U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

CRUISE REPORT

RV INLAND SURVEYER CRUISE IS-98

THE BATHYMETRY OF LAKE TAHOE, CALIFORNIA-NEVADA

August 2 through August 17, 1998

Lake Tahoe, California and Nevada

James V. Gardner 1, Larry A. Mayer 2 , and John Hughes-Clarke 2

Open-File Report 98-509

Conducted under a Cooperative Agreement between the US Geological Survey and the Ocean

Mapping Group, University of New Brunswick

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey

editorial standards or with the North American Stratigraphic Code. Use of trade, product, or firm

names in this report is for descriptive purposes only and does not imply endorsement by the U.S.

Government.

1 U.S. Geological Survey, MS 999 345 Middlefield Road, Menlo Park, CA 94025

2 Ocean Mapping Group, University of New Brunswick, Fredericton, NB E3B 5B3
Cruise Objectives

The major objective of cruise IS-98 was to map the bathymetry of Lake Tahoe, CaliforniaNevada (Fig. 1) to fulfill a commitment made during the Lake Tahoe Presidential Forum in

1997. The only existing bathymetry of Lake Tahoe, collected in 1923, was recently compiled by

Rowe and Stone (1997), but the data density is inadequate for the level of scientific studies

ongoing and anticipated in the near future for Lake Tahoe. Recent advances in marine

multibeam-sonar capabilities now permit a cost-effective way, to precisely map the bathymetry

of large areas of the ocean floor with 100% coverage. Cruise IS-98 applied this state-of-the-art

ocean technology to Lake Tahoe. The newest of these high-resolution multibeam mapping

systems also simultaneously collects backscatter (similar to sidescan sonar) imagery that results

in a complimentary and co-registered data set that is related to the distribution of lake-floor

materials and textures. The two types of maps that resulted from this cruise provide the

multidisiplinary Lake Tahoe research community an unprecedented set of base maps upon which

to build their studies. This report describes the high-resolution multibeam mapping system used

at Lake Tahoe, outlines the data-processing steps used to produce the maps, and includes the

daily log of the cruise.

The Kongsberg Simrad EM1000 High-Resolution Multibeam Mapping System

There are several different brands of high-resolution multibeam mapping systems that are

appropriate for shallow-water surveys. After a review of the currently available systems, we

chose to use the Kongsberg Simrad EM1000 system because of its proven track record, its

demonstrated portability, its ability to map large areas at high speed without compromising data

quality and, most importantly, its ability to simultaneously produce high-resolution side-scan

sonar imagery. For the Lake Tahoe survey, we used an EM1000 system owned and operated by

C&C Technologies, Inc. (Lafayette, LA) installed aboard the 26-ft Inland Surveyor (Fig. 2).

Figure 1. Location map of Lake Tahoe, California-Nevada
An overview of high-resolution multibeam mapping systems can be found in Hughes-Clarke, et

al. (1996). The Simrad EM1000 system operates at a frequency of 95 kHz from a semi-circular

transducer that is mounted on a rigidly attached ram on the bow of Inland Surveyor. The system

was designed to operate in several modes through a range of depths from 5 to approximately

800-m water depth. The shallow (ultrawide) mode, used to maximum depths of about 200 m,

forms 60 receive beams with a spacing of 2.5° distributed across track and 3° wide

along track. This geometry generates a 150° swath that covers 7.4 times the water depth

(Fig. 3). The wide mode, used to maximum depths of about 600 m, uses 48 formed beams with a

spacing of 2.5 ° that results in a swath of 120° or 3.4 times the water depth. The deep

mode, for water depths greater than 500 m, uses 48 formed beams with a spacing of 1.25°

that generates a swath of 60° or 1.1 times the water depth. There are options within these

modes for beam distribution (equiangular or equidistant) and pulse lengths. The specific options

used for the Lake Tahoe survey are discussed in the data processing section below.

Most conventional vertical-incidence echo sounders determine the time of arrival of the returned

pulse (and thus the depth) by detecting the position of the sharp leading edge of the returned

echo, a technique called amplitude detection. In multibeam sonars, where the angle of incidence

increases for each consecutive receive beam to either side of the vertical (nadir), a returned echo

loses its sharp leading edge and the accurate determination of depth becomes inaccurate. To

address this problem, the EM1000 multibeam system uses an interferometric principle in which

each beam is split, through electronic beamforming, into two "halfbeams" and the phase

difference between these "halfbeams" is calculated to provide a measure of the angle of arrival of

the echo. The point at which the phase is zero (i.e., where the wavefront of the returned echo is

normal to the receive-beam bore) is determined for each beam and provides an accurate measure

of the range to the seafloor. Both amplitude and phase detection are recorded for each beam and

then the system software picks the "best" detection method for each beam, based on a number of

quality measurements, and uses this method to calculate depth.
Figure 2. Inland Surveyor with Kongsberg Simrad EM1000 mounted on bow.

 Figure 3. Cartoon of multibeam receive cycle showing 60 beams spread over 150°

swath.
The EM1000 also provides quantitative seafloor-backscatter data that can be displayed in a

sidescan-sonar-like image. The backscatter images can be used to gain insight into the spatial

distribution of seafloor properties. A time series of echo amplitudes from each beam is recorded

at 0.2- to 2.0-ms sampling rate, depending on the water depth. The echo amplitudes are sampled

a much faster rate than the beam spacing and can be processed from beam-to-beam to produce a

backscatter image with the theoretical resolution of the sampling interval (15 cm at 0.2 ms). The

amplitude information can be placed in its geometrically correct position relative to the acrosstrack profile because the angular direction of each range sample is known. The EM1000 software

corrects the amplitude time series for gain changes, propagation losses, predicted beam patterns

and for the insonified area (with the simplifying assumptions of a flat seafloor and Lambertian

scattering). Subsequent processing (see below) uses real seafloor slopes and applies empirically

derived beam-pattern corrections to produce a quantitative estimate of seafloor backscatter

across the swath.

Ancillary Systems

In addition to the multibeam sonar array, a multibeam survey requires a careful integration of a

number of ancillary systems. These include: (1) a differentially corrected Global Positioning

System (DGPS); (2) an accurate measure the heave, pitch, roll, and heading of the vessel, all to

better than 0.01° and the transformation these measurements to estimates of the motion of

the transducer at the times of transmission and reception (motion sensor); (3) a method to

precisely determine the sound-speed structure of the water column, using measurements of

temperature, salinity and depth with a CTD and the calculation of sound velocity profiles [SVP].

The Lake Tahoe survey was navigated with inertial positioning provided by an Applied Analytic

POS/MV motion sensor as well as a Trimble DGPS. The POS/MV records vehicle motion at 100

Hz with an accuracy of 0.01°. USGS benchmarks were located at two sites on the north end

of Lake Tahoe and their positions were determined with a DGPS. These provided a local

calibration for a position accuracy for the survey of ±1 m.

Sound-velocity profiles were calculated several times each day so that ray-tracing techniques

could be used to determine the effect of acoustic refraction in the water, principally caused by

variations in water temperature. A SeaBird CTD was deployed the first day of operations to get a

good reference SVP. Sippican T-7 expendable bathythermographs (0 to 600-m water depth) were

routinely collected anytime it was suspected that the water structure had changed during the

course of the survey. An additional sound-velocity profiler is installed at the transducer array to

determine the speed of sound in water directly at the transducer. All the SVP data are fed directly

into the Simrad EM1000 processor for instantaneous beamforming and raytracing of the

individual beams.

Data Sources and Type

Raw EM1000 data telegrams were acquired over an ethernet on a shipboard workstation. The

data stream include is shown in Table 1. A number of ancillary data sources were also acquired

by C&C Technologies (Table 2).
 Figure 4. Measured offsets required for accurate pitch, roll, yaw, and position.

Table 1. Kongsberg Simrad EM1000 data stream.

• entered static sonar alignment parameters.

• applied sound velocity profiles.

• external navigation data (1-Hz DGPS

• ship-relative bathymetric profile data.

• beam-relative backscatter intensity data
Table 2. Ancillary data sources

• transducer temperature, conductivity and (derived) sound speed data.

• POS/MV 1-Hz position and attitude data (over ethernet).

• independent serial record of DGPS data stream (GPGGA format).

• digital 21-Hz attitude from TSS-335B.

• original SeaBird SVP data

Mounting Alignment Values

The accurate reduction of swath bathymetric data critically depends on a proper knowledge of

the geometry and relative positions of the sonar transducer with respect to the motion sensor, the

ship, and the positioning-system antennae (Fig. 4). C&C Technologies, using standard surveying

techniques, measured these values before the survey began (Table 3). The transducer is at 1.3-m

depth (vessel stationary). All values are relative to the transducer

Table 3. Offsets to sensor alignments on Inland

Surveyor

Sonar to IMU misalignment

X mounting angle

Y mounting angle

Z mounting angle

0.00°

0.30°

3.00°

Ship to IMU misalignment

X mounting angle

Y mounting angle

Z mounting angle

0.00°

0.30°

3.00°
 Table 3 cont. IMU to sonar level arm

X lever arm

Y lever arm

Z lever arm

171 cm

0.00 cm

65 cm

IMU to GPS lever arm

X lever arm

Y lever arm

Z lever arm

-186 cm

0.00 cm

-238 cm

Attitude Compensation

Three Vertical Reference Units were available onboard the Inland Surveyor to measure the pitch,

roll, and yaw. The POS/MV, a full DGPS-aided inertial navigational system, was used as the

primary attitude sensor and directly interfaced to the Simrad EM1000 and provided 100-Hz

measurements of boat attitude to 0.01°. The POS/MV was chosen because it has high

accuracy specifications, it is insensitive to long period horizontal accelerations, and it provides

an inertial position solution that is reliable through DGPS outages of periods of less than about 1

or two minutes.

EM1000 Operational Modes

There are several operational modes available for the EM1000. The differences in the modes are

a function of pulse length, beam spacing, and angular sector. The pulse length controls the

amount of energy transmitted into the water column. The system can be operated in an

"equiangular" (EA) mode in which the beams are spaced at equal angles apart, resulting in a nonlinear (increasing spacing away from nadir) spacing of sonar footprints on the seafloor. The

system can also be operated in an "equidistant" (EDBS) mode in which the beams are spaced

such that the sonar footprints are equally spaced in the across-track profile. The EDBS geometry

is achieved by generating variable beam-angular spacings. Although EDBS has advantages in

data handling (i.e., provides even sounding density), there are two limitations. The beams in the

140° and 150° modes are spaced wider than their beam widths and results in

incomplete coverage that produces a striping close to nadir. This problem disappears as the

swath width closes to ~120°. However, the second limitation occurs because of attitude

uncertainties and imperfect refraction models that can result in sounding errors that grow with

angle from the vertical. Consequently, these limitations render the outermost beams less reliable

than for the EA mode. For the reasons stated above, the EA mode was used for the Lake Tahoe

survey. In the EA mode, the EM1000 was operated with a 0.2 ms pulse length in waters less than

150 m deep, that provided a 150° swath. In waters deeper than 150 m, the EM1000 was

operated with a 0.7 ms pulse length that generated a 120° swath.
Data Transformations

Bathymetry

All bathymetric data were adjusted through Kongsberg Simrad software for (1) transducer draft,

(2) static roll, pitch and gyro misalignments, (3) pitch on transmit, (4) heave at transmit and

reception, (5) roll at reception, (6) refracted ray path, and (7) beam steering at transducer

interface. Post-logging transformations included (1) transformation of navigation from antenna

to transducer, (2) correction for positioning to sonar time shifts, (3) lake surface elevation, and

(4) any unaccounted for static attitude misalignments.

Backscatter

The Kongsberg Simrad EM1000 provides a backscatter-intensity time series for the bottom

insonification period for each of the 60 individual beams. The corrections applied by the

shipboard recording system are listed in the table below in Table 4.

A set of required transformations are performed by specialized software written by the Ocean

Mapping Group at the University of New Brunswick. These transformations include conversion

of each beam time series to a horizontal range equivalent, splicing the 60 beam traces together to

produce one full slant-range corrected trace, and removal of residual beam-pattern effects.

Although the system software corrects for average beam pattern, there are ± 2 dB ripples

in the average beam pattern that vary from transducer to transducer. The beam pattern of the C &

C EM1000 may be slightly different each time the stave connectors are reconnected in the

transceiver.

 Table 4. Corrections applied to each beam for backscatter.

• source power adjustments.

• spherical spreading compensation.

• attenuation compensation (using operator entered 3 dB per km.).

• TVG adjustments.

• designed beampattern compensation.

• calculation of insonified area (assuming a flat seafloor at the nadir depth).

• application of a Lambertian model

• using flat seafloor equivalent grazing angles) to reduce the dynamic

• range of the data (stored at 8 bit (0= -128dB, 255 = 0 dB.).

Our processing approach to backscatter was to stack several thousand pings to view the angular

variation of received backscatter intensity as a function of beam angle. Inherent in this function
is both the transmit and receive sensitivities, as well as the mean angular response of the

seafloor. We then invert this function to minimize the beam pattern and angular variations.

Kongsberg Simrad uses a variable gain within 15° of vertical to reduce logged dynamic

range. The sidescan data at this stage have had a Lambertian response backed out and the beam

pattern has been corrected with respect to the vertical and all receive beams have been roll

stabilized. Consequently, corrections have been made for variations in the beam-forming

amplifiers but not variations in the stave sensitivities of the physical array.

Additional transformations were required to produce calibrated backscatter measurements. These

include (1) removal of Lambertian model, (2) true lake-floor slope correction, (3) refracted ray

path correction, (4) residual beam pattern correction, and (5) aspherical-spreading corrections.

Patch Test

Despite the careful measurements of transducer alignments and offsets, the true geometry of the

installed system can only be determined through the determination of the self-consistency of

seafloor measurements. To facilitate such a determination, we conducted a series of "patch tests"

in Lake Tahoe whereby the system was run back and forth across various features on the lake

floor to determine if there were residual roll, pitch, heading, or timing offsets that required

corection factors.

A full patch test procedure was started prior to data collection on August 2 to calibrate any time

delay, and gyro misalignment. The static adjustments were estimated from the patch test are

listed in Table 5.

 Table 5. Adjustments to shipboard alignments used for Lake Tahoe survey

• time delay (1.25 seconds)

• gyro misalignment (3.0 degrees).

• roll misalignment (0.0 degrees).

• pitch misalignment (0.3) degrees.

• The time delay was entered in to the UNB post-processing software.

• The roll offset was entered into the POS/MV.

• The gyro misalignment was entered into the Simrad OPU.
Navigation Filtering

The 1-Hz DGPS data were logged with the Kongsberg Simrad EM1000 software. The Simrad

Bottom Detection Unit (BDU time stamps the depth and sidescan telegrams and was slaved to a

shipboard SUN Sparc 20 that itself was synched to the GPS 1 PPS. The navigation telegrams

were externally stamped by the Trimble 4000 GPS receiver. The receiver antenna positions were

shifted to the transducer according to the X and Y offsets using the POS/MV output.

Every 1-Hz navigation fix was checked for gross time and/or distance jumps by graphical

examination during data processing. Outliers were interactively interrogated for time, flagged

and rejected (or re-accepted). All navigation jumps greater than 20 s were automatically flagged

as uninterpolatable.

Data Processing

Land-based data processing (Fig. 5) consisted of (1) the editing the 1-Hz navigation fixes to flag

bad fixes; (2) examining each ping of each beam to flagg outliers, bad data, etc.; (3) merging the

depth and backscatter data with the cleaned navigation; (4) reducing all depth values to mean

lake level (1899 m); (5) performing additional refraction corrections, if necessary, for correct

beam raytracing; (6) separating out the amplitude measurements for conversion to backscatter;

(7) gridding depth and backscatter into a geographic projection at the highest resolution possible

with water depth; (8) regridding individual subareas of bathymetry and backscatter into final

georeferenced map sheets; (9) gridding and contouring the bathymetry; and (10) generation of

the final maps. Nearly finalized maps were completed in the field one day after the completion of

the cruise and the final maps that accompany this report were completed one week after the end

of the cruise.

Bathymetric Editing

All swath bathymetric data were passed through a number of simple filters that flagged all beams

that form a spike when compared to their neighbors for which the across-track slope anomalies

exceed 25° and flagged all beams that form a spike when compared to their neighbors for

which the along-track slope anomalies exceed 55°. These two criteria removed the worst of

the spikes. All beams without immediate neighbors fore and aft were rejected and all beams

without immediate neighbors port and starboard were rejected. The two criteria removed isolated

beams but the remaining outliers had to be examined and subjectively examined using the

UNB/OMG SwathEditor tool in a line by line mode.
 Figure 5. Data processing flow chart.

Refraction Issues

The single biggest limitation on the quality of sounding data is water-column refraction.

Refraction-related anomalies grow non-linearly with beam angle and the resulting artifacts can

create short-wavelength topographic features that may be misinterpreted as seabed geology.

There was some fear voiced that the suspected strong lake stratification would present a problem

for the beam steering and ray tracing of individual beams. This did not prove to be the case and
the thermocline and density structure, as repeatedly measured by CTD and XBT, remaimed

remarkable constant over the 16 days of the survey. A representative plot of a water-velocity

profile is shown in Figure 6. In fact, no additional emperical refraction corrections were

necessary during processing. If all of the alignments were correctly determined, Kongsberg

Simrad states that depth "...accuracies better than 0.5% of water depth are achieved across the

entire swath."

Lake Elevation Datum

A lake-elevation station, located on the Coast Guard pier in Tahoe City, is maintained by the

USGS. Readings are taken every four hours so that a detailed time series was available for the

elevation of the lake surface relative to mean sea level. The lake elevation on August 2nd was

1899.08 m and on August 17th it was 1899.02 m. A lake elevation of 1899 m was used as the

reference datum for all depth determinations. However, it appears that there may be a 0.43-m

offset between the datum used for the elevation and the NGVD datum (Gerald Rockwell,

personal commun., 1998). The lake level dropped only 6 cm over the duration of the mapping

survey, but because the drop and the potential datum offset are smaller than the resolution of the

system, so we did not adjust the depth determinations for these offsets. Once each lake depth

determination was calculated, the value was converted to meters above mean sea level. This

allows the entire Tahoe bathymetry to be merged with the regional DEM and to be viewed as a

topographic feature.
Figure 6. Sound-velocity profiles collected throughout the Lake Tahoe survey.
The Maps

The large overview maps of backscatter and shaded relief that accompany this report were

generated from larger-scale subarea maps (Fig. 7 and Table 6). The higher-resolution subarea

maps were regridded at 10 m to produce the maps of the entire lake (Figs. 7 and 8). This

regridding loses resolution in the shallower areas but allows the entire area to be mapped. The

detailed subarea maps were produced at the maximum resolution allowable for the data.

Table 6. Mapsheet sizes in columns (cols), rows, in pixels and resolution (resol) in meters

BATHYMETRY BACKSCATTER

Mapsheets columns rows resolution

(m) columns rows resolution(m)

Area0

Area1

Area2

Area3

Area4

EMBay

661

755

2422

1454

2393

1831

2044

4243

1139

4246

726

1422

12

6

6

6

6

2

1322

1510

4844

2908

2786

3662

4088

8486

2278

8492

1452

2844

6

3

3

3

3

1

Contour maps represent the more traditional method of displaying bathymetry. The contours

were derived from the gridded elevations. The resultant contours were smoothed by a 3-point

running average in the overview maps. Even at the original contour grid, more than 90% of the

data have to be discarded so as to only show some chosen contour interval. A much better

representation of bathymetry, using 100% of the data is a shaded-relief map.
Figure 7. Map sheets used to process at highest resolution. See

Table 6 for details.
Figure 8. Shaded-relief bathymetry of Lake Tahoe.
Figure 9. Backscatter (at 95kHz) of Lake Tahoe
A shaded-relief map (Fig. 8) is a pseudo-sun-illumination of a topographic surface using the

Lambertian scattering law (equation 1), where SI is the pseudo-sun intensity, K is a constant that

allows for even background, and phi is the angle between the pseudo sun and the bathymetric

surface.

SI = K * cos phi (Eq. 1)

The backscatter map (Fig. 9) is a representation of the amount of acoustic energy, at 95 kHz, that

is scattered back to the receiver from the lake floor. The Kongsberg Simrad EM1000 system has

been calibrated at the factory and all gains, power levels, etc. that are applied during signal

generation and detection are recorded for each beam and are used to adjust the amplitude value

prior to recording. Consequently, the backscatter is calibrated to an absolute reflectance of the

seabed. However, the amount of energy, measured in decibels (dB), is some complex function of

constructional and destructional interference caused by the interaction of an acoustic wave with a

volume of sediment or, in the case of hard rock, the rock material. The backscatter from

sediment is volume reverberation to at least 10 cm caused by seabed and subsurface interface

roughnesses above the Rayleigh criteria (a function of acoustic wave length), the composition of

the sediment, and its bulk properties (water content, bulk density, etc.). Although, it is not

possible to determine a unique geological facies from the backscatter value, reasonable

predictions can be made from the backscatter based on the local geology.

It can not be stressed too strongly that one of the great advantages of this survey is that the

bathymetry is completely georeferenced with the backscatter. Consequently, each pixel on the

map has a latitude, longitude, depth, and backscatter value assigned to it.

Daily Log

August 3, 1998 (980803)

The C&C Technology team arrived in Tahoe City and the RV Inland Surveyor was launched at

the boat ramp adjacent to the Coast Guard station. The rest of the day was spent on the water

performing patch tests to ensure all the system alignments were correct. A 3° yaw offset

was noticed, but the other systems appeared aligned. It was determined that the EM1000 could

be run in the shallow and medium modes for optimum resolution and the deep mode is not

required. Weather and lake-state conditions were ideal for the survey. One surprise was that the

measured sound speed at the transducer (~1-m deep) was 1484 m/s, as measured by both velocity

sensors at the transducer. A Seabird CTD was run to ~200 m depth and the sound speed

decreased to a minimum at 50 m of ~1428 m/s.

A second surprise was that the 1900-m elevation of Lake Tahoe rendered the engines of Inland

Surveyor less efficient than was predicted. The result of this inefficiency was that a maximum

speed of only ~8 kts could be maintained. A new set of propellers was ordered to increase speed.

C&C located two USGS benchmarks around the lake and calibrated their navigation systems

with ease. The Coast Guard measures the lake elevation (to the nearest millimeter) once a day

and agreed to provide me with those measurements. These data provided our "tides" for

producing corrected elevations for the lake floor.
August 4, 1998 (980804)

John Hughes-Clarke spent the day on the boat conducting additional patch tests. A USGS

benchmark (Tavern Hole) was located on the west side of the lake for an additional navigation

reference. By noon the systems were operating at normal levels so the survey began with a

complete circumnavigation of the lake following the 100-m isobath. This survey provided the

necessary baseline for separating the medium from shallow modes. No problems were

encountered and the afternoon winds did not materialize.

August 5, 1998 (980805)

Routine day of data collection and processing. It was discovered on the last line of day 980804

that the POS/MV offsets were not properly set in the software. Consequently, we built a new

script (unravel.special) only for day 980804 and reprocessed the entire days data. An additional

line was inserted in mergeNav (-declin -3.0) and a new line was added to the script (unrollOMG

roll_offset -0.3 *.merged) for day 980804. The survey circumnavigated the lake once again and

returned back up the west side to complete the day.

The sediments in the chutes between headlands and in small channels have such low backscatter

(~-55 dB) that the EM1000 could not acquire bottom detect in the areas where the wall of the

channel/chute meets the floor. The result was data dropouts and we could find no solution to this

problem. It is a minor problem, but an interesting one. The new propellers arrrived and were

installed at the end of the day.

August 6, 1998 (980806)

Routine day of data collection and processing. The correction for the POS offset had to be

adjusted again by the script unrollOMG roll_offset 0.3 *.merged for day 980805. All data for day

980805 were reprocessed. The survey continued to fill in the deep areas of the lake. The new

propellers did not improve survey speed.

August 7, 1998 (980807)

Routine day of data collection and processing. Fresh breezes began blowing at about 11:00 am

and picked up to ~25 kts throughout the afternoon. This caused a considerable amount of rolling

and pitching on the boat, but the data did not suffer at all.

August 8, 1998 (980808)

Routine day of data collection and processing. The boat crew discovered that two offsets in the

Simrad software were not properly set. The offset between the POS/MV and the transducer

should be +1.71 m but was set at +0.41 m. The timing offset should be +1.0 s but was set at

+1.25 s. A quick calculation shows that these two errors effectively cancel one another so we

decided not to change the settings, but instead will complete the survey with one set of offset

constants.

August 9, 1998 (980809)

Routine day of data collection and processing.

August 10, 1998 (980810)

Routine day of data collection and processing.

August 11, 1998 (980811)
Routine day of data collection and processing.

August 12, 1998 (980812)

The entire day was devoted to the news media and a visit by Secretary of Interior, Mr. Bruce

Babbitt. The day began at 7:00 am with an open house for the press aboard the Inland Surveyor

tied up at the Tahoe City Marina. News media from San Francisco, San Jose, Sacramento, Reno,

as well as the local Lake Tahoe area, turned out. Promptly at 9:00 the Inland Surveyer left the

dock with Secretary Babbitt aboard and we surveyed an area locally known as the tavern hole. At

10:00 Secretary Babbitt and a host of news media visited the data processing center (Mission

Control) set up in a living room at the Tahoe Marina Lodge.

The afternoon was spent at a "stake-holders conference" where we presented the images on a

large-screen TV connected to the Silicon Graphics workstation and answered questions. Toward

the conclusion of this conference, we asked the stake holders what they wanted us to do with our

remaining survey time. A consensus was reached that they wanted us to complete the mapping to

the 20-m isobath.

While the conference was in session the Inland Surveyor returned to the lake and spent the

afternoon continuing the survey.

August 13, 1998 (980813)

Routine day of data collection and processing until toward the end of the day the computer

aboard Inland Surveyor locked up and we lost all of the days data before they weree backed up.

The day's survey was repeated the next day.

August 14, 1998 (980814)

Routine day of data collection and processing until the mid afternon. Violent thunderstorms with

lightning and hail pelted Lake Tahoe all afternoon. Torrential rain occurred for 30 minutes but

the lightning continued for two hours. All processing computers in Mission Control were shut

down for fear of a power surge; consequently, very little could be accomplished during the

storm. The surveying continued during the storm until aboat 4:00 pm when the storm's intensity

dictated terminating the survey and a return to shore for safety reasons.

August 15, 1998 (980815)

Routine day of data collection and processing. The basin of Lake Tahoe deeper than about 50 m

was completed. The shallow areas along the east and north sides were mapped to at least the 20-

m contour.

August 16, 1998 (980816)

Routine day of data collection and processing. The shallow areas along the west side of the lake

were mapped to at least the 20-m contour. The remaining areas shoaler than 20 m along the

western side of the lake is very difficult to map because of the density of piers and moored boats.

August 17, 1998 (980814)

Routine day of data collection and processing. Around noon one of the outboard motors of

Inland Surveyor quit and it was determined that bad gas caused the problem. New plugs and an

additional fuel filter were added and the survey continued with only an hour lost. The survey

ended at 6:00 pm PDST. All of the lake deeper than ~10 m was surveyed except for those areas
with piers or moored boats.

August 18, 1998 (980818)

The data from August 17th were processed and the final field maps were completed. The

computers were packed and the field program ended.

References

Hughes Clarke, J.E., Mayer, L.A., and Wells, D.E., 1996, Shallow-water imaging

multibeam somars: A new tool for investigating seafloor processes in the coastal zone

and on the continental shelf. Marine Geophysical Researches, 18: 607-629.

Rowe, T.G. and Stone, J.C., 1997, Selected hydrographic features of the Lake Tahoe

Basin, California and Nevada. U.S. Geol. Survey Open-File Rept. 97-384.