I. Marine Geophysical Techniques

A. Seismic Methods

1. Basic Technique

a. sound energy (measured in frequency usually - cycles per second - Hertz)

i. sound energy is transmitted from the surface,

ii. travels to the bottom & is reflected or refracted from various interfaces,

iii. travels back to the surface & is received by transducers or hydrophone arrays, &

iv. converted to electric signals & recorded by computer or graphically

b. The result is an accurate measurement of the TIME it takes sound to travel from the ship to the various interfaces & back to a receiver

c. To obtain DISTANCE (or DEPTH), the VELOCITY of sound in the material through which the sound travelled must be known

i. DISTANCE (or DEPTH) = 1/2 VELOCITY X TIME

2. Echo Sounding - Bathymetry

a. 40 kHz - Fathometer - shallow water (<100 m) only

b. 12 kHz - PDR - Bathymetry in deep water (up to 12 km)

i. no subbottom penetration

c. 3.5 kHz - PDR - Bathymetry in deep water (up to 12 km)

i. up to 100 m of subbottom penetration

3. Seismic Reflection Profiling - Subbottom Geology

a. Uses Large Energy Sources (Explosives, Sparkers, or Airguns)

b. Frequencies are lower than echo sounding (10's to 1000's of Hz)

c. Subbottom penetration is greater (100 to 12000 m)

d. Simple seismic profiling only uses essentially one receiver

i. to convert resulting time section to depth section, velocities must be obtained from some other means

e. Multichannel Seismic Profiling uses a large number of receivers & requires considerable computer processing to get a finished product

i. velocities are obtained during this processing, so depth sections can be produced directly

ii. increased signal to noise ratio

4. Comparison of Echo Sounding with Seismic Reflection Profiling

a. Attenuation of Sound Energy

i. Function of Frequency

high frequency - rapid attenuation

low frequency - slow attenuation

b. Resolution of Reflectors

i. Function of Wavelength

short wavelength - excellent resolution

long wavelength - poor resolution

ii. Related to Frequency & Velocity

Wavelength = Velocity / Frequency , or

Velocity = Wavelength X Frequency

so: 0.571 m. = 2000 m/sec / 3.5 kHz,

2 m. = 2000 m/sec / 1000 Hz, &

20 m. = 2000 m/sec / 100 Hz

thus:

high frequency - excellent resolution

low frequency - poor resolution

5. Seismic Refraction

a. used to obtain velocities & geometries of deep layers

i. sound energy travels parallel to layers during part of its travel path

b. requires stationary receiver - either another ship or a SONOBUOY, an expendible (thrown overboard) free-floating receiver that transmits signals back to the ship by means of radio waves

c. much simpler processing

i. refracted sound waves from a particular layer form a straight line on plots of time of received waves vs. distance between shot & receiver

ii. the slope of the line is proportional to the velocity of sound of the layer through which the sound waves are passing

B. Marine Magnetics

1. Magnetometer

a. big bottle of distilled water surrounded by a coil of wire

b. a large current is passed though the wire, inducing a large magnetic field oriented along the long axis of the bottle

c. the protons in the water also have magnetic fields produced because they are spinning electrically-charged particles

d. the protons align themselves with this induced field

e. when the current is turned off, the induced field disappears, & the protons then align themselves with the Earth's magnetic field

f. the protons achieve this alignment with the Earth's magnetic field by precessing like a top

g. the frequency of the precession of the protons is a function of the strength of the Earth's magnetic field

2. at sea, variations in the strength of the Earth's magnetic field usually are caused by oceanic crust that is either normally polarized or reversely polarized

3. the magnetic character of seamounts can indicate whether the seamount is underlain by magnetic or nonmagnetic materials

C. Marine Gravity

1. Gravimeter

a. essentially a weight on the end of a spring

b. the length of the spring varies as the strength of the Earth's gravity field varies

c. variations of the Earth's gravity field can occur because of varying density of rocks beneath the Earth's surface

II. Marine Geological Techniques

A. Coring

1. Piston corers

a. most useful in obtaining sediment for stratigraphic & paleoceanographic studies

b. a tight-fitting piston inside core barrel configured to be held at or near the sediment water interface creates a suction that holds the sediment in the core barrel & reduces friction between the core barrel & the sediment

c. cores are generally 10-20 m long & are essentially undisturbed

d. special piston corers

i. giant piston core - to get large cross-section cores

ii. hydraulic piston core - used in DSDP & ODP programs to obtain relatively undisturbed cores from the sediment immediately in front of the drill bit (see page 99), in contrast with the highly disturbed sediment obtained only by rotary drilling

2. Gravity corers

a. free fall into bottom

b. generally shorter (<5 m) & more disturbed than piston cores)

c. less complicated, more easily & quickly deployed, so that many more cores can be obtained in a given amount of time

d. some gravity corers have special adaptations to reduce internal friction & obtain relatively long & undisturbed cores

3. Box corers

a. large rectangular box with basal blades for obtaining short (<0.75 m), but voluminous (up to 2 x 105 cm3), surface sediment samples

i. a standard 20 m-long piston core obtains about 1 x 105 cm3 of sediment

B. Dredging

1. a dredge is basically a big rectangular or circular metal frame enclosed at the bottom by a chain bag

a. it is used to get rock samples from the bottom

C. Bottom cameras

1. bottom bounce

2. sleds

D. Submersibles

1. used to carry scientists to the bottom for direct observations & detailed sampling & deployment of instruments

2. example, ALVIN

a. owned by NSF, ONR & NOAA (National Ocean & Atmosphere Administration), & managed by WHOI

b. can dive to 4000 m & operate for up to 8 hours (limited by batteries), although there is enough oxygen on board to remain submerged for 3 days

c. 2 scientists & a pilot in a 6-foot sphere

d. manipulator arms for recovering samples & deploying instruments

e. photographic record obtained by externally mounted bottom cameras (35 mm still & black & white video) & internal hand-held still & video cameras

III. Navigation Methods

A. Astronomical

1. shoot the stars & sun

2. accurate to probably 5 nautical miles (n.m.)

a. 1 n.m. = 1.15 statute mile = 6060 feet = 1814 m = 1.814 km = 1 minute of latitude (60 n.m. = 1 degree of latitude)

3. used mostly during period prior to WWII

B. Sighting on Land Features

1. triangulate on land features

2. useful only nearshore

3. accurate to 1-2 n.m.

C. Radio Methods including LORAN

1. land based master & slave transmitters; ship based receiver capable of tuning in master & one slave transmitter

a. measure difference in time between reception of the two signals

2. accurate to 50-500 m (0.025-0.3 n.m.)

a. accuracy degrades at sunrise & sunset

3. useful only near North America & Europe

D. Satellite Navigation

1. several satellites in polar orbit transmitting a given frequencies

2. satellite receiver on ship linked to computer

3. Doppler effect

4. accurate to 50-200 m (0.025-0.1 n.m.)

a. accuracy improved if ship motion well constrained

E. Transponder Navigation

1. used to navigate instruments & submersibles on the seafloor

2. transponders are similar to PDR transducers

a. transponders transmit in response to signal received from ship's PDR transducer

i. transmit at 8 kKz, 10 kKz, 12 kKz, 15 kKz, or 20 kKz

3. transponders dropped over side to form a transponder network

a. transponder positions are determined by several passes of the ship near & across the transponders

4. additional transponder is attached to the instrument or submersible

5. position calculated by triangulation

6. accurate to 10-50 m (0.0025-0.025 n.m.)

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