The goal of this study is to extract high‐frequency seismic waveforms recorded by both offshore and onshore instrumentations and to quantitatively evaluate the data quality in terms of source‐to‐receiver distance, source parameters, water depth, and sediment thickness. There were two active‐source seismic experiments within the Juan de Fuca plate, MGL1211 and MGL1212, during the first year deployment of the Cascadia Initiative Amphibious Array. In total, we chose 114 ocean‐bottom seismometers and 211 inland stations located around the experiment region. The common receiver gathering analysis shows that most offshore stations located in deep‐water record clear first P‐wave arrivals up to 150 km away from the air‐gun shot. The first arrival transits from crustal phase Pg to head wave Pn at a source‐to‐receiver distance of 25–40 km. For stations located at shallow water, the seismic recordings appear much noisier compared with those of deep‐water stations. Only five inland stations record clear air‐gun shot signals up to 200 km away from the source. The signal‐to‐noise ratio of the first P arrivals generally decreases from a few hundred down to a single number with increasing source‐to‐receiver distance, from deep () to shallow water (), with increasing sediment thickness and decreasing air‐gun shot spacing and time interval. On average, the ratio appears 5–10 times lower at inland stations than at offshore stations. The sediment thickness estimated in this study demonstrates significant along‐strike variations, which is up to 5 km offshore Oregon and 3.5 km offshore Washington. The coincidence of thick sediments with seismicity suggests the importance of sediments on the coupling of the plate interface.