Methods and related systems are described making seismic measurements. Seismic energy is transmitted into the earth using a surface seismic source. The seismic energy is received with one or more downhole receivers located in a borehole. In response to the received seismic energy from the surface seismic source, seismic energy is transmitted into the earth using a downhole seismic source. The seismic energy from the downhole seismic source is then received with one or more surface receivers located on the surface of the earth. A delay interval is measured downhole between the first arrival of the seismic energy from the surface seismic source and the transmission from the downhole seismic source. A surface interval is measured between the transmitting of seismic energy from the surface seismic source and first arrival of the seismic energy from the downhole seismic source using the one or more surface receivers. Travel time can be calculated by subtracting the delay interval from the surface interval.