Nam-Hyung Koo

Acquisition, processing and interpretation of high-resolution seismic data using a small-scale multi-channel system: an example from the Korea Strait inner shelf, south-east Korea

A high-resolution seismic survey was designed to improve the quality of seismic data and study the evolution of inner shelf deposits off the southern coast of Korea. A 0.5 L air gun and 6- or 8-channel streamer cable with a 5 m group interval were used as a seismic source and receiver system, respectively. Data recording was digitally performed at a shot interval of 2 s and a sample interval of 0.1 ms using a personal computer (PC)-based recording system with an analogue to digital (A/D) converter. In the data processing, deconvolution and static corrections were very effective for improving the data resolution. The data resolution and signal-to-noise (S/N) ratio were improved by using multi-channel digital seismic systems as opposed to a single-channel analogue streamer. The results show that a small-scale multi-channel seismic system is an effective way to investigate late Quaternary deposits. Seismic stratigraphic analysis of the high-resolution seismic profiles revealed that inner shelf deposits, up to 30 m thick, consist of three sedimentary units, which comprise transgressive and highstand systems tracts deposited after the last glacial period. The transgressive systems tract includes two units (I and II) separated by a ravinement surface (RS). The lower Unit I, lying below the RS, consists of estuarine sediments left behind by shoreface erosion during transgression, and represents a paralic component. The upper Unit II, including two sub-units (IIa and IIb), lies above the RS and consists of sand produced by shoreface erosion during landward transgression, which corresponds to a marine component. The uppermost Unit III lies above the maximum flooding surface and consists of thick mud clinoforms of the modern Nakdong and Seomjin subaqueous deltas. This unit likely formed during the recent sea level highstand and represents the highstand systems tract. The results of the present study show that a small-scale multi-channel seismic system is an effective way to improve the quality of seismic records and to document the evolution of shallow sediment deposits. The shelf deposits in the Korea Strait inner shelf consist of three distinct seismic units formed during the late Quaternary.

Swell Effect Correction of Sub-bottom Profiler Data with Weak Sea Bottom Signal

A 3.5 kHz or chirp sub-bottom profiling survey is widely used in the marine geological and engineering purpose exploration. However, swells in the sea degrade the quality of the survey data. The horizontal continuity of profiler data can be enhanced and the quality can be improved by correcting the influence of the swell. Accurate detection of sea bottom location is important in correcting the swell effect. In this study, we tried to pick sea bottom locations by finding the position of crossing a threshold of the maximum value for the raw data and transformed data of envelope or energy ratio. However, in case of the low-quality data where the sea bottom signals are not clear due to sea wave noise, automatic sea bottom detection at the individual traces was not successful. We corrected the mispicks for the low quality data and obtained satisfactory results by picking a sea bottom within a range considering the previous average of sea bottom, and excluding unreliable big-difference picks. In case of trace by trace picking, fewest mispicks were found when using energy ratio data. In case of picking considering the previous average, the correction result was relatively satisfactory when using raw data.

Amplitude Variation Analysis for Deep Sea Seismic Data in the Ulleung Basin, East Sea

The amplitude variation with offset of seismic data can detect fluids in the sediment and resolve the petrophysical properties of hydrocarbons in the subsurface. We analyzed and described the amplitude variation in deep sea seismic data obtained from the Ulleung Basin, East Sea. By inspecting seismic CDP-offset and CDP-angle gathers which show a bright reflection event, we decided a target zone for amplitude variation analysis. From the seismic angle gather at the middle of Ulleung Basin, we recognized amplitude increase or decrease versus offset on the intercept-gradient curve. Using the product attribute and Poisson`s ratio change attribute computed in terms of intercept with gradient, the top and the base of gas saturated sediments were described. The area of amplitude variation suggestive of the presence of gas saturated sediments is shown at the depth of 3 s traveltime. Anomalous features of seismic amplitude in the Ulleung Basin were classified by the crossplot of intercept and gradient. The background trend of crossplot between intercept and gradient shows an inverse proportional relation that is common for wet sediments. Anomalous amplitudes of Class III fall into the first and the third quadrants on crossplots. We inferred regional gas/water saturated area with the horizontal dimension of 150 m in the Ulleung Basin by cross-section with respect to cross-plot anomaly.

Swell Effect Correction for the High-resolution Marine Seismic Data

High-resolution marine seismic surveys are used for the imaging of the detailed subsurface geological structure in engineering and marine geological survey. When the sea state gets worse, the quality of the seismic data become worse due to the sea swell. We corrected the swell effect to enhance the quality of seismic data. To remove the swell effect, we picked the sea bottom location automatically, averaged the picked sea bottom times of the adjacent traces and corrected the differences between the calculated and averaged sea bottom location. To make high quality seismic section, we used high-resolution marine 8-channel airgun seismic data acquired off Yeosu, Korea. The energy source was a 30 in3 airgun and the receiver was a 40 m long 8 channel streamer cable with a group interval of 5 m. The offset distance between the source and the first channel was 20 m. The shot interval was 2 seconds corresponding to 5 m in distance, assuming ship’s speed 5 knots. The data were digitally recorded with a sample interval of 0.1 ms and a record length of 1 s. The processing sequence includes basic processing procedures such as gain recovery, deconvolution, frequency filtering, CMP sorting, NMO correction, swell effect correction and stacking. To select sea bottom location for the swell effect correction, we pick maximum amplitude within the expected range including sea bottom location and find the first location at which the amplitude is larger than the threshold that is 40% of the maximum amplitude. We averaged these two-way travel times of sea bottom and corrected the differences. The range of the swell effect correction was -0.5 0.4 ms. After correction the continuity of reflectors were improved and high quality of the seismic data was produced. This study is a part of a Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM), a National Research Laboratory (NRL) project supported by the Ministry of Science and Technology (MOST), and Energy Technology Innovation (ETI) Project of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Ministry of Trade, Industry and Energy (MOTIE). The authors thank the officers and crew of the R/V Tamhae II for their efforts in the field survey.

Source estimation and direct wave reconstruction for the Laplace‐domain waveform inversion of deepwater seismic data

We propose an alternative strategy for overcoming the high sensitivity of the Laplace-domain waveform inversion to early time noises, especially of deepwater seismic data. We estimate the source wavelet from the raw direct wave using the full Newton method in the frequency domain and reconstruct the direct wave without noise using the estimated source wavelet and the Green’s function in a constant velocity medium. The data set adequate for the Laplace-domain waveform inversion can be made by merging the reconstructed direct wave with the original data set without direct wave. Our strategy is applied to field data from deepwater environments and a realistic velocity model can be recovered from the Laplace-domain waveform inversion of the reconstructed seismic data.