Marine sparker source SEG-Y seismic data recorded aboard the R/V Parke Snavely during USGS field activity 2021-619-FA offshore Santa Cruz, California in April of 2021

Aboard the R/V Parke Snavely the Sparker source (either the SIG ELP790 or the Applied Acoustics Delta Sparker) was towed 20 meters from the stern (source layback position = lbsrc). The SIG ELP790 sparker was powered at 700 joules at all water depths. The Delta Sparker was powered at 1000 joules at the 200-meter depth site and powered at 2.4 kilojoules at the 600 meter depth site. Continuous waveform data (TDMS format) was acquired using SignalExpress software sampling at 192 kHz and a Reson TC4034 hydrophone was positioned approximately 1-meter below the sparker source to measure the outgoing source signal. Navigation data was acquired using an Applanix PosMV Elite differential Global Positioning System (DGPS) while the RVPS made several passes along 7 planned survey tracklines and the RVSL (receive vessel) maintained a stationary position at the midpoint of each trackline. Two tracklines were planned for both the 25- and 50-meter depth sites (named L25 and L25A and L50 and L50A, respectively). Both the Delta Sparker and SIG ELP790 minisparker were used as sources at the 200- and 600-meter depth sites and files were named L200_S for the SIG sparker, L200_D10 for the Delta Sparker, L600_S for the SIG sparker and L600_D24 for the Delta Sparker. Passes were lettered sequentially starting at 'a' for all data files. Filenames are defined in the Entity/Attribute section below.

Aboard the R/V San Lorenzo, receive vessel, one or two Cetacean Research CR3 omnidirectional hydrophones (CR3101 and CR3102) recorded continuous waveform data (.wav format) using SpectraPlus software sampling at 192 KHz. Navigation data was acquired with a Garmin GPSMAP 4212 Differential Global Positioning System (DGPS). RBR Solo-D depth loggers were positioned near the hydrophones to record the receive hydrophone depth data (HPdepth_m column when available). The R/V San Lorenzo attempted to maintain a stationary position at the center of each of the 7 planned track lines at 25, 50, 100, 200, and 600-meter water depths as the R/V Parke Snavely (source vessel) made several passes along each navigation line.

Navigational data from both the RVPS source and RVSL receiver vessels were recorded in raw GGA format. The UTC timestamp, latitude and longitude were extracted using a series of Python notebooks. The layback position of the sparker source was estimated to be 20 meters from the stern of the source vessel. The Python tool PyProj.Geod WGS84 was used to merge the data frames of the source layback latitude and longitude positions (lbsrc_lat and lbsrc_lon, respectively) and to calculate and assign the offset distance, in meters, between the sparker source and the RVSL receive hydrophone for each UTC time sample (utc_dt). To orient the offsets from separate passes relative to each other, a signed offset (signoffet) value was assigned which is the source offset value in meters that is positive when the RVPS is northwest of the RVSL and negative when the RVPS is southeast of the RVSL.

SignalExpress software was used to output TDMS float files from the Reson TC4034 hydrophone on the source vessel (RVPS). The Python package npTDMSb (https://nptdms.readthedocs.io/en/stable/apireference.html#module-nptdms) was used to read the recorded hydrophone float voltage and write to an ObsPy trace (https://docs.obspy.org). Aboard the receiver vessel (RVSL), SpectraPlus .wav files were output in 16-bit format from the Cetacean Research model CR3 receiver hydrophones. ObsPy and NumPy (https://numpy.org) Python packages were used to unscale the 16-bit voltage readings to float format ObsPy traces. For each pass at each depth site, the timing of the electromagnetic spike as the power supply (aboard the source vessel) discharges power to the sparker source relative to the nearest offset shot time was used to shift the continuous waveform start time. For both source and receiver ObsPy traces, a 70-7000Hz bandpass filter was applied. ObsPy traces were sliced from the continuous waveform using the shot timing (record lengths for RVPS SEG-Y are 0.2 seconds and the RVSL SEG-Y are 2 seconds with offset dependent start times written to the Delay Recording time DELRT header in header bytes 109-110), resampled to 16 kHz, and standard SEG-Y headers were written (https://seg.org/Portals/0/SEG/News%20and%20Resources/Technical%20Standards/seg_y_rev1.pdf). The source vessel navigation (The position of the sparker source is recorded in SEG-Y header bytes 73/77 in decimal degrees. The position of the hydrophone receiver is recorded in SEG-Y header bytes 81/85 in decimal degrees), shot times, and source vessel hydrophone depths (when recorded) were written to SEG-Y headers (2021-619-FA_RVPS_SrcSegy.zip). A CSV file that contains source vessel shot point navigation, offset, shot times, vessel, device, and SEG-Y filename summary information was created (2021-619-FA_RVPS_SrcSegyNav.csv), along with a shapefile of shot point navigation for visual purposes (2021-619-FA_RVPS_SrcSegyNav_shapefile.zip).