S to P converted waves in very local induced earthquakes used to improve earthquake location

Abstract:

The US midcontinent has seen a large increase in seismicity over the last decade that has been related to large scale wastewater injections in the region. Earthquakes are occurring in the shallow basement, but precise depths are difficult to obtain due to the station geometry of networks monitoring for shallow earthquakes. To obtain a better understanding of the seismicity occurring in south-central Kansas, earthquake monitoring has been performed in the Wellington oil field since early 2015.This catalog revealed the presence of an unexpected waveform in many local earthquakes. A clear waveform arrival can be seen between the direct P wave and the direct S wave. The waveform has particle motion characteristics of a P wave and it is observed predominantly on the vertical channel, but its amplitude ranges from 50%-200% of the direct P wave. Thus, the arrival is believed to be an S to P converted wave. The Wellington oil field has a broad suite of high-quality well logs through the sedimentary section including two wells reaching granitic basement at ~1.6 km depth. Using both P and S wave velocity logs, as well as Vp/Vs logs, the conversion surface was determined as the top of the Arbuckle, a ~300 m thick carbonate overlying granitic basement. This result is contrary to the intuitive expectation of wave conversion occurring at the boundary between the sedimentary section and the underlying granitic basement, but well logs reveal little contrast at the sedimentary/basement interface. Constraining the depth to the S to P conversion surface can provide information on earthquake depth, further constraining earthquake depths in our catalogue to the shallow granitic basement. The S to P wave can also be used to estimate the Vp/Vs ratio of the sedimentary section. Here, well data provide accurate control in all but the shallowest 185m.