SAVI index and agricultural landscapes

SAVI is the acronym for Soil Adjusted Vegetation Index that, as the name suggests, it is one of the many indices for the calculation of the vegetation index.

The general formula for the SAVI index is (indexdatabase.de):

     800 nm - 670 nm
  ________________________  ( 1 + L)

   800 nm + 670 nm + L

   
where L is between -0.9 and 1.6.

In the case of Sentinel 2 data, SAVI uses two bands, B8 and B4, that have the maximum available resolution for Sentinel 2 images, i.e., 10 meters, so the results have a quite high spatial resolution.

B8 is in the NIR (842 nm) while B4 registers the red-NIR transition (665 nm).

For Sentinel 2, the index is therefore calculated as:

     B8 - B4
  _______________   ( 1 + L)

   B8 + B4 + L
   
The default value for L in indexdatabase is 0.5, and in fact testing for a few values (-0.9, -0.5, 0.0, 0.5, 1.0, 1.5) in the prescribed range, the most contrasted results are obtained for L = 0.5.

One information that is obtained from applying such an index to intensive farming landscapes is that it allows to delineate well the agricultural field limits, to compare the texture and the "spatial styles" of farming across different areas, for instances separated by rivers that may have acted as administrative or political borders.

In the following images, I present some examples of textures and styles of limits between fields for different areas in the Piedmont region, derived from a Sentinel 2B image, acquired on 2023/04/04 at 10:25. The SAVI index was calculated using the SNAP software by ESA.

The used L ("soil brightness correction factor") value is 0.5, chosen after some experimentation.

The figures below are generated from within SNAP, using a 8-4-3 composite for infrared view (upper window), while the SAVI index band is represented in the lower window. In the 843 composite, vegetation is red while rivers and roads are black.

Strong agricultural landscapes contrast between the western side (left) and eastern  side (right) of the Sesia river (central, with a North-South orientation). The western landscape is characterized by smaller sizes than in the eastern part, and is also more regular as geometries. Possibly it is the result of different political systems between the two sides in the previous centuries.

 Strong East-West differences in field sizes, orientations and textures without a clear separating physical boundary (such as the Sesia river in the previous example). It could be related to different grown agricultural products between the two zones (?rice to the East), possibly also related to distinct administrative/political situations.  

Irregular field borders probably due to the limiting rivers and their location changes with time.

 

 

GIS evidences for low-angle segments in the Valnerina fault system (Central Apennines, Italy)

A long time ago, my PhD thesis was about the Valnerina line, a Cenozoic structural lineament in the Central Apennines of Italy, that runs parallel to the more important Olevano-Antrodoco line (Fig. 1), that is considered by many Authors to have played an major syn-sedimentary role during the Mesozoic pre-orogenic phase. The Valnerina line was investigated, among others, by Francesco Antonio Decandia (e.g., Decandia 1982), my thesis supervisor in Siena University. 

During the Cenozoic compression phase, both the Valnerina and the Olevano-Antrodoco lines would have been acted as oblique-dextral ramps in the Apenninic thrust-and-fold belt. This role would have derived from the reactivation of syn-sedimentary faults of the Mesozoic Umbrian basin (Decandia, 1982). 

Fig. 1. Map of the described zone. From Fig. 9 in Alberti, 2006.

I remember, in a field trip with students, that Decandia showed us a large fault slickenside between Jurassic Calcari Diasprini/Calcari a Posydonia and Cenozoic Scaglia tectonites in the Schioppo segment of the line. The slickenside was quite high angle, dipping 70° or more to the West (Fig. 2).

 

Fig. 2. Mesofaults with dextral movements in the footwall of the Schioppo fault. From Alberti, 1998.

 

In the Umbrian sector, the Valnerina line is composed of a few segments, mainly with a NNE-SSW trend. I studied two segments at the North of the Schioppo one, the Tassinare and the Grotti faults (Fig. 3). 

 

Fig. 3. Traces of Tassinare and Grotti segments of the Valnerina line. From Alberti, 2006.

Studying the slickensides and shear zones exposed along the trace of the Grotti fault, while top-to-NE movements were common, I didn't  find abundant examples of high-angle meso-faults (e.g., Fig. 4, 5).

Fig. 4. The Grotti faults (left) and observed meso-faults at structural stations (right). From Alberti, 2006.

 

Fig. 5. S-C calcareous mylonites, with calcite shear veins, in a shear zone in the Grotti area. Foto M. Alberti.

At the time, during the first half of '90, I was not aware of GIS tools and related quantitative digital techniques for studying geological surfaces. I just remember, during a stage in Basel University, the geologist Daniel Bernouilli, digitizing a structural surface at the table with the equivalent of a mouse.

Only after the PhD, while working in the Museo dell'Antartide in Siena, I began knowing and working with commercial GIS tools, i.e. ArcView and Arc/Info. Later I began using QGIS, Saga, Grass, i.e, the open source side of the GIS software.

With Python, a scripting language well integrated with QGIS, I started creating plug-ins devoted to structural analysis of geological field data. One of these plug-ins, qgSurf, includes a module, named 'DEM-plane intersection' that allows to calculate the expected intersections between a geological plane and a topography. 

When applying this module to the data of the Grotti fault, I was surprised to find that a very low angle plane (West-dipping and about 7° of dip angle) would approximate in a more than acceptable way the traces of both the Grotti fault and the southern portion of the Tassinare fault, even when considering that the Grotti fault is locally displaced by a few minor NW-SE normal faults  (Fig. 6).

Fig. 6. Map of traces (red lines) of the Grotti (NNE-SSW mean trend, central part) and Tassinare (broadly N-S trending, to the West) faults. The theoretical trace of the inferred geological plane with dip direction 269° and dip angle of 6.7° is superposed (semi-transparent thick orange line).

In Fig. 6 you may note that in the South-Eastern part a large klippe, plus a minor one to the North would be expected. There are no geological evidence of these klippen in the field (cf. Fig. 7), but it could be explained by the fact that the geological surface increases its dip to the South-East.

Fig. 7. Geological sketch of the Tassinare-Grotti zone (from Alberti, 1998).

 

To represent the inferred attitude of the plane with respect to the geological situation, I have modified the gsf and gst Python modules to allow plotting significant planes into parallel profiles, as visualized in the profiles below. 

The input data are geological outcrops, faults and a DEM of the zone. Analyses and plots were made within a Jupyter Notebook.

The five parallel lines in the map (Fig. 8, white lines), from North (# 1) to South (# 5), are shown as topographic profiles in Fig. 9, with geological formations (see legend) and fault traces (red dots) added.

The very low-angle geological plane 269°/06.7° is represented in these profiles by the thick semi-transparent orange line. 

It can be seen that it approximates quite well the mapped traces of the NNE-SSW trending Grotti segment. It is therefore possible that the Grotti segment is a low-angle fault, differently from the Schioppo segment of the Valnerina line.

Fig. 8. Topographic map of the studied zone, with fault traces (red lines) and paralell profiles (white lines). Created with gst and gsf Python modules.

Fig. 9. Topographic profiles as in Fig. 8, with geological formations and fault traces (red dots). The low-angle plane is represented by the thick orange line. Created with gst and gsf Python modules.

References

Alberti, M., 1998. Ruolo cinematico e dinamico di lineamenti sisedimentari mesozoici durante la tettogenesi Appenninica - Linea della Valneria, Umbria. Unpublished Phd thesis.

Alberti, M., 2006. Spatial structures in earthquakes and faults: quantifying similarity in simulated stress fields and natural data sets. Journal of Structural Geology, 28, 998–1018.

Decandia F.A., 1982. Geologia dei Monti di Spoleto (Prov. di Perugia). Boll. Soc. Geol. It., 101, 291-315.