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Chirp Correlation and Acoustic Characterization of Lacustrine Turbidite Deposits in Lake Ozette, Wa Using Ct-Derived Density, Synthetic Seismograms, and Advanced Chirp Processing

Description: 

Lacustrine turbidite deposits provide a high-resolution record of earthquake shaking. Turbidite deposits are characterized by graded, fining-upwards sequences deposited by turbulent flows, which can generate pronounced impedance contrasts in chirp data depending on the composition and thickness relative to background sediment. Turbidite acoustic expression is also controlled by the characteristics of the chirp sound source and the signal processing applied. Identifying turbidite beds within chirp data using a set of specific acoustic properties enables us to map the spatial and temporal distribution of the sandy turbidite beds and allows the selection of coring sites for sampling and dating. We utilize two composite sediment cores measuring 12-14 m in length, additional short cores, and chirp profiles from Lake Ozette in NW Washington to quantitatively characterize and precisely compare proximal and distal turbidite deposits from their source. Advanced processing is applied to the chirp data to broaden the frequency spectrum, allowing for detection of sand beds thinner than 1 cm. Where chirp data intersect the core sites, a stochastically representative wavelet is extracted for synthetic trace modelling. Gas expansion cracking in cores introduces inaccuracies in gamma bulk density measurements, resulting in synthetic seismograms that do not correlate with the chirp data. To obtain reliable density measurements, we use CT scans of the cores to determine the intensities of uncracked sediments, followed by the fitting of a regression curve limited to the gamma density values with void space in CT images < 2.5%. This regression curve predicts bulk density values using the CT intensity values that are combined with p-wave velocities to generate a synthetic seismogram. The resulting synthetics allow use to accurately correlate with the core-chirp data and demonstrate the acoustic variations of turbidites from proximal to distal sites. Additionally, our findings emphasize both the ability and limitation of using chirp data to observe variations in thickness, density, and vertical separation of turbidite deposits.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Jared

Student Presenter: No

Invited Presentation: 

Authors