Introduction to Cartographic Modeling using Global Satellite Gravity


Cartographic modeling is an increasingly common application of remote sensing data. This usually involves a product derived from remotely sensed imagery, (such as a land cover classification, a digital elevation model or a sea surface temperature estimate) in addition to other types of thematic information (like field measurements, environmental variables and socioeconomic parameters). Multiple layers of geographically referenced information are combined to investigate spatially dependent relationships between the parameters represented in the layers. This is usually done with the help of a tool called a Geographical Information System (GIS) which is specifically designed to manipulate combinations of vector (point & line) and raster (grid) information for mathematical and logical calculations. One type of cartographic modeling involves exploratory multidimensional analysis of co-registered spatial data. Other types cartographic modeling involves map algebra in which each layer is treated as a variable in an algebraic or logical expression.


In this lab you will use ENVI to explore relationships between three co-registered global datasets. The exercise will investigate the formation and evolution of ocean basins and the structure of the mantle beneath the seafloor.


Please email short answers to the following questions to Chris & Jeff before 9:00 am Thursday 12 November.





Oceanic plates are formed by accretion at divergent plate boundaries called mid-ocean ridges. The divergence of oceanic plates at mid-ocean ridges causes mantle upwelling which causes decompression melting which results in extrusive volcanism at the midocean ridge. Frequent polarity reversals of the earth's magnetic field are recorded by magnetic minerals in these lavas and can be used to estimate the age of the seafloor. Shipboard measurements of spatial variations in magnetic field intensity (called magnetic anomalies) can be correlated to globally recognized polarity reversals which have been dated with radioactive isotopes and microfossils. We will compare satellite derived marine gravity measurements (discussed in class) with a digital elevation model (derived from shipboard bathymetric measurements and gravimetric predictions) and a seafloor age model (based on shipboard measurements of marine magnetic anomalies) to investigate the sub-seafloor structure and the evolution of ocean basins.


First, load the combined topography/gravity/age dataset:


(File -> External File -> Image Processing Formats -> ERMapper)


The file is: /data/rsvlab/class/LAB8/TGA1.b


Open the Topography band (the first of the surface:TGA5m.b bands) as a grey scale image and replace the grey scale w/ a color scale (Functions -> Color Mapping ->ENVI color table) Choose Rainbow and under Options select "Apply")


Q1-> Why is so much of the image saturated in blue or red?


Generate an image histogram.


Q2-> Why does the image histogram have two modes? Give a physical explanation of what the 2 modes are and why they exist).


Apply a linear stretch to the image and locate the continuous submarine features depicting the mid-ocean ridge system where the tectonic plates diverge. This image shows boundaries between the North American, South American, Cocos, Nazca, Pacific, African, Antarctic and Eurasian plates and several smaller microplates.


Q3-> What is the most obvious difference between the mid-ocean ridge in the Atlantic and Pacific Oceans.

Use the zoom window to enlarge a region of the East Pacific Rise. This is the mid-ocean ridge offshore Peru and Ecuador.


Now look at a comparably sized region of the Mid-Atlantic Ridge at the same latitude.


Q4-> What are the 3 most prominent differences between the East Pacific Rise and the Mid-Atlantic Ridge in these areas?


If it's not obvious to you then apply a new linear stretch to the Mid-Atlantic Ridge region, then look at a similar region on the East Pacific Rise


The differences between the East Pacific Rise and Mid-Atlantic Ridge are believed to be caused by the different rates at which the plates separate.

The Mid-Atlantic Ridge separates at about 30 km/million yrs while the East Pacific Rise separates at about 120 km/million years.


Q5-> What physical consequence might this 4 fold difference in plate separation rate and mantle upwelling rate have for the structure of the ocean basin?


Now open a new window and load the gravity anomaly image in the 2nd band of this file. Apply a linear stretch to the image. Resize the image window and center it over the Mid-Atlantic ridge (w/o including any continents)


Q6-> What differences do you see between the gravity and topography?

Explain why the gravity and topography are so different around the mid-ocean ridges. Link the images to compare them directly.


Open a Scattergram of the topography and gravity bands.

Resize the image window to include only a midocean ridge and its flanks if necessary.


This shows the relationship between gravity and topography in this area. Move the image window w/in the scroll window and notice how the relationship changes. The color indicates the number of pixels with a particular combination of gravity anomaly and elevation.


Q7-> What does this plot tell you about the relationship between gravity and topography in this area?


Now repeat the process with the topography and seafloor age bands.


Q8-> What does this plot tell you about the relationship between seafloor depth and age in this area?


Q9-> What is the prominent spur at the right edge of the plot?


Now move to the Pacific and repeat the above exercises.


Q10-> Does the relationship between gravity and topography seem similar in the Pacific and the Atlantic? How is it different?


Q11-> Does the relationship between depth and age seem similar to that in the Atlantic? In what ways is it similar or different?


Q12-> What features do you see in the gravity that were not apparent in the topography? Why didn't these show up in the topography? What are they?


Q13-> Do the gravity and topography look more similar around the Mid-Atlantic Ridge or the East Pacific Rise? Why?


The most prominent differences between the gravity and topography of a given region of seafloor are seen at long (spatial) wavelengths. These are a result of sub-seafloor structure which is revealed by the gravity anomalies but not reflected by depth variations. This structure is the result of density variations resulting from compositional differences and thermal anomalies.


Most of these structures are still unexplained.