groundwater

seismic waves

impulsive energy sources impose stresses that result in strains on natural materials

what are the two kinds of seismic waves

body and surface

body waves

body waves are waves that are transmitted through the interior of body

what do body waves produce

elastic strain inside a body as they propagate

what are the two kinds of body waves?

P(Primary) waves and S(Secondary) waves

P waves

compressional body waves that pass through a body as a sequence of compressions and dilations parallel to the direction of wave propogation

what do the particles inside the body undergo?P waves?

small displacements(strains ~ 10^-6), oscillate about a fixed point

where do P waves occur?

solids, liquids and gases

s waves

transverse body waves that pass through a body as a sequence of shearing strains perpendicular to the direction of wave propagation

what do the particles inside the body undergo? S waves?

small displacements as the energy packet passes through the body, oscillate about some fixed point

where do s waves occur?

solids NOT liquids or gases

surface waves

seismic waves that are transmitted close to the surface of a body

do surface waves penetrate deeply into the body?

no and their amplitude decreases rapidly with depth

two main types of surface waves?

rayleigh and love

rayleigh waves

surface waves that combine vertically polarized transverse motion (S-wave) with horizontal compressional motion of a P-wave

what do the particles for rayleigh waves undergo?

an elliptical displacment pattern as the two kinds of motion interact, the particles simply circulate about a fixed point

velocity(rayleigh) < Vs

~0.9 Vs

love waves

horizontally polarized waves that pass along the surface of a body as a sequence of shearing strains perpendicular to the direction of wave propogation

passes along the surface of the earth

what do the particles for love waves undergo?

small horizontal displacements as the energy packet passes along the surface and osciallte sideays about a fixed point

resolution formula?

0.25 * wavelength

for reflection the angle of incidence is equal to ____________

the angle of reflection

incident ray

reflected ray and point of incidence are coplanar

seimsic rays undergo both ___________ and ______________

reflection and refraction

where are seismic waves reflected and refracted? what does it act as?

boundary btwn two media, acts as the source of new seismic waves when excited by the incident wave

mode conversion

converting incident P-waves into reflected or refracted S-waves

snells law forumla

sin(theta)/V1 = sin(theta)/V2

critical refraction

where the seismic velocity of the lower medium is greater than that of the upper medium, the refracted ray is deflected away form the normal

critical angle

the angle of incidence for which critical refraction takes place

sin(critical angle)/sin90 = v1/v2 = sin(critical angle)

the geometry and seismic velocity of subsurface layers are determined using what?

time-distance plots

>>>> travel time(time interval btwn source detonation and when the wave arrive at each receiver)

critical distance? what arrives first the direct or refracted wave?

the surface distance from the source at which a critically refracted wave can first be recorded

the direct wave arrives before the refracted wave

corssing distance

the surface distance form source to a point where direct and refracted waves arrive at the same time

at distances close to the source the first wave to arrive at a receiver is the?

direct wave

at distances far away form the source the first wave to arrive at a recevier is the?

refracted wave

low velocity zone

if the velocity decreases with depth then no critically refracted wave is generated and the interface will not be detected

thin layer problem

if the layer thickness is small and/or the velocity contrast across an interface is not substantial then critical refractions form the interface may arrive later than refractions form lower interfaces, making it difficult to detect the top of the layer

seismic reflection and refraction data can be obtained along?

profiles(2D-acquisition) or over areas(3D acquisition)

sources

an energy source is placed near the surface of the earth and "fired" or "shot" to produce an impulse

receivers

devices are positioned in various geometrical patterns around the source which record the motion of the Erath in responseto the arrival of seismic energy that has traveled through various parts of the Erath

describe the setup for a seismic refraction profile

-energy source at one end of profile or several locations along profile

-receivers usually equally spaced in a straight line away from the source

-spacing of receivers chosen such that refracted waves from each horizon are adequately sampled

-length of line governed by the available equipment and the nature of the problem being addressed

what is the most common way of obtaining 2-dimensional multifold seismic coverage

common depth point(CDP)

what does the CDP method assume

horizontal layering

common mid-point profiling method

aka CDP method when layers are not horizontal

common mid point data organization

in common midpoint processing individual traces are organized such that all recordings sharing a common subsurface reflection point are grouped beneath the midpoint separating the source and receiver

ground roll(surface waves) and refracted waves obscure what?

reflected waves

optium window

the range of offset distances that allows reflected waves to arrive before the ground roll and after the critically refracted waves resulting in more clearly identifiable reflected arrivals

two dimensional surveys only adequately map the subsurface where the layers are _______ and ____ along the direction of the profile

planar, dip

elastic properties of rocks

that of rocks are influenced by porosity

highly porous materials are more compressible

seismic velocitites are influenced by porosity

clay and till

have low hydraulic conductivities, forming hydraulic boundaries dividing aquifers

what is till

a mixture of sand, clay and partly chalk, with a wide waviety of grain size distributions

who used artifical sand-clay mixtures for laboratory experiments

Marion, he found a maximum of p-waves velocities with about 40% clay

what does the velocity of p-waves depend on?

porosity and the water saturation

what does the velocity of s-wave or the shear modulus depend on?

stiffness of the rock matrix

p- and s- wave velocities depend on _________

the fracture density of rocks

how can seismic investigations be used in groundwater investigations

find the groundwater table

define bed rock levels

determine the structural and lithological framework of groundwater studies

quantify the heterogeneity of aquifers and aquitards

investigate porosity and fracture density

travel time calculation

aka NMO equation

seismic reflection travel time

equation for travel time vs source receiver offset

t^2 = ((2z)^2 + x^2)/v^2

ray tracing image point approach

the image point is a mirror of the source point and is mirrored about the reflecting layer

current flow in subsurface

>inside the earth the current flows away from the source electrode in all direction

>one current electrode is considered to be the source electrode where current enters the other the isnk electrode where current exits the earth

what does the potential gradient depend on?

on the geometry and electrical properties of layers in the subsurface

whats an equipotential surface

where the electrical potential is exactly the same at every single point

apparent resistivity

the weighted average of the resistivity of each layer,

in a multilayered system we see potential differneces

what are the three most common configurations

wenner, schlumberger and dipole-dipole

describe a wenner array setup

>the potential and current electrodes are all placed in a strsight line

>the potential electrodes are placed between the current electrodes

>the spacing btwn all four electrodes is set to be equal(i.e, a)

>the spacing of the current electrodes(3a) is 3 times that of the potential electrodes(a)

describe a schulberger array setup

>the potential and current electrodes are all placed in a straight line

>the potetnial electrodes are placed btwn the current electrodes

>the spacing btwn the potential electrodes(2l) is usually much smaller than the current electrodes(2L)

dipole array

>potential and current electrodes are placed in various geometrical patterns

>spacing btwn the potential electrodes may be smaller than, equal to or larger than the spacing btwn the current electrodes

> the potnetial and current electrode spacings are equal

electrical resistivity imaging

sophisticated tomographic techniques can be used on pseudosectioins produce a more realisitc picture of subsurface electrical properties

what kind of data can u use to cosntruct a pseudosection

any kind of electrical array survey

where is the apparent resisitivty value placed in a wenner array?

at mid point of the array at a depth equal to the electrode spacing (a)

where is the apparent resisitivty value placed in a dipole-dipole array?

> construct 45 degree lines from the center of the electrode pair into the subsurface

> assigned to the point in the subsurface where these 2 lines meet

forward modeling - non uniqueness

> by increasing the thickness and decreasing the resistivity of individual layers it is possible to produce similar or identical responses

> for very conductive layers where most of the current flow is concentrated it is possible to increase the resistivity and increase the thickness and yet maintain the same response

> for the depper layers in a model the thickness and/or the resistivity is generally poorly constrained as variation in either property produces very little change in the apparent resistivity

how can problems with non-uniqueness be resolveddd

> geologic information can be used to constrain possible interpretations

> the interpretation is enhanced if some subsurface measuremnts are made

> a well may be used as a source or receiver and electrodes placed on the surface

vadose zone def? water name?

> btwn the ground surface and the permanent water table

> the water here is called vadose or suspended water

what is the vadose zone reffered to as? why?

unsaturated zone bc it has low water content relative to the saturated zone below the water table

above the capillary fringe, vadose zone pore spaces are generally ____________?

air-filled, there are thin water films coating the solid particles

applications of resistivity surveys

> broad range of geologic, environemntal and engineering problems

INCLUDES

> gorundwater exploration and water quality

> pollution problems where chemicals in solution produce significant changes in electrical properties

> landfill sites - layered structure thickness variation

> landslides > stability, water saturation etc

> archeological mapping

> geothermal areas

> permafrost thickness variation

elecricaly resitivity survey problem

>not a high resolution tecnique and therefore has limitations in defining precise boundaries or depths of target