Marine sparker SEG-Y seismic data recorded aboard the R/V San Lorenzo using receive hydrophones during USGS field activity 2021-619-FA offshore Santa Cruz, California in April of 2021

A sparker source (either the SIG ELP790 or the Applied Acoustics Delta Sparker) was towed on a 20-meter cable from the stern of the source vessel (R/V Parke Snavely; RVPS) along seven planned survey tracklines in water depths ranging from 25 to 600 meters. The SIG ELP790 sparker was powered at 700 joules at all water depths. The Applied Acoustics 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. A Reson TC4034 hydrophone and an RBR depth logger were towed on a second cable and positioned in the water just below the sparker to measure the near-field outgoing source signal and depth. Continuous waveform data (TDMS format) from the near-field hydrophone were acquired using SignalExpress software sampling at 192 kHz. A unique code name was given to each navigation line where PS=the source vessel R/V Parke Snavely; L=line followed by 2- or 3-numerals representing the water depth (the water depth may be followed by an A denoting a planned replicate transect line); S=SIG ELP790 700-joule minisparker source, D10=1.2 kilojoule Delta sparker source, or D24=2.4 kilojoule Delta sparker source; subsequent passes are followed with a, b, c, d, and a numeral as needed. Navigation data aboard the source vessel were acquired using an Applanix PosMV Elite differential Global Positioning System (DGPS). The source vessel navigation data (SrcNav) and outgoing source signal data (SrcSegy) are provided elsewhere in this data release.

The receiver vessel (R/V San Lorenzo; RVSL), maintained a nearly stationary position at the midpoint of each trackline and recorded the signal on one or two Cetacean Research CR3 omnidirectional hydrophones (CR3101 and CR3102) deployed mid-vessel between 10 and 20-meters water depth. The CR3 hydrophones recorded continuous waveform data (.wav format) using SpectraPlus software sampling at 192 KHz. RBR Solo-D depth loggers were positioned near the hydrophones to record the receiver hydrophone depth data (HPdepth_m column when available). Navigation data for the receiver vessel (RecNav) were acquired with a Garmin GPSMAP 4212 Differential Global Positioning System (DGPS) and are provided elsewhere in this data release.

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 receive 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 receiver vessel navigation, shot times, and RBR hydrophone depths (when recorded) were written to SEG-Y headers (2021-619-FA_RVSL_RecSegy.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_RVSL_RecSegyNav.csv), along with a shapefile of shot point navigation for visual purposes (2021-619-FA_RVSL_RecSegyNav_shapefile.zip).