This optical soil survey devices comprise two parts, a soil excavation unit, which goes under the surface and excavates it and a sensing unit attached to the rear portion of the excavation u nit, which collects the data. The present invention provides a particular design of an optical soil survey device, whereby the sensing unit is mounted on the excavation unit such that the lower part of the sensing unit penetrates the soil below the lower part of the excavation unit. When using such a survey device the sensing unit penetrates the trench bottom that was opened by the excavation unit, assuring that the lower part of the sensing unit is continuously submerged in the soil. In a second object the invention provides a sensing unit specially designed for penetrating the trench bottom and for preparing the bottom of the obtained trench for optical surveying.