Monday PM: Dan and Doug

Glacial Seismology Basics

Its about earth movement

Then there was the smell of heather crushed and the roughness of the bent stalks under her head and the sun bright on her closed eyes . . . and for her everything was red, orange, gold-red from the sun on the closed eyes. . . . For him it was a dark passage which led to nowhere, then to nowhere, then again to nowhere, once again to nowhere, always and forever to nowhere, heavy on the elbows in the earth to nowhere . . . now beyond all bearing up, up, up and into nowhere, suddenly, scaldingly, holdingly all nowhere gone and time absolutely still and they were both there, time having stopped and he felt the earth move out and away from under them.

- Ernest Hemingway, For Whom The Bell Tolls.

Great authors have worried about earth movement

glacial seismology is about feeling movements:

the ice move as part of the earth (tectonics and tomography)

the ice move despite the earth staying still (icequakes, ice flow)

the ice move as part of the ocean surface (sea swell)

the ice move into the ocean (iceberg calving)

the earth move because of the ice (Greenlands glacial earthquakes)

the water move inside or below the ice (subglacial hydrology, hydrofracture)

the movements, in turn tell us about parts of the ice wecannot see:

(active source seismology) to measure basal conditions

finding the ocean bottom below ice shelves, subglacial lakes

determining the competence the ice and sediment below the ice

glacial seismologists worry about earth movement too:

Instruments that do the feeling

up and down motion back and forth motion

what you need to know about these instruments:

observed

desired :

what you need to know about these instruments:

if this term isnt important (vibration is very fast compared to swing frequency)

ground displacement sensor

X

what you need to know about these instruments:

if this term isnt important (vibration is very slow compared to swing frequency)

ground acceleration sensor

X

Instrument response has to be either:

accounted for mathematically

eliminated by instrument design ($$$)

both (what seismologists do)

The magic of the Laplace Transform:

look! no time derivatives!

generalized instrument

where:

Poles and Zeros and flat response

zeros

poles

instrument designers arrange poles and zeros carefully:

Trillium T120

1

100 s 100 Hz

flat

0 phase shift

what basic seismometer data looks like:

what basic seismometer data looks like:

seismogram

what basic seismometer data looks like:

all the science

Skill set for a glacial seismologist:

interface with IRIS data types

deconvolve instrument response to motion

filter data to specific ranges of frequency

create spectrograms,

recognize various types of waves

recognize dispersion

recognize anthropogenic vs. natural signals

make cross and auto correlations

visualize particle orbits

differentiate teleseisms from local signals

plan active source geometries

it goes on, and it goes on, and so forth

glacial seismology deals with lots of different kinds of waves:

Aberdeen Bestiary, 12th century

glacial seismology deals with lots of different kinds of waves:

Aberdeen Bestiary, 12th century

P waves

S waves

Rayleigh waves

flexural gravitywaves

sea swell

air waves

waves in layered media

Where to learn about the bestiary?

http://levee.wustl.edu/seismology/book/

https://carmenconnect.osu.edu/p6ylzk69blh/?launcher=false&fcsContent=true&pbMode=normalhttps://carmenconnect.osu.edu/p6ylzk69blh/?launcher=false&fcsContent=true&pbMode=normalhttp://levee.wustl.edu/seismology/book/

a few examples of whats out there in the bestiary

flexural gravity wave (C17 landing on sea ice):

this thing is called a spectrogram

Just when you get used to waves dispersed one wayanother will come along that is dispersed the opposite way.

Some stuff you wont be able to find without spectrograms.

Some stuff is just too strange

Eventually you will see things everywhere

whats that?!

and that?!and that?!and that?!and that?!

and that?!

and you think you will be able to make sense of it

Lets give it a try!

Exercise with McMurdo Ice Shelf data*

strange event on 18 December 2017

24 hours of LHZ, LHE, LHN data from 2 ice-shelf stations: BB01 & BB02

24 hours of LHZ data from nearby (land based) SBA (a GSN station)

either a .mat file, or .csv file (limited)

sensor: Trillium T120 | sensor constant: 1201 V/(m/s)

digitizing constant: BB01=419430 counts/V | BB02=629327 counts/V

skill goal: Create ground motion seismogram

skill goal: Create spectrogram

skill goal: Compare stations (travel time? dispersion?)

thought goal: is the event an ice-related phenomenon?

thought goal: what kind of wave? does it disperse? what is source?

related goal for Wednesday: did the GPS record the event?

* If you find something cool, I plan to brazenly steal it from you!

you will be given data from these three stations:

You will be provided starter code

Goal is to make this seismogram:

and figure out what this is

Is it a teleseism?

See if you can figure it out.

make this spectrogram

figure out what this slope means

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