Open source code for downward continuation and redatuming of marine seismic data

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 waveform attributes. In wide-angle reflection/refraction (WAS) experiments, arrival times of seismic phases identified in Ocean Bottom Seismometers (OBS), are used to image relatively deep structures. Most experiments have relatively low redundancy and produce limited resolution velocity models. As alternative to WAS experiments, the shallow subsurface is commonly studied with Multi-Channel Seismic (MCS) data collected in towed streamers. In this case, the recording of refractions as first arrivals is limited primarily by the streamer length and by structural features like water depth and P-wave velocity structure and, in general, a considerable amount of the refractions are masked by reflections and noise. The most widely used tool to extract refraction information from MCS data is the so-called downward continuation technique, which is designed to redatuming streamer field data to the seafloor. In this new virtual configuration, the early refractions transform to first arrivals becoming visible from zero offset, which facilitates identification and use in travel-time tomography. 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 redatumed, virtual recordings.