A seismic source is a device that releases energy in the form of an acoustic wave. Basically there are two types of seismic sources, an impulsive seismic source of which the waveform cannot be controlled and a waveform- controllable seismic source. Examples of an impulsive seismic source are an explosive device, a dropping mass or an implosive device, and the signal generated by an impulsive seismic source is a pulse. The waveform of an impulsive seismic source cannot be controlled. A waveform-controllable seismic source is a vibrator that generates a controlled wave train. The seismic sources are used to make a seismic survey of the structure of an underground formation.

Making a seismic survey of the structure of an underground formation comprises activating the seismic sources to generate seismic waves; allowing the seismic waves to travel through the underground formation in which they are reflected by acoustic-impedance contrasts and refracted through members having a variable velocity; and receiving and recording the seismic waves by seismic receivers. The arrival times of the seismic waves are observed to obtain a result that can be interpreted.

It is an object of the present invention to use the information obtained from the seismic survey to determine how to activate a seismic source such that the seismic energy received in a target point in the underground formation has a prescribed shape.

As observed above, there are two kinds of seismic sources, impulsive seismic sources and waveform- controllable seismic sources.. For the impulsive seismic sources it-is an object to concentrate the seismic energy in a target point.

To this end the method of providing a set of seismic sources in a focusing array to enable concentrating seismic energy in a selected target point in an underground formation according to the present invention comprises the steps of: (a) performing a standard seismic survey of the underground formation using a set of impulsive seismic sources i and a set of downhole seismic receivers j, and recording the signals uij (t) ; (b) selecting a position k to be the target point ; (c) determining from the signals ui (t) the one-way travel times Tik from a number of sources i to the target point at the location k; and (d) calculating ti=to-Tik to obtain the activating times ti at which the impulsive seismic sources i have to be activated, in order to get a focusing array of the seismic sources i of which the seismic waves arrive at the target point k in phase at time to.

This method is suitably applied in case the target point coincides with a seismic receiver k. However, in case the target point 1 does not coincide with a seismic receiver k, a correction has to be applied. The correction on the activation time of the sources i is Ati=Tik-Til, wherein Til is the one-way travel time from source i to the target point 1. The corrected activation time for the seismic source i is then tCorri=ti+Ati.

Suitably the one-way travel times are calculated with a velocity model that is obtained from signals uim (t), wherein m refers to a set of seismic receivers, at least one of which is located at surface.

For the waveform-controllable seismic sources it is an object of the present invention to enable concentrating seismic energy in a selected target point in an underground formation, wherein the wave at the target point has a predetermined desired waveform.

To this end the method of providing at least one focused waveform-controllable seismic source to enable concentrating seismic energy in a selected target point in an underground formation according to the present invention comprises the steps of: (a) performing a standard seismic survey of the underground formation using the at least one waveform- controllable source i, and a set of downhole seismic receivers j, and recording the signals uij (t), wherein each source is activated by a signal si (t) ; (b) selecting a downhole seismic receiver k from the set of seismic receivers to be the target point ; (c) taking the direct part udik (t) of the signal uik (t) and making an inverse filter (udik (t))-1 of the direct part of the signal; (d) selecting a desired waveform fk (t-to) to be focused at the location of the downhole seismic receiver k at time t=to ; and (e) calculating the new signal with which the seismic source i is to be activated in accordance with snewi (t) =si (t) 0 (udj (t))"l0fk (t-to) to get a focused source i of which the waves arrive at the downhole seismic receiver k with the desired waveform fk (t) starting at time is to.

This method is applicable in case the target point coincides with a seismic receiver k. However, in case the target point does not coincide with a seismic receiver k, a correction has to be applied. To this end the method of providing at least one focused waveform-controllable seismic source to enable concentrating seismic energy in

a selected target point in an underground formation according to the present invention comprises the steps of: (a) performing a standard seismic survey of the underground formation using the at least one waveform- controllable source i, and a set of downhole seismic receivers j, and recording the signals uij (t), wherein each source is activated by a signal si (t); (b) selecting a position 1 to be the target point, wherein the position 1 is near a downhole seismic receiver k of the set of seismic receivers, and determining from the signals uij (t) the one-way travel times Tik from a number of sources i to the seismic receiver k near the target point at the location li (c) taking the direct part udik (t) of the signal uik (t) and making an inverse filter (udik (t))-1 Of the direct part of the signal ; (d) selecting a desired waveform fk (t-to) to be focused at the location of the downhole seismic receiver k at time t=tp ; and (e) calculating the new signal with which the seismic source i is to be activated in accordance with sneWi (t) =si (t) 0 (udik (t))-lXfk (t-to) to get a focused source i of which the waves arrive at the downhole seismic receiver k with the desired wave form fk (t) starting at time is to, and applying a correction on the signal with which the seismic source i is activated, wherein the correction is given by AtCorri=zik-Til being the estimated one-way travel time from source i to the target point 1, such that snewci (t) =snewi (t+Ati).

Suitably, the one-way travel time to the target location is calculated using a velocity model obtained from signals uim (t), wherein m refers to a set of seismic receivers, at least one of which is located at surface.

In the specification and the claims the symbol 0 denotes convolution.

The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein Figure 1 shows an embodiment of the invention; and Figure 2 shows schematically how the activation times are calculated.

Reference is now made to Figure 1, showing schematically and not to scale a section 1 of the earth having a surface 2. At the surface 2 are located impulsive seismic sources 5, and in a borehole 6 are located downhole seismic receivers 8.

The first step according to the method according to the present invention is performing a standard seismic survey of the underground formation using a set of impulsive seismic sources i and a set of downhole seismic receivers j, and recording the signals uij (t). Thus for each pair i, j there is one seismic signal uij (t). In the embodiment shown the set of impulsive seismic sources i consists of one or more of the impulsive seismic sources 5. For the sake of clarity we have indicated one of the sources with the reference i. The set of downhole seismic receivers j consists of one or more of the seismic receivers 8, and for the sake of clarity we have indicated one of the receivers with the reference j.

The next step is selecting a position that is to be used as a target point. In this case we selected the dot k to represent the position of the target point, which coincides with one of the receivers 8.

The seismic signals uij (t) are used to make a velocity model, and that velocity model is used to calculate the one-way travel times Tik from a number of sources i to the target point at the location k (which coincides with one of the seismic receivers 8). Please

note that making a velocity model and calculating the one-way travel times from it is well known, and will therefore not be discussed in this specification. The velocity model can also be obtained from signals uim (t), wherein m refers to a set of seismic receivers, at least one of which is located at surface.

The one-way travel times Tik are now used to calculate the activation times ti at which the impulsive seismic sources i have to be activated, in order to get a focusing array of the seismic sources i of which the seismic waves arrive at the target point k in phase at time to. The activation times ti are calculated according to the expression ti=to-Tik- Figures 2a and 2b show how the activation times for three impulsive seismic sources arranged at the surface 2 of the formation 1 are calculated. The wave paths from the sources to the receiver are referred to with' reference numerals 10,11 and 12. Reference numeral 15 refers to a fictitious trace that is a combination of the signals pull, u21 and u31. When the impulsive seismic sources i=1, i=2 and i=3 are activated at the same time, the signal recorded with seismic receiver k=1 shows the arrivals 21,22 and 23 of the three seismic waves. We do not show the remainder of the signals consisting of reflections.

The one-way travel times are T11, T21 and T31, respectively, wherein (in this example) x21<T11 <T31. To ensure that the seismic waves arrive at the target point k in phase at time to-signal 25 in Figure 2b- the activation times must be calculated as shown in Figure 2b, wherein t3<t1<t2.

Because the target point k coincides with a seismic receiver, the activation times are so calculated that the seismic waves arrive at the seismic receiver at k in phase at time to. However if the target point k does not

coincide with a seismic receiver the one-way travel times Tik have to be estimated from the velocity model, or a correction has to be applied.

Thus the invention gives a simple method for concentrating the seismic energy of a set of impulsive seismic sources in a target point.

An application of this method is creating a virtual source at the position k, which may coincide with a seismic receiver 8. The waves emitted from the virtual source at the position k can be used to get a seismic image of a formation layer 30, see Figure 1. Moreover the signal from the receiver 8 at the position k will provide a check on the validity of the velocity model used to calculate the one-way travel times. Thus the method of the present invention offers the opportunity to check the validity of the velocity model, and it provides a method for providing an image of the formation layer 30 using a source k that is closer to it then the sources 5.

Assume now that this formation layer 30 is an oil- bearing layer from which oil is produced via a well that is not shown in Figure 1. If now a target point 1 is selected inside the formation layer 30 the concentrated seismic energy can be used to mobilize the oil in the formation layer 30 and thus the oil production is augmented.

The activation times used to ensure that the seismic waves arrive at the location 1 in phase at a time to can be estimated from the velocity model.

However, if we already know the activation times ti that are calculated to activate the impulsive seismic sources such that the seismic waves arrive at the seismic receiver at k in phase at time to, the activation times used to ensure that the seismic waves arrive at the location 1 in phase at time to can be found by a applying the following correction Ati=Tik-Til, wherein Til is the

estimated one-way travel time from source i to the target point 1. Adding the correction to the above equation for the activation times gives tCorri= ti+Ati=to-Tik+Ati= to-xil, which is the activation time that has to be used in order that the seismic waves arrive at the location 1 in phase at time to.

We will now discuss an embodiment of the present invention that is applicable to the second group of seismic sources, the waveform-controllable seismic sources. In this embodiment a mathematical method is used to determine the signal with which the seismic source has to be activated in order that a desired waveform fk (t-to) is focused at the location of the downhole seismic receiver k at time t=to. The method will be discussed with reference to Figure 1, wherein a difference with the above-described method is that the sources are waveform- controllable seismic sources.

The first step of the method is as discussed above, performing a standard seismic survey of the underground formation using the at least one waveform-controllable source i, and a set of downhole seismic'receivers j, and recording the signals uij (t), wherein each source is activated by a signal si (t). Then a downhole seismic receiver k is selected from the set of seismic receivers to be the target point.

The next step is taking the direct part udik (t) of the signal uik (t) and making an inverse filter (udik (t))-1 Of the direct part of the signal.

Then the desired waveform fk (t-to) has to be selected.

The new signal with which the seismic source i is to be activated is calculated in accordance with the expression sneWi (t) =si (t) 0 (udik (t))-1Sfk (t-to). When the seismic source with this input signal, one obtains a focused, source i of which the waves arrive at the

downhole seismic receiver k with the desired wave- form fk (t) starting at time is to.

When more than one seismic source is used, the above described method is applied to calculate for each seismic source i the new input signal sneWi (t).

This method can be applied to create a virtual source at the position k, which may coincide with a seismic receiver 8. The waves emitted from the virtual source at the position k can be used to get a seismic image of a formation layer 30, see Figure 1. Moreover the signal from the receiver 8 at the position k will provide a check on the validity of the velocity model used to calculate the one-way travel times. Thus the method of the present invention offers the opportunity to check the validity of the velocity model, and it provides a method for providing an image of the formation layer 30 using a source k that is closer to it then the sources 5.

The step of taking the direct part of the signal and making an inverse filter of the direct part of the signal can be performed in the time domain or in the frequency domain using Fourier Transforms.

The Fourier fransform of the direct part of the <BR> <BR> <BR> #<BR> signal is Udik(f) = #udik(t).e-2i#ftdt. The inverse<BR> <BR> -00 filter is (udik (t))-1 so selected that its Fourier Transform is 1/Udik (f).

For the sake of completeness, we would observe that ideally a spike is obtained at t=to when the inverse filter is convolved with the delayed direct part of the signal. In equations: the convolution is (udik (t))-l (udlk (t-to)), the Fourier Transform of the delayed direct part udik (t-to) of the signal is (e-i2fto). (Udik (f)), and the Fourier Transform of the convolution is e~i2sft0. The inverse Fourier transform of

e-2i#ft0 is #(t-t0), which is the Dirac delta function at t=to.

The desired waveform at t=t0 can be obtained by convolving the Dirac delta function at t=tp with the desired waveform, in equation: fk (t-to) = 6 (t-t0) 0fk (t).

Given the limited bandwidth of the data, the inverse filter, (udik (t))-1, is suitably stabilized by convolving it with a band limited desired response.

The above-described method is applicable in case the target point coincides with a seismic receiver k.

However, in case the target point does not coincide with a seismic receiver k, a correction has to be applied. And this embodiment of the invention will now be discussed.

The first step of the method wherein the correction is applied is performing a standard seismic survey of the underground formation using the at least one waveform- controllable source i, and a set of downhole seismic receivers j, and recording the signals uij (t), wherein each source is activated by a signal si (t). Then a position 1 is selected to be the target point, wherein the position 1 is near a downhole seismic receiver k of the set of seismic receivers, and determining the one-way travel times Tik from a number of sources i to the seismic receiver k near the target point at the location 1. Subsequently the direct part udik (t) of the signal uik (t) is used to make an inverse filter (udik(t))-1 of it.

A desired waveform fk (t-to) to be focused at the location of the downhole seismic receiver k at time t=tp is selected. The new signal with which the'seismic source i is to be activated is calculated in accordance with the expression sneWi (t) =si (t) 0 (udik(t))-1#fk(t-t0).

The result is a focused source i of which the waves

arrive at the downhole seismic receiver k with the desired waveform fk (t) starting at time is to. However, because the target point 1 does not coincide with the seismic receiver k, a correction on the signal has to be applied. The correction on the signal of the source (s) i is a time shift AtCorri=Tik-Til, wherein Til is the estimated one-way travel time from source i to the target point 1. The corrected new signal becomes snewci (t) =snewi (t+Atcorri).

Suitably, determining the one-way travel time to the target point 1 comprises obtaining from the signals uij (t) a velocity model, and calculating the one- way travel times Til with the velocity model.

This method can for example be applied to concentrate seismic energy in a target point 1 in an oil-bearing formation layer 30 in order to mobilize the oil in the formation layer 30.

The focused set of seismic sources is suitably used to provide high energy at the target point. Applications may include enhancing oil production by increased mobilization in area around the target point; enhancing the signal-to-noise ratio of seismic imaging ; enhancing the signal to noise ratio of seismic-electric effects in which the seismic signals are used to generate detectable electromagnetic responses; and transmission of signals or energy to remote devices.