1. Marine Reflection Seismology - Geometry
The streamer records waves reflected from interfaces
Concept of reflection. Ship steams in a straight line firing airguns every s. Reflection recorded at various distances along the streamer

2. Marine Reflection Seismology - Data
The seismic data recorded for a particular shot will look display a geometric effect termed “normal moveout” (NMO) which reflects the increased distance the wave travels for as the source-receiver offset increases
Time of reflections will increase with distance along streamer since the path is longer
Offset X
Time, s
Time

3. Marine Reflection Seismology - Sorting Records
The records are sorted so that they all have the same mid-point (Common Mid-Point - CMP)
To process data, reflections are reordered (demultiplexed) so that that they are in sets with common mid-points and increasing horizontal offsets.

4. Marine Reflection Seismology - Airgun Sources
The seismic records can be corrected for geometric affects and stacked (summed) to produce a single record for the reflections below each each point
Before Geometric Correction
After Geometric Correction
Stacked (summed)
A geometric correction is applied to align trace. Depends on velocity but one can get this by trying different values to align reflection.
Traces are now summed (stacked) to increase signal to noise. Up to ~100 traces may be stacked. The stacked trace is a record of the reflections for a vertical ray at the mid-point

5. Marine Reflection Seismology - Filled Wiggle Plots
Stacked records are plotted on the same plot with the horizontal axis showing position along the profile. Rather than showing lines for each record the plots often show filled regions for positive (or negative) displacements
Stacked traces at different mid-points are now plotted on a reflection profile. Go from wiggle plots to filling in positive deflection
Time, s
Position

6. Reflection Profile CMP Number 1 km 300 400 500 600 700 800 900 1000
The resulting profile shows the two way time (up and down of reflection) as function of distance (or the number of the common mid-point). Want to interpret two-way times in terms of depth
300
400
500
600
700
800
900
1000
CMP Number

7. Fast/Intermediate Spreading-Rate Ridges3 6
Depth, km
Ridge
Axis
Crustal formation at spreading rates of >~5 cm/year. Basic structure of ocean crust. 6 km thick (key #), melt in magma chamber – crust below forms by crystallization of gabbros (coarse grained basalt). Crust above by diking-eruptive events which give a dike layer with volcanics on top
Sinton & Detrick, 1992

8. Water Basalts – Layer 2A Sheeted Dikes - Layer 2B Gabbro – Layer 3
(1.5 km/s; 1000 kg/m3)
Reflector
Ophiolites
Basalts – Layer 2A
(2.5 to 4 km/s; 2200 kg/m3)
Reflector
Sheeted Dikes - Layer 2B
( km/s; 2800 kg/m3)
Gradational
(No reflection except when magma chamber is present)
4 major geological units
3 units form the crust
Basalts - basaltic rocks that erupted on the seafloor as pillow lavas and sheet flows
Sheeted Dikes - Magma solidified in 1-m-wide vertical cracks extending along the ridge axis. These cracks transported lavas from the magma chamber to seafloor eruptions.
Gabbros - Course grained rocks formed by slow cooling in the crustal magma chamber
1 unit forms the mantle
Harzburgite - Mantle rocks which partially melted to produce the crust.
Gabbro – Layer 3
(6.5 to 7 km/s; 2800 kg/m3)
Structure of crust from ophiolites, drill holes, seafloor scarps/cliffs
Reflections from seafloor, base of volcanics (change in porosity), Moho. Dike/gabbro boundary is gradational so only a reflector when magma is present
Layer 2A, 2B and 3 were names given by seismologists before it was known what the layers were (Layer 1 is sediment and Layer 4 is mantle but these terms are no longer used)
Reflector
Moho
Mantle
(8 km/s; 3400 kg/m3)

9. molten 2A
gabbro
Shows seismic velocity versus depth and correspondence of seismic layers to rock units
Moho

10. A reflection profile across the East Pacific Rise
Relection profile from East pacific Rise. Seafloor, base of layer 2A, AMC, Moho (M)
Reflections come from the seafloor, the base of layer 2A (pillow basalts), the axial magma chamber (AMC) and the Moho (M)

11. Intersecting Record Sections from the East Pacific Rise
Higher quality for upper crust. Along and across rise axis.
Lab you will look at a profile for the Juan de Fuca Ridge