New Orleans is often rumored to have above ground graves because of the city's water tables; supposedly dug graves wouldn't survive long in such damp soil and the bodies would float up. This is mostly an urban legend: in fact, it's more to do with the city's traditional French heritage. Here are some photos from the famous Pere Lachaise Cemetery in Paris to show French graveyard tradition:
It's interesting to note that while New Orleans is considered the "Voodoo Capital" of the US, a lot of the magical traditions concerning the dead in Voodoo and hoodoo obviously originate with other cultures: the habit of collecting graveyard dirt for example cannot be easily achieved in above-ground sarcophagi. Scratching or drawing an X into the concrete of the tomb and making a petition seems to be the tradition, instead.