a#general_region

Kreemer and Holt, 2000

Das and Henry, 2003

Kreemer and Holt, 2000

GEOL 460 LECTURE 5 TODAYS MATERIAL: Magnetometry Magnetometry PREVIEW MATERIAL: Gravity Gravity Note Supplemental Reading on website:

Potential Paths in a Refraction Survey When doing a seismic refraction survey, a recorded ray can come from three main paths The direct ray The direct ray The reflected ray The reflected ray The refracted ray The refracted ray Because these rays travel different distances and at different speeds, they arrive at different times. The direct ray and the refracted ray arrive in different order depending on distance from source and the velocity structure. Scott Marshall

The Time-Distance (t-x) Diagram Think about: What would a fast velocity look like on this plot? Why is direct ray a straight line? Why must the direct ray plot start at the origin (0,0)? Why is refracted ray straight line? Why does refracted ray not start at origin? Why does reflected ray not start at origin? Why is reflected ray asymptotic with direct ray? Scott Marshall

The Direct Ray The Direct Ray Arrival Time: Simply a linear function of the seismic velocity and the shot point to receiver distance Scott Marshall

The Reflected Ray The Reflected Ray Arrival Time: is never a first arrival is never a first arrival Plots as a curved path on t-x diagram Plots as a curved path on t-x diagram Asymptotic with direct ray Asymptotic with direct ray Y-intercept (time) gives thickness Y-intercept (time) gives thickness Why do we not use this to estimate layer thickness? Extra credit! Why do we not use this to estimate layer thickness? Extra credit! Scott Marshall

The Refracted Ray The Refracted Ray Arrival Time: Plots as a linear path on t-x diagram Plots as a linear path on t-x diagram Part travels in upper layer (constant) Part travels in upper layer (constant) Part travels in lower layer (function of x) Part travels in lower layer (function of x) Only arrives after critical distance Only arrives after critical distance Is first arrival only after cross over distance Is first arrival only after cross over distance Travels long enough in the faster layer Travels long enough in the faster layer Scott Marshall

or Refracted Ray Arrival Time, t Making a t-x diagram Scott Marshall Branch 1 Slope = 1/α 1 Branch 2 Slope = 1/α 2

Making a t-x diagram Dentith and Mudge, 2014 Branch 1 Slope = 1/α 1 Branch 2 Slope = 1/α 2

Seismic Reflection: The Basics In the simplest sense seismic reflection is echo sounding. Echoes come from layers in the Earth, not fish or the sea floor Echoes come from layers in the Earth, not fish or the sea floor E.g. a ship sends out a seismic pulse The pulse is reflected back to a receiver on the ship’s bottom after some time has passed The pulse is reflected back to a receiver on the ship’s bottom after some time has passed The various arrivals can be used to map out subsurface “reflectors” or layers The various arrivals can be used to map out subsurface “reflectors” or layers

Seismic Reflection Caveats 1.The vertical scale on seismic reflection profiles is time, not depth Velocity varies with depth, so time cannot be easily converted to depth Velocity varies with depth, so time cannot be easily converted to depth 2.Reflections may not come from directly below the source Reflections occur perpendicular to the interface. Receivers / Geophones / Seismometers cannot directly detect this. Reflections occur perpendicular to the interface. Receivers / Geophones / Seismometers cannot directly detect this. 3.There may be multiple reflections off of single interfaces Called multiples Called multiples

Non-Vertical Reflection Reflected rays travel back to the source following a path perpendicular to the interface The receiver will record an arrival time that is too short and a dip that is too shallow The receiver will record an arrival time that is too short and a dip that is too shallow We’ll talk more about this later… We’ll talk more about this later…

Multiples On their return to the surface… On their return to the surface… Reflected rays can also reflect back down and then later be reflected back up Reflected rays can also reflect back down and then later be reflected back up This causes a single reflector to potentially produce several “multiples” This causes a single reflector to potentially produce several “multiples” Short path (less reflections) multiples are usually stronger Short path (less reflections) multiples are usually stronger These artifacts can be removed by migration These artifacts can be removed by migration We’ll talk about migration later… We’ll talk about migration later…

Multiples in a Seismic Section The sea floor is commonly a strong reflector, so it commonly produces multiples

Velocity Determination Using Normal Moveout To deduce the velocity structure, multiple receivers are needed so that most rays do not travel vertically To deduce the velocity structure, multiple receivers are needed so that most rays do not travel vertically For a horizontal reflector… For a horizontal reflector… The shortest path is the vertical one The shortest path is the vertical one Rays that reach receivers to each side travel increasingly longer distances Rays that reach receivers to each side travel increasingly longer distances This is the Fixed Source Method There are also other methods: e.g. the Common Midpoint Method

Velocity Determination Using Normal Moveout Normal Moveout = The later time of arrival of the reflected rays at receivers offset from the source for a horizontal reflector. Normal Moveout = The later time of arrival of the reflected rays at receivers offset from the source for a horizontal reflector. On a t-x diagram, the Normal Moveout (NMO) produces a hyperbola. On a t-x diagram, the Normal Moveout (NMO) produces a hyperbola.

Using the Fixed Source Method, we can estimate seismic velocity Using the Fixed Source Method, we can estimate seismic velocity These parameters are read off of the t-x diagram These parameters are read off of the t-x diagram If layer thickness is large compared to receiver offset… If layer thickness is large compared to receiver offset… X But where does this come from? Velocity Determination Using Normal Moveout

Multiple Horizontal Layers When multiple reflectors are encountered, multiple NMO hyperbolas are produced When multiple reflectors are encountered, multiple NMO hyperbolas are produced The shallowest reflector arrives first at t1,0 The shallowest reflector arrives first at t1,0 Each reflector’s TWTT for vertical reflection (t1,0, t2,0, t3,0, etc…) and NMO (Δt) can be read off of the graph. Each reflector’s TWTT for vertical reflection (t1,0, t2,0, t3,0, etc…) and NMO (Δt) can be read off of the graph. We can then easily calculate the one-way travel times in each layer, τn We can then easily calculate the one-way travel times in each layer, τn

Curved Reflectors If the reflector is curved, distortions may be more complex If the reflector is curved, distortions may be more complex There may be many paths between the reflector and receiver There may be many paths between the reflector and receiver

Anticline: Santa Barbara 2004 Pacific Cell Friends of the Pleistocene

%20I/Compl%C3%A9ment%20au%20cours/la%20terre/Orog%C3%A9n%C3 %A8se/Orog%C3%A9n%C3%A8se.htmSyncline: Sideling Hill Road cut I-68 near Hancock, MD

Fold Trains Folds exposed along a cliff eastern Ireland Anticlines and synclines are commonly found together in trains of folds. Anticlines and synclines are commonly found together in trains of folds. Note that in these particular folds, the fold axes are not vertical. Note that in these particular folds, the fold axes are not vertical.

Curved Reflectors If the reflector is curved, distortions may be more complex There may be many paths between the reflector and receiver There may be many paths between the reflector and receiver The vertical ray is generally not first to arrive Only true if the center of curvature is below surface of earth Only true if the center of curvature is below surface of earth Common for tight folds Common for tight folds Open folds don’t usually produce multiple arrivals Open folds don’t usually produce multiple arrivals

Synclines Raw data from a multi-shot reflection survey produce… Raw data from a multi-shot reflection survey produce… A “bow tie” pattern for synclines A “bow tie” pattern for synclines If center of curvature is above the surface… If center of curvature is above the surface… A narrowed syncline is produced A narrowed syncline is produced (stacked)

Anticlines For subsurface anticlines… For subsurface anticlines… A broadened or widened anticline pattern with multiples near the anticline margins A broadened or widened anticline pattern with multiples near the anticline margins

Refraction vs. Reflection So, what are the main take-home differences between these two seismic techniques? RefractionReflection Resolves gross crustal velocities Resolves fine subsurface details critical refraction requires large v gradient requires a change in v or density X=5-10x the depth of interest X=1x the depth of interest Processing is relatively easy Processing can be very CPU intensive ~30% of global CPU time is spent on processing seismic reflection data

Magnetic Surveys! 14/ /?type=3&theater

Inverse Theory We think that gold is buried under the sand so we make measurements of gravity at various locations on the surface.

Newtonian gravitation relates the gravitational potential, φ, to density, ρ. So although we don't know how to turn our gravity measurements into direct information about the density in the earth beneath us, we do know how to go in the other direction: given the density in the earth beneath us, we know how to predict the gravity field we should observe.

An idealized view of the beach. The surface is at and the subsurface consists of little blocks containing either sand or gold.

Our preconceptions as to the number of bricks buried in the sand. There is a possibility that someone has already dug up the gold, in which case the number of gold blocks is zero. But we think it's most likely that there are 6 gold blocks. Possibly 7, but definitely not 3, for example. Since this preconception represents information we have independent of the gravity data, or prior to the measurements, it's an example of what is called a priori information.

Pirate chests were well made. And gold, being rather heavy, is unlikely to move around much. So we think it's mostly likely that the gold bars are clustered together. It's not impossible that the bars have become dispersed, but it seems unlikely.

A stickier issue is that the real beach is definitely not one of the possible models we consider. The real beach is three-dimensional, is three-dimensional, has an irregular surface, has an irregular surface, has objects in addition to sand and gold within it (bones and rum bottles, for example) has objects in addition to sand and gold within it (bones and rum bottles, for example) has an ocean nearby, and is embedded in a planet that has lots of mass of its own and which is subject to perceptible gravitational attraction by the Moon and Sun has an ocean nearby, and is embedded in a planet that has lots of mass of its own and which is subject to perceptible gravitational attraction by the Moon and Sun etc. etc.

An unreasonable model that predicts the data. The true distribution of gold bricks.

The magnetic flux surrounding a bar magnet. Schematic representation of an element of material in which elementary dipoles align in the direction of an external field B to produce an overall induced magnetization.

Histogram showing mean values and ranges in susceptibility of common rock types.

The geomagnetic elements.

The variation of the inclination of the total magnetic field with latitude based on a simple dipole approximation of the geomagnetic field.

Vector representation of the geomagnetic field with and without a superimposed magnetic anomaly.

The horizontal (DH ), vertical (DZ), and total field (DB) anomalies due to an isolated positive pole.

Principle of the proton magnetometer.

A typical flight plan for an aeromagnetic survey.

The removal of a regional gradient from a magnetic field by trend analysis. The regional field is approximated by a linear trend.

Gravity (Dg) and magnetic (DB) anomalies over the same two- dimensional body.

An example of ambiguity in magnetic interpretation. The arrows correspond to the directions of magnetization vectors, whose magnitude is given in Am-1. (After Westbrook 1975.)

Continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and forearc mantle, cooled by subduction, should contribute to long-wavelength contribute to long-wavelength magnetic anomalies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and aeromagnetic anomalies and constrained by seismic velocities, and find that hydrated mantle explains an important disparity in potential-field anomalies of Cascadia. A comparison with aeromagnetic data, thermal models, and earthquakes of Cascadia, Japan, and southern Alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide.

Cascadia potential field anomalies and geology

B: Stacked magnetic profile, average of 11 profiles shown in A. C: Stacked gravity profile. D: Crust and upper-mantle model.