The L-CHEAPO is a completely autonomous seafloor data logging system based
on 2 decades of development at Scripps Institution of Oceanography. It incorporates
a custom acoustic navigation and release system with a modern, high capacity
digital data logger, and can be configured for a variety of scientific tasks.
To date, these instruments have been used for passive seismology, active-source
seismology, borehole seismometers/tiltmeters, magnetotelluric sounding,
monitoring of whale vocalizations, and ocean acoustic studies.
Logging electronics reside in a 14.6 cm inside diameter 7075-T6 aluminum
tube which is anodized and painted to resist corrosion by seawater and terminated
by two end-caps sealed with O-rings. One endcap has ports to start the computer,
purge damp air from the instrument, and to connect to the SCSI disk drive
inside the instrument. The other endcap has high-pressure, underwater connectors
for linking the sensors to the logger inputs. The entire system is capable
of resisting water pressure to depths of 7,350 m.
The logger pressure case is supported in a polyethylene framework which
protects the instrument from damage during handling and supports three to
five glass flotation spheres, the acoustic release package, and sensors.
The acoustic release unit serves both to locate the instrument underwater
and to release the package from the seafloor at the end of the recording
period.
During marine operation, a steel or concrete anchor is attached to the plastic
frame by means of a release containing a short, nylon insulated, stainless
steel wire (burn-wire). The insulation is cut to bare a 2 mm section of
wire which, on receipt of the release command, is supplied with +18V from
internal batteries in the acoustic release package. Within 5 to 10 minutes,
this positive voltage causes the steel wire to electrolysize away, releasing
the anchor from the instrument and allowing the positively buoyant package
to float to the surface for recovery.
Instrument components:
Sensors - A variety of sensors can be connected via up to four 4-pin
underwater bulkhead connectors, and even powered from within the L-CHEAPO.
To date we have used hydrophones, the broadband pressure variometer developed
by Chip Cox and Spahr Web (sometimes called DPGs or differential pressure
gauges), DC or AC coupled electric dipoles, magnetometer coils, and bubble
inclinometers.
Postamplifiers - An analogue card cage, which houses the analogue
to digital converter (ADC), has provision for up to 5 amplifer, signal conditioning,
or driver cards for sensors such as the DPG. Alias filters are switched
by replacing an integrated 16-pin DIP module. We have a 2-channel AC-coupled,
fixed gain postamplifier of relatively simple design, a gain-ranging amplifer
for use with hydrophones, and very high gain (120 dB) amplifier for active-source
EM studies or high-frequency magnetotelluric sounding.
Data logging system - The data logging system is controlled by Onsett
Corporation's Tattletale 8 (TT8) microcomputer. We have equipped this device
with 4 Mbytes of PCMCIA ram storage and developed a digital backplane that
allows this computer to control the various timing, logging, storage and
power switching operations required for low-power, autonomous data logging.
The TT8 acquires data in RAM by controlling a 16 channel, 16 bit analog-digital
converter (ADC) via a parallel interface port. Once the RAM buffer is filled,
a small computer systems interface (SCSI) disk drive is switched on by sending
a control signal to the power card, which provides the regulated 12 and
5 Volts required by the disk drives. Data are then transferred over the
SCSI interface to the disk, and the disk powered down. Disk spinup and transfer
of 2 Mbytes of
data takes less than 60 seconds. The entire logger, amplifiers, and sensor
systems can be powered by means of rechargable NiCad batteries or lithium
oxyhalide cells.
Clocks - Since all the seafloor instruments are autonomous, and beyond
the reach of radio communication, accurate timing must be accomplished by
on-board quartz clocks. The data logger is timed by a custom low-power oscillator
built for SIO with a timing accuracy of about 1 part in 10^8. Phase locked
loops provide all the frequencies required by the system, such as the 40
kHz CPU clock, the 2 kHz for the E-field chopper amplifier, the ADC clock,
sample interrupts, and the software real time clock of the TT8. The on-board
clocks are started using a GPS time standard, with initial timing accurate
to 1-10 microseconds. After recovery, clocks are again checked against the
GPS standard to estimate drift or error. Drift rates are typically 1 ms
per day or so.
Logger mass storage - The data primarily reside on the SCSI disk
drive in the logger, in a binary format of SIO standard. Since the logger
disk drives are easily removable, instruments are turned around between
deployments by swapping recently used disk drives with ones that have already
been transcribed. If data volumes are modest (less than 500 Mbyte), instruments
can be turned around even more quickly by reading the data through the encap
SCSI port, without opening the pressure case. In shallow-water MT deployments,
instruments can be routinely returned to the ocean within an hour of sending
the release command.
Coming attractions - By the end of 1998 we expect to have implemented faster SCSI transfers, a hardware real-time clock, a larger memory board, a 4-channel 24-bit ADC, and GPS timing on land-based logging systems.