Lecture 15 Resistivity-GPR

Death Valley Basin

Furnace Creek Fault

Resistivity SYSCAL R1 Plus – resistivity meter for medium-depth exploration can be used for: geological mapping groundwater exploration

Put 48 stainless steel electrodes in the ground every five meters Attach the electrodes to the cable with clips Press start on the box

Ohm’s Law + - Battery current resistance + - V I R

Resistivity Double length double R Double area halve R R proportional to length/area Area current length

Archie’s law

Wenner Array a=2πa ΔV/I Ohm’s law V=IR Apparent resistivity Voltage (V) Current (I) Resistance (R) a=2πa ΔV/I Electric Sounding Shallowest = a Deepest = spacing*(round down((number of electrodes -1) /3))

Field Data

Theory r1 r2 V I r3 r4

Wenner array

Two Layer Model Wenner Spread

Mtlab Program for 2 Layer Resistivity %Resistivity function from Pages 529 Telford et al., Applied Geophysics %Second Edition clear f aarray=x; %meters rho1=a(1); rho2=a(2); z=a(3); k=(rho2-rho1)/(rho2+rho1); %Equation 8.38 for i=1:length(aarray); aa=aarray(i); m=1:10000; D=sum( k.^m./((1+(2*m*z./aa).^2).^0.5) )-sum( k.^m./((4+(2*m*z./aa).^2).^0.5) ); rhoa=rho1*(1+4*D); f(i)=rhoa; end figure(1) plot(log10(aarray),log10(f),log10(aarray), log10(y),'*')

Pseudosections Roughly how resistivity varies vertically and laterally Electrical imaging creates ‘true’ section using tomographic theory

Turkey Flat Modeled Resistivity

Middle Mountain Resistivity Line 1 Line 2 Two resistivity lines were laid out for Middle Mountain The anomaly colored yellow is most likely the water table dammed by the faults From the 1st line to the 2nd line the water table increases in size likely due to the increase in distance between the fault splays

Furnace Creek Fault

SAF Splay Map view Model to explain increase in size of water table