Seismic measurements are taken during drilling operations by an array of seismometers placed on the earth's surface about the axis of a drillstring connected to a conventional drillbit and wherein the drillstring has an arrangement of strain gages and accelerometers mounted thereon in such a way as to measure axial and torsional vibrations of the drillstring. Signals indicating vibrations induced by the drillbit or another source and transmitted through the drillstring are compared with signals received by the seismometer array to determine the characteristics of the formation based on measurement of acoustic velocity from direct and reflected transmission of acoustic vibrations through the formation.
A non-contact vibrometer is employed to measure seismic energy imparted to the earth by a land vibrator. The non-contact vibrometer can be a laser vibrometer operable to measure the motion of a baseplate of the land vibrator. A computing device can be used to record both the imparted seismic energy detected by the non-contact vibrometer and reflected seismic energy sensed with geophones.
A drill string in a fluid-filled borehole acts as a waveguide for an extensional wavefield that is generated in the drill string when it is excited by the vibrations of a drill bit secured to the lower end of the drill string. The waveguide acts as a distributed acoustic energy source which initiates head waves in the surrounding formation. The head waves are detected at one or more discrete locations near the surface of the earth. The extensional wavefield is detected by a sensing means mounted at the top of the drill string. The detected head waves are cross-correlated with the detected extensional waves to create a cross-correlogram which may be displayed.
This invention is a method of conducting seismic prospecting operations using a drilling tool (1) acting at the bottom of a well (2), which creates very powerful seismic vibrations, transmitting reference signals representing the vibrations of the tool in real time or later, and correlating the seismic signals picked up by a receiving assembly (12) with the reference signals. The reference signals are furnished by sensors (7) in contact with the drill string and at a distance from the down-hole assembly constituted by tool (1) and associated drill collars (4). In order for these reference signals to be truly representative, a damping means (8, 19) and at least one filtering element (20) designed to attenuate to a great degree the multiple reflections in a frequency range useful for seismic prospecting are interposed into the drill string. The invention has application to the making of seismic recordings.
The method consists in performing seismic prospecting operations by using a drill bit (1) working at the bottom of a well (2), which generates very powerful acoustic jogs, in transmitting in real time or in delayed time reference signals representative of the jogs of the drill bit picked up in the immediate vicinity thereof by bottomhole sensors and in correlating the seismic signals picked up by a reception set with the reference signals coming from the bottom. The reference signals may be transmitted to the surface station through a line (4) included in a drill string (3). Absorbing elements are preferably interposed on the string to damp the vibrations propagating directly towards the surface along the drill string. Resonant elements and possibly acoustic discontinuity elements favouring resonances in the seismic frequency band may also be interposed.
The present invention relates to a method for the acquisition of signals during a well drilling operation, wherein the following stages are carried out: shakes are generated by means of a vibration source, the signals resulting from the shakes are detected by downhole and surface measuring means (23; 24) connected to a control unit (22), a code for synchronizing the signals acquired by the downhole (7) and surface means (23; 24) is emitted, the signals supplied by the downhole measuring means (7) are stored in a storage module (11), the signals contained in storage module (11) are collected at the surface, then processings are performed on the synchronized downhole and surface signals.
