Method and apparatus for estimating a fluid driven fracture volume during hydraulic fracturing treatment of a ground formation. A series of tiltmeters are positioned at spaced apart tiltmeter stations at which tilt changes due to the hydraulic fracturing treatment are measurable by those tiltmeters. Tilt measurements obtained from the tiltmeters at progressive times during the fracture treatment are analysed to produce estimates of the fluid driven fracture volume at each of those times as the treatment is in progress. The analysis may be performed sufficiently rapidly to provide real time estimates of the fluid driven fracture volume and may also produce estimates of fracture orientation. The estimates of fracture volume may be compared with the volume of fluid injected to derive an indication of treatment efficiency.