Seismic Wave Attenuation in the Unconsolidated Sediments of the Upper Mississippi Embayment

The attenuation characteristics of P waves in the Upper Mississippi embayment were estimated using U.S. Geological Survey explosion data recorded by 17 Portable Array for Numerical Data Acquisition (PANDA) stations deployed in the New Madrid Seismic Zone. Seven explosive sources, ranging from 500-lbs to 1000-lbs, located west of the PANDA stations were used. Head waves with an apparent velocity of 6km/s and the large amplitude wave train with an apparent velocity of 1.8 to 2km/s are identified from the seismogram. They were interpreted as a P-wave refracted from the top of Pleozoic basement and reverberating P-waves trapped in the unconsolidated embayment sediments, respectively. The P wave quality factor (Qp) of the unconsolidated sediments and the top of Paleozoic basement were estimated by taking the spectral ratio of coherent phases between two stations with similar azimuths from the source. A total of 21 station pairs were identified for spectral ratio analysis. The Qp values estimated were assumed to be frequency independent. The estimated Qp ranges from 113 to 280 for the embayment sediments. The mean Qp for the refracted phase is 290. Results show a lateral variation of the estimated Qp for the embayment sediments. Results are consistent with a previous study by Langston et al. (2005) which reported an average Qp~200 in the unconsolidated embayment sediments