High-frequency Acoustic Recording Package (HARP) for broad-band, long-term marine mammal monitoring

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Title

High-frequency Acoustic Recording Package (HARP) for broad-band, long-term marine

mammal monitoring

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https://escholarship.org/uc/item/0p6832s1

Journal

International Symposium on Underwater Technology 2007 and International Workshop

on Scientific Use of Submarine Cables & Related Technologies 2007, UT07

Authors

Wiggins, Sean M

Hildebrand, John A

Publication Date

2007

Peer reviewed

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University of California

High-frequency Acoustic Recording Package (HARP) for broad-band,

long-term marine mammal monitoring

Sean M. Wiggins

Scripps Institution of Oceanography, La Jolla, CA 92093-0205 USA

John A. Hildebrand

Scripps Institution of Oceanography, La Jolla, CA 92093-0205 USA

Abstract - Advancements in low-power and

high-data-capacity consumer computer technology during the

past decade have been adapted to autonomously record

sounds from marine mammals over long periods. Acoustic

monitoring has advantages over traditional visual surveys

including greater detection ranges, continuous long-term

monitoring in remote locations under various weather

conditions and independent of daylight, and lower cost.

However, until recently, the technology required to

autonomously record whale sounds over long durations has

been limited to low-frequency (< 1000 Hz) baleen whales. The

need for a broader-band, higher-data capacity system capable

of autonomously recording toothed whales and other marine

mammals for long periods has prompted the development of

a High-frequency Acoustic Recording Package (HARP)

capable of sample rates up to 200 kHz. Currently, HARPs

accumulate data at a rate of almost 2 TB per instrument

deployment which creates challenges for processing these

large data sets. One method we employ to address some of

these challenges is a spectral averaging algorithm in which

the data are compressed and viewed as long duration

spectrograms. These spectrograms provide the ability to view

large amounts of data quickly for events of interest, and they

provide a link for quickly accessing the short time-scale data

for more detailed analysis. HARPs are currently in use

worldwide to acoustically monitor marine mammals for

behavioral and ecological long-term studies. The HARP

design is described and data analysis strategies along with

software tools are discussed using examples of broad-band

recorded data.

I. INTRODUCTION

Used as a method for monitoring marine mammals,

underwater acoustic recordings have provided ecological,

geographical, and behavioral information on a variety of

species. For example, recorded calling patterns have given

clues to whales’ daily calling behavior and seasonal

presence (e.g., [1],[2]). Also, acoustic monitoring can aid

in studying behavioral responses of calling animals to

acoustic events, either anthropogenic or natural. When

combined with other data such as visual, environmental

and satellite measurements, long-term acoustic monitoring

can be an especially powerful observational tool for

studying marine mammals.

Since the 1990’s several forms of autonomous acoustic

monitoring systems with various capabilities have been

developed and used in various settings throughout the

world’s oceans for recording whale sounds

(e.g.,[1],[3]-[7]). However, these systems have been

limited in sample rate and record only low-frequency

baleen whales (<1000 Hz) such as blue (Balaenoptera

musculus), fin (B. physalus), humpback (Megaptera

novaeangliae), and right (Eubalaena spp.) whales. More

recently, autonomous acoustic systems capable of higher

sample rates have been used in very small packages

attached to whales for behavioral studies [8] and in larger

packages for seafloor deployment in very shallow water

[9], but these newer systems are not capable of providing

both long-term (months) and broadband (up to 100 kHz or

more) recordings which are required for monitoring

toothed whales (odontocetes) (e.g., [10]). As the need

grows to conduct long-term studies on odontocetes to learn

about their behavior and population dynamics, including

understanding their responses to anthropogenic sounds,

autonomous recorders with enhanced capabilities are

required.

Until recently, long-term high-bandwidth acoustic

monitoring was not feasible using autonomous instruments

because of data acquisition hardware limitations.

However, with the proliferation of new, low-power

consumer electronics such as laptop computer hard disk

drives, low-power microprocessors, and high-speed

digitizers, these limitations can be overcome. In this paper,

we describe an instrument called HARP (High-frequency

Acoustic Recording Package) which is capable of

recording long-term, high band-width acoustic data. We

present example data to show some of the challenges and

current solutions for processing these high-capacity data

sets.

II. METHODS

A. Data Logger Design

To provide long-term acoustic records of odontocete

calls from an autonomous instrument, there are three main

requirements for the data acquisition electronics:

low-power, high-speed digitizing, and high-capacity data

storage. As with any battery-powered autonomous

instrument, low-power components are essential for long

duration deployments. High-speed digitizing is needed to

record broad-band odontocete calls and to provide enough

bandwidth for call differentiation. High-speed digitizing

coupled with long duration recordings requires

high-capacity data storage be used. Various forms of

digital data storage devices are currently available, but

laptop computer disks were chosen because their

widespread use in the consumer electronics market allows

them to be cost effective and readily available.

Furthermore, their design provides a rugged, small form

factor (high capacity density), and most importantly,

low-power device; all desired characteristics which will

likely continue to improve.

We use 16 integrated drive electronics (IDE) laptop

disk drives (2.5” form-factor) for our high-capacity data

storage. The disks are arranged in a block and are

addressed sequentially with all disks connected to a

common 50-pin bus (Fig. 1). Only one disk is addressed

and powered at a time. To allow for efficient instrument

refurbishment, the block of 16 full disks can be easily

removed and replaced with a new block of recently

formatted disks upon instrument recovery. The removed

disk block then can be connected to a computer and the

data can be uploaded to a smaller number of large capacity

desktop computer disk drives for data backup and analysis.

In 2003, 40 GB laptop computer disk drives were readily

available and provided a total of 640 GB per deployment.

In early 2006, 120 GB disks became cost effective and

allowed for a total of 1.92 TB of data space per

deployment. It is anticipated that the disk capacity

increasing trend will continue for the near future.

Fig. 1. HARP data logger mounted on aluminum pressure

case end cap (7” diameter x 2” thick) with underwater

connectors. The data logger consists of a backplane

populated with five primary printed circuit boards (clock,

A/D, CPU, RAM, Ethernet/IDE controller), a disk block

with 16 laptop computer disk drives and 48 D cell alkaline

batteries. Another pressure case filled with alkaline

batteries can be included for long-term deployments.

In addition to the disk block, the data logger consists of

five printed circuit boards (PCBs) connected via 96-pin

connectors to a backplane circuit board (Fig. 1). The

backplane has two additional connectors for future PCB

enhancements. The five PCBs are identified as the

central processing unit (CPU), analog-to-digital converter

(ADC), static random access memory (SRAM) buffer,

Ethernet/IDE communication, and clock cards. The

primary component on the CPU card is a 32-bit, 20 MHz

microcontroller from Motorola (http://www.motorola.com)

which controls all data logging operations. Also included

on the CPU card are FLASH memory chips for data

buffering and a RS232 transceiver for user communication

with the data logger via a standard computer terminal with

a serial communications port. For converting the analog

sensor signal into digital data, a low-power, low-noise

ADC from Analog Devices (http://www.analog.com) was

chosen that provides 16-bit resolution and up to 250,000

samples/second sample rate. Eight sample rates ranging

from 2000 to 200,000 Hz were chosen to be available for

various mission configurations and are software selectable

prior to mission initiation. Also included on the ADC card

are a power supply for the hydrophone sensor and a 4-pole

anti-alias filter for the analog signal that can be easily

modified for the various sampling corner frequencies. The

SRAM card consists of 32 MB of data buffer space in the

form of sixteen 2 MB chips. After digitizing, the data are

stored in the SRAM buffer until about 30 MB is occupied,

at which point one of the disk drives is turned on and the

data are flushed to the disk while the ADC continues to fill

up the free buffer space on the back end. The disk writing

process requires about one minute to complete and then the

disk is turned off. The Ethernet/IDE card provides

10BaseT file transfer protocol (FTP), telnet connectivity,

and IDE communication between the data logger and the

disk drives. The FTP functionality can be used to upload

individual 30 MB files from the data logger disks (i.e.,

individual SRAM buffer flushed disk writes) without

connecting the disk block directly to a computer.

Uploading these files through the data logger housing

allows the data acquisition system to be evaluated and

tested prior to deployment and allows the data quality

recorded on the seafloor to be evaluated immediately after

recovery without opening the housing. The final card is the

clock card which is populated with a temperature

compensating phase-lock circuit and a low-power Seascan

clock oscillator module which provides low, long-term

clock drifts on the order of 1 part in 10

-8

. Precise clocks are

needed for tracking calling whales with multiple

instruments deployed in an array configuration.

Data storage capacity dictates monitoring duration and

sample rate. The 1.92 TB data storage capacity allows for

approximately 55 days of continuous sampling at 200 kHz

or about one year continuously at 30 kHz. To extend

monitoring durations when using high sample rates,

non-continuous sampling can be used; for example, record

five continuous minutes on a 10 minute duty cycle.

However, there is a tradeoff between minimizing the

non-sampling period and maximizing the monitoring

duration which must be considered along with the

scientific monitoring requirements and species temporal

acoustic behavior. For example, a sampling scheme of 12

hrs on and 12 hrs off each day would not provide adequate

ta coverage to test for a diel calling pattern hypothesis.

The rate at which power is consumed by the data logger

is dependent on sample rates and data acquisition sampling

schemes (i.e., continuous or non-continuous).

Approximately 250 mW is required by the data logger

during sampling at the maximum sample rate, but only

about 25 mW when in the non-sampling mode. The disks

require an additional 2.2 W for one minute while writing

data and peak near 5 W upon initial disk spin up.

Batteries are required to operate HARPs autonomously,

and the longer the deployment and the higher the sample

rate, the larger the number of batteries that are needed to

accomplish the mission. To a large extent, batteries drive

the design of an autonomous instrument packaging, for

instance, pressure cases are often used to house the

batteries and additional instrument flotation is required to

buoy the weight of the batteries during instrument recovery.

In the current HARP seafloor instrument configuration, a

total of 192 D size alkaline cells (140g each) are arranged

in four sub-packs. Each 14.5 cm diameter sub-pack has 48

D cells arranged in four layers of 12 cells. Twelve Volts

per sub-pack are provided by six parallel strings of 8 cells

in series, and all sub-packs are connected together in

parallel between pressure case housings via underwater

cables and bulkhead connectors. One sub-pack is housed

with the data logger electronics and provides power for

testing the data acquisition system without the need of the

additional sub-packs packaged in the battery-only pressure

case. For this configuration of twenty-four 12 V strings,

an estimated 330 Amp-hours are available per deployment.

When recording continuously at the maximum sample rate,

the disks become full before the battery pack capacity is

reached, but as disk capacity continues to increase,

additional alkaline batteries or higher energy capacity

batteries (e.g., lithium chemistry) will be required. An

alternative approach to housing the batteries with the

instrument package would be to jettison the battery pack

during instrument recovery resulting in less required

buoyancy and smaller instrument packaging.

B. Hydrophone Design

The HARP acoustic sensor is a broad-band (10 Hz -

100,000 Hz), low-power (50 mW), high-sensitivity (more

than -120 dB re 1V/µPa) hydrophone which includes two

types of transducers and signal conditioning preamplifiers,

pre-whitening filters and anti-alias filter electronics. To

produce a hydrophone that has low self-noise, high gain

(over 80 dB), and can pre-whiten the ocean ambient noise

across four frequency decades, we developed a

hydrophone with two separate stages of signal conditioning,

one for the frequency band from 10 Hz to 2000 Hz, and the

other from 1000 Hz to 100,000 Hz. After signal

conditioning, the signals for the two stages are added

together via a differential receiver before being digitized

by the ADC and stored on disk.

The two stages use different transducers and provide

the ability to record both baleen whale low frequency

sounds and high frequency sounds from odontocetes. The

high frequency stage uses a spherical omni-directional

transducer (ITC-1042, www.itc-transducers.com) which is

constructed from lead zirconate titanate ceramic and has an

approximately flat (+/- 2 dB) sensitivity response of about

-200 dB re 1Vrms / µPa from 1 Hz to 100 kHz. The low

frequency stage uses six cylindrical transducers (Benthos

AQ-1, www.benthos.com) connected in series to provide a

total sensitivity of about -187 dB re 1Vrms / µPa with a

flat response (+/- 1.5 dB) from 1 Hz to 10,000 Hz. The

signals from the transducers are fed into preamplifiers with

approximately 40 dB of gain for the low frequency stage,

and about 80 dB for the high frequency stage. The

signals are pre-whitened with a frequency response similar

to the reciprocal of the ocean ambient noise as a function

of frequency (i.e., ocean ambient noise decreases as

frequency increases; [11]). The pre-whitening filter

flattens the response of the hydrophone system in the

presence of ocean ambient noise, adding more gain at

higher frequencies where ambient noise levels are lower

and sound attenuation is higher. The pre-whitening is

accomplished through the two preamplifiers and through

the low-end rolloffs of the high-pass filters of the two

stages (i.e., below 30 Hz on the low frequency stage and

below 10 kHz on the high frequency stage). After

pre-amplifying and pre-whitening, a 4-pole low-pass filter

is used to reduce high-frequency aliasing effects (above 2

kHz for the low frequency stage and above 100 kHz for the

high frequency stage). Line drivers send the signals

separately through a 10m cable to a differential receiver in

the data logger which combines the signals, and another

4-pole low-pass filter with a -3 dB point at 100 kHz is

included in the data logger to further reduce high

frequency aliasing effects. Fig. 2 shows the hydrophone

system sensitivity as a function of frequency.

Fig. 2. Hydrophone sensitivity plot showing two stages of

preamplification, pre-whitening, and anti-aliasing filters.

The shape of the hydrophone sensitivity was designed to

follow the reciprocal of ocean ambient noise so that the

sensor’s response would allow for large amplitude signals

across the wide band of frequencies above ambient noise.

The transducer and signal conditioning electronics are

packaged in a soft, oil-filled polyurethane tube to provide

good acoustic coupling with the seawater. The signal

conditioning surface mount electronics are populated onto

a 2 cm x 8 cm, two-sided printed circuit board which is

mounted on a bulkhead connector to allow easy electronics

changing based on experimental requirements (i.e.,

different sample rates require different anti-alias and

pre-whitening filters).

C. Seafloor Package Design

The size of a HARP seafloor package is dictated by the

requirement to buoy and bring back to the sea surface the

data logger and acoustic release electronics, pressure cases,

frame, and about 27 kg of batteries. The batteries are the

major weight component of the system, but also control the

deployment duration. Currently, the HARP seafloor

package has about 60 kg of buoyancy in the form of six

30.5 cm diameter glass spheres rated to 6600m (Fig. 3).

The seafloor HARP has an acoustic release system

which utilizes an EdgeTech (www.edgetech.com)

electronic board and ITC transducer to receive acoustic

commands from a support ship and in turn power a motor

activated release of the ballast weights. The ballast

weights are standard athletic weight lifting plates and are

readily available worldwide. In addition to being lower

cost than fabricating comparable ballast weights, these

plates come with a center mounting hole, are smooth and

appropriate size for easy handling and stacking, and are

painted which reduces rust accumulation during transport

and storage. Two acoustic release systems can be used on

one seafloor package to provide a redundant system and

increase the likelihood of instrument recovery in the event

of failure of one of the release systems.

Fig. 3. HARP Seafloor package including data logger

and acoustic release electronics pressure cases, ballast

weights, glass flotation sphere in yellow hard hats, and

hydrophone tethered ~ 10m above seafloor.

High-density polyethylene (HDPE) plastic panels and

tubes are used to provide the framework for the seafloor

package design. HDPE is low cost, easily machined,

durable, and buoyant in sea water. The panels include

holes for direct mounting to a ship’s deck, for lifting points,

and for lines when controlling the package during

deployments and recoveries. The panels also act as runners

allowing a fully loaded frame to be easily slid on deck into

launch position by two people. The HDPE cross-frame

tubes house the aluminum pressure cases and provide

structural strength for the frame. All frame-fastening

hardware is unalloyed titanium to minimize corrosion

problems.

The hydrophone sensor is tethered to the seafloor

seafloor by floats. The floats are either glass spheres as

used to buoy the frame or syntactic foam-filled plastic

tubes which are more durable than glass during

deployment and recovery operations. The hydrophone is

buoyed off of the seafloor to minimize noise from spinning

disk drives in the data logger or flow noise from the

seafloor package, and to provide reception of acoustically

refracted sound waves arriving at low incident angle.

instrument via polypropylene line and buoyed 10 m off the

. Data Processing

ng-term, high-frequency (200 kHz)

aco

e 16 raw laptop disks in the

dis

essing files are generated in a format we

cal

ment results in about 2000

Working with lo

ustic data collected by HARPs can be challenging

because of the large size (2- 12 TB/yr per instrument) of

these data sets. Just the process of uploading from the

raw disks and backing up the large amounts of data can be

difficult and time intensive.

Data are uploaded from th

k block to a smaller number of larger form-factor (3.5”),

higher capacity disks to make data handling more

manageable. The larger disks are also more cost effective

than the laptop disks, do not need to be low-power, and

typically operate at faster rates. During the uploading

process, the data are copied from the HARP specialized

file system to a standard file system so that the data can be

read by a desktop computer. Each raw HARP disk is

copied to a single file (e.g., 120 GB) to provide a complete

backup of the original disk. The size of these backup

files requires the use of a 64-bit computer so that locations

within the file can be addressed to generate smaller and

more manageable working files. We use a computer

running a Linux operating system and the dd command to

upload the raw data into the backup files. The backup

files are then parsed into smaller (~1 GB) processing files

using MATLAB (www.mathworks.com). We found this

size to be a good optimization of the tradeoff between

having the fewest number of files per

instrument-deployment to manage and file sizes small

enough to easily process. Also, standard computers and

readily available software are currently limited to 32-bit

addressing, which prohibits easily working with files larger

than about 4 GB.

The 1 GB proc

l XWAV which is similar to a WAV formatted file but

which include additional information in an expanded

header. For example, the XWAV header also includes data

timing information (i.e., start and stop times), latitude,

longitude and depth of instrument deployment, and other

experiment specific information. The raw data are

evaluated for timing accuracies before including in the

XWAV file header. XWAV files have a single header

followed by a stream of data as in WAV files to allow for

more efficient data processing than a file format with

timing headers interleaved throughout the data. XWAV

files can be viewed and played with standard audio

software that can read WAV files. The XWAV header

also can be modified to adjust the gain and speed at which

the file is played in standard audio software, but still retain

the original amplitude and sample rate for processing with

XWAV-capable software.

Each instrument deploy

AV time series files, and viewing and analyzing each

one of these files in a non-automated way is not practical,

so we use a means of file compression for data overview

based on long-term spectral averages (LTSA).

Spectral-averaging is a method of searching for acoustic

events such as whale calls in long-term data sets (e.g., [12]

– [14]). Instead of inspecting short duration spectrograms

for individual calls, successive spectra are calculated and

averaged together. These averaged-spectra are arranged

sequentially to provide a time series of the spectra. The

averaging time determines the resolution of the resulting

plot and the data compression factor. Essentially,

spectral-averaging is a spectrogram over long time periods

and provides a coarse map or table of contents to groups of

events in the finer time scale XWAV data. Depending on

number of samples used for the spectra and the averaging

time used, data compression factors of 4000 or more are

possible while still providing enough resolution to observe

short-term events above the ambient noise.

III. RESULTS

s an example of our processing technique, an LTSA

was

and, offshore of southern

SA in Fig. 4 denoted

y thin vertical black lines at approximately 0.4, 1.2, and

1.5

Fig. o

s were

generated from HARP data sampled at 200 kHz

offshore of southern California in the fall of 2006 and is

shown in Fig. 4. The two hour LTSA was calculated using

1000 point fast fourier transforms (FFTs), Hanning

windows, no overlap, and averaged over 5 seconds.

Figure 4 shows quiet periods for approximately the first

and last one-third hours, with sounds of various intensities

and frequencies in-between. In this data set, marine

mammal sounds with similar characteristics lasting one to

two hours occur only two to four times per day, so a large

portion (70% – 90%) of these data do not contain calls, but

with the LTSA technique, the quiet times can be easily

passed over during analysis.

a site south of Santa Catalina Isl

Fig. 4. Two hour long-term spectral average (LTSA) fro

California and approximately 330m deep. The LTSA was

generated using 1000 point FFTs, Hanning windows, no

overlap and averaged every 5 seconds on HARP data

recorded at 200 kHz. Sections A, B and C are shown as

uncompressed spectrograms in Fig. 5

Three different sections of the LT

hours and are labeled A, B, and C, respectively. The

corresponding short time-scale spectrograms are calculated

and shown in Fig. 5. These spectrograms use 1000 point

FFTs, Hanning windows, and no overlap. Fig. 5a shows

mostly dolphin whistles from around 5 – 30 kHz, whereas,

Fig. 5b is full of broadband clicks from about 20 kHz to

presumably beyond our recording Nyquist frequency, and

many intense whistles at lower frequencies. Matching

these characteristics back to the LTSA plot shows most of

the energy from the whistles is clustered between 8 – 18

kHz and clicks are broad-band events primarily between

20 and 70 kHz. The spectrogram of Fig. 5c is not from

dolphins but from a passing boat using a 50 kHz echo

sounder which is easily observed in both Fig. 4 and Fig. 5c

in addition to the wideband noise at low frequency (< 2

kHz) presumably from the boat propulsion system.

sections A, B and C from Fig. 4. Spectrogram

5. Short time-scale spectrograms corresponding t

calculated using 1000 point FFTs, Hanning windows, and

no overlap. Note A and B are recordings of dolphins where

as C is from a passing boat with a 50 kHz echosounder.

IV. DISCUSSION

The motivation for HARP development is based on

providing enhanced capabilities to record mid-frequency

whistles and high frequency clicks from toothed whales

over long periods. While the preceding examples show

we have accomplished these goals along with the ability to

record low frequency baleen whales and anthropogenic

noise, we are still striving to improve the HARP’s

capabilities by increasing its sample rate, data storage

capacity and deployment duration.

Currently, HARP deployments are limited in duration

by the amount of data storage available and can record for

almost two months at maximum sample rate. However, as

larger capacity disks become available, longer

deployments will be possible with additional batteries.

These additional batteries may require HARPs to be

deployed as part of large oceanographic moorings where

the additional weight can be easily compensated with

additional buoyancy. On the other hand, lower power

electronics and faster data transfer rates from the memory

buffer to data storage disks (i.e., disks are powered for

shorter periods) also could provide for longer deployments

with the same or fewer batteries. Data compression

schemes may provide a means in which to decrease power

consumption rates and therefore increase deployment

duration, so these approaches also should be investigated.

Perhaps in the near future with the advancement of digital

cameras and other similar memory devices, low-power,

solid-state memory costs will decrease and their storage

capacities will increase enough to make it feasible to use

these devices to replace the energy-intensive, motorized

disk drives currently used in HARPs.

V. CONCLUSION

Long-term, broad-band, ocean acoustic recordings

from HARPs can provide detailed information on a variety

of sources including natural sounds from baleen and

toothed whales, other marine mammals like pinnipeds and

sirenians, fish, wind, rain, earthquakes, and from

anthropogenic sources such as ships, sonars, and seismic

exploration. Not until recently has an autonomous

acoustic system been capable of recording mid- to high-

frequency sound over long periods and if the trend in

consumer computer electronics continues as it has for the

past 30 years, we should expect longer-term, higher sample

rate, larger capacity, lower cost, and smaller instrument

packages to evolve.

ACKNOWLEDGEMENTS

We thank Chris Garsha, Greg Campbell, Ethan Roth,

Graydon Armsworthy, and Kevin Hardy for their

excellence in providing design and technical assistance

with development and deployment of HARPs throughout

the world’s oceans. Thanks also go to Erin Oleson, Melissa

Soldevilla, Jessica Burtenshaw, Lisa Munger, Marie Roch

and Mark McDonald for discovering new information on

marine mammals by processing HARP data. We thank

our funding sources and collaborators for their support of

HARP development and deployments: Center of Naval

Operations N45 Frank Stone, Ernie Young and Linda

Petitapas; Office of Naval Research Ellen Livingston and

Bob Gisner; Naval Postgraduate School Curt Collins;

National Oceanographic and Atmospheric Administration

Sue Moore, Brad Hanson, and Jay Barlow; Alaska

Department of Fish and Game Bob Small; Universidad

Autónoma de Baja California Sur Jorge Urban.

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