Downward continuation of marine seismic reflection data: an undervalued tool to improve velocity models

Abstract

The purpose of marine seismic experiments is to provide information of the structure and physical properties of the subsurface. The P-wave velocity distribution is the most commonly modelled property, usually by inversion of arrival times or waveform attributes. In wide-angle seismic reflection/refraction (WAS) experiments, arrival times of seismic phases identified in data recorded by Ocean Bottom Seismometers (OBS), are used to image relatively deep structures. Most WAS experiments have relatively low redundancy and produce robust velocity models of limited resolution. The shallow subsurface is also commonly studied with Multi-Channel Seismic (MCS) data recorded by towed streamers, a technique that is highly complementary to WAS. In this case, the recording of refractions as first arrivals is limited primarily by the streamer length and by features like water depth and the velocity structure and, in general, most refractions are masked by reflections and noise. However, MCS data of variable quality are available in many regions where no other data exist and previous work has shown that these data can also be used to retrieve velocity models through travel-time and full waveform inversion provided that first arrival information is properly extracted from the record sections. The most widely used tool to extract refractions as first arrivals from MCS recordings is the so-called downward continuation technique, which is designed for redatuming streamer field data to the seafloor. In this new virtual configuration, the early refractions transform to seismic phases that are becoming visible as first arrivals from nearly zero offset, facilitating its identification and use in travel-time tomography. However, there is limited literature, let alone available codes, to be used with the available MCS data sets. This work presents a user friendly open source HPC software for redatuming 2D streamer field data to the sea bottom for any seafloor relief. The main ingredient is the acoustic wave equation used backward in time, allowing first the redatuming of the receivers, and after, the redatuming of the sources. Assessment tools are provided to evaluate the information available after redatuming for specific data acquisition configurations. Also, we present a step-by-step analysis that defines the most important features that influence the quality of the virtual, redatumed recordings.

Evidence of Strong Upper Oceanic Crustal Hydration Outboard the Alaskan and Sumatran Subduction Zones

“…To unravel upper crustal seismic arrivals, and improve the resolution in the upper oceanic crust through a better distribution of turning rays, we employ downward continuation of the seismic wavefield (e.g., Arnulf et al., 2011; Audhkhasi & Singh, 2019; Jimenez‐Tejero et al., 2022; Kardell et al., 2019). We utilize an efficient downward continuation method based on the Kirchhoff integral theorem (Berryhill, 1979) that has been successfully adapted to operate on prestack data (Arnulf et al., 2011; Berryhill, 1984; Jimenez‐Tejero et al., 2022). Downward continuation allows us to bypass most of the water layer by extrapolating the data recorded near the sea surface to a datum closer to the seafloor.…”

“…These include the extrapolation, or downward continuation, of the recorded wavefields to a datum near the seafloor, which have facilitated the recovery of upper crustal velocity structure on the length scales of geologic processes (e.g., 500–1,000 m resolutions; Arnulf et al., 2011). Downward continuation exposes valuable sedimentary and crustal refractions useful for upper crustal travel‐time tomography (Berryhill, 1979, 1984; Jimenez‐Tejero et al., 2022). Most of the studies employing this method focus on or near spreading ridges (e.g., Arnulf et al., 2011, 2014a; Harding et al., 2016; Henig et al., 2012), with only a few sampling mature oceanic crust (Audhkhasi & Singh, 2019; Kardell et al., 2019).…”