The solution of subsurface geological problems requires an analysis ofvertical and horizontal dimensional relationships. For many, the ability tovisualize structures in three dimensions is not easily acquired, and for areaswhere the structural conditions are complicated by faulting the need arises forillustrative models. This has led to the many forms of peg models, in which theusual course of construction includes a peg for the well bore, and thesuspension of strings from peg to peg to illustrate the formation correlationbetween wells, thus developing the usual folds and flexures. Such models provedunsatisfactory for the analysis of fault problems. Consideration was then givento the use of transparent materials. A model of the Dominguez oil field was constructed in 1935, which consisted ofa series of glass panels from well to well on which the formations were shownin color. This model, though serving its purpose at the time, proved to beimpractical from the standpoints of cost, breakage hazard, and extremedifficulty in maintenance. The cost of materials was estimated to beapproximately $300, and the construction required the services of one man for aperiod of from 10 to 12 months. Maintenance after actual construction was anadditional expense.Consideration was then given to the use of celluloid, andafter some experimental work it was found that the advantages in its use were:a short construction time,ease of handling the material,sectionsare easily reworked,low cost. Construction It would be of little value to recount here the difficulties encountered in thebuilding of the first celluloid model. The conclusions drawn, however, suggestthe following procedure for those who contemplate the construction of such amodel:Determination of the Scale.-Accurate geological interpretation requires theuse of the same scale horizontally and vertically. It should be emphasized thatthe limited size of the celluloid sheets (20 by 48 in. maximum dimensions), thearea to be studied, the maximum depths to be plotted, and area of possiblefuture development are the controlling factors in adopting a scale to beused.Construction of the Base.-A satisfactory base may be constructed by usingtwo pieces of three-ply wood separated by strips of I by 2-in. material. Thistype of construction prevents warping of the base. The size of the base isdetermined by the dimensions of the area to be studied plus 5 in. on all sides.A map of the same scale as that adopted for the model, indicating the lines ofsection to be used, should be fastened to the base. T.P. 1271