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.

We measure gz, the vertical component of gravity: Integrate over all mass in a distant volume to get the anomalous gravity at a point, P:

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 (+/ mgal) latitude (+/ mgal) elevation above mean sea level (+/- 0.1 mgal) local/regional deviations in topography Thus total error is ~ +/ milligal d/d(horizontal) +/ 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