Stratigraphic characterization of the Heunghae Basin, Korea, using horizontal-to-vertical spectral ratio of microtremor records

Abstract

The Heunghae area of the Cenozoic Pohang Basin, located in the southeastern part of the Korean Peninsula, is a small-scale sub-basin covered with alluvium. The Jurassic granite is overlaid by the Cretaceous sedimentary and volcanic rocks, which form the basement of the basin composed of the Miocene non-marine and marine sediments. Therefore, the vertical distribution of strata in the Heunghae Basin can be summarized as a sequence of Quaternary alluvium, Tertiary and Cretaceous sedimentary layers, and Jurassic granite. Depending on each layer's formation time, a distinct difference in the physical properties of each layer may occur, which mechanically results in the contrast of acoustic impedance of elastic wave energy. The resonant frequency measured from the horizontal-to-vertical spectral ratio (HVSR) curve of microtremor records at a seismograph station is known to be an effective value for determining the depth to the basement with strong contrast in acoustic impedance. Based on the assumption that the boundaries formed by each layer in the Heunghae Basin have a distinct difference in acoustic impedance, we tried to estimate the resonant frequencies corresponding to each boundary from the HVSR. A total of 114 three-component geophones with a natural frequency of 5 Hz were evenly installed to obtain microtremor records over the Heunghae Basin. The distance between geophones is approximately 500 meters. The installation period is from September 24 to November 24, 2021, and the recording time varies from a minimum of 2 hours to a maximum of about 26 hours, depending on the measurement site. The recording was made at a sampling rate of 500 samples per second. The HVSR analysis used two-hour long recordings for all sites. One or more peaks can be identified in the HVSR curve of most sites. Since the resonant frequency that can be confirmed through the HVSR curve is related to the depth of the boundary between the layers where strong impedance contrast occurs under each geophone, the boundary at various depths can be determined from these frequencies of peaks. The range of resonant frequencies was found to be approximately 0.3 – 26 Hz. To compare the resonant frequency with the known geological information, the HVSR curve near the borehole site was compared with the geological logging information. In the case of some measurement sites, it was difficult to specify other peaks because one resonance frequency peak was dominant over the HVSR curve. Multiple resonant frequencies can be assumed to correspond to major layer interfaces. Due to the uncertainty of the velocity structure model, it was difficult to accurately determine the depth to the interface from these resonant frequencies. Nevertheless, the results show that the multiple resonant frequencies of the HVSR curve indicates the layer boundaries with a strong impedance contrast, and thus it can contribute to reveal the sequence stratigraphy of a basin with multiple episodes of deposits.