Seismic Acquisition part 2

Question 1

Describe operations in a mountainous region

Answer 1

• No access for heavy equipment, need to use dynamite
• Geophone system used but fewer number, can be difficult to plant spikes, spike can be replaced by base plate
• To minimise extent of cables geophones sent to recording instruments by telemetry

Question 2

Describe operations in a jungle

Answer 2

Possible to bulldoze and clear survey lines for conventional shooting techniques

Dynamite sources tend to be used in shallow holes rather than vibroseis and recording often by telemetry

Question 3

What potential issues are there in planning marine operations?

Answer 3

• Naval restricted zones
• Constructions (oil platform)
• Areas containing fish traps, reefs
• Needs to give adequate warning to fishermen, shipping authorities so other ships avoid it
• Vital to ensure survey is designed to intersect as closely to any exploration wells drilled as possible

Question 4

What two kinds of boats are used? Briefly describe them

Answer 4

Multi purpose vessel: Designed for seismic, recording and positioning unit, houses all crew and equipment, small safety boats to rescue and warn off other ships

Draught boats: used for near shore surveys

Question 5

What is often used to generate signal during marine ops? How do they work?

Answer 5

Air guns: Compressed air shot rapidly from a cylinder into the sea. Frequency of signal depends on volume of cylinder

Question 6

How can airguns be made less noisy?

Answer 6

Simultaneous firing of differently sized air guns

Question 7

What is the issue with airguns and how is this mitigated?

Answer 7

Issue: bubbles formed produce pulses of noise after shot fired

Mitigation: bubble noise is predictable and easilt eliminated by deconvolution at later stage, also if different sized air guns fired then bubbles tend to be destructive and noise reduced

Question 8

Waterguns run on same principle as air guns, why are they used less frequently?

Answer 8

Low frequency content of the signal is limited

Question 9

Can vibroseis be used at sea?

Answer 9

Occasionally can be used as marine or shallow marine source

Question 10

How is signal recorded off shore?

Answer 10

Same as land, common depth point method

Question 11

What receiver is used off shore and how does it work? Why does it not pick up s-waves and boundary waves?

Answer 11

Hydrophones: measure variations in water pressure as changes in voltage by means of two matched piezo-electric crystals

S-waves and boundary waves are not transmitted through water

Question 12

What are hydrophones arranged within?

Answer 12

A cable or streamer

Question 13

How is streamer kept at correct depth?

Answer 13

Oil filled neutral buoyancy, kept at the correct depth by vaned depth controllers

Question 14

How is the position of streamer tracked? Why does the boat have to turn?

Answer 14

Streamer end is tagged by radar reflector and GPS

Meeds to turn to ensure streamers are straight for next run

Question 15

What are benefits of the new synthetic solid streamer?

Answer 15

• More robust and unaffected by noise from towing, critical on near traces
• Reduced noise capabilities make it more versatile in collecting data in bad weather

Question 16

When are dual vessel operations used?

Answer 16

• When undershooting man made obstructions
• Making up coverage across a patch of shallower water
• Split spreads for very long offsets to undershoot salt domes
• Wide swathe surveying with two vessels running side by side

Question 17

During dual vessel operations, what are the roles of the master and slave vessels?

Answer 17

Master: line selection and shot numbering
Slave: transfers positioning to master

Question 18

How are Ocean Bottom Cables used during deep water acquisition?

Answer 18

• Laid on sea bed

- Stay in position while a seismic vessel travels overhead completing a series of shots

Question 19

What do OBC hydrophones have? How do they transport info received? What are the benefits?

Answer 19

• Each source equipped with GPS and each receiver has its own high frequency acoustic positioning system
• Info received transported back to surface using telemetry
• Cable stationary so no noise and below wave base so weather independent

Question 20

What makes it easier to remove multiple noise effects in OBC?

Answer 20

Can use both pressure sensitive hydrophones and motion sensitive geophones

Question 21

What techniques can be used in transisiton zones?

Answer 21

Combination: geophones laid out on beach and in shallow water and recorded into by an airgun source in the shallow water

Bay cable: similar to OBC in that it contains hydrophones and designed to rest on seabed, bekow wavebase so reduced noise

Remote detectors can be laid out in muddy estuaries using inflatable craft at high water, data transported by telemetry