Maybe a clue is that toward the east of the field, we found a tall metal post in the sand, about 3m high with numbers and letters: on one side: AL91-306-VP1291+7 and on the other side: AL91-581-VP1619+6.
Amazing what you find on PPRuNe. My day job is in oil exploration, currently offshore exploration, but I've had plenty of experience with onshore exploration and I know exactly what this metal post is.
I agree with ML's conclusion in Post #4.
It is a "permanent marker" placed by a surveyor to mark the intersection of two seismic lines. In this case AL is either the name of the oil company or of the exploration concession. 91 is the year of the survey and 306 and 581 are the numbers of the two intersecting seismic lines. VP1291 and VP1619 are the numbers of the closest "Vibration Points" on each line and the +6 and +7 refer to the offset distance in metres of each VP from the marker.
It is called a VP instead of a shotpoint (SP), because the seismic crew were using vibrators as an energy source instead of dynamite.
Before my use of the V word get this thread delegated to Jet Blast, I should point out that I am referring to 20 tonne vibrators like these:
http://www0.cgg.com/proserv/EyeD/eyedmaking216.html
Looking at the image on Google Earth, you can see many straight lines, some parallel to each other and some at angles. These are seismic lines and I expect that Lawrence's metal post would be at the intersection of two of the visible lines.