non-quadrupole modes as well as flexural and screw modes. The strongest of non-dipole/non-quadrupole modes is the Stoneley mode, whose amplitude increases with increasing mismatch. However, we can recover the flexural mode signals by A-C processing, and the screw mode by A-B+C-D processing, respectively. The Stoneley mode, which has the same amplitude at the same radial distance from the borehole axis, is cancelled out by A-C and A-B+C-D processing as long as the tool is placed at the centre of the borehole. The responses from a time-mismatched LWD multipole source look like the summation of responses by two or four monopole sources off the borehole axis. However, we can avoid the misinterpretation of the formation velocities by referring to the computed dispersion curves, which are independent of the arrival times of the modes, on the frequency semblance plot.

Sonic wave dispersion characteristics are one of the most important targets of study, particularly in estimating shear wave velocity from borehole sonic logging. We have tested dispersion characteristics using monopole and dipole sources. Theoretical dispersion curves were computed for tool-absent and tool-included models having the same physical properties but different diameters (including Φ520 mm, Φ150 mm, and Φ76 mm). Comparisons were made between boreholes of different sizes and between tool-absent and tool-included models. Between the tool-included and the tool-absent boreholes, a close similarity in dispersion curve shape was revealed for the monopole source, and a significant difference was shown for the dipole source. However, for the cut-off frequency, particularly in the engineering boreholes (Φ76 mm and Φ50 mm), a significant difference was observed for signals from the monopole source, but approximately the same cut-off frequencies were found with the dipole source. This indicates the need of careful choice of source frequency in monopole-source sonic logging, particularly in an engineering borehole.

The results of numerical experiments show that cut-off frequency is exponentially proportional to the inverse of borehole radius, irrespective of the mode type and the presence of a tool, and that the cut-off frequencies for each borehole environment could be expressed as an exponential function, rather than the inversely proportional relationship between the cut-off frequency and the borehole radius that was previously generally recognised. From the direct comparison of dispersion curves, the effects on the dispersion characteristics of borehole size and the presence of the tool can be revealed more clearly than in previous studies, which presented the dispersion curve and/or characteristics for each borehole environment separately.

Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. In the present study, we investigated subcritical crack growth in rock in distilled water (pH = 5–7) and in an aqueous solution of sodium hydroxide (NaOHaq, pH = 12), comparing the results to those in air. We also investigated the effect of the pH in an aqueous environment. We used andesite and granite for all our tests. We determined the relationship between the crack velocity and the stress intensity factor using the double-torsion test under conditions of controlled temperature. We showed that crack velocities in water were higher than those in air, in agreement with other research results indicating that crack velocity increases in water. When we compared our results for NaOHaq with those for water, however, we found that the crack velocity at the same stress intensity factor did not change even though the pH of the surrounding environment was different. This result does not agree with the accepted understanding that hydroxide ions accelerate subcritical crack growth in rocks. We concluded that the pH at the crack tip influences subcritical crack growth, and not the bulk pH, which has little effect.