Marine Reflection Seismology - Geometry The streamer records waves reflected from interfaces Concept of reflection. Ship steams in a straight line firing airguns every 30-60 s. Reflection recorded at various distances along the streamer

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

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.

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

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

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

Fast/Intermediate Spreading-Rate Ridges 3 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

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 (5.5-6.5 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)

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

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)

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