Joint SH-wave seismic reflection and VSP imaging of shallow subsurface structure in an urban area

Shallow subsurface imaging of lithologies and structures is important for development and utilization of deep underground space. However, it is a challenging task for geologists and geophysicists in areas with dense buildings and high population. Compared with compressional wave reflection methods, shear waves generally have low frequencies and shorter wavelengths and can often provide higher lateral and vertical resolution for identification of small-scale subsurface features. To validate the shear wave reflection method to map subsurface structures and to help to build a 3D geological model in an area with few boreholes, a 3 km 2D SH wave seismic reflection profile was acquired in Shanghai, China, in October 2019. Additionally, vertical seismic profile (VSP) measurements were performed in a c. 152 m deep borehole about 120 m offset from the seismic profile. We report here on some results from the 2D survey and from the VSP measurements.

A 5 ton vibrator truck operated in S-wave mode with a sweeping frequency from 10 to 100 Hz and sweep length of 10 s was used as a source. An S-Land seismic recording system with 168 microelectromechanical systems (MEMS) were available for recording the SH wave seismic data. Out of these, 96 units were used to record at a sample rate of 0.5 ms and 3 s of data, and 72 units were rolled to the far end of the line during data acquisition. In total, 1032 receiver locations were occupied during acquisition and 304 source points, with 9 m source spacing, were activated along the survey line. VSP data were recorded with a GEODE seismic recording system and one 28-Hz 3C receiver over the depth interval 3 m to 152 m in the borehole. The data were spatially sampled at 1 m intervals and recorded at a sampling rate of 0.25 ms. The vibrator was activated at 4.5 m offset from the borehole.

Numerous continuous reflection horizons consistent with VSP data are observed in the c. upper 2 s after stacking and migration. A particularly strong reflection at  1.5 to 1.6 s likely originates from the bedrock, which was not penetrated by the borehole. Quaternary deposits consist of sand interlayered with clay that are also reflective in the upper 0.25 s, as well as a semi-continuous reflection with relatively low amplitude between 0.25 s to 1.5 s. The VSP was used to calibrate the seismic data and to improve the geological interpretation down to total drilled depth. The SH wave imaging extrapolated the imaging from the bottom of the borehole to bedrock. Our results not only provide high-resolution imaging of subsurface structures, but also show the potential of the method to help building 3D geological models for development and utilization of deep underground space.