A downhole seismic source (20) capable of generating seismic forces in excess of 1000 newtons. The source produces seismic waves for seismic applications, particularly cross borehole measurements and vertical seismic profiling. Coupled with motion sensing devices, the present invention also performs as a seismic logging tool. The source is comprised of an outer housing (32), a means (60) for clamping the source securely to the wellbore, and a linear electromagnetic actuator (30) which utilizes permanent magnetic material having remanent magnetic field exceeding about .9 T such as rare permanent magnetics. The source has a diameter of about 12.5 cm to fit within a typical wellbore and the actuator has a length of about 1 meter.