A satellite-enabled peek at Saudi oil.

Wednesday, January 23, 2008

Dating Google Earth Imagery

[Edit 03/25/2009: The current version of GE simplifies the task of dating images. When you zoom into an area, the photograph date is displayed at the bottom of the window. In addition, there is a "Historical Imagery" feature for going back in time to load earlier hi-res images (if available). But, this blog post describes how much more difficult this was when I began this research.]

After figuring out what you are looking at, the next question is: when?. Google Earth stitches together multiple photographs from several vendors taken on different dates. This gives a rather patchwork appearance to most areas of the world:

Dating a particular place you are looking at consists of 1) identifying the vendor, and 2) identifying the image by that vendor currently used by Google Earth. The first step is easy, as the copyright notices displayed over the lower part of the image window indicate which vendors contributed to the current view. If more than one is visible, zoom in until only one vendor is displayed.

Step two takes a bit more effort. First, if the imagery is from TerraMetrics (low resolution), stop. I don't know how to determing when that image was taken. Erstwhile, open up the lower left panel in Google Earth (Layers), and expand the (as of this writing) last item (More). By checking the last two items, available images from those two vendors are indicated in the view area. Those from DigitalGlobe can be narrowed down further by date and quality. You can usually assume that the imagery if from DigitalGlobe, unless none of the choices seem to fit. Start with the most recent images of good quality and work your way down.

By comparing the overlayed boundaries of where an image is located with the patchwork boundaries on the underlying image, a quick identification can often be made. If not obvious, the next step is to preview the image in question (at low resolution) and compare with what you see at high resolution. Note that the preview images have not been corrected for skew (perspective of the satellite camera) .

Example: Dating Ghawar Images

Show below (left) is the Google Earth image covering the Ghawar oil field, the world's largest. The field and subfield boundaries are shown below. There are two broad areas of low-resolution coverage: in the north covering the center of Shedgum and a corner of Uthmaniyah, and in the south covering the SE corner of Hawiyah and the northern half of Haradh.

Shown above to the right are the dates corresponding to the various DigitalGlobe images identified as indicated by polygon overlays in the appropriate locations. Note that a huge swath of Ghawar was photographed on a single day (this was not by accident, of course), but that the high-resolution coverage of Haradh (except for a small sliver) dates to 2004.

The area indicated in purple and dating to August 2006 is rather interesting. When the screenshot corresponding to the above-left image was made, the purple area was only covered at low resolution. However, in August 2007, the image set was updated to include the new photograph taken a year earlier. There are only two DigitalGlobe images taken on that date in the vicinity, corresponding to the area in purple and an area immediately below. This latter one had some cloud cover, and so for this or some other reason, the "deciders" at Google Earth have not included it in what is visible.

Well, DigitalGlobe didn't take those images just for fun: somebody paid for it. My guess is a contractor to Saudi Aramco. It costs a few thousand dollars if the image has already been taken, perhaps quite a bit more for a new shoot, but it might prove rather valuable. For me, the interest is due to the fact that those areas cover regions of the Uthmaniyah part of Ghawar that have been rather problematic, with uneven waterflooding, a high degree of fracturing, and the presence of a tar mat near the original oil-water contact (see Simmons pp. 173-175). I will cover this in an upcoming section on recent activity in Uthmaniyah.

Besides that, it also provides a fortuitous opportunity to observe a small area at high resolution on two dates separated by three months and a somewhat larger area by 3 years. In the former, a well being (re)drilled in May 2006 was still being worked on in August. It the latter, I was lucky enough to have captured the "before".

1 comment:

Dating said...

The area indicated in purple and dating to August 2006 is rather interesting.