Problem 7 Time section does not perfectly image depth section Dipping reflectors are incorrectly located Moho displays “velocity pull-down” beneath the low-velocity basin

Time section Depth section Reflector is flatReflector is “anticlinal” Velocity pull-up beneath a high-velocity carbonate reef

We plot reflection data directly below the commom Why are the dipping reflectors incorrectly located? We plot each trace at the common midpoint of the gather, hence reflections from dipping reflectors appear down dip and shallower than they are in real life.

Unmigrated time section Migrated time section (computationally expensive to do properly) In unmigrated sections, anticlines appear wider and synclines appear narrower than they really are. Real situation Seismic section Migration

Pg = V 1 = km/s Pn = V 2 = 8 – 9 km/s T 1 = 5.3 – 7.2 km/s Z 1 = 25-30km Pn is not straight because Moho has some topography

Today’s lecture Electrical data Resistivity Theory Data acquisition Plotting and interpretation Examples Induced polarization Theory Example Problem 9

Resistivity is a measure of resistance to the passage of electrical current Dry rock usually has a high resistivity Most rocks can have a range of resistivities Clay is unique – the resistivity of fresh clay is 1-15 Ωm Where rocks are porous and filled with saline water, the rock resistivity is low and resistivity decreases with increasing porosity. Fresh water, gas and oil have a higher resistivity than saline water Contaminated water can have an unusually low or high resistivity

We define the resistivity (ρ) of a conducting cylinder of resistance δR, crossectional area δA and length δL as: ρ = δRδA/δL where δR = δV/δI The units of resistivity are Ωm

Current flow in homogeneous subsurface Current flow in two-layers Resistivity layer 1 (ρ 1 ) > resistivity in layer 2 (ρ 2 ) Current takes “least resistant” path Resistivity equipment consists of a voltmeter, ammeter and battery. Current is sent through current electrodes (C 1, C 2 ) and Voltage drop is measure between two potential electrodes (P 1, P 2 )

2D profile usually has 4 electrodes 3D profile can have large number electrodes

There are many possible electrode configurations; the Wenner and Schlumberger arrays are the most common ρ a = 2πaR ρ a = πL 2 R/2l We measure R, and use this to determine ρ a (apparent resistivity)

VES surveys – detects change in resistivity with depth As we increase the electrode separation (a) the depth of current flow penetration increases ρ1ρ2ρ1ρ2 When a (electrode spacing) is small ρ a = ρ 1 ρ1ρ2ρ1ρ2 When a (electrode spacing) is large ρ a = ρ 2

ρ1ρ1 ρ2ρ2 Depth of penetration (depth of measurement) ~ electrode separation (a) At point of inflexion Z 1 ~ a ρ2ρ2 ρ1ρ1

Can construct curves for 2, 3 or more layers

Resistivity increases with depth Standard curves for 2 layers Wenner array Resistivity decreases with depth ρaρa a Compare real data to standard curves

Plot on log paper (resistivity varies by orders of magnitude) ρaρa a

CST surveys - use to detect sub-vertical boundaries Keep electrode spacing constant Move whole array gradually across vertical contact If contact is buried, electrode separation must be greater than depth of burial of contact ρ2ρ2 ρ1ρ1 ρ2ρ2 ρ1ρ1 Boundary Horizontal distance (m)

Example

Can generate pseudo-depth section (Double dipole array)

Landfill site, Sweden The low resistivity zone (blue) indicates the contamination from landfill Ground water recharge has caused contamination from the landfill to spread in the E-W direction Example of pseudo- depth section

Geotechnical investigation, New Zealand Basalt flows overlie Tertiary sediments Aim was to determine basalt thickness to guide future construction Basalts have high resistivity, sediments have low resistivity pink line = interpretation of base of basalt 2 boreholes helped calibrate data

Problems, assumptions and non-uniqueness Layers may be undulating, dipping or gradational Resistivity in each layer may not be constant The position of the water table may show seasonal changes Thin beds may not be detectable There are many possible solutions to any dataset (You will always get a better fit with many layers than just a few)

Induced polarization Apply a voltage at surface for a short period of time and measure potential drop between 2 electrodes (same equipment as resistivity) The Earth has some capacitance, so when you switch the voltage off, the potential drops and then decreases gradually. This slow decrease is a measure of the “chargeability” of the rocks

Some typical values: Chargeability minerals rocks Pyrite13.4 msec sandstone msec Cu12.3 msec 8-20% sulphides msec Magnetite 2.2 msec 2-8% sulphides msec Haematite 0.0 msec limestone10-20 msec Best method of detecting disseminated sulphides

One problem rocks with clay particles are also chargeable Can’t tell the difference between membrane polarisation (caused by clay particles) and electrode polarisation (caused by conductive minerals)

Example