2 data acquisition

8/3/2019 2 Data Acquisition

http://slidepdf.com/reader/full/2-data-acquisition 1/12

Data Acquisition

Chapter 2



Data Acquisition

• 1st step: get data – Usually data gathered by some

geophysical device – Most surveys are comprised of

linear traverses or transects• Typically constant data spacing• Perpendicular to target• Resolution based on target• Best for elongated targets

– When the data is plotted (aftervarious calculations have beenmade): Profile



Grids

• When transects are combineda grid can be formed.

– Good for round or blob-shapedtargets

• Or if target geometry is unknown – Useful for making contour

maps – Allows transects to be created

in multiple directions



Data Reduction• Often the raw data collected is

not useful. – Data must be converted to a useful

form• Removing the unwanted signals in

data: Reduction• Targets are often recognized by

an “anomaly” in the data – Values are above or below the

surrounding data averages.• Not all geophysical targets

produce spatial anomalies. – E.g. seismic refraction produces

travel time curves depth tointerfaces

• Also a type of reduction.



Signal and Noise• Even after data is reduced, a

profile may not reveal a clearanomaly due to noise.

– Noise: Unwanted fluctuations inmeasured data.

• May be spatial or temporal• What causes noise?

– Signal: The data you want, i.e. nonoise.

• Noise can be removed using

mathematical techniques – Stacking – Fourier Analysis – Signal Processing

Magnetic or Gravity profile



Stacking• Stacking is useful when:

–

Noise is random – Signal is weak – Instrument is not sensitive

• If noise is random – Take multiple readings – Sum the readings – Noise cancels out

• Destructive Interference – Signal should add

• Constructive Interference• Stacking improves signal to

noise ratio – Commonly used with numerous

techniques.



Resolution

• Even if you have a goodsignal to noise ratio,detection of your targetdepends on your

resolution. – Know what you are looking

for before you begin – Know the limits of your

data resolution



Modeling

• Most geophysical data istwice removed fromactual geologicalinformation

– Reduced data is modeled• Models

– Aim to describe a specificbehavior or process

– Are only as complex as thedata allows

• Occam’s Razor: “Entitiesshould not be multipliedunnecessarily”



Model Types

• In the most basic sense models come in two flavors: – Forward model

• Given some set of variables, what is the result.• I.e. you input the “cause” and some “effect” is produced

– Inverse model• Given some measurements, what caused them• You know the “effect”, try to determine the “cause”•

Often involves mathematical versions of “guess and check”

Depth = DFault Slip

GPS Station Motions



Model Types• Models also come in several flavors

based on technique – Conceptual Model

• Models an idea…no math/physical parts – Analog Model

• A tangible model “scaled” to reproducegeologic phenomena

– Empirical Model• Based on trends in data

– Analytical Model• Solves an equation• Usually deals with simple systems

– Numerical Model• Computer-based approximations to an

equation. – Thousands, millions, or billions of

calculations• Can handle complex systems.

Analog Model

Empirical Model

From Wells & Coppersmith 1994



Non-Uniqueness of Models• Typically, multiple models

can fit data – So any given model is non-

unique – Distinguish between models

based on• Match with geologic data• Model with least

parameters (most simple)

• Data has limited resolution – Surveys must be finite – “Blurs the picture” – Omission of detail

emphasizes key features



Geologic Interpretation• After data is collected and

modeling is complete theresults must be interpretedinto the geological context.

• Use all available data. – Don’t only look, when you can hear

and touch!• Interpretations are also typically

non-unique – Many geologic materials have similar

properties. – Best interpretations use all available

data, geologic, geophysical, chemical,etc…

Material Density (gm/cm 3)

Air ~0

Water 1

Sediments 1.7-2.3

Sandstone 2.0-2.6

Shale 2.0-2.7

Limestone 2.5-2.8

Granite 2.5-2.8

Basalts 2.7-3.1

MetamorphicRocks

2.6-3.0

2 data acquisition

Documents