Mapping the floor of Lake Mead (Nevada and Arizona): Preliminary discussion and GIS data release, USGS Open-File Report 03-320

METHODS

Three years of geophysical surveys were conducted by the U.S. Geological Survey in Lake Mead totaling over 1398 km of survey lines (Fig. 1).

Figure 1. Map showing the location of the study area and survey tracklines.

These surveys included sidescan-sonar and high-resolution seismic-reflection data collection. The first year of surveying, 1999, focused on the Boulder Basin area of Lake Mead. In 2000, the surveying efforts were focused in the shallower westernmost part of the lake which included Las Vegas Bay, Las Vegas Wash, Gypsum Wash, and Government Wash. The remainder of the lake, east of Boulder Basin to the eastern end of Gregg Basin, was surveyed in 2001. This final year of surveying included Virgin Basin, Overton Arm, Iceberg Canyon and Gregg Basin. Although the methodology was very similar for the three years of surveying, equipment and processing techniques did vary between survey years. In all cases of the sidescan-sonar acquisition, a strong acoustic return, also referred to as high backscatter, appears as white and light gray tones within the imagery. A weak acoustic return, low backscatter, appears as black or dark gray. The methodology information will be summarized here, but more details of the 1999 and 2000 surveys are available from Twichell and others, (1999), and in Twichell and others, (2001), respectively.

During 2001, the Bureau of Reclamation conducted a swath bathymetry survey using a Reson SeaBat multibeam echosounder. The survey covered the areas of post-impoundment sediment accumulation. The system consisted of a single transducer mounted on the center of the bow of the boat. From the single transducer, a fan array of narrow beams generated a detailed cross section of bottom geometry as the survey vessel passed over the areas to be mapped. The system used for this survey transmitted 80 separate 1.5-degree slant beams resulting in a 120-degree swath from the transducer. The massive amounts of data collected using this system were then used to generate 10 TIN (triangulated irregular network) surfaces covering different areas of the lake floor. This high resolution data (less than 1 m/pixel) was downsampled to 10 m and 30 m cellsize grids to be included in this GIS data release.

1999 Survey:
This survey was conducted aboard a 19-m houseboat. Both the sidescan-sonar and chirp seismic-reflection data were acquired with a Benthos SIS-1000 acquisition system. This particular towfish system contains the sidescan-sonar transducers as well as a chirp seismic-reflection system with a central frequency of 3.5 kHz (2-7 kHz band). These data were logged digitally using a Triton-Elics digital acquisition system (ISIS). Two fire-rates were used for the system: one second and half second. For sidescan-sonar imagery, a one-second fire rate translates to a total swath coverage of 1500m. A half-second fire rate translates to a 750m swath coverage area. All navigation was acquired with a P-Code GPS receiver and logged to PC running an in-house software package. Single beam bathymetry data, acquired with the Odom fathometer, were recorded with the navigation.

Both the sidescan-sonar and seismic-reflection data had initial processing completed in the field. The one-second fire-rate data were primarily used for the sidescan-sonar image. Where necessary, half-second fire rate data were used to fill data gaps of the one-second data. The sidescan-sonar imagery was demultiplexed, corrected for slant-range distortions and signal attenuation using XSonar and ShowImage software packages developed at the US Geological Survey. These processing techniques are summarized in Danforth and others, (1991). Due to the stark contrast between highly reflective rock outcrops and fine-grained sediment deposits, XSonar software was modified in 2001 to incorporate the ability to exclude portions of the imagery from the beam angle correction routine. Since this software modification was not available until 2001, the 1999 survey data, along with the 2001 acquired data were reprocessed in late 2001. The individual files of image data were then geographically mapped using software described by Paskevich (1996). Once the individual files were mapped, then were then mosaicked digitally using Geomantic PCI remote sensing software. The techniques for generating the composite digital sidescan-sonar mosaic are summarized by Paskevich (1992).

Processing the seismic data involved two steps. The first step was to extract the seismic data from the raw SIS-1000 data files to a SEG-Y format. This utility was developed by the US Geological Survey to convert the data to a standard 16-bit unsigned integer SEG-Y format described in detail by Barry and others (1975). After the cruise, all of the seismic-reflection SEG-Y files were imported into Landmark Graphics Corp. Seisworks software package (both one-second and half-second fire rates). Seisworks enables a digital interpretation of the seismic-reflection profiles.

Details of the data acquisition are contained in the log books maintained during the cruise. These log books include the ISIS log book (for sidescan-sonar and seismic-reflection data), and the navigation log book. Because the sidescan- sonar and seismic-reflection data were acquired with the same system, only one log book was maintained for the two data types.

2000 Survey:
Due to the shallow nature of the survey area, different equipment had to be used to complete this survey. This survey was conducted aboard an 8-m pontoon boat. All navigation were acquired with a P-Code GPS receiver and logged to a PC running Coastal Oceanographics Hypack navigation software. A single beam Garmin fathometer logged depth information to the Hypack system as well. For this survey, two different instruments were used to acquire the sidescan-sonar and seismic-reflection data.
The sidescan-sonar was acquired using an Edgetech DF-1000 system and logged to a Triton Elics digital acquisition system (ISIS). Data were collected at a 200m swath. The processing of the sidescan-sonar data was completed in the same manner as described for the 1999 survey. However, due to the small aerial coverage of this dataset and the temporal nature of the area surveyed (a large portion of this survey area is sub-aerially exposed in 2002 due to a large drop in lake level), these data did not undergo reprocessing in 2001 with the XSonar software modification.

The seismic-reflection data were acquired with a Knudsen Chirp subbottom profiling system. These data were logged to a PC lap top running Knudsen acquisition software. This particular system uses range, as opposed to fire-rate, to describe the data acquisition. The ranges used on this cruise were 50m, 100m, and 200m. After the cruise, these data were processed with SIOSEIS seismic processing software to rectify the data and convert it to "true" Chirp data containing instantaneous amplitude data. In addition, shot numbers were renumbered to start at 1. As with the 1999 data, once the adjustments to the SEG-Y data were completed, all of the seismic data collected were loaded into Landmark Graphics Corp. Seisworks software package in order to digitally interpret the data.

Details of the data acquisition are contained in the log books maintained during the cruise. These log books include the ISIS log book (for sidescan-sonar acquisition), the seismic log book (for the chirp seismics), and the navigation log book.

2001 Survey:
This survey was most similar to the 1999 survey. A 19-m houseboat was used as the survey platform. Both the sidescan-sonar and chirp seismic-reflection data were acquired with the Benthos SIS-1000 acquisition system and logged digitally using a Triton-Elics digital acquisition system (ISIS). All navigation was acquired with a P-Code GPS receiver and logged to PC running Hypack navigation software. Single beam bathymetry data acquired with an Odom fathometer was also recorded by the Hypack system.

The basic processing of the sidescan-sonar and seismic-reflection data was the same as in 1999. The acquisition system had undergone modifications, which meant some of the acquisition parameters changed. Data were collected at a 1-second and 0.53 second fire rate, which translates to a 1500m and 800m sidescan-sonar swath respectively. Also, due to system problems associated with the 1500m-swath imagery, most of the data comprising the sidescan-sonar mosaic is based on the 800m swath data. All of the sidescan-sonar data were reprocessed in 2001 to incorporate the new beam-angle parameters available in XSonar.

In addition to the seismic data acquired as part of the SIS-1000 system, several lines of single channel "boomer" seismic-reflection data were collected. This system was comprised of a Benthos streamer and a GeoPulse "boomer" sound source. This system was fired at a half-second fire rate.

Details of the data acquisition are contained in the log books maintained during the cruise. These log books include the ISIS log book (for sidescan-sonar and chirp seismic data), the seismic log book (for chirp and boomer seismic data), and the navigation log book. Because the sidescan-sonar and chirp seismic-reflection data were acquired with the same system, information pertaining to the chirp seismics tended to be written in both the ISIS and the seismic log books. To glean all the pertinent information for the chirp seismic, both the ISIS and the seismic log book should be used.

All of the systems used by the USGS during the three years of surveying are further described on the Seafloor Mapping web page (http://woodshole.er.usgs.gov/operations/sfmapping/). A summary for the systems used by the USGS, and other acquisition parameters are presented in Table 1.

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