Depth imaging with multiples is a prestack depth migration method that uses
multiples as the signal for more accurate boundary mapping and amplitude r
ecovery. The idea is partially related to model-based multiple-suppression
techniques and reverse-time depth migration. Conventional reverse-time migr
ation uses the two-way wave equation for the backward wave propagation of r
ecorded seismic traces and ray tracing or the eikonal equation for the forw
ard traveltime computation (the excitation-time imaging principle). Consequ
ently, reverse-time migration differs little from most other one-way wave e
quation or ray-tracing migration methods which expect only primary reflecti
on events. Because it is almost impossible to attenuate multiples without d
egrading primaries, there has been a compelling need to devise a tool to us
e multiples constructively in data processing rather than attempting to des
troy them. Furthermore, multiples and other nonreflecting wave types can en
hance boundary imaging and amplitude recovery if a full two-way wave equati
on is used for migration. The new approach solves the two-way wave equation
for both forward and backward directions of wave propagation using a finit
e-difference technique. Thus, it handles all types of acoustic waves such a
s reflection (primary and multiples), refraction, diffraction, transmission
, and any combination of these waves.
During the imaging process, all these different types of wavefields collaps
e at the boundaries where they are generated or altered. The process goes t
hrough four main steps. First, a source function (wavelet) marches forward
using the full two-way scalar wave equation from a source location toward a
ll directions. Second, the recorded traces in a shot gather march backward
using the full two-way scalar wave equation from all receiver points in the
gather toward all directions. Third, the two forward- and backward-propaga
ted wavefields are correlated and summed for all time indicts. And fourth,
a Laplacian image reconstruction operator is applied to the correlated imag
e frame.
This technique can be applied to all types of seismic data: surface seismic
, vertical seismic profile (VSP), crosswell seismic, vertical cable seismic
, ocean-bottom cable (OBC) seismic, etc. Because it migrates all wave types
, the input data require no or minimal preprocessing (demultiple should not
be done, but near-surface or acquisition-related problems might need to be
corrected). Hence, it is only a one-step process from the raw field gather
s to a final depth image. External noise in the raw data will not correlate
with the forward wavefield except for some coincidental matching; is usual
ly unnecessary to do signal enhancement processing before the depth imaging
with multiples. The input velocity model could be acquired from various me
thods such as iterative focusing analysis or tomography as in other prestac
k depth migration methods.
The new method has been applied to data sets from a simple multiple-generat
ing model, the Marmousi model, and a real offset VSP. The results show accu
rate imaging of primaries and multiples with overall significant improvemen
ts over conventionally imaged sections.