Nyack - Geophysical Characterization

Problem - determine subsurface parameters, relevant to fluid flow and basin evolution, from non-invasive observations made on the surface.

Expectations - simple visual observation indicates a bed load dominated, glacial fluvial system. Thus one expects a fairly thin valley fill dominated by non-continuous, coarse grained fluvial components.

Two Geophysical Methods

• Gravity for bedrock configuration. Gravity is the measure of the mutual attraction of masses. The valley fill is significantly less dense than the surrounding bedrock.

• Radar for characterization of very shallow stratigraphy. Different mixes of valley fill have slightly different dielectric properties which affect the propagation of radar waves according to Maxwell’s equations.

Take a couple of Newton's laws:

f mg fGMm

r2

Equate them:

mgGMm

r2

and you get:

gGM

r2

Think of g being the gravitational effect of some mass M at a distance r from the point of observation.

We measure gz, the vertical component of gravity:

Integrate over all mass in a distant volume to get theanomalous gravity at a point, P:

g P( ) zyxGr 2

dM d d d

Gravity anomaly from equal bodies, different depths. Area under the curves is equal.

Here’s where we currently have gravity observations

Gravity Observations - collect and process

•Collect observations, GPS gives +/- 30 cm elevation control

•Correct observations for:

• instrumental and tidal drift (+/- 0.002 mgal)

• latitude (+/- 0.001 mgal)

• elevation above mean sea level (+/- 0.1 mgal)

• local/regional deviations in topography

Thus total error is ~ +/- 0.11 milligal

d/d(horizontal) +/- 0.05 mgal/km for terrain correction

Color is terrain correction; contours are topography

Terrain corrections are largest source of error but not random error

Complete Bouguer Anomaly on Topography

• Processing ends and interpretation begins

• A subjective step

• Probably the most important step in gravity methods

Observed Gravity - Regional Gravity = Residual Gravity

Knowns for the Nyack Valley:

• We are looking for the anomaly caused by the lower density valley fill. Thus at the bedrock contacts at the valley’s edge, the residual gravity must be near zero

• Bedrock density is around 2800 kg/m^3 (experience)

• Glaciation post dates faulting - valley is roughly U-shaped

• Model results must fit gradients and volume of anomaly values

Gravity from Beyond ~ Planar

Regional Gravity as Best Fit Plane

Residual at Small Scale

Residual, from shallow sources, centers on zero milligals.

Residual - After Adjusting for Known Zeroes

Magenta = zero residual contour; green = known zeroes; cyan = data

Color is Residual; Contours on Topography

Compare Complete Bouguer Anomaly and Residual

Removed planar regional

Residual’s zero (faint magenta line) approximates bedrock contact

Poor fit - density contrast too low

High gradients and short-radius curvature require high density contrast and help bound density contrast. This was delta-rho = -250 kg/m^3; higher delta rho means shallower basin

Profile A-A’; with delta rho = -700 kg/m^3

Maximum depth ~ 109 meters

Profile B-B’; with delta rho = -700 kg/m^3

Maximum depth ~ 150 meters; close to gravity minimum

Gravity - that’s where we are

Maxwell’s Equations

Maxwell’s Equations