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A two-soil-moisture-reservoir model is developed to improve the estimate accuracy of g runoff-volume-prediction model. Soil moisture accounting in the two reservoirs is an intermediate step to runoff prediction. A decay-type function describes the moisture depletion between days of rainfall. The moisture depletion constant in the function varies by season with soil moisture, pan evaporation, and mean daily temperature. The runoff-prediction equation relates runoff to rainfall and soil moisture at the beginning of the storm. Computed runoff volumes are compared with values observed on a 3-acre native grass-meadow watershed for an 11-year period. Accumulated computed amounts for the period agree within 1% of the accumulated observed amounts. INTRODUCTION Prediction of surface runoff volumes fromungaged watersheds is a major concern to hydrologists. Several methods and techniques for estimating storm runoff have been published. Among these is a runoff volume prediction method developed by Hartman et al. [1960] for the heavy clay soils in the Blacklands of Texas. The purpose of this paper is to present a refined model of this method, which uses two moisture reservoirs instead of one. Climatic, runoff, and soil moisture data from the Blacklands Experimental Watershed near Riesel, Texas, were used in developing the prediction model. Results of a test of the model are given for g native grass-meadow watershed. PREVIOUS WORK Numerous models have been developed to predict surface runoff [Chow, 1964; Committee for Hydrological Research, 1966]. Schulze [Committee •or Hydrological Research, 1966] started with the simple soil moisture bookkeeping method and worked up to the more sophisticated models. He noted the principal feature Copyright • 1969 by the American Geophysical Union. 84 for each model and pointed out the limitations that necessitated improved models. The present stage of development includes two-level and multicapacity accounting models. The two-level and multicapacity models take into account the complex effects of soils. In these models, the soil mantle is divided into layers, and each layer is represented by a reservoir. Hamon [1964] used two layers: one the active root zone of most rapidly changing moisture and the other a lower zone that contributed some moisture to evapotranspiration. However, most of the multireservoir models consider only one reservoir operative at any one time. That is, the first is completely filled or entirely empty before the second is operative, or vice versa. Yet, under field conditions, unsaturated flow and capillary moisture movement take place, and there can be exchange between reservoirs without requiring saturated flow. Reservoir simulation largely neglected land use. Land use, used herein to mean crop, affects lhe rate of moisture depletion. Hartman [1960], Hartman e• al. [1960], and Richardson [1966] recognized potential differences due to land use and developed moisture-depletion equations based on land use and season. Hartman et al.

There have been several major blowouts in the marine environment due to shallow gas or shallow faults carrying gas to the surface. In addition numerous drilling sites have been abandoned due to sediment failure and lost circulation above 3000 feet subsea level. Millions of dollars have been spent on these problems. Each could have been avoided with prior knowledge of the hazard. Drilling engineers can design their drilling plan to compensate for potential hazards. A potential hazard is anything that will endanger or will cause delays to the drilling operation. These are shallow gas near surface faults hard and soft sediments old river and glacier channels waterbottom anomalies and man-made objects. Loss of life loss of the rig, loss of materials, wasted rig time, adverse publicity and government control can be avoided. The geophysical prediction of potential hazards at depths above 3000 feet is the subject of this paper. Recent case histories are used as illustrations. High resolution engineering geophysical methods can locates potential hazards mud lumps, acoustic voids, mud volcanoes, and gas-charged sediments do not blow rigs out of the water. Usually only digital common-depth-point, multifold, acoustic data delineates hazardous shallow gas. If careful attention is paid to the data collected by high resolution engineering surveys, the risk to offshore drilling operations will be reduced. INTRODUCTION The only way to forecast potential drilling problems besides drilling a well is by geophysical surveying. Conventional CDP exploration seismic surveys are focused below 2000 feet and cannot define the shallow drilling and foundation zones adequately. A combination of engineering geophysical tools are required to "see" drilling hazards. Drilling is vulnerable at shallow depths without the interpretation of shallow potential hazards. The geophysicist sees such potential problems as shallow gas, near surface faults, old river and glacier channels, sediment strength, waterbottom anomalies, and man-made objects. The drilling engineer knows he has low overburden gradient, a low fracture gradient, no blowout preventors no casing, and low mud weight. He has to jack-up successfully the first time, anchor his floater and stabilize the wellhead BOP stack, or set his platform with precise judgment. Drillers are scared of shallow gas and uncontrollable situations. These problems are predictable from High Resolution Geophysical data; shallow gas and uncontrolled situations can be either planned for in the drill plan or simply avoided by a change of location. Lack of adequate site investigations and proper interpretation of geophysical drilling data can lead to drilling failures. Drilling permits have been delayed by Government authorities due to the lack of adequate drilling forecast information. A number of jack-up rigs and floaters have been severely damaged or destroyed when the well blew out or when subsurface sediments gave way. In recent years, we have been finding more and more applications for marine seismic technology that involve high resolution and relatively shallow depths of interest. The main purpose of this paper is to illustrate the capabilities of high frequency sources in this kind of work.

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