Electrical Resistivity Lab HWR 532 Josh Clemons

The following graphics were generated using “Model 41” consisting of 41 electrodes spaced 1 meter apart with 4 nodes per electrode spacing.

Buried Target Small target, shallow depth

Buried Target Large target, shallow depth

Buried Target Small target, intermediate depth

Buried Target Large target, intermediate depth

Buried Target Small target, deep depth

Buried Target Large target, deep depth

2 Targets, Different r Small, high r target above large, low r target

Note how the following two graphics are almost identical, although the underground situation is different. The small target below the large target is virtually invisible.

2 Targets, Different r Small, high r target below large, low r target

1 Target Large, low r target

1 Target Small, high r target

Values Used for Clay/Sand Slides EC of water = 800 mmho/cm Porosity of sand = 0.4 Porosity of clay = 0.45 Archie equation: ECbulk = (ECwater)(water content)2(porosity)1.2

1 m sand above clay q = 0.05

2 m sand above clay q = 0.05

3 m sand above clay q = 0.05

4 m sand above clay q = 0.05

1 m sand above clay q = 0.15

2 m sand above clay q = 0.15

3 m sand above clay q = 0.15

4 m sand above clay q = 0.15

1 m sand above clay q = 0.25

2 m sand above clay q = 0.25

3 m sand above clay q = 0.25

4 m sand above clay q = 0.25

1 m sand above clay q = 0.35

2 m sand above clay q = 0.35

3 m sand above clay q = 0.35

4 m sand above clay q = 0.35

Notice in the following four slides that it is difficult to tell the difference between various water contents in the same beds.

2 m sand above clay q = 0.05

2 m sand above clay q = 0.15

2 m sand above clay q = 0.25

2 m sand above clay q = 0.35

Inversion Modeling Wenner array, 1 m spacing

Inversion Modeling Wenner array, 1 m spacing RMS error after 3 inversions is 29.7% - not very good. The model indicates a relatively resistive formation or group of formations just below the surface, underlain by less resistive layers.

Inversion Modeling Wenner array, 3 m spacing

Inversion Modeling Wenner array, 3 m spacing RMS after 3 inversions is 35.7% - even worse than for the 1 m spacing. Drawing conclusions from this model would be risky. However, it does appear that there is something relatively resistive just below the surface, underlain by less resistive material.

Inversion Modeling Wenner array, 5 m spacing

Inversion Modeling Wenner array, 5 m spacing RMS after 3 inversions is 64.1% - the worst yet. The RMS has increased with electrode spacing for these models. As with the 3 m spacing, drawing a conclusion is risky. However, the apparent relatively resistive body appears again at right.