• Geophysics and Geophysical Exploration
• /
• v.25 no.1
• /
• pp.1-13
• /
• 2022

For surveying shallow gas reservoirs in the Pohang basin, we proposed a seismic exploration method applicable to the transition zone in which land and marine areas are connected. We designed the seismic acquisition geometry considering both environments. We installed land nodal receivers on the ground and employed vibroseis and airgun sources in both land and marine areas. For seismic exploration in the transition zone, specific acquisition and processing techniques are required to ensure precise matching of reflectors at the boundary between the onshore and offshore regions. To enhance the continuity of reflection events on the seismic section, we performed amplitude and phase corrections with respect to the source types and applied a static correction. Following these processing steps, we obtained a seismic section showing connected reflectors around the boundary in the transition zone. We anticipate that our proposed seismic exploration method can also be used for fault detection in the transition zone.

• Geophysics and Geophysical Exploration
• /
• v.26 no.3
• /
• pp.138-149
• /
• 2023

In geoscience and engineering the geological characteristics of sediment strata is crucial and possible if reliable borehole logging and seismic data are available. To investigate the characteristics of the shallow strata in the Korea Strait, laboratory sonic logs were obtained from deep borehole data and seismic section. In this study, we integrated and analyzed the sonic log data obtained from the drilling core (down to a depth of 200 m below the seabed) and multichannel seismic section. The correlation value was increased from 15% to 45% through time-depth conversion. An initial model of P-wave impedance was set, and the results were compared by performing model-based, band-limited, and sparse-spike inversions. The derived P-impedance distributions exhibited differences between sediment-dominant and unconsolidated layers. The P-impedance inversion process can be used as a framework for an integrated analysis of additional core logs and seismic data in the future. Furthermore, the derived P-impedance can be used to detect shallow gas-saturated regions or faults in the shallow sediment. As domestic deep drilling is being performed continuously for identifying the characteristics of carbon dioxide storage candidates and evaluating resources, the applicability of the integrated inversion will increase in the future.

• Geophysics and Geophysical Exploration
• /
• v.6 no.1
• /
• pp.28-34
• /
• 2003

A problem of ground subsidence has been a focus of our research over the past 3 years. The purpose of this study is to investigate the disturbed stratigraphic structure by mining and to separate the possible ground subsidence area using shallow seismic reflection survey and processing. To overcome the problems such as the distortion and attenuation of seismic signal caused by ground disturbance and to acquire the high frequency data, an array with short spacing (0.3m) for both the shot and receivers, yielding near-offset (<30m) and CMP spacing of 0.15m was implemented. Data were acquired along the survey line with length of about 43m by fixed receiver array. By considering statics caused by the ground disturbance and offset distribution of data, careful processing steps such as muting and residual statics correction were applied for successful shallow reflection imaging. By correlating the ground subsidence data and stack section, possible subsidence zone could be interpreted quantitatively.

• Journal of the Korean Geophysical Society
• /
• v.6 no.1
• /
• pp.31-38
• /
• 2003

Multi-channel seismic survey, which has been mainly employed in oil prospecting, is carried out as a high resolution shallow marine seismic exploration. Fault drop as small as 1 m can be resolved by employing high-resolution seismic survey. Similar to the effect of shallow inhomogenities in the land seismic data, due to occurrence of swell quite often higher than 1 m, shallow marine seismic data tend to be severely degraded. Suppression of such a swell effect is critical in processing of steps of marine seismic shallow high-resolution data. Compared to the moving average depth method, a newly developed method using cross-correlation technique is found out to be very effective in increasing the resolution of the shallow reflection events by accuratly elucidating the depth of sea bottom.

• Geophysics and Geophysical Exploration
• /
• v.9 no.2
• /
• pp.148-154
• /
• 2006

The effect of small-scale near surface inhomogeneity on Rayleigh wave propagation and dispersion has been investigated in this study using two-dimensional FEM elastic modeling. Various inhomogeneity models with a variety of geometrical shape and embedment depth which exist in homogeneous half-space and two-layered media are considered. Results show that any near surface inhomogeneity greater than one wavelength in terms of minimum wavelength of Rayleigh wave shows dispersion characteristics. Such dispersion effect become stronger as the dimensions of the inhomogeneity increase. The effect of horizontal dimension is more dominant factor governing the dispersion characteristics than vertical dimension. However, the dispersion effect can not be identifiable in seismogram if the horizontal dimension is not wide enough. Nonetheless, even in this case, the existence of inhomogeneity can be inferred by the reflection or transmission event of Rayleigh wave. The results can be expected to provide insights on the behavior of Rayleigh wave which may be helpful for designating field work or new processing scheme to detect near surface inhomogeneity by surface wave method.

• Geophysics and Geophysical Exploration
• /
• v.13 no.1
• /
• pp.24-30
• /
• 2010

This study utilises repeated numerical tests to understand the effects of variable near-surface conditions on time-lapse seismic surveys. The numerical tests were aimed at reproducing the significant scattering observed in field experiments conducted at the Naylor site in the Otway Basin for the purpose of $CO_2$ sequestration. In particular, the variation of elastic properties of both the top soil and the deeper rugose clay/limestone interface as a function of varying water saturation were investigated. Such tests simulate the measurements conducted in dry and wet seasons and to evaluate the contribution of these seasonal variations to seismic measurements in terms of non-repeatability. Full elastic pre-stack modelling experiments were carried out to quantify these effects and evaluate their individual contributions. The results show that the relatively simple scattering effects of the corrugated near-surface clay/limestone interface can have a profound effect on time-lapse surveys. The experiments also show that the changes in top soil saturation could potentially affect seismic signature even more than the corrugated deeper surface. Overall agreement between numerically predicted and in situ measured normalised root-mean-square (NRMS) differences between repeated (time-lapse) 2D seismic surveys warrant further investigation. Future field studies will include in situ measurements of the elastic properties of the weathered zone through the use of 'micro Vertical Seismic Profiling (VSP)' arrays and very dense refraction surveys. The results of this work may impact on other areas not associated with $CO_2$ sequestration, such as imaging oil production over areas where producing fields suffer from a karstic topography, such as in the Middle East and Australia.

• The Journal of Engineering Geology
• /
• v.22 no.4
• /
• pp.459-466
• /
• 2012

A difficulty encountered in engineering seismic mapping is that reflection events from shallow discontinuities are commonly overlapped with coherent noise such as air wave, direct waves, head waves, and high-amplitude surface waves. Here, the radial trace transform, a simple geometric re-mapping of a trace gather (x-t domain) to another trace gather (v-t domain), is applied to investigate the rejection effect of coherent linear noises. Two different types of data sets were selected as a representative database: good-quality data for intermediate sounding (hundreds of meters) in a sedimentary basin and very noisy data for shallow (${\leq}50m$) mapping of the weathered zone and bedrock surface. Results obtained with cascaded application of the radial transform and low-cut filtering proved to be as good as, or better than, those produced using f-k filtering, and were especially effective for air wave and direct wave. This simple transform enables better understanding of the characteristics of various types of noise in the RT domain, and can be generally applied to overcoming diffractions and back-scatterings caused by joints, fractures, and faults commonly that are encountered in geotechnical problems.

• Geophysics and Geophysical Exploration
• /
• v.14 no.4
• /
• pp.282-288
• /
• 2011

One of the most challenging issues facing shallow seismic survey is how to generate large amplitude of high frequency signal with small seismic sources. We tested the performance of the most commonly used shallow seismic source, hammer, with four plates: PE, nylon, aluminum, and steel plates. We compared their signal powers in terms of impulsive forces, accelerations, and ground vibration velocities caused by hammer impacts. According to a previous work, hammer blowing to an aluminum plate would generate the largest amplitude among four combinations. However, it was found in this experimental research that aluminum plate delivers seismic wave energy to the ground less than that generated with steel or PE plate. Even though the amplitude is relatively small, plastic plates could provide seismic pulses of 180 ~ 200 Hz in the bandwidth, and it seems to be very hard to generate seismic energy over the frequency of 250 Hz.

• Geophysics and Geophysical Exploration
• /
• v.14 no.2
• /
• pp.127-132
• /
• 2011

We acquired and interpreted more than 650 km of high-resolution seismic reflection profiles in the Hupo Basin, offshore east coast of Korea at $37^{\circ}N$ in the East Sea (Japan Sea) to image shallow and basement deformation. The seismic profiles reveal that the main depocenter of the Hupo Basin in the study area is bounded by the large offset Hupo Fault on the east and an antithetic fault on the west; however, the antithetic fault is much smaller both in horizontal extension and in vertical displacement than the Hupo Fault. Sediment infill in the Hupo Basin consists of syn-rift (late Oligocene. early Miocene) and post-rift (middle Miocene.Holocene) units. The Hupo Fault and other faults newly defined in the Hupo Basin strike dominantly north and show a sense of normal displacement. Considering that the East Sea has been subjected to compression since the middle Miocene, we interpret that these normal faults were created during continental rifting in late Oligocene to early Miocene times. We suggest that the current ENE direction of maximum principal compressive stress observed in and around the Korean peninsula associated with the motion of the Amurian Plate induces the faults in the Hupo Basin to have reverse and right-lateral, strike-slip motion, when reactivated. A recent earthquake positioned on the Hupo Fault indicates that in the study area and possibly further in the eastern Korean margin, earthquakes would occur on the faults created during continental rifting in the Tertiary.

• Journal of the Korean earth science society
• /
• v.32 no.6
• /
• pp.674-686
• /
• 2011

Choosing a seismic source and geophone type including a coupling method can be the most important factor in shallow seismic surveys. We studied the characteristics of seismic signals by analyzing 6 different seismic data sets that collected from several sources and geophone conditions. Geophones attached to weight plate (1.8 kg) can be easily and economically installed on the paved road where geophones with spikes would cause the coupling problem. In addition, experiments in this study revealed that a small handy hammer can be used as a seismic source by striking the paved road to generate the seismic signals within 200 ms two-way travel time. Attaching weight plates to geophones may change the geophone response curve which generally depends on the geophone mass, but the change seems not to give significant differences in the first arrival of refracted wave and in the pattern of reflection events. Consequently, using weight plates on paved roads can be an efficient and cost-saving method in the near-surface high-resolution seismic surveys.