Yamasaki et al. Progress in Earth and Planetary Science (2017) 4:25
DOI 10.1186/s40645-017-0140-y

Progress in Earth andPlanetary Science

METHODOLOGYOpen AccessAcoustic water bottom investigation with aremotely operated watercraft survey
systemShintaro Yamasaki1*, Tomonori Tabusa2, Shunsuke Iwasaki2 and Masahiro Hiramatsu1

Abstract
This paper describes a remotely operated investigation system developed by combining a modern leisure-use fish finderand an unmanned watercraft to survey water bottom topography and other data related to bottom materials. Current
leisure-use fish finders have strong depth sounding capabilities and can provide precise sonar images and bathymetric
information. Because these sonar instruments are lightweight and small, they can be used on unmanned small watercraft.
With the developed system, an operator can direct the heading of an unmanned watercraft and monitor a PC display
showing real-time positioning information through the use of onboard equipment and long-distance communication
devices. Here, we explain how the system was developed and demonstrate the use of the system in an area of submerged
woods in a lake. The system is low cost, easy to use, and mobile. It should be useful in surveying areas that have heretofore
been hard to investigate, including remote, small, and shallow lakes, for example, volcanic and glacial lakes.
Keywords: Aquatic investigation, Echo sounding, Bathymetry, Fish finder, Remote operation, Unmanned watercraft

Introduction
Acoustic investigation by sonar is a basic scientific methodthat has been used to measure depth and investigate
geological conditions under water. However, using thismethod has historically been costly and required
researchers to have special training. Acoustic methods
have also been used for commercial and leisure fishing,whether to find fish and fish beds or to prevent ships andboats from grounding. The performance of leisure-use fish
finders has improved considerably recently. They provide
high-quality depth measurements and high-resolution
sonar images. In addition, the data are recordable withsimultaneous positional information, so a wide range ofscientific data can be collected at a relatively low cost;moreover, the equipment can be used on small boats
because of their small size and light weight. Thus, thesemodern fish finders have been used in a variety of research
fields, including marine engineering (Uchida et al. 2008;
Tabusa et al. 2013), glaciology (Sugiyama et al. 2015),* Correspondence: yamasaki@mail.kitami-it.ac.jp
1
Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507,
Japan
Full list of author information is available at the end of the article

marine biology (Heyman et al. 2007), and archeology andgeology (Yamasaki and Kamai 2015).
Because leisure-use fish finders are light and small, they
have been used in trials on remote-controlled watercraft(e.g., Tedesco and Steiner 2011; Purdie et al. 2016). The
surveys using them can perform acoustic investigations ofsmall areas, including small lakes, ponds, and other areas
unsuitable for normal manned vessels. Previous studies
using remote-controlled watercraft and leisure-use fish
finders mainly focused on measuring depth to produce
bathymetric maps. By combining the use of a modern
high-performance fish finder, which provides information
on bottom materials and topographic details, and aremote-controlled watercraft, more detailed and broadbased analyses can be conducted in small water areas. In
addition, these types of remote control systems can be
developed by using recently developed, inexpensive
commercial electronic devices. In this study, we developed
a low-cost remote control system and evaluated its use ona remote-controlled watercraft equipped with a highperformance fish finder.
The developed watercraft is easy to produce, so we
expect it could be used in a wide array of earth science© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Yamasaki et al. Progress in Earth and Planetary Science (2017) 4:25

fields. In this paper, we present a method of producing thesystem, a procedure for using it, and a case study of its use.

Methods/Experimental
Sonar systems

We used fish finders to provide sonar images, depth
data, and synchronized positioning information via
GNSS (global navigation satellite system). Sonar images
differ depending on acoustic sources; 50 to 200 kHz
acoustic sources have usually been used in traditionalfish finders. Although these frequencies emphasize fish
finding, shapes are indefinite in the sonar images. Some
GNSS fish finders, however, provide not only traditional
sonar images (Fig. 1a) but also side-scan images or highresolution sonar images with higher frequency acoustic
sources, for example, 455 kHz (Fig. 1b). Side-scan
imaging is a technology that can be used to survey wide
areas and that can produce images of shape and dimensions (Fig. 1c). In this study, we looked for a sonar
system that would transmit acoustic beams on two

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different frequencies as well as provide traditional acoustic images, high-resolution sonar images, and side-scan
images simultaneously. We adopted two low-cost systems that consisted of the following components: a combination of a HDS®-5 Gen 2, StructureScan® HD scanner(Lowrance Electronics, US), and a B60-12° transducer(Airmar Technology Corp., US) or a combination of the
HDS®-7 Gen 2 touch scanner (Lowrance Electronics)
and a P319 transducer (Airmar Technology Corp.).
StructureScan® HD scanner is a machine of side-scanand high-resolution sonar to use with HDS®-5 Gen 2.
HDS®-7 Gen 2 touch is a sonar machine that has functions of normal, high-resolution and side-scan sonars.
The performance of both systems was quite similar, but
the B60-12° transducer includes a function such that the
acoustic beam is always shot straight down regardless ofwhether the vessel is rolling or not.
The frequency of the acoustic source is related to thesounding limit at a given power level. For example, lower
frequencies reach a greater depth, but the resolution ofFig. 1 Sonar images obtained by a a 200 kHz acoustic source (P391 transducer, Airmar), b 455 kHz acoustic source (StructureScan® HD, Lowrance), andc 455 kHz side-scan (StructureScan® HD, Lowrance) for the same track. The red arrows indicate the submerged trees; X and Y on a and b are the same
objects. Brighter images suggest stronger acoustic reflection. c Side-scan image shows the areal distribution of acoustic reflections on the lake floor;
prominent materials on the floor are bright and their shadows are dark. Thus, the shape of materials on the floor is identifiable; for example,
submerged trees (yellow arrows) have long shadowsYamasaki et al. Progress in Earth and Planetary Science (2017) 4:25

the sonar image worsens at lower frequencies (Fig. 1a, b).
With the combination of the HDS®-5 Gen 2, StructureScan® HD scanner, and B60-12° transducer, we could
detect objects at a depth of less than about 150 m with a
200 kHz acoustic source, but we obtained 455 kHz highresolution images only at depths of less than about 80 m.
Side-scan imaging by StructureScan® HD with a 455 kHz
acoustic source is effective at depths of less than 60 m andwidths narrower than 100 m.
Acoustic beams are transmitted and received throughtransducers, and there are many types of transducers interms of both power and acoustic frequency. An acoustic beam has a radiation angle, and the width of thescanned area at the bottom surface depends on depth.
For example, if the radiation angle is 12°, the area of
scanning is a circle 6 m in diameter at a depth of 30 m,
but it is 21 m in diameter at 100 m. Because sonar systems measure distance from the closest point to thetransducer, the detected depth is not always the true
depth at the position of transducer. Thus, the radiation
angle of transducer, depth range, and irregularity of topography should be considered in effective surveys.
We confirmed the accuracy of the previously notedfish finders by using a hand lead. The depths measured
by the two systems were 0.2 m deeper than the true
depth for a depth of 32.0 m and a water temperature of
8.2 °C. However, we measured depth accuracy only
under this one condition for each system. Therefore, theaccuracy of each system still needs to be examined

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under a greater variety of conditions because acoustic
velocity varies with water temperature and density.
Remote-controlled vessel

The developed remote-controlled vessels were equippedwith the sonar system, GNSS receivers and a heading
sensor, and an electric motor system (Fig. 2). Positioning
and heading information were transmitted to the vessel
from the operator on land, who could remotely control
the heading and speed of the watercraft while monitoring its real-time position and path on a PC display. The
concept of the overall system is based on Tabusa et al.
(2013), but the developed system is simpler. Tabusa
et al. (2013) developed a completely automated system
in which watercraft pass through a number of GPS way
points. For surveying shallow water areas and hazardous
water areas, however, a semi-automated system such as
ours can be more effective. For example, there are generally many obstructions in small water areas, and small
watercrafts are easily affected by wavy and windy conditions, so having an operator remotely direct the headingof the vessel allows for more flexibility in avoiding
obstructions. This flexibility allows the operator to select asafer or more easily navigable route when the watercraft is affected by obstacles, waves, or wind.
We used 12 V electric motors (Model 50700-120,
Haswing, China), weighing about 15 kg, on the remotecontrolled watercraft. Although each electric motor had
a wireless controller, the communication distance of theFig. 2 The concepts behind the remote-controlled watercraft survey system. The watercraft has four systems: motor (blue), sonar (red), GNSS receiverand heading sensor (green), and transceiver (black). The positioning and heading information are transmitted to a PC on land, and the operator canwatch the trace and real-time position on a PC display and transmit a signal to a radio controller to operate the craftYamasaki et al. Progress in Earth and Planetary Science (2017) 4:25

controller was less than 50 m. We attached a ZigBee
2.4 GHz device (XBee-PRO® 528, Digi International Inc.,
US) to the motor’s wireless controller to increase thecommunication distance to about a maximum of 1.6 km
in open air conditions.
To monitor position, speed, and heading direction of
the watercraft in real time, we equipped the vessels with
GNSS and a heading sensor device (Point-1 Antenna,
Lowrance Electronics, US) and received the data througha similar long-distance transmission mechanism as
described above. The real-time position, heading, andspeed data (described in NMEA 0183 protocol) were
processed by a microcomputer, then transmitted througha serial communication method using a ZigBee device.
The total weight of the sonar, communication system,
and battery was about 30 kg. The data were received onland by another ZigBee device and transmitted to a PC
through a serial port. The data were output andrecorded in real time to show the path of the vessel on a
PC display. We developed the monitoring software with
Google™ Maps JavaScript API v3.
In designing the watercraft, the balance between
weight and sturdiness against waves and winds needed
to be considered. Unbalanced watercraft cannot run
straight, which causes a loss of time and power in thewatercraft’s battery. We first used a box boat and an inflatable boat. They were easy to use and could carry a
lot of materials on their decks. They were, however, unstable in wavy conditions and difficult to control. We
then developed a twin-hulled watercraft to improve stability by using wood panels and plastic sheets (Fig. 3aand model A in Table 1). In this case, the transducers
can be placed on the center of the watercraft, which isadvantageous for both stability and data quality. We also
developed a twin-hulled watercraft made with plastic

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floats, a wood board, and steel pipes (Fig. 3b and model
B in Table 1). This watercraft could easily be disassembled into small components for transportation.
Analysis of subaqueous topography and surface geology

The procedure to make bathymetric maps and analyze
the bottom surface geology consists of three steps: (1)field work, (2) data processing, and (3) visualization.
Field work

To make a bathymetric map, the watercraft has to cruise anumber of parallel tracks. Because fish finders use asingle-beam echo sounder, a moving watercraft equippedwith sonar can only make a depth profile. Thus, theresolution of the bathymetric map depends on the interval
between the tracks. The length of the interval, however, isrestricted by the scanning range of the acoustic beam,
which increases with depth. The researcher has to adjust
the cruise speed appropriately. If the speed is too high, themeasured depth may be mismatched with the true depth,
particularly when measuring steep slopes.
Data processing

Sonar data were recorded in the SD card in the fish finder.
To make a bathymetric map, it is necessary to extract position and depth data from the sonar data. Recently, lowcost GIS software that displays and analyzes sonar image
data has become commonly available (e.g., ReefMaster
Pro, ReefMaster Software Ltd., UK; SonarTRX, LeraandEngineering Inc., USA). These software packages havefunctions that synchronize position and sonar images,
detect depth from sonar images, and generate matrices
including positions and depth as CSV files.
With some processing software, an analysis of bottom
materials is also possible by using the properties ofFig. 3 The developed twin-hulled watercraft: a a model made with a wood frame and waterproof plastic sheets and b a model made with plastic
floats, a wood platform, and steel pipes

Yamasaki et al. Progress in Earth and Planetary Science (2017) 4:25

Table 1 Specifications of the developed unmanned watercraft
Watercraft

Model A

Model B

Length

1.2 m

1.3 m

Width

1.0 m

1.1 m

Total weight

60 kg

Propulsion
EnergyOperational speed

45 kg
Electric motor

2 × 12 V lead battery

1 × 12 V lead battery

3–5 km/h

return signals. The return signal detected by a fish finder
is a combination of multiple echoes. The first echo isstrongest at the bottom surface, and the echo graduallyattenuates. The second echo, which is caused by a reflection on the water surface or boat hull, appears fromabout twice the depth of the first echo and it also attenuates (Fig. 4). Chivers et al. (1990), Kloser et al. (2001),
Lawrence and Bales (2001), Collier and Brown (2005),
Penrose et al. (2005), and others have extracted the E1
and E2 indices from these echoes and successfully classified bottom materials by comparative analysis. As shownin Fig. 4a, the tail part of the first echo is E1 and theentire second echo is E2. The E1 index is a measure of
the roughness of the bottom surface, and the E2 index isa measure of hardness of the bottom surface. E1 and E2
are recognized as layers on the sonar images (Fig. 4b).
Bottom analysis using E1 and E2 layers has been semiempirically established. Chivers et al. (1990) and Lawrence
and Bales (2001) explained E1 and E2 as follows: in the casewhere the bottom is rough, the reflections transmit fromvarious angles and form the E1 part. The first part of first

Page 5 of 9echo is not included for the calculation, because it contains
ambiguous sub-bottom reverberations. The second echo(E2) is formed by the signal that is reflected twice from thebottom and once from the water surface. Through theseprocesses, specular reflections to the transducer from thebottom are formed and the reflection energy is a direct
measurement of acoustic impedance. Because acoustic
impedance is the product of density and sound velocity atthe bottom surface, E2 is a measure of hardness.
We analyzed the layers automatically by using the software package ReefMaster Pro ver. 1.8. This software canalso calculate a peak reflection strength in the first echo asa hardness value (in this software package, this value is referred as peak SV, Fig. 4). The E1, E2, and peak SV values
are relative values within the same sonar image, and they
are not comparable with other values from other images.
We confirmed by underwater camera observation thatrugged bottom surfaces showed higher E1, E2, and peakSV values than smooth sandy surfaces. However, because
few studies using leisure-use fish finders have conductedsonar analysis with ReefMaster Pro, more study isneeded to confirm reliability under various conditions.
Visualization

The CSV data include longitude, latitude, depth, E1 value,E2 value, and peak SV value. To visualize them, we used
3D visualization software (Surfer 12, Golden Software
LLC, USA). Since latitude and longitude are in degrees,
we converted latitude and longitude into an equidistantprojection that interpolates discrete data by the Kriging
method to create grid data and the contour maps.

Fig. 4 Visualization of E1, E2, and peak SV. a Conceptualized E1, E2, and peak SV indices for a single acoustic reflection (modified from Penrose
et al. 2005). The first echo appears at the bottom depth, and the second echo appears at double that depth. Peak SV is an index for the strongest
reflections in the first echo. b The E1 and E2 layers and the position of peak SV in a part of a 200 kHz acoustic profile (based on the ReefMaster
reference manual, ReefMaster Software Ltd.)Yamasaki et al. Progress in Earth and Planetary Science (2017) 4:25

A case study of the Ashinoko submerged wood area
Background

Lake Ashinoko (Fig. 5) is a dammed lake formed by thecollapse of a volcano. Researchers generally agree thatlake formation started about 3000 years ago on the basisof geologic evidence and carbon dating, for example,
Oki and Hakamada (1975) dated a piece of wood in thedammed deposit at 3100 ± 90 year B.P. However, many
submerged trees have been found in the areas that havemuch younger 14C dates. For example, in the north partof the lake, a number of trees standing and lying on thelake bottom at a depth of about 30 m have been dated;their 14C dates are 1610 ± 35, 1530 ± 56 (Kihara Institute of Biology 1974), and 2110 ± 110 year B.P. (Oki et al.
1988). Oki et al. (1988) and Oki (1993) inferred that thehills and submerged trees were deposited by a landslide.
We presumed the distribution of trees would have distinctive characteristics and that the base geologic materials would differ from those of muddy lake sediments if
a landslide had occurred and investigated the area with
our remotely operated watercraft system.
We used the watercraft made with a wood frame andwaterproof plastic sheets (model A in Table 1 and Fig. 3a)
and a combination of the HDS®-7 Gen 2 touch and P319
transducer. The watercraft scanned about 2.5 km, collecting
200 and 455 kHz high-resolution sonar images and 455 kHz
side-scan sonar images. The investigation covered about 4 ha,and the watercraft’s speed was maintained at about 3 km/h.
The duration of survey using the system was about 1 h.

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Results
We obtained high-quality sonar images similar to thosewould be obtained in a manned operation (Fig. 1). Sidescan sonar imaging and high-resolution sonar imaging
clarified that most of the submerged trees were distributed on the topographic rises (Figs. 1 and 5). The sidescan sonar covered the entire area, but it was difficult to
identify the trees using side-scan sonar only because
many of the trees were too small to be clearly identifiable. Thus, the distribution map of the submerged trees
in Fig. 5 is not completely comprehensive, but it does
show general tendencies.
Although some of the contour lines in the bathymetricmap have unrealistic curvature, the topography of therises is clearly identifiable (Fig. 5). A comparison of bottom material data (Fig. 6) shows the E1–roughness value
is generally higher on steep slopes, whereas the E2–hardness and peak SV–hardness values are generally higher on
the rises. By the way, those values are numerical, but theactual bottom material data are qualitative and show
somewhat discontinuous features. We therefore displayedbottom material data by using color gradations on themap. Because acoustic beams reflect off the bottom with acircular form, the bottom material data are represented by
the colored circles shown in the figure. There are no bottom material data for the gaps between the circles. We did
not obtain the data to fill the gap; however, narrower scanning than the acoustic reflection area could provide
complete coverage of the bottom. Although there were

Fig. 5 The location of the study area and the bathymetric map (0.2 m interval in depth) obtained by our method. On the bathymetric map, the red
dots are the depth measurement points, the blue solid triangles are the submerged trees detected by the high-resolution 455 kHz sonar imaging, andthe blue open triangles are the trees detected by the side-scan 455 kHz sonar imaging (based on the ninth Japanese rectangular coordinate system;
the top is the north and the original point is 36° 00′ 00″ N, 139° 50′ 00″ E)

Yamasaki et al. Progress in Earth and Planetary Science (2017) 4:25

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Fig. 6 The results of acoustic bottom analyses obtained by using a 200 kHz acoustic source: a E1–roughness value, b E2–hardness value, and cpeak SV–hardness value. The results are shown with colored solid circles 6 m in diameter (the area of scanning at a depth of 30 m, the average
depth in this area)

many anomalies in the values, the bottom materials on
the rises were generally observed to be harder than thosein the other parts of the lake. The fact that the rises andbases of submerged trees consist of hard materials is
consistent with that of the idea that they had been transported via a landslide. The area with high E1 values could
have been formed by secondary small landslides on steep
slopes, and/or the deposition of lake sediments could havebeen slow on steep slopes.

Discussion
We introduced an investigation method using the combination of a remote-controlled watercraft survey system

and a modern leisure-use GNSS fish finder. Here, we
discuss prospects for future studies and problems.
As we mentioned, the greatest advantage of this method
is its mobility and much lower cost than existing specialized
scientific investigative methods. The investigation of smalllakes and ponds has heretofore been difficult, but it nonetheless can be important for hazard prediction if abnormal
phenomena are thought to occur in these areas. For example, the color of crater lake water suddenly changed inan active volcano (Mt. Zao, Japan) in 2014, but it has beendifficult to distinguish whether the phenomenon was related to a volcanic event. Gas emissions from the sea or lake
floor have been observed by using fish finders (e.g., Merewether et al. 1985), so it is possible that observation of gas

Yamasaki et al. Progress in Earth and Planetary Science (2017) 4:25

emissions from volcanic lakes could be feasible by using
our proposed method. In addition, observations and investigations of glacial lakes and lakes dammed by landslides
are also feasible with this method. These types of lakes canpose a high risk of flooding for residents, so that prompt
investigation of the topography of the dams is needed forhazard mitigation. However, water in volcanic lakes andglacial lakes is often turbid and from the air, so acoustic
sounding is necessary for these types of investigations. In
addition, these small lakes are generally remote and inaccessible, so conducting investigations with our proposedsystem should prove useful and effective.
The watercraft of Tedesco and Steiner (2011) and Purdie et al. (2016) used small water jet systems, so the vessels could be relatively flat and have a small risk ofgrounding. Smaller vessels also have a lower risk ofgrounding, and smaller sonar systems can be mounted
on them. We used normal 12 V electric motors designed
for use on manned vessels, so their drafts were slightly
deeper, and unsuitable for use at depths of less than
1 m. We also used a high-performance fish finder systemwith two transducers, which consumed more electricity
than a smaller fish finder would. The developed system
prioritizes not only cost-effective and productive vessels
but also the collection of plentiful acoustic data.
The utilization of fish finders does have limitations. The
beam width of fish finders is larger than that of expert-use
echo sounders or multi-beam echo sounders, thereby
restricting the resolution of the obtained bathymetry. The
resolution and accuracy of the bathymetry also depends
on the accuracy of GNSS positioning. However, thismethod provides sonar images with a high enough resolution to detect trees as small as several tens of centimeters in diameter. In our investigation (data not reported),
we also detected ropes and nets during surveys.
Improved high-performance fish finding systems arecontinuously coming onto the market. Before they are
installed for this type of use, researchers need to checktheir accuracies and sonar imaging properties.

Conclusions
We described an investigation method for subaqueous
topography and geology by using a remote-controlled
unmanned watercraft and modern leisure-use GNSS fish
finders. In our method, the watercraft has an electricmotor controlled by a land-based operator via a longdistance communication device. The operator directs
the heading of the watercraft while monitoring a realtime positioning and a trace on a PC display. The
obtained sonar images detect objects as small as several
tens of centimeters in diameter with a true shape. By
processing the data, we were able to produce a bathymetric map and maps of hardness and roughness ofbottom materials in a case study area. The method isPage 8 of 9

simple, is mobile, and of quite low cost, and it allows investigations of inaccessible water areas or high-riskareas, including volcanic lakes and glacial lakes.
Acknowledgements
We developed the method presented in this paper with the assistance of
the Technical Division and Engineering Center of the Kitami Institute ofTechnology. Shin Sugiyama of the Institute of Low Temperature Science,
Hokkaido University, gave us useful information and assisted our study inmany ways. We are grateful for their assistance.
Funding
This work was supported by the Japan Society for the Promotion of Science
KAKENHI Grant Numbers 23710206 and 26560187 and the Arctic Challenge
for Sustainability Project, Japan.
Authors’ contributions
SY proposed, conceived, and designed the study and analyzed the data. TT
mainly developed the long-distance communication and controlling systemand collaborated with SY in writing the manuscript. SI and MH mainly developed the watercraft. All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507,
Japan. 2National Institute of Technology, Yuge College, 1000 Shimoyuge,
Yuge, Kamijima-cho, Ochi-gun, Ehime 794-2593, Japan.
Received: 6 January 2017 Accepted: 21 August 2017

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