Course Module: 36 – Crustal Model-Moho

The boundary between the Earth’s crust and the mantle has been defined as the Mohorovicic discontinuity or known for short as the Moho. This boundary interface involves a large change in both velocity measured directly using refraction data and density contrast measured using gravity data. Due to this contrast, undulations in the Moho surface generate anomalies of regional scale producing gravity gradients that can conceal and hide geological features of interest. Therefore, gravity anomalies sourced within the sedimentary section must be enhanced by: 1) removing the gravity effect of the Moho surface, and 2) attenuating the gravity effect of shallow topographic features; and what remains is a crustal model that reflects gravity anomalies of exploration significance from the Earth’s surface to the base of the crust.

The surface which separates the Earth’s crust from the mantle can be configured by seismic or gravimetric methods. The seismic method is expensive and suffers the scarcity of available seismic data. On the other hand, the gravity method is less costly and regional gravity public domain data sets are easily accessible. The interpretation of the surface using gravity is carried out in three steps: a) compute an estimated depth-to-Moho surface using Airy-Heiskanen model, b) incorporate the surface as a layer in a regional 3D gravity EarthModel, then invert on the Bouguer gravity field, and c) calibrate and adjust the inverted depth surface to available refraction or reflection data sets.

Learning Objectives:

After completing this Module, you will be able to:
• Describe the isostasy principal.
• Describe the gravity crustal model.
• Describe a Depth-to-Moho surface
• Describe a calibrated and adjusted Depth-to-Moho Surface.
• How to generate an isostatic residual map.
• Identify the anomaly zone of interest: from earth surface to the base of crust.
• Example area is used to illustrate the procedure.

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