INSTRUMENTS &

INDIRECT SAMPLING

GEOL 1033

Lesson 7 in the Study Guide

INDIRECT STUDY & SAMPLING• Photography

• Nets

• Dredges

• Grab samplers

• Corers

• Echo sounding

• Side-scan sonar

• Towed magnetometer

INSTRUMENT DEPLOYMENT

• Most instruments are attached by a cable to a winch– Data or samples must be retrieved

– Not expendable

• Different types:– Towed

– Suspended

– Self-powered

PINGERS• Used to position instruments suspended from a ship

• Usually, 1 second between sounds from pinger

• Detected with echo sounder

• Difference in arrival time between direct signal &

reflected signal from bottom determines depth

of instrument

CAMERAS for BOTTOM PHOTOGRAPHY• Diagram of a deep-sea camera system as it

takes a picture of the seafloor

• Camera sled

NETS• Plankton nets

– Microscopic organisms

– Floating (planktic or planktonic) pelagic forms

– Base of the marine food web

– Major groups• Phytoplankton

• Zooplankton

• Nets for larger organisms, e. g., fish– Various kinds for various depths and organisms

Bottom trawl net used on the Challenger

ROSETTE• Lowered on a cable

• Electronically determines:– Temperature

– Salinity

– Depth

– Other data

• And, it collects a water samples

BOTTOM SAMPLES• Getting bottom samples without having to be there:

– Surface materials• Dredges

• Grab samplers (=clamshell samplers & snappers)

• Box cores

– Subsurface sediments• Gravity corers

• Piston corers

• Drilling ships

DREDGE• Dragged along seafloor

• Strong, open "mouth" scoops materials into rear

• Size of rear openings determines minimum size of collection

• Smaller sizes wash out

SAMPLING MARINE SURFACE SEDIMENTS

• “Grab sampler” (="clamshell sampler")– Lowered until it strikes bottom

– A release mechanism then closes "cups"

– Heavy enough to stay closed when raised

– Covers prevent sediment from washing out

cover

Scissor-like

SAMPLING MARINE SURFACE SEDIMENTS• How a Van Veen-type grab sampler works

DOWN TOUCH & RELEASE CLOSE & UP

BOX CORERS

• Box-like metal container

• Forced down into upper half metre of surface sediment

• Closed, then raised back to surface

• Sample is relatively large– Little disturbance of structures

– Enough for different studies

– Shows megascopic sedimentary structures

GRAVITY CORER• Main components

– 1. Nose cone

– 2. Core catcher

– 3. Inner plastic liner

– 4. Steel core barrel

– 5. Weights (up to a ton or more)

– 6. Flap (on gravity corer) or piston

– 7. Tripping arm (on piston corer)

– 8. Cable to ship

FREE-FALL GRAVITY CORER• Expendable

• Empty glass spheres– Buoyant

– Pull out core liner

– Float to surface

– Flashing light signal

PISTON CORER

DEPLOYING & RETRIEVING PISTON CORERS

CUTTING CORE LINER• 40-foot long core liner with core is being cut

– Transversely

– 1.5 m lengths

– Before splitting

– (How do you tell which one is a crewman?)

Magnetic Intensities & Directions of Rocks & Sediments in Cores

• JOIDES Resolution": the paleomagnetic equipment on board can record the magnetic intensities and directions of rocks & sediments as cores are recovered. Distinctive patterns of magnetic field reversal records are used with other stratigraphic data to determine the geologic age of rocks and sediments.

SPLIT CORE for Study• Core is cut in half lengthwise (longitudinally)

• Placed in trays & photographed

• Stored in hermetically sealed cold rooms.

• One of the halves is studied & sampled

• Other half is archived

Turbidites deposited by turbidity currents

CORE LIBRARIES• Gulf Coast Repository (pictured here) of the Ocean Drilling Program, located

at Texas A & M University, stores about 75,000 sections taken from more than 80 kilometers (50 miles) of cores recovered from the Pacific and Indian oceans.

• Smaller core libraries are maintained at – Scripps

– Lamont-Doherty Earth Observatory.

Stopped here on Thursday’Oct. 15th

CLIMATE CHANGES INTERPRETEDFROM DEEP-SEA CORES

• Identification of foraminiferal species (planktic) helps to date layers in deep-sea cores.

• Plotting the abundance of cold and warm climate Foraminifera from core samples indicates climatic changes through time.

ECHO SOUNDING• Determining water depth

• Old methods– Ropeline

– Rope with weight

– Rope with weight release

– Steel line & weight release

• Sound echo– Formula:

Depth = 1/2 (travel Time) x (Speed of sound in water)

or

Depth = 1/2 Time x Speed = (sec) x (m/sec) = m

(Speed of sound in water is ~1500 m/s)

• Continuous echo sounder (PDR)– German ship Meteor 1925-27

– Permanent graphic record

CONSTRUCTING DEEP-SEA PHYSIOGRAPHIC DIAGRAMS

SIDE-SCAN SONAR• SONAR = Sound Navigation And Ranging

– High frequency sound waves (`1500 m/s in seawater)

– Detect reflections back off objects under water

• Conventional echo sounders– Miss features with steep slopes

– Cannot detect small features

• Side-scan sonar– Does not emit signal straight down from ship

– Emits signal • At an angle to seafloor

• From a towed streamlined device ("fish") at depth

– Differences in intensity of returning (reflected)

signal can be used to distinguish details, such as– Downed aircraft

– Sunken ships

– canyon walls, etc.

Towed Magnetometer• JOIDES Resolution": the Underway Geophysics Laboratory is on the fantail under the

helipad. While the ship is in transit between drilling sites, digital single-channel seismic reflection profiles are collected and processed along with echo soundings, and magnetometer traces. This view shows a towed magnetometer package.

GRAVITY METERS

• Measure the variations in the strength of Earth’s gravity field which is partly dependent on the underlying sediment properties and those of the underlying basalts of the oceanic crust

End of File

• Use of a diving bell, ca. 1752 -- an early attempt at marine exploration. From an engraving in Univ. Mag., Mar. 1752, v. 10, opp. p. 113.

BASAL SEDIMENT AGES• Core studies permit dating of sediments

• Basal sediments increase in age away from mid-ocean ridges

• Confirm seafloor spreading

Direct signal from pinger

Pinger signal reflected off seafloor

time

Sidescan Sonar "Fish"• Georges Bank (offshore New England), U.S.A. Studying the ocean floor: Cruise 94-11 of

the RV "Delaware II" in 1994.

• Sidescan sonar "fish" has a winged "depressor" to get it down to depth with the minimum amount of wire out. It has transducers port and starboard and they can be checked with a rub test (rub briskly for about 30 seconds and look for noise on the record) before it is deployed to see if it is sending data to the recorder.

• Side scan sonar "fish" is lowered over the stern of the RV "Delaware II" for imaging the bottom in swaths that will be digitally merged into a mosaic of the seafloor.

• Towed behind the ship at about 4 knots, by a cable and winch and "flown" about 12 meters above the seafloor.

• Photo shows torque-balanced wire (prevents twisting) being used to lower the sidescan sonar "fish" to the proper depth. The winch can be controlled remotely in the lab and is hauled in or payed out to keep the "fish" at the proper altitude off the seafloor.