Perpustakaan Digital ITB

Forward modeling program has been developed to understand the response characteristics of vertical gradient borehole gravity and its Second Vertical Derivative (SVD) anomaly. The program is tested using synthetic data of borehole gravity and ABC field data in United States. The results show that SVD anomaly response could identify the subsurface layer boundaries and its density better than the vertical gradient response of borehole gravity. In the vertical gradient response curve, a layer will be in between a positive maximum point and a negative maximum point. Whereas in the SVD response curve, a layer will be in between two positive maximum points or two negative maximum points. In addition, the vertical gradient response in a layer model with positive density contrast will have a negative anomaly trend to the depth, where top boundary of the layer will have the maximum positive value, and the bottom boundary will have the maximum negative value. This is contrary to the model layer with negative density contrast. In the SVD response curve, a layer model with positive density contrast will have a negative maximum value, and vice versa for a layer model with negative density contrast will have a positive maximum value. Those response characteristics are then used as a guide in determining the layer boundaries. Furthermore, based on borehole gravity anomaly response, it could be estimated the value of reservoir rock porosity. The results of interval density and porosity estimation derived from borehole gravity data at ABC field show the suitability of response with the trend of density and porosity logs data. The average error value generated is around 1.7% for interval density and around 7.7% for interval porosity. Variations on estimation result appear as there is difference on interval distance between measurement data of borehole gravity and density and porosity logs.